Sintered Reaction-Bonded Silicon Nitride with High Thermal Conductivity and High Strength

Sintered reaction-bonded silicon nitride (SRBSN) materials were prepared from a high-purity Si powder doped with Y₂O₃ and MgO as sintering additives by nitriding at 1400°C for 8 h and subsequently postsintering at 1900°C for various times ranging from 3 to 24 h. Microstructures and phase compositions of the nitrided and the sintered compacts were characterized. The SRBSN materials sintered for 3, 6, 12, and 24 h had thermal conductivities of 100, 105, 117, and 133 W/m/K, and four-point bending strengths of 843, 736, 612, and 516 MPa, respectively. Simultaneously attaining thermal conductivity and bending strength at such a high level made the SRBSN materials superior over the high-thermal conductivity silicon nitride ceramics that were prepared by sintering of Si₃N₄ powder in our previous works. This study indicates that the SRBSN route is a promising way of fabricating silicon nitride materials with both high thermal conductivity and high strength.     

Enhanced thermal conductivity and flexural strength of sintered reaction-bonded silicon nitride with addition of (Y0.96Eu0.04)2O3

Sintered reaction-bonded silicon nitride (SRBSN) with high thermal conductivity was obtained using (Y_0.96Eu_0.04)₂O₃ and MgO as sintering additives. Green compacts were nitrided at 1400°C for 4 h. Post-sintering was carried out at 1850 and 1900°C for 4 h, respectively. In reaction-bonded silicon nitride (RBSN) doped with Y₂O₃ and MgO, the β-Si₃N₄ content and nitridation degree were 51.1% and 93.8%, respectively. However, the β-Si₃N₄ content and nitridation degree were 72.6% and 96.7% in a nitrided compact doped with (Y_0.96Eu_0.04)₂O₃ and MgO. After post-sintering, the phase composition, microstructure, mechanical properties, and thermal conductivity were investigated. After sintering at 1900°C for 4 h, the thermal conductivity of SRBSN doped with (Y_0.96Eu_0.04)₂O₃ and MgO was increased by 16.5% compared to that of the samples doped with Y₂O₃ and MgO. The highest hardness of 1639 HV and the good flexural strength of 776.4 MPa were also achieved in the sample doped with 2-mol.% (Y_0.96Eu_0.04)₂O₃ and 5-mol.% MgO.     

Electrical resistivity of silicon nitride produced by various methods

It has been shown that the grain growth and amount of the glass phase influence the electrical resistivity of pressureless sintered and spark plasma sintered silicon nitride. Sintering additives strongly affect the impurity conductivity of pressureless sintered silicon nitride and slightly influence the intrinsic conductivity due to the longer sintering process as compared with the spark plasma sintering. It was demonstrated that Al₂O₃-Y₂O₃ lead to decrease in the electrical resistivity of SPSed silicon nitride due to increase in the band gap width as opposed to Al₂O₃-MgO. Effect of the sintering additive on the impurity conductivity is practically absent but there is a strong dependence of the sintering temperature for reported spark plasma sintered silicon nitride. However, intrinsic conductivity of SPSed silicon nitride is affected by both sintering temperature and sintering additive. It was also shown that electrical resistivity of produced ceramics is linearly depends on the content of β-Si₃N₄ and microhardness. Electrical resistivity of manufactured silicon nitride varied from 3.16·10⁹ to 1.73·10¹¹ Ω?m. It has been observed strong influence of the sintering additive and sintering temperature on the electrical properties of SPSed and pressureless sintered silicon nitride.     

Effects of microstructure and intergranular glassy phases on thermal conductivity of silicon nitride

In this study, the binary sintering additives Y₂O₃-Sc₂O₃, were first applied to the Si₃N₄ system to investigate their effects on microstructure and thermal conductivity. The microstructure and thermal conductivity of both sintered silicon nitride (SSN) and sintered reaction-bonded silicon nitride (SRBSN) were found to be significantly dependent on the additive composition. Among various combinations of Y₂O₃ and Sc₂O₃, 1 mol% Y₂O₃−3 mol% Sc₂O₃ prominently enhanced thermal conductivity, and the enhancement could not be attributed to any difference in microstructure or lattice defects. TEM observation revealed that this composition was more liable to devitrify the glassy phase with a lower degree of stress accumulation, and to possibly produce a grain boundary that was cleaner or with a higher order of atomic arrangement. A microstructure model for thermal conductivity was proposed which took the thermal resistance of the grain boundaries into account. The grain boundary state exerted a remarkable influence on the thermal conductivity of fine microstructures, and the experimentally measured thermal conductivity values were consistent with those given by the proposed model.     

