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425

� Corresponding author: Prof. Dr. Ralf Riedel

Petersen Strasse 23, D-64283 Darmstadt

Tel.: �49–6151–16–6347, Fax: �49–6151–16–6346

riedel�materials.tu-darmstadt.de

Scheme 1. The Yajima-Process.20�,21�

425

Journal of the Ceramic Society of Japan 114 [ 6 ] 425–444 (2006)Special Article Review

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Ralf RIEDEL,�Gabriela MERA, Ralf HAUSER and Alexander KLONCZYNSKI�

Institut fur Materialwissenschaft, Technische Universitat Darmstadt

Petersenstrasse 23, D-64287 Darmstadt

�Robert Bosch GmbH, Stuttgart, Germany

This review presents the synthesis, characterization techniques, processing and potential applications of silicon-

based ceramic materials derived from organosilicon polymers. The Si-ceramics are prepared by thermolysis of

molecular precursors. The influence of the initial molecular structure of the precursor on the properties of the

final ceramic material and its applications is discussed. The thermolytic decomposition of suitable Si-based poly-

mers provides materials which are denoted as polymer-derived ceramics �PDCs�. In particular, this procedure is

a promising method for the preparation of ternary and multinary silicon-based ceramics in the system SiCNO.

There is no other synthetic approach known to produce e.g. SiCO or SiCN based ceramics. In the case of PDCs

route, common preceramic polymers are poly�organosilazanes�, poly�organosilylcarbodiimides� and poly�or-

ganosiloxanes�. One basic advantage of the PDC route is that the materials can be easily shaped in form of

fibers, layers or bulk composite materials by applying processing techniques established in the plastic industry.

The PDCs in general exhibit enhanced thermomechanical properties, i.e., temperature stabilities up to approxi-

mately 1500�C. Recent investigations have shown that in some cases the high temperature stability in terms of

decomposition and�or crystallization can be increased even up to 2000�C if the preceramic polymer contains

some amount of boron. The composition and microstructure of the PDC are a result of the molecular structure

of the preceramic polymer. Therefore, the observed differences in the macroscopic properties are also closely

related to the variation of composition and solid state structure of these materials.

�Received March 1, 2006�

Key-words : Si-based Polymers, SiCO, SiCN, SiBCN, SiBCO, Synthesis, Applications, Solid state thermolysis

�SST�

1. Introduction

THE synthesis of ternary and multinary ceramics by ther-molysis of molecular precursors �PDCs route� has gained

substantial interest in the last years. Many examples of ultrahigh temperature stable polymer-derived ceramics have beenpublished1�–10� although the process is a relatively young areaof research. A continuing series of workshops related to thistopic take place regularly since 1998 at the University ofColorado at Boulder �see http:��me-www.colorado.edu� Ã rajr�ultratemp��. Presently, PDCs are advertised as�Polymer-Derived Ceramics Flow out of the Laboratory and into theMarkets�by the company Starfire System Inc.11� which givesan enormous impact to further research and development inthis field.

In the early 1960s, Ainger and Herbert,12� Chantrell andPopper13� reported the production of non-oxide ceramicsstarting from molecular precursors. The first practical trans-formation of polyorganosilicon compounds �polysilazanes,polysiloxane and polycarbosilanes� to ceramic materials wasdeveloped by Verbeek, Winter and Mansmann14�–16� in theearly 1970s, primarily to manufacture small-diameter Si3N4�

SiC ceramic fiber for high-temperature use.

The first synthesis of a SiC ceramic material from polycar-bosilanes precursors was based on the work of Fritz17� and, atabout the same time, the early work of Yajima.18� The YajimaProcess for the synthesis of SiC ceramic materials �developedat the end of 1970s�19� by the thermolysis of polycarbosilanesis shown in Scheme 1.

Until then, significant improvements were made in thedevelopment of novel synthesis routes to preceramic polymerswith controlled microstructure and processing behavior.22�–26�

The silicon-based polymers have proven to be promisingprecursors for the production of technologically importantceramic components such as fibers, coatings, infiltratedporous media or complex-shaped bulk parts. In recent years,many examples of polysilanes,27�–32� polycarbosilanes,25�,33�–38�

426

Scheme 2. Synthesis routes to polycarbosilazanes starting from

chlorosilanes.

426 Silicon-Based Polymer-Derived Ceramics: Synthesis Properties and Applications–A Review


poly�organosilazanes�39�–50� and polysiloxanes51�–54� as pre-ceramic polymers have been reported.

In order to be competitive with traditional ceramics, mole-cular-derived ceramics have to be either cheap or their synthe-sis has to be selective to give the desired product with novelcomposition and exceptional properties. In past few years,many efforts have been made not only for the synthesisof classical binary ceramics such as silicon nitride Si3N4 orsilicon carbide SiC but also to produce multinary componentceramics. The quaternary Si-B-C-N materials produced frompolyborosilazanes at the beginning have become highly in-teresting due to their exceptional high temperature and oxida-tion stability.

The structural basis of these compounds is provided bypolymeric or cyclic silazanes that are cross-linked via C–B–Cbridges8�,55�–60� or N–B–N units.61�–67� Another route for thesynthesis of Si-B-C-N ceramics is by means of thermolysisof borazine-based oligosilazanes2�,6�,68�–73� or boron-modifiedpolysilylcarbodiimides.74�–76� Thus, a great variety of pre-ceramic organosilicon polymers have been developed thatcould be used to produce ceramic materials with a wide rangeof compositions in the system Si-B-C-N-O and additionalmetallic constituents, such as Ti, Al and Zr.77�,78� In order tounderstand the materials properties of PDCs at a fundamentallevel, several modelling and computational studies have beenreported,79�–85� not at least to derive a meaningful and under-standable interpretation of the experimental data.

The organosilicon polymers are materials which can beprocessed or shaped using conventional polymer forming tech-niques such as polymer infiltration pyrolysis �PIP�, injectionmolding, coating from solvent, extrusion, or resin transfermolding �RTM�. Once formed, objects made from thesepreceramic polymers can then be converted to ceramic compo-nents by heating to temperatures high enough to consolidatethe elements contained in the polymer structure.

2. Synthesis of preceramic Si-based polymers

2.1 Molecular precursors for the synthesis of ternary

ceramics

2.1.1 SiCO molecular precursors

Polysiloxanes are versatile materials, many having excellentchemical, physical, and electrical properties. Polysiloxanes areusually inexpansive and commercially available. Since a varie-ty of recent review articles has already reported on the featuresof polysiloxanes such as in references,86�,87� their synthesis isnot covered in our present review. A class of presently interest-ing polysiloxanes are the silicon-rich polymers �poly-silaethers�.88� They combine the properties of the polysilanesand polysiloxanes in a hybrid form. There are two generalroutes to form polysiloxanes: the polycondensation of a,v-functionalized linear silanes89�–91� and the ring opening poly-merisation �ROP� of cyclic silaethers.92�,93�

2.1.2 SiCN molecular precursors

Chloro-organosilicon compounds are important startingmaterials for the synthesis of polysilanes, polycarbosilanes,poly�organosilazanes�, polyborosilazanes, polysilylcarbo-diimides, polysilsesquioxanes, polycarbosiloxanes and othersilyl-containing polymers.20�,40�,78�,94�–109�

Several polymers derived from vinylsilanes,98� disilyla-ryls,110� allylsilanes and cyclocarbosilanes111� can be used asprecursors for SiC-based ceramics.

The first publication on polyorganosilazanes synthesized bymeans of ammonolysis of organosilicon chlorides appeared in1964 by Kr äuger and Rochow.112�

Synthesis of SiCN-precursors can be made starting from

chlorosilanes by means of ammonolysis reactions withammonia or aminolysis with different amines, as shown inScheme 2. The main disadvantage of these routes is thedifficult separation of the polymeric reaction product fromthe solid byproducts NH4Cl or H3NRCl.

These types of reactions normally yield a mixture ofoligomers and low molecular weight polymers which caneasily volatilize and depolymerize giving low ceramic yields.Therefore, cross-linking must be performed prior to pyroly-sis.105�,113�–115�

The attachment of various organic groups to the siliconatoms allows adjustment of their physicochemical propertiesto control the thermolysis chemistry and also to influence theresulting materials properties. The nature of the substituentsdetermines the potential cross-linking chemistry. Trichlorosi-lanes yield highly branched poly�silsesquiazane�s.116�–118� Theproperties of the resulting polymers and that of the finalceramics can also be influenced by the organic substituentsattached to nitrogen in the case the polycondensation isperformed by aminolysis with different alkylamines. Illustra-tive examples are the very recently published results on am-monolysis and aminolysis of dichlorosilanes,119� as presentedin Scheme 2.

