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Available online at www.sciencedirect.com

Journal of the European Ceramic Society 32 (2012) 3297–3301

Effects of MgO addition on the microwave dielectric properties of highthermal-conductive silicon nitride ceramics sintered with ytterbia as

sintering additives

Hiroyuki Miyazaki ∗, Kiyoshi Hirao, Yu-ichi YoshizawaNational Institute of Advanced Industrial Science and Technology (AIST), Anagahora 2266-98, Shimo-shidami, Moriyama-ku, Nagoya 463-8560, Japan

Received 17 October 2011; received in revised form 10 April 2012; accepted 16 April 2012Available online 7 May 2012

bstract

mol% of MgO was added together with 7 mol% of Yb2O3 as sintering additives to silicon nitride powder to fabricate advanced silicon nitrideeramics with both high thermal conductivity and low dielectric loss at 2 GHz. The mixed powder was CIPed at a pressure of 120 MPa and was gas-

◦ −1 −1

ressure sintered at 1900 C to >98% of theoretical density. The sintered Si3N4 sample exhibited a high thermal conductivity of ∼100 W m Knd a loss tangent (tan δ) of ∼4 × 10−4, concurrently. The tan δ was further reduced by half after the heat treatment at 1300 ◦C for 24 h. Themprovement in tan δ due to the annealing was explained from the point of crystallization of the intergranular glassy phase.

2012 Elsevier Ltd. All rights reserved.

eywords: Sintering; Dielectric properties; Si3N4; Insulators

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. Introduction

Both high thermal conductivity and low dielectric loss aressential for advanced ceramic insulators used in microwaveevices for semiconductor processing industries since the lowhermal conductivity and the microwave absorption of the com-onents result in heterogeneous temperature profile which canamage the quality of the products. Among the several struc-ural ceramics, silicon nitride (Si3N4) is the most attractive

aterial because it possesses superior thermal conductivityf >100 W m−1 K−1 and excellent mechanical properties.1–3

oreover, the loss tangent (tan δ) of the chemically vapor-eposited Si3N4 is very low, ∼10−4.4,5

However, the reported microwave tan δ for silicon nitrideeramics varied notably from 2 × 10−3 to 2 × 10−1 dependingn the composition and the sintering method,4,6–13 all of which

4,5

ere much higher than those of the pure ones, while the varia-ion in the permittivity was not so wide and in the range of 3–10.hese literature’s data imply that reducing the tan δ for sintered

∗ Corresponding author. Tel.: +81 52 736 7486; fax: +81 52 736 7405.E-mail addresses: h-miyazaki@aist.go.jp (H. Miyazaki), k-hirao@aist.go.jp

K. Hirao), y.yoshizawa@aist.go.jp (Y.-i. Yoshizawa).

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955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jeurceramsoc.2012.04.025

i3N4 to the intrinsic value is a main subject to minimize theicrowave absorption since the absorption of microwave is the

roduct of the permittivity and the tan δ. Authors investigated theffect of heat treatment on the microwave dielectric properties ofi3N4 sintered with Yb2O3 and SiO2 as sintering additives andhowed that crystallization of the intergranular glass phase wasffective for lowering tan δ to 3 × 10−4.14 Furthermore, Si3N4ith high thermal conductivity of ∼100 W m−1 K−1 and low

an δ of 1.4 × 10−4 was successfully gas-pressure sintered bydding 7 mol% of Yb2O3 to the starting powder.15 However,n extremely high cold isostatic pressure (CIP) of 450 MPa isecessary to densify the sample fully, which hinders industrialroduction of the large components. Therefore, densification ofhe compacts CIPed at a lower pressure has been required forhe practical applications of the high thermal-conductive Si3N4ith low dielectric loss.The lack of sinterability of the Si3N4-Yb2O3 system, which

s thought to originate from the high viscosity of the glasselt at the sintering temperature,16 has been overcome by hot

ressing17–19 or by an addition of a small amount of MgSiN21,2

1,2,20 21,22

r MgO or Al2O3 for gas-pressure sintering. Mechani-al and thermal properties of those densified silicon nitrides haveeen extensively investigated. These studies have shown thathe high-temperature strength could be improved by increasing

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he addition of Yb2O3 to 5–7 mol%16–18 and that the ther-al conductivity could be also enhanced up to 93 W m−1 K−1

hen 5 mol% of Yb2O3 and 5 mol% of MgO were addedoncurrently.20 However, few systematic studies have been per-ormed on the microwave dielectric properties of those excellentilicon nitrides. In this study, a small amount of MgO was addeds the second sintering aid to increase the density of the gas-ressure sintered silicon nitride with high thermal conductivity.oth effects of the MgO addition and the thermal annealingn the microwave dielectric performance were evaluated andiscussed in conjunction with the chemical composition of thelassy phase and its microstructural evolution due to the heatreatment.

