multifunctional properties of alumina composites reinforced by a hybrid filler

Multifunctional Properties of Alumina CompositesReinforced by a Hybrid Filler

Kaleem Ahmad, Wei Pan* and Zhixue Qu

State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science andEngineering, Tsinghua University, Beijing, 100084, China

Hybrid microstructure design has been used to fabricate alumina composites reinforced by 5 vol% of multiwalled carbonnanotube (MWNT) together with different (1, 2, 3 vol%) contents of SiC nanoparticles by spark plasma sintering. Themechanical, thermal, and electrical properties of the composites were determined as a function of the SiC volume fraction.The thermal conductivity decreased for 1 and 2 vol% of SiC, while for 3 vol%, it increased. Substantial improvements inthe fracture toughness, bending strength, and electrical conductivity were observed and attributed to a synergetic effect of theMWNT and SiC nanoparticles in the hybrid microstructure design.

Introduction

Since their discovery, carbon nanotubes (CNTs)have attracted considerable attention due to their out-standing electrical, mechanical, and thermal properties.1

In particular, a novel experiment suggests that the ten-sile strength and Young’s modulus of the outer layer ofan individual multiwall carbon nanotube (MWNT) isin the range 11–63 and 270–950 GPa, respectively.2

Further, experimental observations at room temperatureon individual MWNT show a high thermal conductiv-ity value of 43000 W/mK.3 The electrical propertiesdemonstrate a multichannel quasiballistic conductingbehavior attributed to multiple walls with a high elec-

trical conductivity value along the long axis and hugecurrent density, about 1.85� 103 S/cm and 107 A/cm2,respectively.4,5 In addition, cost-effective mass produc-tion, high aspect ratio, nanosize, and low density makeMWNTs ideal candidates for development of advancedengineering composite materials. Alumina/CNT com-posites are the focus of recent investigations6–9 based ontheir potential or already used applications in variousfields such as armor systems, wear resistance products,electronic substrates, cutting tools, automotive parts,turbine hot section components, power generator com-ponents, furnace elements, and components.10,11 How-ever, the present alumina-based ceramics have lowperformance, limited by their fracture toughness.11,12

Furthermore, there are some special uses of alumina,where high electrical conductivity is required, such asheating elements, electrical igniters, antistatic, and elec-tromagnetic shielding effectiveness of electronic com-ponents.13–15 The present organic electroconductivematerials have significant shortfalls in their mechanical

Int. J. Appl. Ceram. Technol., 6 [1] 80–88 (2009)DOI:10.1111/j.1744-7402.2008.02257.x

Ceramic Product Development and Commercialization

Presented in part at the 31st Annual International Conference on Advanced Ceramics and

Composites, Daytona Beach, FL, January 21–26, 2007.

Supported by the National Natural Science Foundation of China (Grant No. 50232020

and 50572042) and Higher Education Commission of Pakistan.

*[email protected]

r 2008 The American Ceramic Society

i:/BWUS/IJAC/02257/[email protected]

properties, heat resistance, and chemical resistance thatlimit their usefulness, while the electroconductive metalceramic composites possess poor mechanical propertiesdue to high loadings of imbedded metallic particles andmay not retain refractoriness, stiffness, and hardness. Noattempts have been made to take full advantage of theunique properties of MWNTs in alumina-based com-posites. Various efforts were made to improve mechan-ical properties, especially the fracture toughness ofalumina using CNT. However, the improvements are,in general, disappointing for MWNTs/alumina com-posites,7,16–18 while a few conflicting reports have beenpublished for single-wall CNT/alumina compos-ites.9,12,19 Most of the researchers showed no or someimprovements either in the fracture toughness or bend-ing strength, while others even showed a slight decreasein one property with an increase in the other property orvice versa.6,7,18,19 Perhaps one of the main reasons whydramatic improvements in mechanical properties havenot been achieved so far is that the presence of CNTs atthe grain boundaries makes them mechanicallyweak.9,19 This may be due to the agglomeration ofCNTs and/or the existence of thermal residual stress-es,20,21 which result in fracture along the grain bound-aries (intergranular fracture mode).9,12,19 Generally, thefracture toughness of grain boundaries is lower thanwithin the grains and polycrystalline alumina exhibitsan intergranular fracture mode. Therefore, a majorimprovement in the mechanical properties of aluminausing MWNTs has not been realized so far.