Because the thermal polymer to ceramic conversion involvesa series of individual reaction steps that have to be consideredcarefully,120�–123� �see also Chapter 3 of this review�, the inten-tion was to design precursors in a way that hydrogen alone isthe only volatile thermolysis product �Scheme 3�. After sub-sequent cross-linking of the low viscosity polymers using n-BuLi, the ceramization leads to SiC�Si3N4 ceramics without�free�carbon.119�,124�,125�

Besides poly�organosilazanes�, polysilylcarbodiimides areintensively investigated as precursors for SiCN ceramics.

The dimeric bis�trialkylsilyl�carbodiimides and H3Si-N�

C�N-SiH3 have been known since 1960s.126�–131� Since then, agreat variety of monomeric and polymeric silylcarbodiimideshave been prepared and characterized.

It has been found that organosilylcarbodiimides can beapplied, for example, as stabilizer for polyurethane and poly�vinylchloride�, for high-temperature and radiation-resistantdyes and sealing materials, as well as for the synthesis oforganic cyanamides, carbodiimides, and heterocycles.132�,133�

The early work on the synthesis of silylcarbodiimides wassummarized in a comprehensive review article published byGordetsov et al. in 1982.134�

Drake et al.,135� Reischmann et al.,136� and several otherauthors134� have found that bis�trimethylsily�carbodiimide�R3Si-N�C�N-SiR3, with R�CH3� itself is an efficientstarting material for the synthesis of other element carbo-diimides. Thus, organoelement halides as well as pure element

427

Scheme 3. Synthesis of Si-based ceramics free of excess carbon.

Scheme 4. Carbodiiminolysis of chlorosilanes with bis�trimethyl-

silyl�carbodiimide.

Fig. 1. Comparison of the non-oxidic sol–gel process �left� with the

aqueous counterpart �right� and the final products obtained after

pyrolysis of the dried gels.

427Ralf RIEDEL et al. Journal of the Ceramic Society of Japan 114 [ 6 ] 2006


halides can be reacted with bis�trimethylsilyl�carbodiimide toform novel carbodiimides as shown in Scheme 4.134�,136�–143�

The trimethylchlorosilanes byproduct can be eliminated fromthe reaction mixture by means of distillation.

Polysilylcarbodiimides containing alternating Si-N�C�N-units were first synthesized by Pump and Rochow144� bymetathesis reactions of dichlorosilanes and disilylcyanamide.Three years later, Klebe and Murray reported the synthesisof several polysilylcarbodiimides by means of the reactionbetween chlorosilanes and bis�trimethylsilyl�carbodiimide.145�

The applicability of polysilylcarbodiimides as precursors forSiCN ceramics was discovered in our group137�,146�,147� twentyyears after the patent of Klebe and Murray.

In addition to the well-known binary phases SiC and Si3N4,silicon dicarbodiimide with the stoichiometry SiC2N4 nowrepresents the first crystalline ternary Si-C-N phase.137�,148�

These findings clearly indicate the high potential of elementcarbodiimides for the synthesis of advanced non-oxide materi-als. The synthesis of polysilylcarbodiimides starting fromchlorosilanes and bis�trimethylsilyl�carbodiimide in the pre-sence of a catalytical amount of pyridine occurs similarly tothe aqueous reactions of alkoxysilanes by means of sol–gelprocess �Fig. 1�.

In the non-oxidic sol–gel process, bis�trimethylsilyl�car-bodiimide adopts the role of H2O applied in the conventionaloxidic sol–gel route as indicated in Fig. 1. The first reactionstep involves the substitution of the silicon-bonded chlorineatoms by hydroxyl groups in the oxidic sol–gel path whilechlorine is replaced by the silylcarbodiimide unit in the non-

oxidic counterpart. Subsequent polycondensation results inthe formation of polymeric gels. Finally, calcination and�orpyrolysis of the gels up to 1000–1200C in Ar or N2 yield sili-con oxycarbide �SixOyCz� glasses149� and silicon carbonitride�SixCyNz� ceramics, respectively.

The carbodiimidolysis reaction has important advantagesover aminolysis or ammonolysis because it is a one-step salt-free reaction under inert atmosphere and catalyzed by pyri-dine, it uses inexpensive educts and has quantitative yields.148�

Depending on the chlorosilane used, cyclic monomers as wellas linear or highly branched polymers can be obtained.

2.2 Molecular precursors for quaternary ceramics

2.2.1 SiBCN precursors

The introduction of reactive substituents to silicon in poly�organosilazanes� and polysilylcarbodiimides such as hydro-gen and vinyl groups enables the modification of the polymerswith boron or aluminium for the production of quaternarySi-E-C-N ceramics �E � B, Al�. It was found that the inser-tion of boron or aluminium at the molecular level generallyimproves the high temperature stability, oxidation resistanceand high-temperature mechanical properties of the PDC.5�

The first example of a molecular precursor for SiBCNceramics was reported by Takamizawa et al. in 1985.150�–152�

Since then, several reaction pathways have been described inthe literature which lead to polymers containing Si, B, Cand N. The most important types of precursors which canform Si-B-C-N ceramics are borazines, polyborosilazanes andpolyborosilylcarbodiimides. The first borazine-based SiBCNpolymers were published by N äoth153� in 1961 but no investiga-tions of these polymers related to their use as ceramic precur-sor were reported. The first polymeric SiBCN precursor,obtained by a dehydrocoupling of borane dimethylsulfideBH3SMe2 and cyclotri�methylsilazane� �SiMeH-NH�3 wasreported by Seyferth in 1990.2�,68�

In 1993, Sneddon et al.3�,69�,70�,72�,73� reported and investiga-ted in detail the syntheses of SiBCN polymers in which bora-zine units are directly bonded to poly�organosilazanes� aspresented in Scheme 5.

By means of dehydrocoupling reactions, the borazine ringhas been reacted with oligomeric or polymeric silazanes toform poly�organosilazanes� with pendant borazine units.Alternative synthetic approaches to SiBCN polymers startingfrom functionalized borazines were published by Srivastava6�

and Haberecht10�,154� as well.Polyborosilazanes are the second class of precursors capa-

ble to form SiBCN ceramics. Their synthesis was first reportedby Jansen et al.63�–66�,155� in 1992 in a two-step reaction startingfrom hexamethyldisilazane, SiCl4 and BCl3 as shown in

428

Scheme 5. Examples of boron-substituted poly�organosilazanes�

starting from borazine-based precursors.

Scheme 6. Synthesis of polyborosilazanes starting from hexa-

methyldisilazane.

Scheme 7. The synthesis of C-B-C bridged polyborocarbosilazanes.

Scheme 8. Dehydrocoupling reactions of hydrosilanes with ammo-

nia and amines to synthesize SiBCN molecular precursors.

Scheme 9. Sol–gel synthesis of polyborosilylcarbodiimides.



Scheme 6.The resulting polymer is obtained in 80� yield and has a

structure in which Si3�NCH3�3 six-membered ring systems areconnected via HN-B and N�CH3�B units.63�–66�

The thermolysis of the polymer gave a black amorphousSiBN3C ceramic in 50� yield and no crystalline phase wasobserved below annealing at 1900C.67�

Two years later, Riedel et al.5�,56�,156�,157� and Kienzle55�

reported the synthesis of polyborosilazanes by ammonolysisof tris�dichloromethylsilylethylene�borane, �B�C2H4Si�CH3�

Cl2�3 �C2H4�CHCH3, CH2CH2��. The C-B-C bridged poly-borocarbosilazane obtained in 85� yield produced a Si3BC4N2

ceramic in 50� yield after pyrolysis. The SiBCN ceramichas an excellent thermal stability and the crystallization isretarded to �1750C, the degradation not being observedbelow 1950C �Scheme 7�.

More highly branched nitrogen-rich polymers can beobtained performing hydroboration of �H2C�CH�Si�CH3�

Cl2 with H2BClSMe2 or with HBCl2SMe2, followed byammonolysis.158�–160� Thus, the ceramics yields increase to56� and 76�, respectively, and the B Si ratio to 1 2 and 1 1,respectively. In the recent years Weinmann et al. reported

other derivatives of these polyborocarbosilazanes having addi-tional cross-linking motifs instead of the chemically inert Si-bonded methyl group, replacing the methyl group with chlo-rine and hydrogen.57�,59�,161�,162� As a result, the ceramic yieldsincrease to 82� and 85�. In order to avoid the difficultpurification of the polymer from the NH4Cl salt, the hydrobo-ration at the vinyl substituent was performed after ammonoly-sis of the initial chlorosilanes.