Cost effective preparation of Si3N4 ceramics with improved thermal conductivity and mechanical properties

High-purity silicon powder is used as the starting material for cost-effective preparation of silicon nitride ceramics with both high thermal conductivity and excellent mechanical properties using RE₂O₃ (RE=Y, La or Er) and MgO as sintering additives. Nitridation is a key procedure that would affect the properties of green bodies and the sintered samples. The β: (α+β) ratio can be increased as the samples nitrided at 1450ºC and a large amount of long rod-like β-Si₃N₄ grains were developed in the samples. It was found that the addition of Er₂O₃-MgO could help to improve the mechanical properties of the sintered Si₃N₄ ceramics, the thermal conductivity, flexural strength and fracture toughness of the sample were 90 W/(m∙K), 953±28.3 MPa and 10.64±0.61 MPa·m^1/2, respectively. The RE³⁺ species with larger ionic radius tended to increase the oxygen of nitrided samples and decrease N/O ratio (triangle grain boundary) of sintered samples.     

Investigation on thermal conductivity and mechanical properties of Si3N4 ceramics via one-step sintering

High thermal conductivity Si₃N₄ ceramics were fabricated using a one-step method consisting of reaction-bonded Si₃N₄ (RBSN) and post-sintering. The influence of Si content on nitridation rate, β/(α+β) phase rate, thermal conductivity and mechanical properties was investigated in this work. It is of special interest to note that the thermal conductivity showed a tendency to increase first and then decrease with increasing Si content. This experimental result shows that the optimal thermal conductivity and fracture toughness were obtained to be 66 W (m K)^-1 and 12.0 MPa m^1/2, respectively. As a comparison, the nitridation rate and β/(α+β) phase rate in a static pressure nitriding system, i.e., 97% (MS10), 97% (MS15), 97% (MS20) and 8.3% (MS10), 8.3% (MS15), 8.9% (MS20), respectively, have obvious advantages over those in a flowing nitriding system, i.e., 91% (MS10), 91% (MS15), 93% (MS20) and 3.1% (MS10), 3.3% (MS15), 3.3% (MS20), respectively. Moreover, high lattice integrity of the β-Si₃N₄ phase was observed, which can effectively confine O atoms into the β-Si₃N₄ lattice using MgO as a sintering additive. This result indicates that one-step sintering can provide a new route to prepare Si₃N₄ ceramics with a good combination of thermal conductivity and mechanical properties.     

Effects of yttria and magnesia on densification and thermal conductivity of sintered reaction-bonded silicon nitrides

A variety of combinations of Y₂O₃ and MgO were used as additives in preparing Si₃N₄ ceramics by the sintering of reaction-bonded silicon nitride (SRBSN) method. By varying the amount of Y₂O₃ in the range of 0-5 mol% and that of MgO in the range of 0-8 mol%, the effects of Y₂O₃ and MgO additives on nitridation and sintering behaviors as well as thermal conductivity were studied. It was found that appropriate amount and combination of Y₂O₃ and MgO additives were essential for attaining full densification and achieving high thermal conductivity. The sample doped with 2.5 mol% of Y₂O₃ and 5 mol% of MgO attained a thermal conductivity of 128 Wm⁻¹K⁻¹ when sintered at 1900°C for 6 hours, and the sample doped with 2 mol% of Y₂O₃ and 4 mol% of MgO achieved a thermal conductivity of 156 Wm⁻¹K⁻¹ when sintered for 24 hours.     

Effects of processing method and additive composition on microstructure and thermal conductivity of Si3N4 ceramics

A comparative investigation of sintered reaction-bonded silicon nitride (SRBSN) and sintered silicon nitride (SSN) was carried out by using two additive compositions of Y₂O₃–MgO and Yb₂O₃–MgO. The effects of additive composition on the processing, microstructure and thermal conductivity of both SRBSN and SSN materials were evaluated. It is shown that the replacement of Yb₂O₃ by Y₂O₃ has no significant effect on the densification, microstructure and thermal conductivity of SSN materials, but it has a large effect on the processing, microstructure and thermal conductivity of SRBSN materials. The replacement of Yb₂O₃ by Y₂O₃ results in highly bimodal microstructural development during post-sintering and this may be attributed to the formation of large rodlike β-nuclei during nitridation. Although the replacement of Yb₂O₃ by Y₂O₃ leads to the production of fully dense SRBSN materials, but it leads to a decrease in thermal conductivity, which is consistent with microstructural difference. More importantly, the present work implies that SRBSN materials with comparable thermal conductivity to SSN materials could be achieved.     

Defect structures and dopant solution states of Hf-doped Si3N4 ceramics

Transition-metal-doped silicon nitride ceramics have attracted much attention as gate materials for semiconductors because of their electrical properties as well as chemical and thermal stability. The present study aims to clarify the defect structures of Hf-doped β-Si₃N₄ by theoretical calculations and scanning transmission electron microscopy (STEM). First-principles calculations based on a hybrid functional method were performed. It was found that Hf dopants are mainly substituted for the Si sites and can be occasionally located at interstitial sites in the lattice of β-Si₃N₄. The substitution sites of Hf dopants predicted by the first-principles calculations were also confirmed by the high-resolution STEM images.     