Recently, it has been reported that the unique grainboundary structures in CNT/alumina composites have astrong influence on the mechanical, electrical, thermal,and thermoelectric properties of the composites.9 How-ever, at present, no attempts have been made to studythe effect of grain boundaries strengthening, whilesimultaneously reinforcing the matrix by MWNTs onelectrical, mechanical, and thermal properties of al-umina composites. In this work, the hybrid microstruc-ture design has been used to fabricate thenanocomposites. In this design, the alumina matrix isreinforced by different (1, 2, and 3) vol% of SiC nano-particles, and 5 vol% of MWNTs, concurrently. It iswell established through several studies that addition ofSiC in alumina results in a change of fracture modefrom intergranular to transgranular, which implies grainboundaries strengthening. Several advantages of usingSiC in combination of MWNTs have been foreseen.The incorporation of SiC nanoparticles into alumina

results in the removal of residual stresses at the grainboundaries,22–24 because residual stresses are undesir-able in optimizing the cracking behavior in MWNT/alumina composites20,21 and thus helpful in improvingthe mechanical properties. Unlike the previous nano-composites reinforced by either nanotubes or nanopar-ticles, the hybrid filler provide a mutually redundanttoughening mechanism: first, by strengthening the grainboundaries and toughening the matrix by nanosize SiCnanoparticles and second, by fiber-toughening mecha-nisms through MWNTs. In addition, the hybrid fillerwas also suggested in providing enhanced connectivityand has been found to be effective in improving thermalconductivity of the composite.25 Furthermore, SiC is asemiconductor material and has high thermal and elec-trical conductivities relative to alumina. Addition of SiCis expected to further improve the electrical and thermalproperties of the composites in addition to improvingmechanical properties. In the present study, the multi-functional properties of hybrid alumina compositeswere investigated to achieve most of the exceptionalcharacteristics of MWNTs through strengthening thegrain boundaries by nanosize SiC particles and tough-ening the matrix by MWNTs concomitantly. The re-sults have shown that the electrical conductivity andmechanical properties have significantly improved si-multaneously in hybrid alumina composites withoutdeterioration its intrinsic properties. These kinds ofcomposites have potential applications in aerospaceand the defense industry. Their resistance to chemical,corrosion, and, generally, to severe weather conditions,including extreme radiation and ultraviolet exposure,makes them suitable for manufacturing enduring bodyparts in railway trains, space facilities, and ruggedizedcircuitry substrates, and so forth. The electrostatic dis-sipative property look promising for automotive fuellines, O-rings for the pumps in gas stations, filters, andfor the plastic body parts that require electrostatic paint-ing. The potential military applications include electro-magnetic absorbing materials for aircraft, missiles, andportable electronic devices that need to prevent inter-ference from other electronic equipment.

Experimental Procedure

MWNTs were dispersed carefully to ensure maxi-mum homogeneity of the composites using the processdescribed by Zhan et al.12,26 In brief, the starting pow-der alumina (99.9% in purity, Chong Qing Tuo Yuan,

www.ceramics.org/ACT Multifunctional Properties of Alumina Composites 81

China) was ultrasonically mixed with 5 vol% ofMWNT (Green Chemical Reaction Engineering andTechnology, Beijing, China) and with different (1, 2,and 3) vol% of nanosize SiC (99.9% in purity, Shijiaz-huang High-Tech Ceramics Material, Shijiazhuang,China) particles in ethanol. For further mixing, ballmilling was performed for 24 h. After drying, powdermixtures were spark plasma sintered (Dr. Sinter 1050,Sumitomo Coal Mining, Tokyo, Japan) at 15501C invacuum under a pressure of 50 MPa in 20 and 12.5-mminner diameter cylindrical graphite molds. X-ray diffrac-tion analysis showed absence of any new phase. Thebulk densities of sintered samples were measured by theArchimedes method at a temperature of 201C with deion-ized water as the immersion medium. The relative den-sities were obtained as the ratio of the Archimedesdensities, and the theoretical densities calculated by therule of mixtures. Fracture toughness was measured by thedirect toughness measurement technique, that is, the sin-gle edge notch beam method on rectangular bars with asize of about 3 mm� 2 mm� 15 mm containing a pre-notch of length r1.5 mm and width � 0.25 mm at across head speed of 0.05 mm/min. The bending strengthwas measured by a three-point bending test on rectangularbars with a size of about 2 mm� 3 mm� 12 mm at across-head speed of 0.5 mm/min as reported in the liter-ature.27,28 These tests were performed at room tempera-ture using a universal testing machine (Shimadzu ServoPulser EHF- EG50KNT-10L, Tokyo, Japan). Four barswere tested for each material and the mean value andstandard deviations were obtained. The hardness wasmeasured by Vickers indentation method with a load of5 kg. Cracks on polished surfaces were induced by Vickersindent. Silver paste was painted on both sides of the disc-shaped samples of diameter � 12.5 mm and room-tem-perature dc electrical conductivity was measured by thetwo-point probe method. The thermal diffusivity wasmeasured along the thickness of the disc-shaped samples� 12.5 mm in diameter using the laser flash technique