In another work focused to eliminate long and difficultfiltration processes for the separation of solid by-productssuch as NH4Cl �in the case of ammonolysis of chlorosilanes�,a novel synthetic approach for the SiBCN-precursors has beendeveloped.58�,163� In the first step, chlorosilanes are treatedwith LiAlH4 to form the corresponding organosilanes. Thesecond step is an nBuLi catalysed dehydrocoupling reaction.Polyborocarbosilazanes can be obtained by this new syntheticpathway as shown in the example given in Scheme 8.

Many examples of polysilazanes and polycarbosilazanessynthesized by means of dehydrocoupling reaction of hydrosi-lanes have been reported in recent years,58�,164�,165� showingthe importance of hydrosilanes as suitable starting materialsin these processes. Polyborosilylcarbodiimides are anotherimportant class of precursors for SiBCN ceramics. These poly-mers can be successfully synthesized by the reaction of tris�chlorosilyl-ethylene�boranes B�C2H4SiRCl2�3 with an excessof bis�trimethylsilyl�carbodiimide �Scheme 9�:74�,75�,166�

Unfortunately, the ceramics obtained from these polymersdo not possess satisfactory ultra high-temperature properties,since the thermal degradation already starts at T�1500C.Preliminary studies suggested that the degradation tempera-ture strongly depends on the nitrogen content of thesamples.163� In order to reduce the amount of nitrogen, a de-hydrocoupling reaction of tris�hydridosilyl-ethylene�boraneswith cyanamide, H2N-CN was developed.167�,168� The obtained

429

Scheme 10. Synthesis of polyborosilylcarbodiimides precursors by

dehydrocoupling reactions.

Scheme 11. Hydroboration reactions of borazine with vinylsilanes

and trivinylcyclotrisilazane for the synthesis of SiBCN precursors.

Scheme 12. Synthesis of polyboro-organosiloxanes by means of

hydroboration reaction of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcy-

clotetrasiloxane with borane dimethylsulphide.

Scheme 13. Synthesis of molecular precursors suitable to form

SiAlCO ceramics upon pyrolysis.



ceramics do not decompose below 1900C �Scheme 10�.Further synthesis routes for SiBCN ceramics were reported

by Kim et al. �Scheme 11�.In the first reaction shown in Scheme 11, a novel preceramic

SiBCN polymer was successfully prepared by hydroborationof dimethyldivinylsilane with borazine, resulting in the for-mation of a processible colorless liquid polymer with a syn-thetic yield�85 wt�.169� The polymer showed a relatively lowceramic yield below 45 wt�, compared to that of other pre-ceramic polymers which is in the range between 60 and 80wt�. This behaviour was attributed to thermally weak B–Cbridges formed by the hydroboration without an alternativecrosslinkable route.

In the second reaction �see Scheme 11�, a soluble gel-likepreceramic SiBCN polymer with a high synthetic yield of 95wt� was synthesized by hydroboration of cyclotrisilazanecontaining vinyl groups with borazine in thf solvent withoutcatalyst and without formation of byproducts.9� The subse-quent transformation of the polymer to the ceramic phasestarting at 400C was accompanied by thermal decompositionof the organic groups with a major weight loss up to 700C,finally resulting in 75 wt� ceramic yield at 1000C. Moreover,polymer-derived SiCBN films derived therefrom retained anamorphous ceramic phase up to 1400C, and had extremelyhigh oxidation resistance with no weight change at elevatedtemperatures. A polycrystalline composite comprised of SiC,Si3N4 and BN phases was formed at 1800C. In addition, itwas found that the polymer could be easily processed to formhigh quality SiCBN films by spin-coating. Such performancecharacteristics make these ceramic films excellent potentialcandidates for composite materials and for high-temperatureprotective coating applications.

2.2.2 SiECO molecular precursors

Similar to the findings in the field of SiCN-derived cera-mics, the insertion of aluminium or boron in polysiloxane isexpected to furnish a higher temperature and oxidation

resistance of SiCO ceramics derived therefrom. There are twomain methods for the synthesis of Al or B-containing poly-siloxanes. The sol–gel process is an attractive synthetic ap-proach to produce silicon oxycarbide glasses, as revealed by anumber of studies published in this field.170�–172� The sol–geltechnique can be applied for the synthesis of SiAlCO-precur-sors as well.173�,174� An alternative method for the synthesis ofSiBCO-precursors by the hydroboration reaction of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane with boranedimethylsulphide was developed in our group175� and ispresented in Scheme 12.

Modification of the molecular precursors was achieved bythe reaction of triethoxysilane and methlydiethoxysilane withaluminium or boron containing alkoxides forming a sol–gelprocess in the presence of a solvent. This method allows incor-poration of Al or B at a molecular level.175�

For the synthesis of SiAlOC ceramics we developed thesol–gel reaction of a commercial polysiloxane containingalkoxy or hydroxy substituents with alumatrane as presentedin Scheme 13.172�



Silicon oxycarbide glasses can be synthesized as presentedabove from modified silica gels through a pyrolysis process ininert atmosphere. A critical analysis of the literature suggests aclose relationship between oxygen content in the gel precursorsand the amount of carbon atoms covalently bonded to siliconatoms in the corresponding silicon oxycarbide phase.

3. Applications of Si-based polymers

Organosilicon polymer materials take a universal place inour daily life due to their specific properties such as lowweight, mechanical strength and processability.176� In additionto their widespread use as ceramic precursor materials, theseinorganic polymers have now demonstrated wide utility inmany other applications.

Organosilicon polymers can be considered as hybrid materi-als and can be systematically modified in terms of physical�optical, magnetic and electric� and chemical �catalytic andselective separation� properties mainly by the variation of theorganic part.

3.1 Polysiloxanes

Silicones are odorless, colorless, water resistant, chemicalresistant, oxidation resistant, stable at high temperatures, anddo not conduct electricity. They have many uses, such aslubricants, adhesives, sealants, gaskets, breast implants, pres-sure compensating diaphragms for drip irrigation emitters anddishware.177� Due to their thermal stability and relatively highmelting and boiling points, silicones are often used whereorganic polymers are not applicable. Their unreactivity gene-rally makes them non-toxic. Simethicone, a silicone-basedanti-foaming agent, has remained available as an over-the-counter substance and food additive.

Millions of modern products rely-in some way-on siliconesfor performance and reliability.178�

For example, silicones give modern personal care productsessential qualities which we take for granted today. But theyare also found in thousands of industrial applications. Inaerospace applications, for instance, silicone products in-crease the lifespan of vital components, while in railwaylocomotives they provide tough, long-lasting motor insulationand lubricants for bearings.

Silicones are used as coatings to protect facades and histori-cal monuments and are also used for window and bathroomseals. Silicones are the basis for coolants in transformers, pro-tective encapsulating material for semiconductors in com-puters and foam-control agents in laundry detergents.

The electronics and telecommunications industries need sili-cones to produce optical fibers and silicon wafers and chips.Some of their many other uses include adhesion promoters inglues, sealants, pigments, paints, textiles and wire and cables,and as strengthening agents to reinforce rubber.

Polysiloxanes have attractive technological properties suchas low surface tension, low glass transition temperature, theyare liquid even with high molecular weight, unusually lowhydrophobicity, etc.. It is expected that continuous develop-ment of this polysiloxane family of materials will further growand find new applications179� beyond the present claims as syn-thetic fabric,180� adhesive foam,181� high oxygen permeablecontact lens,182� waterproof membranes,183� process aids,184�

self lubricants185� and improved toughness.186�

Linear polysiloxanes have many attributes that make themattractive for lithographic applications, such as oxidative andthermal stability, good adhesion properties, solubility in com-mon solvents and resistance to etching in oxygen plasma.187�

3.2 Polysilanes and polycarbosilanes

Polysilanes are a class of materials with a one-dimensional

Si backbone and organic substituted side chains which havebeen subjected to a large number of investigations, partiallybecause of their intriguing optoelectronic properties thatoriginate from so called s-conjugation. An extensive reviewcovering the literature up to 1988 on the synthesis and proper-ties of polysilanes was given by Miller and Michl.30�

Peralkyloligosilanes have attracted interest because theyexhibit potential applications in photoconducting and charge-transporting materials.30� They show unique electronic, physi-cal and chemical properties which distinguish these com-pounds from saturated catenates of carbon.188� In particular,cyclic oligosilanes resemble in their behavior aromatic hydro-carbons, in that they have electronic transitions in the nearUV-VIS region, form anion radicals upon reduction andcation radicals upon oxidation, in which the odd electron isdistributed over the ring, form charge-transfer complexes withp-acceptors, and exhibit substituent effects.