. Experimental procedures

The starting powders used in this study were �-Si3N4 pow-er (SN-E10, Ube Industries, Ltd., Japan), MgO (1000A, Ubendustries, Ltd., Japan) and Yb2O3 (Nihon Yttrium Co. Ltd.,apan). Two types of Si3N4 ceramics, one with 7 mol% of Yb2O3hereafter 7Yb) and the other with 1 mol% of MgO and 7 mol%f Yb2O3 (hereafter 7Yb1M) were fabricated by the follow-ng procedure. The starting powders were mixed by planetary

illing in methanol using Si3N4 balls and a Si3N4 pot for 1 h.he slurry was dried, and then passed through 60 mesh sieve.he mixed powders were formed into compacts of about Ø5 mm × 100 mm in size under an isostatic pressure of 120 MPa.hese compacts were set in a BN crucible with BN packing pow-er and sintered at 1900 ◦C for 3 h in a 0.9 MPa N2 atmospheresing a graphite resistance furnace. For comparison, 7Yb pow-er was also compacted using a CIPing pressure of 450 MPand sintered by the same procedure (hereafter 7Yb+), whichas reported in our previous paper.15 Heat treatments of the

s-sintered bodies were performed subsequently at 1300 ◦C for4 h using the same equipment, since it has been reported that theevitrification of intergranular glass phase took place between250 ◦C and 1450 ◦C for 12–24 h.21,22

The crystalline phases in the specimens were identified by-ray diffraction (XRD). The densities of the as-sintered bodies

nd the annealed samples were measured using the Archimedesechnique on the machined samples. Relative densities werevaluated using theoretical density calculated from an arithmeticean of the starting composition. The machined samples were

olished and plasma etched in CF4 gas before microstructuralbservation by scanning electron microscopy. For the measure-ents of dielectric properties, specimens with the dimensions

f 1.5 mm × 1.5 mm × 80 mm were prepared from the centralart of each sample. Dielectric constant (ε′) and dielectric losstan δ) were measured at 2 GHz by the perturbation method usingcavity resonator and a vector network analyzer (HP8722ES).isk specimens (10 mm in diameter and 3 mm thick) were

lso machined from both sintered and annealed bodies. Both

ides of the pellet samples were coated with a 60 nm thickayer of gold using a sputter coater (SC-701AT, Sanyu Electrono. Ltd., Tokyo), followed by a subsequent graphite spray-oating (Bonny-phite, Nippon Graphite Industries, Ltd., Japan)

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Ceramic Society 32 (2012) 3297–3301

o increase the amount of energy absorbed. The thermal diffu-ivity of the samples was measured using the laser-flash method.hermal conductivity was obtained by the product of the ther-al diffusivity, density and specific heat of the samples. In this

tudy, a constant value of the specific heat, ∼0.68 J g−1 K−1 wassed for the calculation.1

. Results and discussion

.1. As-sintered specimens

Both weight loss and shrinkage during the sintering forach sample are presented in Table 1. The weight losses forll samples were not so significant, in the range of 1.7–2.1%.he shrinkages of two samples without MgO addition (7Ybnd 7Yb+) were about 17%. However, the sample CIPed at20 MPa (7Yb) could not be densified to more than 87% ofheoretical density, while the sample CIPed at 450 MPa (7Yb+)eached 97.6% of theoretical density due to its high green den-ity. The shrinkage increased up to 21% when 1 mol% MgOas added to 7Yb1M sample, leading to enhanced densifica-

ion despite of the lower green density in the CIPed compact.t is clear that the MgO addition was effective for sinteringf silicon nitride, which is in good agreement with the earlytudies.1,2,20 In the following, the properties of 7Yb sampleere not evaluated due to its poor relative density. Fig. 1 showsEM micrographs of 7Yb+ and 7Yb1M samples. Microstruc-

ures of both samples were almost the same. Fig. 2 shows XRDatterns of the two as-sintered samples. The diffraction peaksf the MgO added sample (7Yb1M) resembled to that of non-gO added sample (7Yb+) and Yb4Si2O7N2 was identified as

he secondary phase in both samples. Previous observations ofhe microstructure of silicon nitrides sintered with 5–10 mol%f Yb2O3 by using transmission electron microscopy revealedhat crystalline Yb4Si2O7N2 was formed in the major triple-oint pockets but the crystallization was incomplete.17,18,21,22 Its natural to suppose that a small amount of glass phase remainedn our samples as well and Mg2+ ion existed in the glassy phase inhe 7Yb1M sample since no crystalline phase containing Mg wasetected.