(NETZSCH Laser Flash Apparatus LFA 427, Selb, Ger-many) in the temperature range from 251C to 5001C inan argon atmosphere.

Result and Discussion

Mechanical Properties

Hybrid microstructure design is shown schemati-cally in Fig. 1. In brief, the nanosize SiC particles relieve

the residual stresses in the matrix and along the grainboundaries by generating dislocations around the parti-cles.24,29 This reduces the defects on the grain bound-aries and strengthens them by impeding theintergranular fracture mode of alumina. In addition,the hybrid design also provides two different mecha-nisms to improve the fracture toughness. Any crack, re-sulting either from processing flaws or otherwise presentthat can propagate toward the SiC nanoparticle due tointernal tangential tension and, would be blunted bythe particle. The ropes, like intertwining networks ofMWNT at the grain boundaries, bridge the crack prop-

Fig. 1. Schematic of alumina matrix reinforced by hybrid filler(MWNTs and SiC nanoparticles). MWNTs, multiwall carbonnanotube.

82 International Journal of Applied Ceramic Technology—Ahmad, Pan, and Quzhi Vol. 6, No. 1, 2009

agation at the intergranular positions through fiber-toughening mechanisms. As a result, the networksrestrict the fracture of grains and thus improve the tough-ness of the composites (Fig. 1). This new hybrid class ofceramic nanocomposite provides mutually redundantmechanisms to improve the strength and toughness ofthe alumina matrix. More details of the hybrid nano-composite have been described in the reference.30

An optimum value of 5 vol% has been used forCNTs in this study. In general, higher contents ofCNTs tend to agglomerate as the probability of ag-glomeration increases, while lower contents of CNTsshow insignificant toughening effects in the resultingcomposites.7,16 Sun et al.17 used only 0.1 wt% of CNTsto reinforce alumina and reported a 32% improvementin the fracture toughness. The enhancement appears tobe substantial for only 0.1 wt% of CNTs. However,they performed the indentation method to estimate thefracture toughness. This method has been reported toproduce overestimated results due to shear deformationof CNTs under the indenter.19,31 In contrast, in thepresent study, a relatively high (5 vol%) content ofCNTs was used to observe the substantial and cleareffect of toughening on the composites. The fracturetoughness was measured using a more reliable and directtoughness measurement technique. The improvementswere corroborated by potential toughening mechanisms.SiC was used in 1, 2, and 3 vol% for comparison. How-ever, 1 vol% is sufficient to change the fracture mode ofalumina from intergranular to transgranular,32,33 as oneof the main toughening mechanisms in SiC/aluminacomposites is to change the fracture mode, which im-plies a reinforcement of grain boundaries.34 The secondadvantage of SiC is that radial cracks were observed byVickers indentation. The cracks in samples not contain-ing SiC could not be observed due to accommodationof shear deformation of CNTs under the inden-ter.14,19,20,31 These cracks helped to uncover the actualmechanism for toughening by providing direct evidencethrough MWNTs in the nanocomposites. The thirdbenefit is that the hardness is almost unaffected.