There is a growing interest in the chemistry of organosiliconpolymers, like polysilanes and polycarbosilanes, since they areincreasingly used as functional materials, e.g. as photoresists,semiconductors, hole-transporting materials, and they arevaluable as precursors for silicon carbide.189�,190�

A major factor in the growth of interest in polycarbosilanesin recent years has been the potential for their use asprecursors to silicon carbide, mainly as sources of continuousceramic fibre. This has led directly or indirectly to the synthe-sis of many new polycarbosilanes. However, polycarbosilaneshave also become a focus of research efforts in recent yearsthat have little or nothing to do with the prospect of develo-ping ceramic precursors.120�,191�–194� These efforts appear to bestimulated by a more general interest in polycarbosilanes as anovel class of polymers that have potential for use in a muchwider range of applications, as well as from a more fundamen-tal perspective. In particular, it appears that chemists havebegun to look at carbosilane chemistry for answers to im-portant fundamental questions regarding the structures andproperties of compounds and polymers that contain both car-bon and silicon in their backbone structures.

Unsaturated polycarbosilanes

Unsaturated polycarbosilanes are used in modern electronicapplications. Conductive polymers have been developed formany uses, such as in corrosion inhibitors,195� compact capaci-tors,196�,197� antistatic coatings,195� electromagnetic shielding ofcomputers,198� piezoelectric sensors196�,197� and in smart win-dows199� that can change colours and transparency. Further-more, research is being done on the more sophisticated appli-cations of conductive polymers in transistors,30� light-emittingdiodes,200� lasers,201� solar cells202� and flat television screens.Low-cost and flexibility are some of the reasons why conduc-tive polymers are attractive.

These materials have applications such as deep-UV photo-resists or precursors for silicon carbide polymers and are beinginvestigated because of their semiconducting and non-linearoptical properties. The electronic structures of these copoly-mers have been theoretically studied on the basis of the one-dimensional tight-binding self-consistent field crystal orbitalmethod by Tanaka and co-workers.203�–205� Because themechanism for electronic delocalization is based on s�pmixedconjugation, these systems can be classified as a third type ofconjugation involving both p conjugation �of polyethylene�and the s conjugation of polysilane.

Polymers of this type serve as an important bridge betweenthe organic and inorganic materials areas. From the viewpointof properties, they span the gap between classical biologicaland petrochemical polymers on the one hand, and mineralogi-



cal materials and synthesized ceramics on the other.Unsaturated polycarbosilanes compounds are ideal systems,

in order to receive organic semiconductors or metal-like con-ductive organic compounds, which, at the same time, possessthe typical opto-electronic characteristics of polysilanes �pho-toresists for microlithography�. The nonconjugated chainallows increased solubility in common solvents and improve-ment of the stability in the doped form.

The application of unsaturated polycarbosilanes as precur-sors for conductive films has been reported recently.206� Afterdoping with iodine, the conductivity values have been similarto that of conjugated organic polymers. These results206� indi-cate that, inspite of nonconjugated structures, the respectiveproducts display an accentuated s–p delocalization. In addi-tion, after oxidative doping the resulted oligomers are conduc-tive. This fact reveals that the electronic conductivity ofoligomers is not limited only to conjugated systems. Theseresults strongly support the conclusion that�conjugation isnot a pre-requisite for a polymer to be conductive.�207�

3.3 Poly�organosilazanes�

Poly�organosilazanes� are the answer for a broad range ofchallenging applications that cannot be satisfied by conven-tional materials. The polymers enable long-term, cost-effec-tive and�user friendly�solutions in situations where high tem-perature stability, corrosion resistance, or long term durabilityare critical factors. Poly�organosilazanes� are extremely ver-satile materials. They are quite useful as coatings, infiltrantsfor porous bodies, or as components of molding composi-tions.208�

Poly�organosilazanes� can be used in protective coatingformulations, in ceramic matrix and metal matrix composites,as a component in certain organic�inorganic�hybrid�poly-mers, and in a wide range of other applications, such as clearcoats. These polymers adhere tenaciously to a spectrum ofsubstrates, including metals, composites, graphite and glass.

The poly�organosilazanes� have now demonstrated wideutility in the clear-coat market segment, where they offer acombination of oxidation and corrosion resistance along withUV stability and high hardness. They can be used in a numberof applications such as clear, protective coatings for both fer-rous and non-ferrous metals and alloys, clear automotivefinishes, thermally durable clear coats, tarnish-resistant clearcoats for electrical and plumbing fixtures, industrial, infras-tructure, and marine coatings, anti-graffiti coatings for buil-ding facades and highway signage.

Polysilazane-based coatings can be used for metal corrosionprotection for salt water and other salt environment condi-tions such as those encountered in marine or automobile com-ponent use; for corrosive environments in industrial applica-tions such as in pump and engine components, pipelines, andtanks; for aerospace applications such as structural compo-sites and radome, for electrical insulation such as on wiring,for waterproofing of surfaces such as fabric and buildingmaterial, and for mechanical protection of optical surfaces,wear surfaces, indoor flooring, etc. Additionally, polysilazanecoatings may be used as, for example, floor waxes, emulsionor latex paints with increased temperature resistance withoutreducing transparency �e.g., baseball bats, fence posts, tim-bers, fence rails, decking�. Furthermore, when the coatingproperties are tailored to include a non-stick attribute, usesmay include, for example, non-stick cooking utensils �e.g.,frying pans, pots, spatulas� or mold release coatings.

Molded objects prepared from poly�organosilazanes� areuseful in sporting goods applications such as golf clubs, tennisracquets, skate wheels, watercraft bodies, housing, and

propellers, snowmobile bodies, sail boards, automotive appli-cations such as fenders, hoods, and body panels, aerospaceapplications such as structural composites; industrial usessuch as wear parts in mining, coal, or ore handling such aspump and chute liners.

Poly�organosilazanes� may also be used as surface modifi-ers for compatibilizing inorganic� organic interfaces in com-posite materials, or as binders for polymer, mineral, ceramicor metal filler for fabricating either monoliths or compositematerials.

Poly�organosilazanes�may also be used as adhesives, or forthe feedstock in spinning fibers. They are also ideally suitedfor use as, for example, clear or colored transparent or trans-lucent bodies including, for example, hard contact lenses,automotive lenses �e.g., headlights, taillights�, safety and�orsecurity glazing, skylights, illuminated signs, optical fibers,optical fiber coatings, windshields �e.g., automotive, con-struction equipment, motorcycles�, guards �e.g., industrialmachining equipment, commercial appliances, consumerappliances�, mirrorized sheets, double extrusion panels �e.g.,solar energy applications�.

Additionally, poly�organosilazanes� may be applied ascements �e.g., glues, contact adhesives� possessing propertiesfor combining chemically and�or microstructurally and�orstructurally similar, or dissimilar materials including, forexample, metals, minerals, ceramics �e.g., dental adhesives,ceramic paper�, plastics or polymers, natural materials �e.g.,to form plywood, particle board�, metal matrix composites,ceramic matrix composites, plastic or polymer matrix compo-sites and combinations thereof.

Moreover, poly�organosilazanes� can be used to producenetwork polymers that form, for example, gels having goodoxygen permeability uses may include, for example, soft con-tact lenses, materials for gel chromatography and membranes.Moreover, poly�organosilazanes� may be used in, for exam-ple, rigid pipes or fittings for the construction industry, ther-moformed liners �e.g., for refrigerator doors�, small boathulls, telephone machine housings, business machine housings�e.g., typewriters, facsimile machines, printers, monitors,computers�.

Poly�organosilazanes� can also be used in photographicfilms, transparency sheets, blister packaging, outdoor signs,metallized decorative parts, film packaging for the foodindustry �e.g., boil-in or bake-in bags or pouches�, etc. Poly�organosilazanes� can also be used as slides, guides or geartrains in mechanical or chemical processing equipment, bear-ings, valves, impellors, propellers, housings �e.g., for portableappliances including circular saws, power drills, sanders, mitersaws and the like.