The thermal conductivity and dielectric constant (ε′) arehown in Table 2. The thermal conductivities of both samplesere more than 94 W m−1 K−1, which was almost the same

s that of silicon nitrides sintered with 5 mol% of Yb2O3 andmol% of MgO.20 The variation of ε′ between the samples was

mall, 7.6–7.8, indicating that ε′ was unsusceptible to the compo-ition of secondary phases, which is consistent with our previousorks.14,15

By contrast, the dielectric loss (tan δ) increased significantlyrom 1.4 × 10−4 to 4.1 × 10−4 when MgO was added to theample (Fig. 3). It is likely that the compositional change inntergranular glassy phase (Mg2+ ion and the ratio of oxygen to

itrogen) deteriorated the tan δ of the 7Yb1 M sample since theicrostructure and the crystalline phases of both samples were

lmost identical. Clarke and Ho found that the loss tangent of theot-pressed Si3N4 with MgO was controlled by the dielectric

H. Miyazaki et al. / Journal of the European Ceramic Society 32 (2012) 3297–3301 3299

Table 1Weight loses (�w/w0) and shrinkages (�l/l0) during the sintering and relative densities of silicon nitride samples sintered with Yb2O3 and MgO as sintering additives.

Sample CIPing pressure (MPa) Relative density ofthe compact (%)

�w/w0 (%) �l/l0 (%) Relative density (%)a

7Yb+ 450 55.4 1.7 16.8 97.67Yb 120 47.3 2.1 17.8 86.57Yb1M 120 45.6 1.9 21.0 98.4

a Relative density was obtained using the machined sample.

Table 2Thermal conductivity and dielectric constant at 2 GHz of the densified silicon nitride samples both before and after the heat treatment.

Sample Thermal conductivity (W m−1 K−1) Dielectric constant

As-sintered Annealed As-sintered Annealed

7Yb+ 97 98 7.6 7.67Yb1M 94 97 7.8 7.7

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ig. 1. SEM micrographs of the Si3N4 samples sintered with (a) 7 mol% ofb2O3 (7Yb+) and (b) 7 mol% of Yb2O3 and 1 mol% of MgO (7Yb1M).

roperties of the intergranular glass phase, and presented theollowing relationship,4

an δ ∝ nq2

η(1)

here n is the number of mobile ions in the glassy phase, q ishe charge of the ion, and η is the viscosity of the glassy phase.pparently, the glassy phase in the 7Yb1M sample possessed

Fig. 3. Loss tangent of the Si3N4 samples sintered with 7 mol% of Yb2O3

(7Yb+) and with 7 mol% of Yb2O3 and 1 mol% of MgO (7Yb1M).

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ore mobile ions due to the addition of MgO as compared withhat in the 7Yb+ sample. However, the increments in mobile ionslone cannot account for the increased tan δ since Kume et al.eported a decrease of tan δ of sintered AlN by the addition of

g3N2.23 The glassy phase was composed Mg3N2, Y2O3 andl2O3 (as an impurity of the surface of AlN). More mobile ions

n the glassy phase introduced by the addition of Mg3N2 shouldeteriorate the tan δ of sintered AlN. However, it was reportedhat the viscosity of the Y–Si–Al–O–N glass increased by >2rders of magnitude when 18 equivalent percent nitrogen wasubstituted for oxygen.24 It is natural to suppose that the viscos-ty of glassy phase in AlN was also increased significantly by the

ore nitrogen content from the Mg3N2 additive. Accordingly,he improvement of tan δ of the sintered AlN could be reason-bly explained by the enormous reducing effect of the viscosityf glassy phase.

By contrast, it is expected that the ratio of oxygen to nitrogenn the glassy phase of our 7Yb1M sample was increased by theddition of MgO, leading to the lower viscosity of the glass. Theffect of MgO addition on the viscosity of the glassy phase canlso be deduced on the analogy of the effect of Al2O3 additionn the viscosity of the glassy phase in silicon nitrides sinteredith 5–10 mol% of Yb2O3. Table 3 shows room and high tem-erature strength of silicon nitrides with Yb4Si2O7N2 reportedreviously.17,18,22 Heat resistance is given by dividing high tem-erature strength (σHT) by room temperature strength (σRT). Theeat resistance was higher than 0.75 when no Al2O3 was added,hereas that of Al2O3 added sample was reduced significantly to.38. The results suggest that viscous flow occurred at elevatedemperature by the addition of Al2O3 into the glassy phase andhus deteriorated the high temperature strength. It is reasonableo suppose that the addition of MgO into the glass decreased theiscosity of the glassy phase just like Al2O3 did. Therefore, thencrease in tan δ due to the addition of MgO could be reasonablyxplained by using Eq. (1). The result indicates that the low vis-osity of the glass melt by the addition of MgO was favorableor the easier sintering but was adversely harmful for loweringhe dielectric loss, so that it seems difficult to combine sinter-bility and low tan δ value for as-sintered silicon nitrides withigh thermal conductivity.