Mechanical properties and relative density of al-umina and hybrid nanocomposites as a function of SiCvol% are shown in Fig. 2. The relative densities of 1, 2,and 3 vol% of SiC along with 5 vol% of MWNTs ofeach alumina composite were reached at around 98.0%,96.46%, and 96.40%, respectively. The addition of SiCinhibits the densification of alumina and lowers thesintering rate.35,36 Therefore, higher sintering tempera-

tures are required to obtain the fully dense materials for2 and 3 vol% of SiC-reinforced hybrid alumina com-posites and that may have adverse effects on CNTs. Themechanical properties show some dependency on thedensity (Fig. 2) and the high values are obtained for5 vol% of MWNTs and 1 vol% of SiC reinforced hy-brid alumina composites. For 2 and 3 vol% of SiCreinforced hybrid alumina composites, the residualporosity probably affected the improvement in thebending strength, fracture toughness, and hardness.The bending strength and fracture toughness most like-ly did not improve much due to the presence ofapproximately 3.5% residual porosity. In the presenceof porosity, the cracks formed more easily due to highconcentration of stress generated around the pores andthat may result in small fracture energy and consequent-ly less improvement in these properties. The improvedmechanical properties for 1 vol% of SiC-reinforced hy-brid alumina composites may be correlated to lower re-sidual porosity. The hardness did not improve butremained almost unaffected due to addition of SiC.

Recently, Fan et al.6 reported a 4% decrease inbending strength for 12 vol% MWNT/alumina com-posites. The decrease in bending strength was probablycaused due to weakening of grain boundaries byMWNT agglomerates.6 In contrast, our study shows46% improvement in the bending strength, clearly in-dicating that grain boundaries have been strengthened.This is also supported by the fracture mode in themicrograph that showed predominantly the transgran-ular fracture mode (Fig. 3). The intragranular fracture

Fig. 2. Mechanical properties and relative density of alumina andhybrid nanocomposites as a function of SiC volume %.


mode indicates strengthening of the grain boundaries bynanosize SiC particles, resulting in improvement of thebending strength as reported earlier.23,34 The thermalexpansion mismatch between alumina and SiC nano-particles plays an important role in strengthening andtoughening the alumina matrix. The addition of nano-size SiC particles yields dislocations around the parti-cles, which relieve residual stresses in the matrix andalong the grain boundaries and thus reduces the flaw sizealong the grain boundaries.24 The increase in bendingstrength (46%) by addition of 1 vol% of SiC in hybridnanocomposites may be attributed to a decrease in crit-ical flaw size and reduction of tensile residual stresses inthe matrix.23,24,37 The fracture toughness improved� 104% over monolithic alumina for 1 vol% of SiC-reinforced hybrid nanocomposites. The toughnessenhancement by SiC in hybrid nanocomposites maybe ascribed partly to the change in the fracture modefrom intergranular to transgranular and crack deflectiondue to internal stress around the SiC particles.37 Thesignificant toughness enhancement in hybrid nanocom-posites can be dominantly attributed to MWNTsthrough fiber toughening mechanisms. The scanningelectron microscopy observation of fiber toughening(crack bridging, crack deflection, and MWNT pullouts) by the MWNTs provides direct evidence for sig-nificant improvement in the toughness of the compos-ites (Fig. 4). The crack bridging by MWNTs restrainthe crack opening and reduces the driving force forcrack propagation. The marked rectangular area in

Fig. 4a has been magnified in Fig. 4b and shows thatMWNTs remain intact to bridge the crack by providingsufficient toughening. The debonding occurs at theatomic scale and the work requires pulling MWNTsout against residual sliding friction at the interface, im-parting significant fracture toughness to the aluminamatrix. The crack bridging and crack deflection byMWNTs lead to improvements in extrinsic and intrin-sic toughness of the alumina matrix, respectively.37 Thepull-outs suggest that MWNTs bear significant stressesby sharing the portion of the load. The pull-outs aremore conspicuous due to load transfer of the aluminamatrix to the outer shell of MWNTs. Figure 4 showsthe underlying fiber toughening by MWNT in a highlydisordered situation. Xia et al.20 also reported fiber

Fig. 3. Uniform distribution of multiwalled carbon nanotubes inthe hybrid nanocomposites.

Fig. 4. Fiber toughening by multiwalled carbon nanotubes in thehybrid nanocomposites.


toughening on a specially designed highly ordered par-allel array of MWNTs on an amorphous nanoporousthin alumina matrix (20 and 90 mm thickness). How-ever, in their study, the overall composite toughness wasdifficult to measure due to complex nature of residualstresses and crack bridging.20 There are possibly twofactors complementing each other in toughening andstrengthening the hybrid alumina composites. Firstly,the low volume fractions of SiC nanoparticles thatstrengthen the grain boundaries and impede the inter-granular fracture mode. Secondly, the good bondingbetween the CNT graphene wall and alumina12,38 aswell as the presence of a network structure of CNTs atintergranular positions, as reported in several studies,9,26

may result in strengthening and toughening of thenanocomposites by MWNTs.