Poly�organosilazanes� may also be used for film packagingfor food �e.g., dairy products, meat products�, as for exam-ple, boil-in or bake-in bags or pouches. Other uses includepaperlike sheet as electrical insulation �e.g., for transformers,electrical motors, generators, alternators�. Furthermore, poly�organosilazanes� may be applied as protective fabric orclothing �e.g., gloves, jackets, leggings, aprons, and headgear�, conveyor belts, textile fibers as tire cords, ropes, cables,coating fabric for inflatable structures. Wire or cable insula-tion and�or coatings, solder resistant printed circuit boards,encapsulation or potting compounds for integrated circuits,filters, temperature and flame resistant fabrics are also possi-ble.

4. Polymer-to-ceramic transformation

The objective of the precursor route is the design of

432

Fig. 2. Routes for the transformation of precursors into ceramic

materials.Scheme 14. PDCs route for the synthesis of amorphous SiCN and

SiCO ceramics.

Fig. 3. Composition regimes �shaded area� for SiCO and SiCN

polymer-derived ceramics.220� The isothermal sections are valid at T�

1300�C for SiCO and at T�1450�C for SiCN.



advanced ceramics based on molecular units. In principle wedistinguish between three main methods for the transforma-tion of molecular precursors to ceramics materials �Fig. 2�:

�solid state thermolysis �SST��chemical vapor deposition �CVD�

�chemical liquid deposition �CLD�

The transformation of molecular precursors into the cor-responding ceramic materials via solid state thermolysis �SST�can be applied for all starting materials independently fromtheir physical properties.39�,78�,189�,194�,209�–210�,211� If the precur-sors are adequately volatile, they can be deposited via vaporphase �CVD� to give super hard materials like for examplec-BN.212� The deposition of precursors from the liquid statecan be made via chemical liquid deposition �CLD� usedpredominantly for the synthesis of inorganic materials in bio-logical systems.213�–218�

By means of these three methods, high-temperatureresistant ceramics can be obtained. The in-situ crystallizationof the initially amorphous phases allows the synthesis ofnanocrystalline materials by a powder-free process. Theproperties �structure and composition of the grain bounda-ries� of the obtained nanocrystalls depend on the molecularstructure of the initially used precursors. Therefore, theprecursor route can be also applied for the architecture ofgrain boundaries.

The solid state thermolysis �SST� is a suitable processto synthesize oxide and nonoxide-based ceramic materials.The present review highlights the synthesis of silicon basedceramics via the SST method starting from poly�organosila-zanes�, polysilylcarbodiimides and polysiloxanes. The ob-tained ceramics are based on the SiCN and SiCO systems aspresented in Scheme 14.

Until now, there are no reported examples of SiCN ceramicsproduced by the thermolysis of Si-rich poly�organosilazanes�and polysilylcarbodiimides as shown in Scheme 14 on the righthand side. Although the synthesis of the disilane-containingpolysilazane with R1, R2, R3, R4�methyl has been publishedsince 1998,219� no thermal investigations have been done.

The disilane-containing polycarbodiimide–�SiMe2-SiMe2-NCN�n- was synthesized for the first time by Razuvaev etal.132� Similar to the case of the polydisilazane, the ceramiza-tion of the polymer was not studied so far. By increasing thenumber of silicon atoms in the monomeric units, an increase

in the temperature stability and in the ceramic yield is expec-ted. For that reason, silicon-rich poly�organosilazanes� andpolysilylcarbodiimides are interesting potential molecularprecursors suitable for the formation of SiCN ceramics.

The composition regimes where SiCN, SiCO and SiBCNceramics were successfully synthesized are shown in theisothermal sections of the phase diagrams represented inFigs. 3 and 4.

The SiCN and SiCO ceramics apparently belong to the sameclass of materials denoted as polymer derived ceramics orPDCs. They are both distinguished from their binary con-stituents �that is, by mixtures of SiC, Si3N4 and SiO2� by thepresence of excess, or�free�carbon, as pointed out in Fig. 3�albeit there are on-going attempts to create PDCs that liealong the tie-lines connecting SiC to SiO2 or to Si3N4�.

In the case of the SiCN composition diagram, stablecrystalline phases up to 1450C �0.1 MPa N2� are silicon car-bide and silicon nitride, exclusively. In general, the composi-tions of silicon carbonitride ceramics SixCyNz obtained frommolecular precursors are located either on the tie line Si3N4-Cif obtained from nitrogen-rich polymers or in the three-phasefield SiC�Si3N4�C. At temperatures above 1876C, Si3N4

dissociates into silicon and nitrogen. Between 1484C and1876C, the systems SiC�C�N2, SiC�Si3N4�N2 and SiC�Si3N4�Si are stable. In the first two systems, loss of nitrogen

433

Fig. 4. Composition diagram of SiBCN-ceramics at 0.1 MPa N2.159�

The marked compositions P1-P4 and P9 represent SiBCN materials

derived from different poly�organoborosilazanes�.

Table 1. A Summary of the Structural and High-Temperature

Properties of SiCN and SiCO Polymer-Derived Ceramics

Fig. 5. Methods for the powder-free processing of silicon-based

ceramics.



at this temperature is observed due to the loss of nitrogen ac-cording to the reaction Si3N4�3 C�3 SiC�2 N2. Silicon car-bonitride prepared e.g. from polyhydridomethylsilazane221�

decomposes at around 1500C giving off N2 and simultaneousformation of polycrystalline Si3N4�SiC micro�nano-compo-sites.60�,222�,223�

Both SiCN and SiCO show higher resistance to crystalliza-tion than the binary amorphous compositions of silicon-nitride, silicon-carbide or silicon-dioxide. Both possess aremarkable resistance to creep at ultrahigh temperatures, eventhough they are ostensibly amorphous. Indeed the fact thatthe presence of some oxygen �1–5 wt�� in SiCN has nodeleterious effect on the kinetic behavior also suggests thatthese two amorphous ceramics may share essentially the samebasic nanostructural framework. Structural and some selectedmaterials properties of PDCs SiCN and SiCO are listed inTable 1.

In the discussed cases, the amorphous phases are formed incarbon-rich regimes, relative to the stoichiometric mixtures ofthe crystalline forms. A key question in understanding thetransformation of the polymeric state to the nanostructuredceramic is the role of the�free�carbon.

The introduction of further elements �like boron oraluminium� into polymeric precursors can increase the hightemperature stability, creep and oxidation resistance. Theseproperties of PDCs are characteristic for the�amorphous�state which can persist at a temperature as high as 1800C. Thethermal decomposition of the molecular precursors leads tomass loss and shrinkage. In consequence, the final microstruc-ture of the material may present pores and cracks which leadto a deterioration of the mechanical properties. To preventcracking, the thermolysis can be influenced by the use of addi-tional fillers. In general, the powder-free processing of silicon-based polymers can be performed in three different ways: i�polymer without filler,239� ii� polymer with passive filler or iii�with passive and active filler242� as represented schematically inFig. 5.

In the case of the processing of polymers with passive filler,no reactions occur between the filler and the matrix or thedecomposition products. At the same time, the crack forma-tion due to the decrease in volume is prevented. As passivefillers, different metal carbide, oxide and nitride powders can

be used. Active fillers such as metals or intermetallic com-pounds react either with the matrix and�or with the thermoly-sis products to form the appropriate nitrides or carbide.

Moreover by the use of fillers, the resulting mechanical,electrical and magnetic properties of the ceramics can be effec-tively controlled. Other advantages of the active filler-con-trolled pyrolysis process are the possible decrease in shrinkageand porosity, the possibility to synthesize ceramic micro- andnanocomposites and the improvement of the corrosionresistance. The powder-free processing of poly�organosila-zanes� is illustrated schematically in Fig. 6. Polysiloxanes canbe processed analogously.

The polymer-to-ceramic conversion, as shown in Fig. 6,

434

Fig. 6. Powder-free processing of polysilazanes.

Fig. 7. Nanodomain structure of SiCN ceramics obtained from

polysilazanes �left� and polysilylcarbodiimides �right�.

Scheme 15. Structural rearrangement of polysilylcarbodiimides

during thermolysis forming SiN4 tetrahedra.

Fig. 8. Microstructure of SiBCN ceramic �TEM micrograph�.



involves a series of individual reaction steps that have to beconsidered carefully.120�–123� In particular, homogenisationand cross-linking are essential steps for the further processingof bulk PDCs.