.2. Annealed specimens

Fig. 4 shows XRD patterns of the samples after the annealing.he Yb4Si2O7N2 peaks became larger in both samples after theeat treatment. Thus the glass phase in the as-sintered samplesas crystallized by the thermal annealing for 24 h. The weak

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ot pressing – –ot pressing – –as press. sint. 0.5 –as press. sint. 0.5 1450 ◦C for 24 h

a) 7 mol% of Yb2O3 (7Yb+) and (b) 7 mol% of Yb2O3 and 1 mol% of MgO7Yb1M).

eaks of Yb2SiO5 appeared in the 7Yb+ sample, but not in theYb1M sample. The reason of the absence of Yb2SiO5 in theYb1 M sample was unclear but it could be deduced that theresence of small amount of Mg2+ ion in the intergranular glasshase may prevent the crystallization of the Yb2SiO5 phase.able 2 demonstrates a slight increase in thermal conductivityor both samples. It is conjectured that the increment in ther-al conductivity of the two samples could be attributed to the

rystallization of the glassy phase. The dielectric constants wereardly affected by the annealing, suggesting again the lack ofusceptibility to the secondary phases.

By contrast, the tan δ for the 7Yb1M sample was dimin-shed by half from 4.1 × 10−4 to 2.1 × 10−4 after the annealingFig. 3), while that for the heat treated 7Yb+ remained almosthe same. It is obvious that the significant decrease of tan δ forhe 7Yb1M sample was associated with the crystallization of Mgontaining glassy phase which has the great contribution to tan δ

s described above. Huang et al. reported that by post-sinteringhe major part of amorphous phase was completely crystallizedn the silicon nitride sintered with 10 vol% Yb2O3 and 0.5 vol%l2O3, which improved the heat resistance to 0.75 as shown

n Table 3.22 It seems reasonable to suppose on the analogy ofuang’s study that the residual glass phase in the triple-pointockets in the 7Yb1M sample was also entirely devitrified by

he heat treatment, which decreased the tan δ markedly.

The mechanism of the slightly increased tan δ in 7Yb+ sam-le after annealing is not clear at this point. It is obvious that

4Si2O7N2 and silicon nitrides with Yb4Si2O7N2 and Al2O3.

Strength (MPa) σHT/σRT Ref.

RT HT

1080 890(1400 ◦C) 0.82 [17]977 728(1350 ◦C) 0.75 [18]776 293(1350 ◦C) 0.38 [22]690 515(1350 ◦C) 0.75 [22]

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he contribution of glassy phase in 7Yb+ to tan δ was negligibleince tan δ of the sample was almost identical to that of pureilicon nitride fabricated by CVD.4,5 Thus the crystallization ofuch glassy phase will hardly cause any reduction in tan δ. Byontrast, it is likely that the valence of small amounts of Yb ionsn the glassy phase may be decreased from 3+ to 2+ after anneal-ng in the reducing atmosphere. Hopping conduction betweenons with different valences is the major origin of the electricalonductivity of glasses containing transition metal oxides. It isonjectured that the hopping conduction might occurred slightlyn the intergranular glassy phase of 7Yb+ sample after annealing,hich resulted in small increments in the tan δ. Further study iseeded to clarify this point.

. Conclusions

Silicon nitrides could be densified to more than 98% ofhe theoretical by adding both Yb2O3 and MgO as sinter-ng additives regardless of the poor compact density due toIPing at 120 MPa. The MgO added Si3N4 showed high tan δ of.1 × 10−4 as compared to the sample without MgO addition,hich could be attributed to both the presence of Mg2+ ion and

he increased ratio of oxygen to nitrogen in the intergranularlass phase. However, the thermal annealing at 1300 ◦C for 24 heduced the tan δ by half, which could be explained by the crys-allization of the glassy phase. Both high thermal conductivityf ∼100 W m−1 K−1 and low dielectric loss of 2 × 10−4 wereuccessfully achieved for the silicon nitrides ceramics by bothhe addition of MgO and the heat treatment.

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