The hardness of the hybrid nanocomposites re-mains almost unchanged. Zhan et al.12 showed a sharpdecrease in hardness with an increase of fracture tough-ness, while some other authors have reported an increasein hardness, but in their work there is no consistent re-lationship between hardness and fracture toughness.16,39

The addition of SiC in hybrid composites maintainshardness almost unchanged even with a decrease of den-sity (Fig. 2). The best result reported so far for MWNT/alumina composites is an 80% improvement in thefracture toughness at the cost of 4% decrease in bendingstrength.6 However, the enhanced toughening was notsupported by key evidence of potential tougheningmechanisms. In the present study, the hybrid nanocom-posites have shown an improvement not only in thefracture toughness (� 104%) but also in the bendingstrength (� 46%). Furthermore, the observed signifi-cant enhancement in the fracture toughness has beensubstantiated by direct evidence of a fiber-tougheningmechanism through a nanoscale dispersion of highlydisordered MWNTs.

Thermal Conductivity

The temperature dependence of thermal conduc-tivity of alumina and hybrid nanocomposites is present-ed in Fig. 5. The thermal conductivity (lTC) of thesamples was calculated from the thermal diffusivity co-efficient k, bulk density r, and specific heat Cp using thefollowing standard equation.

lTC ¼ krCp ð1Þ

In a multiphase material, the specific heat can beapproximated using the rule of mixture. In the presentstudy, the specific heat Cp of the composites was calcu-lated using the rule of mixture and the effect of tem-perature on Cp was also taken into account. The specificheat values for the MWNTs, SiC, and alumina wereobtained from the thermodynamic database40,41 and thespecific heat of graphite was used for MWNTs due tothe structural similarity of both carbon materials.42,43 Itis surprising to find that the thermal conductivity ofhybrid alumina composites first decreases up to 2 vol%of SiC and then increases for 3 vol% of SiC (Fig. 5).The effective thermal conductivity of a composite is re-lated to the conductivities of the matrix and inclusions.In the present study, firstly, the decrease in thermalconductivity by addition of 5 vol% of MWNT for 1 and2 vol% of SiC may be attributed to the low thermalconductivity of MWNT ropes, because the effective re-sistance between two nanotubes drastically reduces thethermal conductivity of the ropes in comparison withindividual MWNT. Secondly, the interactions betweennanotubes and the surrounding matrix provide suffi-cient scattering of phonons to reduce the effective con-ductivity of the composites. It has been reported that thethermal conductivity is controlled by interface thermalconductance, which can significantly alter the effectivethermal conductivity of CNTs reinforced composites.44

Because of interfacial thermal resistance, a decrease inthermal conductivity by addition of CNTs has been re-ported in ceramics by other authors.8,45 The addition of

Fig. 5. Thermal conductivity of alumina and hybridnanocomposites.


1 and 2 vol% of SiC nanoparticles further decreases thethermal conductivity of the composites possibly due todominant effect of interface thermal resistance. In ad-dition, interfacial imperfections, induced by the mis-match of the thermal expansion coefficient betweenalumina and SiC nanoparticles, also affect the thermalconductivity of the composites. The scattering of pho-non from these imperfections, such as internal residualstresses, dislocations, and microcracks, also reduces thethermal conductivity as reported by others.46,47 Thedecrease in the thermal conductivity of the compositesby addition of SiC due to interface thermal resistancehas also been reported earlier.48 The combined effect oflow thermal conductivity of CNT ropes, interface ther-mal resistance, or Kapitza resistance between the al-umina matrix and MWNTs/SiC due to scattering ofphonon may be the possible causes of decreases in ther-mal conductivities for 1 and 2 vol% of SiC-reinforcedhybrid alumina composites. At 3 vol% of SiC, the in-trinsically high thermal conductivity of SiC may dom-inate the interfacial resistance and thermal conductivitystarts increasing. Another possibility of increase in ther-mal conductivity at 3 vol% of SiC may be that the al-ready existing percolating network of MWNTs maycome in contact with SiC nanoparticles and may form alarge network of heat conduction paths by hybrid filler.Thus lowering the interface density or in turn interfaceresistance that may result in increase of thermal con-ductivity as reported for 30 vol% SiC/MoSi2 compos-ites.48 In our study, the high aspect ratio of MWNTsand probably the enhanced connectivity achieved in ahybrid filler have reduced this value to only 3 vol% forthe alumina matrix. Because of the intragranular frac-ture mode of the hybrid nanocomposites, it is somewhatdifficult to show the hybrid percolation network at thegrain boundaries.