The microstructure of the amorphous Si-based ceramicscan be analysed by means of solid state NMR studies.29Si MAS NMR measurements have shown that ceramics deri-ved from poly�organosilazanes� have to be considered assingle phase amorphous SiCN while polysilylcarbodiimide-derived SiCN consists of a two phase matrix comprised ofamorphous Si3N4-nanodomains and carbon segregations asillustrated in Fig. 7.243�

The 29Si MAS NMR analysis of the amorphous Si-C-Nexhibited two different structures: The polyorganosilazane-derived Si-C-N ceramics were composed of SiCxNy �x�y�4�units, and the polyorganosilylcarbodiimide-derived ceramicsmainly consisted of Si3N4 units interconnected with amor-phous carbon. The formation of these particular structuresretard the crystallization and phase partitioning processes thatlead to the thermodynamically stable phases Si3N4 and SiC.

The thermal decomposition of polysilylcarbodiimides148�

occurs in two steps as shown in Scheme 15.Silicondicarbodiimide, Si�NCN�2, decomposes at 960C

under evolution of cyanogen and nitrogen to form Si2CN4.The thermical transformation of polysilylcarbodiimides incyanamides and finally in silicon nitride is supported by TGA�

MS and 29Si MAS NMR measurements.The insertion of boron in poly�organosilazanes� results in a

drastic increase in the temperature resistance which in turn isresponsible for the retarded crystallization at about 1800C.

The pyrolysis of the polymers leads to black, glass-likeceramics with excellent thermal stability in terms of decompo-sition up to 2000C in protective atmospheres.5�,158� Crystalli-zation and phase separation start at 1800C and no significantchanges of the elemental composition were found in the tem-perature range between 1100 and 1800C. The ceramic yield ofthe as-synthesised materials amounts to 50–60 wt�.

A possible explanation for the exceptional temperatureresistance is the special network rearrangement in PDCs asshown in the TEM microstructure in Fig. 8.

The amorphous Si�B�CN phase encapsules the formed a-Si3N4 crystals and hinders their thermal decomposition even atelevated temperatures.

Besides of the polymer-derived non-oxide SiCN materials,the SiCO system is intensively investigated.149�,244�,245� Siliconoxycarbides are reported to be prepared by the pyrolysis of

435

Table 2. Some Selected Properties of SiCO-Based Ceramics in Com-

parison with Quartz Glass �Pure Silica�

Fig. 9. Microstructural analysis of Si�B�CO and SiCO ceramics

pyrolyzed at 1300�C in Ar by means of Raman spectra. Fig. 10. 29Si MAS NMR of SiCO and SiBCO samples.

Fig. 11. TEM micrograph of a boron containing SiCO after anneal-

ing at 1400�C for 20 h in air.255�



polysiloxanes230� in inert or reactive atmospheres. In compari-son with the non oxide system, silicon oxycarbide glasses showa lower oxidation resistance and limited high-temperaturestability. However, polysiloxanes are commercially availablein large quantities, can be processed in air, and the finalproperties of the ceramics produced therefrom can be adjustedby the addition of different inert or reactive fillers.246� Siliconoxycarbide-derived ceramics exhibit an amorphous micro-structure in which the silicon is simultaneously bonded tocarbon and oxygen and have technologically interestingproperties149� as presented in Table 2.

Silicon oxycarbide is often described as black glass247� be-cause of their distinctive black color after pyrolysis of theprecursor resin. The black colour is due to the presence ofa free carbon phase. The separation of free carbon248�,249�

and the microstructural development of the SiCO ceramicswere reported in different studies.240�,250� The influence of thefree carbon phase on the electrical properties of oxycarbideglasses with and without fillers was studied by Engel251� andCordelair.252�

In the last decade much concentration was devoted tothe modification of SiCO glass by boron and alumi-nium.173�,174�,253�,254� The microstructural analysis of theceramics can be quantitatively analyzed by means of Ramanspectroscopy. The Raman spectra of the SiCO productsobtained at 1300C are shown in Fig. 9.255� SiCO and Si�B�CO ceramics have been prepared by pyrolysis of commerciallyavailable poly�methylsilsesquioxane� filled with SiO2 andSiO2�B2O3 powders, respectively. The absorption bands locat-ed at 1360 cm�1 and 1600 cm�1 �D- and G-band� are charac-teristic for sp3- and sp2-carbon, respectively, present in SiCOceramics. The boron-free ceramic clearly exhibits two strongpeaks while only weak absorption bands are seen for theboron containing products synthesized at the same tempera-ture. The results of the Raman studies indicate the formationof an amorphous silicoboron oxycarbide for SiBCO while theboron free silicon oxycarbide network contains significant

amounts of segregated amorphous carbon characterized bythe carbon D- and G-Raman bands.

Figure 10 compares the 29Si MAS NMR spectra of a boron-free SiCO sample with that of a SiBCO sample.

In the case of boron-free sample, a nearly random distribu-tion of different silicon sites can be found. In the 29Si MASNMR-spectrum of SiBCO the presence of SiO4 and SiC4 sitesis analysed exclusively.

TEM studies of SiBCO ceramics �Fig. 11�255� show that thepresence of boron leads to an enhancement of b-SiC crystalli-zation in the oxycarbide matrix. Furthermore, the separationof free carbon is inhibited in the boron containing materialproviding a higher electrical resistivity. Moreover, no crystalli-zation of a-cristobalite takes place in the SiBCO ceramics.

436

Fig. 12. Temperature dependence of the electrical resistance of

SiCO and SiBCO ceramics annealed at 1300�C for different time.

Fig. 13. Temperature dependence of the viscosity h of SiAlOC glass.

Data concerning vitreous silica, SiCO glasses from various authors are

superimposed.172�,257�

Fig. 14. SiCO based ceramic glow plug �source: Robert Bosch

GmbH�.



The addition of boron leads to fine dispersed nano b-SiCparticles embedded in a high temperature stable amorphousborosilicate glass structure. Therefore, SiBCO is a candidatematerial with high temperature resistance and stable electricalproperties which are required for example in ceramic heaters.

The crystallisation behavior of SiBCO obtained from poly-silsesquioxane filled with SiO2�B2O3 powders correspondsnicely to that of SiBCO gels.256� This feature may becomeimportant for the further development of less expensive syn-thesis routes to produce amorphous or nanocrystalline SiBCOceramics.

Figure 12 shows the difference between the temperaturedependence of the electrical resistance of SiCO and SiBCOceramics.

As can be seen from Fig. 12, the boron-containing samplesreveal a significantly higher electrical resistance than that ofSiCO. Moreover, the electrical resistance of SiCO decreaseswith increasing annealing time of the sample while SiBCOdoes not show such an aging effect.

The aging of SiCO is due to enhanced carbon segregationwith increasing annealing time resulting in a higher electricalconductivity.

The presence of aluminium in the preceramic polysiloxaneinfluences the viscosity of the ceramics �Fig. 13�.

The viscosity of SiAlCO is higher than the viscosity of SiO2

and at the same time lower than the viscosity of SiCO. It couldbe shown that an increase in the aluminium content leads to adecrease in the viscosity for the respective SiCO ceramics. Thisfeature is in accordance with the behaviour of Al containingsilica as can be also taken from Fig. 13.

In conclusion, the presence of B and Al in the PDCs strong-ly influences the thermomechanical properties of the resultingamorphous ceramic. This property variation allows producingSiCN and SiCO materials for different applications

5. Applications

Due to their particular electrical, thermal, chemical, mecha-nical and biological properties polymer-derived ceramics�PDCs� have a great variety of potential applications.Numerous publications have reviewed the molecular tailoringof PDCs with improved properties compared to conventionalceramic materials. Also important to mention is the possibilityto adjust the required property profile by addition of metallicor other ceramic filler particles. By this means, the materialsproperties of PDCs related to shrinkage, bending strength,thermal expansion, electrical conductivity, thermal conducti-vity, oxidation resistance, heat capacity, thermal shock resis-tance and others can be modified.

5.1 Ceramic heater

Owing to their high temperature and oxidation resistancePDCs find applications as ceramic heating elements. This typeof ceramic heater is mounted in a glow plug for diesel enginesproduced by BOSCH GmbH, Germany, as shown in Fig. 14.The ceramic glow plug is the first industrial application ofpolymer-derived ceramics made from polysiloxanes and a var-iety of filler materials. These types of glow plugs show a lon-ger lifetime, lower input currents, significantly higher heatingrates and working temperatures in comparison with traditio-nal metallic glow plug systems.

Especially the high heating rate and working temperatureled to a strong reduction of soot particles and polycarbons atthe starting phase of a diesel engine. This particular behaviouris an important step to reduce pollution caused by diesel carsand to reduce fuel consumption.

A second example of PDC application is the ceramic micro-igniter presented in Fig. 15. The device is based on a SiCONmaterial and promises a combination of high heating rates,short response times and a high number of heating cycles.

5.2 Polymer-derived ceramic coatings

The precursor technology opens the possibility to formceramic coatings and membranes by using liquid phase deposi-tion �LPC�. This is a low-cost alternative to PVD and CVDmethods known from the lacquer technology as well as dip-coating, spray-coating and spin-on-coating. All methods useliquid precursors and�or solutions of inorganic polymers fordeposition of substrates with subsequent ceramisation. Be-cause of their isotropic shrinkage behavior, it is not possible to

437

Fig. 15. SiCON micro-igniter �source: Rishi Raj, University of

Colorado at Boulder, USA�.

Fig. 16. SEM micrographs of a 50 mm thick oxidation resistant SiCN

coating on a C�C�SiC substrate with a magnification of 1000 �a�,

4000 �b� and the cross-section. The nickel film is for protection of the

ceramic during the ceramographic process.

Fig. 17. SEM micrographs of 200 nm �single coated, a� and 500 nm

�twofold coating, b� SiCN porous coatings on Si3N4 substrates.

Fig. 18. Polymer-derived C�C�SiC ceramic brake disk STAR-

BLADETM produced by Starfire Systems, Inc. for motorbikes.



produce ceramic coatings with a thickness higher than 100–200nm. The isotropic shrinkage leads to stresses at the interfacesubstrate�coating and formation of crack systems.258� In orderto form thicker coatings, multiple deposition steps or usage offiller particles is required to minimize the amount of shrink-age.

SiCN films have potential for applications in wear or corro-sion protection259� as well as microelectronic and optoelectro-nic devices.260� Most metals have relatively low melting points;therefore, coatings on metals must be prepared at significantlylower temperatures than the melting point of the metal in or-der to avoid thermal damage. Ceramic SiCO coatings are usedas high-temperature resistant coatings for glass and oxidefibers. In this manner, the fibres are protected against fireburn-out and corrosion.11�

A SiCN ceramic coating on C�C�SiC substrate for oxidativeand mechanical protection is presented in Fig. 16. The methodused was spray coating and the procedure was repeated 4times. The result is a dense, nearly crack-free coating with athickness of 50 mm.261�

The porous SiCN membrane shown in Fig. 17 is fabricatedby spin-coating on a porous Si3N4 substrate and has a thick-ness of 200 nm for single and 500 nm for twofold coating.Such membranes can be used as selective membranes for hotgas separation. The advantage of SiCN membranes is theirhigh temperature stability in comparison with conventionaloxidic glass or ceramic membranes. Presently, in cooperationwith the Japan Fine Ceramics Center, Nagoya, Japan, we de-velop PDC membranes suitable for the high temperature sepa-ration of hydrogen.262� The separation of hydrogen is e.g.

technologically important for the production of H2 by the CH4

reforming reaction:

CH4�H2O� 3 H2�CO

5.3 C�C�SiC brakes for motorbikes

The ceramic brake disk STARBLADETM �Starfire SystemsInc., USA� for motorbikes presented in Fig. 18 is a C�C�SiCcomposite fabricated by infiltration of a C�C matrix with acommercial polysiloxane �Starfire Systems Inc., USA� andsubsequent pyrolysis. Several infiltration steps are necessaryto form a dense ceramic body. Such ceramic brake systemspromise a better brake performance with a longer lifetime ofthe disks, higher brake forces and delay as well as the exclu-sion of fading. These advantages result in better driving com-fort and more security for motorcyclists. A further advantageof the ceramic brake disk is their low density in comparisonwith metallic disks. Hence, it is possible to reduce the weightof the motorbikes.11�,263�

5.4 Micro electro mechanical systems �MEMS�

A potential application of polysiloxane derived SiCO isin the field of ceramic micro electro mechanical systems�MEMS� which are presently under development for microoptics or micro motors in optical or mechanical systems. Forthe formation of MEMS, the silicon-based polymer shouldhave the following requirements: sufficient wetting of themicro mold by the polymer, solidification of the polymer e.g.by chemical or thermal cross-linking after complete filling ofthe mold to retain the shape of the mold cavity and easydemolding of the green micro component. Subsequent poly-mer pyrolysis gives the ceramic MEMS.

To fabricate micro-component ceramics �MEMS�, variousactive �Ti, TiH2, Al etc.� and passive �SiC, Al2O3, TiB2�

fillers are homogenized together with the preceramic poly-mer. Subsequently, the polymeric compound mixture is con-verted into the SiCO–ceramic in various atmospheres.264�,265�

Figure 19 displays a micro gear derived from a polysiloxane254�

438

Fig. 19. Polymer-derived SiCO micro gear after pyrolysis of the

demolded polymeric green body.254�

Fig. 20. �a� Ceramic matrix composite �SiOC�SiC� formation with

complex structure: �a� from left to right: Euro coin as a mold, Euro

green form after warm pressing and final CMC Euro coin with

detailed surface topography after pyrolysis in argon at 1100�C. �b�

and �c� SEM micrographs of the crack-free SiCO�SiC ceramic matrix

composite showing the accuracy of the Euro coin topographic details

in the pyrolyzed state.257�

Fig. 21. SEM micrographs of SiCO foams with cell sizes of ca. 1 mm

�left� and 10–20 mm �right� �source: P. Colombo, University of

Padova, Italy�.



by micro molding.To demonstrate the precision of the micro molding process,

a coin was used as a mold. Figure 20�a� shows the coin, theshaped green body and the ceramic CMC, derived from poly-siloxane filled with 30 vol� SiC powder with an average parti-cle size of 0.5 mm. Details from the surface of the PDC aspresented in Figs. 20�b� and �c� exhibit the precise filling ofthe mold cavities by the preceramic polymer and their formstability during cross-linking and pyrolysis.257�

5.5 SiCO ceramic foams

Cellular ceramics possess unique combination of favourableproperties, such as low density, low thermal conductivity, lowdielectric constant, high thermal shock resistance, high speci-fic strength and high chemical resistance, which make themgood candidates for both structural or functional applications�e.g., thermal insulation, liquid metal filtration, impact ab-sorption, catalyst supports, light weight structures�.266�–268�

A novel process for obtaining silicon oxycarbide �SiCO�

ceramic foams from a preceramic polymer was recently deve-loped by Colombo et al. The method involves the foaming of ahomogeneous solution comprised of a thermosetting siliconeresin with or without polyurethane precursors �polyols andisocyanates�.269�–271� The process is simple, economical andversatile, and large bodies with various shapes �tubes, plates,and blocks� can be produced. The green porous bodies areconverted into SiCO ceramic foam by pyrolysis at 1000 to1200C in nitrogen. Both, open or closed macro-cellular �cellsize ranging from about 100 to 800 mm� foams can be

achieved. Produced SiCO foams have bulk densities rangingfrom about 0.15 to 0.60 g�cm3 �7 to 28� relative density�.Crushing strength, flexural strength and Young's modu-lus vary with the morphology and density of the foams, andmaximum values �at room temperature� of about 10 MPa, 18MPa and 6 GPa, respectively, have been obtained so far. Thefoams possess a low thermal expansion coefficient �3.5�10�6

K�1�, and display excellent thermal shock resistance as well asdimensional stability up to their pyrolysis temperature. Theintroduction of filler powders allowed for the synthesis offunctional ceramic foams, possessing electrical conductivity ormagnetic properties.

Novel micro-cellular ceramic foams with a cell size in therange between 1 and 100 mm have also been produced from apreceramic polymer and sacrificial fillers.272�,273� The fillers,comprised of PMMA spherical micro beads with a dimensionof about 1.5, 5, 10, 25, 50 or 100 mm, are dry-mixed with athermosetting silicone resin, and burned out in air at 250–350C before pyrolysis at 1200C in N2. The polymer-to-ceramic conversion yields a highly porous SiCO ceramic, withopen porosity and relative densities between 0.15 and 0.3�bulk density �0.3 to 0.6 g�cm3�, depending on the amountof micro beads introduced. The compressive strength of themicro cellular foams is 2 to 10 times higher than that of macrocellular foams of similar density and composition. Morpho-logical investigations also reveal that the cell size distributionis more homogeneous than that of SiCO macro cellular foamsproduced by direct foaming of a silicone resin. Foams with agraded cell size or density can be produced by varying thePMMA micro beads' size or amount along one axis of thematerial �Fig. 21�.