Electrical Conductivity

MWNTs have been used successfully to improvethe electrical conductivity of insulating alumina in ahybrid combination. The room-temperature dc electri-cal conductivity improved around 12 orders of magni-tude from monolithic alumina to 1 vol% of SiC and5 vol% of MWNTs reinforced hybrid alumina compos-ites and achieved a maximum value of � 9 S/m for3 vol% of SiC and 5 vol% of MWNTs (Fig. 6). Thisvalue is higher than the previously observed value inCNT/alumina composites for the same volume% of

MWNTs.49 It is well known that the 5 vol% of CNTs iswell above the percolation threshold in insulating al-umina due to their high aspect ratio. The switchingfrom electrically insulating alumina to electrically con-ducting alumina is mainly due to the addition ofMWNTs, which provides a spanning network ofCNTs traveling along the grain boundaries throughthe alumina matrix. Furthermore, SiC is a semiconduc-tor and its room-temperature electrical conductivity var-ies in a wide range, and is highly microstructuredependent.50,51 The results suggest that the additionof SiC may contribute to the improvement of the elec-trical conductivity of the composites. The conductivityincreases slowly with an increase of SiC content from 1to 2 vol%, while at 3 vol% of SiC a sharp increase hasbeen observed. The key difference between thermaltransport and electrical transport is the value of theconductivity ratio between the matrix and the filler. Forthermal transport, even for very conductive CNTs, theratio between CNTs and matrix is about 103–104, whilefor electrical transport the ratio of conductivities can beof the order of 1012–1016. Therefore, in case of electricalconductivity with such a high ratio, the effective path isthrough CNTs, while in case of thermal conductivity,the dominant channel of heat energy flow always in-volves the matrix.52 The 5 vol% of MWNTs alreadymaintained an effective percolation conductive pathspanning through the matrix. The addition of SiC upto 2 vol% increased the electrical conductivity slowly,while at 3 vol% a sharp increase was observed. This maybe attributed to the combined effect of MWNTs and

Fig. 6. Variation of electrical conductivity of alumina and hybridnanocomposites with volume fraction of SiC at room temperature.


SiC nanoparticles. At 3 vol% of SiC, the hybrid filler maybecomes a percolating network with a combination ofMWNTs and SiC nanoparticles, which means the attach-ment of isolated SiC particles with the already existingpercolating networks of MWNTs. This may result in areduction in the interfacial electrical resistance. It has beenreported recently that when the CNT content is largerthan the percolation threshold, a small decrease in theinterface resistance has a large effect on the effective elec-trical conductivity of the composites53 and this may resultin a sharp increase in the conductivity for 3 vol% of SiC-reinforced hybrid nanocomposites.

Conclusions

Hybrid alumina composites reinforced by different(1, 2, and 3 vol%) contents of SiC nanoparticles andwith concurrent reinforcement of MWNTs (5 vol%)were fabricated by spark plasma sintering. The signifi-cant improvements in fracture toughness and bendingstrength are substantiated by fiber toughening mecha-nisms through MWNTs and grain boundary strength-ening by SiC nanoparticles, respectively. The additionof low volume fractions of SiC nanoparticles and 5 vol%of MWNTs results in conversion of insulating aluminainto electrically conducting composites, with a maximumvalue of � 9 S/m. The thermal conductivity decreaseswith an increase of SiC contents (1 and 2 vol%), sug-gesting the dominance of interfacial thermal resistanceand imperfections caused by SiC nanoparticles. Only3 vol% of SiC is sufficient to diminish the interfacial re-sistance effects for both thermal and electrical conduc-tivities of the composites. The concurrent improvementsin the mechanical and electrical properties of the com-posites without a deterioration in their intrinsic proper-ties, have numerous applications in various fields.

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multifunctional properties of alumina composites reinforced by a hybrid filler

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