Several applications have been envisaged and tested forPDC foams, including impact absorption, thermal manage-ment �e.g., thermal insulation at high temperatures in aero-space applications� and functional substrates.274�–278�

5.6 SiCO precision components

Filled preceramic polymers allow the fabrication of SiCOceramic precision components. Such components can be usedfor pump devices that can be operated in harsh environmentsin which metallic components cannot be used e.g. in chemicalpumps. Figure 22 presents an excenter for a pump device deri-ved from a filled polysiloxane. The device demonstrates thepossibility to form compact polymer-derived ceramic bodieswith larger dimensions and with a high dimensional accuracy.Different filler systems open the possibility to adapt thecoefficient of thermal expansion �CTE� to the other metalliccomponents of the pump system. The SiCO ceramic filled withAl2O3 reaches a CTE value compatible with steel and a mix-ture of Al2O3 and SiO2 allows to adjust a CTE matching thatof aluminium alloys. This is an advantage in comparison withcommon ceramics such as silicon nitride, aluminium oxide or

439

Fig. 22. Left: High precision device for vacuum pump systems with

a similar thermal expansion coefficient to steel, right: dimensional

accuracy during fabrication of the devise and tailoring of the CTE

�source: P. Greil, University of Erlangen, Germany�.



zirconium oxide and can reduce the stress between the ceramicand the metal components of a device like a chemicalpump.279�

5.7 Polymer-derived ceramic fibres

The formation of carbon fibres derived from organic poly-mers is an established technological procedure. Carbon fibresshow high strength and are used for reinforcement of ceramicmaterials e.g. brake disks or components for space applica-tions. The main drawback of carbon fibres is their sensitivityto oxidation and corrosion. To combine the high strength ofthe carbon fibres with the corrosion and oxidation stability ofa ceramic, polymer-derived ceramic fibres are considered aspossible alternative materials. The first commercially availablepolymer-derived ceramic fibre is the SiC �High� Nicalon�fiber from Nippon Carbon Co., Ltd., Japan. Jansen et al.reported the synthesis and properties of polymer-derivedSiBCN fibres.67�,280�–282� SiBCN fibres were also fabricated byBernard et al.283�,284� Polymer-derived BN fibres can beformed by using polyborazines as starting materials.285�–288�

The advantages of the PDC fibres are their high strengthcombined with oxidation and corrosion resistance. This com-bination allows the reduction of corrosive and oxidative at-tacks on fibres occurring during fabrication of fibre-rein-forced ceramics. The process involves infiltration of corrosiveinorganic polymers or silicon into porous fabrics.

5.8 Other applications

A polymer of Starfire Systems is currently being consideredas a first aid kit for thermal protection of ceramics for re-entryspace systems such as a space shuttle. The polymer will be usedto seal damaged parts of the thermal protection shield and canbe directly applied in space. The pyrolysis takes place in situduring the re-entry into the atmosphere.11�,289�

Another possible application of polymer-derived ceramicsis the functionalisation of surfaces and building of micro-structure systems by lithographic methods. After coating asubstrate with a preceramic polymer, selected parts of thepolymer coating can be cross-linked by irradiation with UVlight. Dissolving of the unexposed part of the polymer andsubsequent pyrolysis leads to the formation of complex struc-tures such as functional microstructures, MEMS or microdevices.290�

6. Conclusions

The polymer pyrolysis route gives the exceptional oppor-

tunity to generate novel multi component amorphous materi-als denoted as PDCs. Ternary and multinary PDCs aremetastable up to extraordinary high temperatures and cannotbe produced by traditional synthesis methods. Multiphaseceramic composites can be formed by crystallization of theas-pyrolyzed polymers revealing tailor-made microstructures.Additionally, PDCs are additive-free ceramic materials pos-sessing excellent oxidation- and creep-resistance up to excep-tionally high temperatures. The manufacture of differentceramic shapes like fibres, powders, coatings and bulk materi-als is possible with polymer-derived ceramics. Moreover, thecovalently bound non-oxide Si-based ceramics can be obtainedat moderate temperatures, thus requiring less energy com-pared to conventional densification techniques. The molecularstructure of the starting organosilicon polymers strongly de-termines the solid state structure and microstructure of thereceived inorganic solid ceramic.

Because of their excellent properties and processing beha-viour, the preceramic polymers can be employed as high-per-formance ceramics. Through the variation of the chemicalstructure and by using filler particles, materials with differenttailor made properties can be produced.�Polymer-DerivedCeramics Flow out of the Laboratory and into the Markets,�this slogan characterizes the present technological status ofPDCs. Fibres, glow plugs �ceramic heaters� and motorbikebrakes based on PDCs have emerged as innovative technologi-cal products in the recent years from basic university research.Other applications are expected to be transferred into industri-al products in the near future. National and internationalscientific research programmes such as the German PriorityProgramme�Nanomat��see www.spp-nanomat.de � fundedby the German Science Foundation �DFG�, the ResearchTraining Network �RTN��PolyCerNet�funded by the Euro-pean Union and a binational programme�Unusual Stabilityof Amorphous Polymer Derived Ceramics at High Tempera-tures�funded by the German DFG and by the NSF of theUSA have been established in 2005 and 2006 and will contri-bute to the further development of PDCs in the forthcoming4–6 years.

Acknowledgement The authors thank the Deutsche For-

schungsgemeinschaft, Bonn, Germany �DFG–SPP 1181 and

DFG–NSF research initiative�, and the European Union �RTNproject�PolyCerNet�� for financial support. The authors ac-

knowledge also the Humboldt Foundation for a research fellow-

ship, and the generous support of the Robert Bosch GmbH,

Germany, the Fonds der Chemischen Industrie, Frankfurt�Germany�, KiON Inc., USA and Starfire Systems Inc., USA. A

special thank is given to the continuous scientific cooperation

with Dr. Y. Iwamoto, and to the financial support by the Japan

Fine Ceramics Center, Nagoya, Japan. We also gratefully ac-

knowledge discussions and input of various examples of PDC

applications from P. Colombo, P. Greil, R. Raj.

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Prof. Ralf Riedel studied chemistry and got his Ph.D. in Inorganic Chemistry in 1986.Between 1986 and 1992 he joined the Max-Planck-Institute for Metals Research and theInstitute of Inorganic Materials at the University of Stuttgart. In 1992 he finished his habilita-tion in Inorganic Chemistry. Since 1993 he is Professor at the Institute of Materials Science atthe Darmstadt University of Technology. Prof. Riedel is Fellow of the American CeramicSociety and was awarded with the Dionyz Stur Gold Medal for merits in natural sciences. Heis a member of the World Academy of Ceramics and Guest Professor at the JiangsuUniversity in Zhenjiang, China. His current research interest is focused on i� polymer derivedceramics and on ii� ultra high pressure synthesis of new materials.

Gabriela Mera is scientific assistant in the Department of Dispersive Solids, Material Scienceat Darmstadt University of Technology since 2005. She received the Ph.D. in inorganicchemistry in 2005 at the Ruhr-University Bochum, Germany with the thesis�Contributions tothe Synthesis of Silicon-Rich Oligocarbosilanes and Their Use as Precursors for ElectricallyConductive Films�and the Magister of Science in 2000 at the Institute for Inorganic andAnalytical Chemistry, Technical University of Braunschweig, Germany as Socrates fellow.She graduated in 1998 in Chemistry and Physics at the University of Bucharest, Romania.Her current research interest is focused on the molecular design of PDCs by design andsynthesis of the organic precursors, in terms of the DFG-NSF research collaboration.

Ralf Hauser is the leader of the�processing-group�of the Department of Dispersive Solids,Materials Science at the Darmstadt University of Technology since 2002 and scientificassistant since 2001. After graduation in 1995 at the Institute of Inorganic Chemistry at the�Martin-Luther-University�Halle-Wittenberg, he received the Ph.D. in inorganic chemistryin 1999 with the thesis�Synthesis, Structure and Reactivity of New Organometallic Substitut-ed Tin and Germanium Chalcogen Compounds�at the same university. From 1995 to 2000,he continued to work as a scientific assistant. His major research fields include the synthesisand processing of PDCs for high-temperature applications and plasma-enhanced chemicalvapour deposition of single-source precursors.



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