Improved thermal conductivity of sintered reaction-bonded silicon nitride using a BN/graphite powder bed

Sintered reaction-bonded silicon nitride (SRBSN) with improved thermal conductivity was achieved after the green compact of submicron Si powder containing 4.22 wt% impurity oxygen and Y₂O₃-MgO additives was nitrided at 1400 °C for 6 h and then post-sintered at 1900 °C for 12 h using a BN/graphite powder bed. During nitridation, the BN/10 wt% C powder bed altered the chemistry of secondary phase by promoting the removal of SiO₂, which led to the formation of larger, purer and more elongated Si₃N₄ grains in RBSN sample. Moreover, it also enhanced the elimination of SiO₂ and residual Y₂Si₃O₃N₄ secondary phase during post-sintering, and thus induced larger elongated grains, decreased lattice oxygen content and increased Si₃N₄-Si₃N₄ contiguity in final SRBSN product. These characteristics enabled SRBSN to obtain significant increase (∼40.7%) in thermal conductivity from 86 to 121 W ∙ m⁻¹ ∙ K⁻¹ without obvious decrease in electrical resistivity after the use of BN/graphite instead of BN as powder bed.     

Effect of Sintering Additive Composition on the Processing and Thermal Conductivity of Sintered Reaction-Bonded Si3N4

Two compositions of the Y₂O₃–MgO (YM) and Yb₂O₃–MgO (YbM) systems were chosen to study the effect of the sintering additive composition on the processing and thermal conductivity of sintered reaction-bonded silicon nitride (SRBSN). The nitridation, postdensification, microstructural evolution, and thermal conductivity of SRBSN were found to depend strongly on the sintering additive composition. The RBSN materials with YbM exhibited a poor sinterability, whereas those with YM exhibited an excellent sinterability. However, the SRBSN materials with YbM showed a higher thermal conductivity than those with YM. This was associated primarily with the isolated distribution and lower amount of secondary phase and the higher percentage of large grains in the former materials.     

Enhanced thermal conductivity in Si3N4 ceramic by addition of a small amount of carbon

In order to fabricate Si₃N₄ ceramic with enhanced thermal conductivity, 93 mol%α-Si₃N₄-2 mol%Yb₂O₃-5 mol%MgO powder mixture was doped with 5 mol% carbon, and sintered firstly at 1500 °C for 8 h and subsequently at 1900 °C for 12 h under 1 MPa nitrogen pressure. During the first-step sintering, the carbothermal reduction process significantly reduced the oxygen content and increased the N/O ratio of intergranular secondary phase, resulting in the precipitation of Yb₂Si₄O₇N₂ crystalline phase, higher β-Si₃N₄ content and larger rod-like β-Si₃N₄ grains in the semi-finished Si₃N₄ sample. After the second-step sintering, the final dense Si₃N₄ product acquired coarser elongated grains, lower lattice oxygen content, tighter Si₃N₄-Si₃N₄ interfaces and more devitrified intergranular phase due to the further carbothermal reduction of oxynitride secondary phase. Consequently, the addition of carbon enabled Si₃N₄ ceramic to gain a significant increase of ∼25.5% in thermal conductivity from 102 to 128 W∙m⁻¹ K⁻¹.     

Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride. I: Effect of Si Powder Characteristics

This paper will present sintered reaction-bonded silicon nitride (SRBSN) material with a high thermal conductivity of 121 W·(m·K)⁻¹, which has been successfully prepared from a coarse Si powder with lower levels of oxygen and aluminum impurities, using a mixture of Y₂O₃ and MgSiN₂ as sintering additives, by nitriding at 1400°C for 8 h and subsequent post-sintering at 1900°C for 12 h at a nitrogen pressure of 1 MPa N₂. This thermal conductivity value is higher than that of the materials prepared from high-purity α-Si₃N₄ powder (UBE SN-E10) with the same additive composition under the same sintering conditions. In order to study the effects of Si powder characteristics on the processing, microstructure, and thermal conductivity of SRBSN, the other type of fine powder with higher native oxygen and metallic impurity (typically Al and Fe) contents was also used. The effects of Si particle size, native oxygen, and metallic impurities on the nitriding process, post-sintering process, and thermal conductivity of the resultant SRBSN materials were discussed in detail. This work demonstrates that the improvement in thermal conductivity of SRBSN could be achieved by using higher purity coarse Si powder with lower levels of oxygen and aluminum impurities. In addition, this work also shows that the nitriding temperature has no significant effect on the microstructure and thermal conductivity of SRBSN during post-sintering, although it does affect the characteristics of RBSN formed during nitridation.     

Processing of micro-components made of sintered reaction-bonded silicon nitride (SRBSN). Part 1: Factors influencing the reaction-bonding process

In order to establish a process for the manufacturing of injection moulded micro-components of sintered reaction-bonded silicon nitride (SRBSN) several process parameters were investigated with regard to their influence on the reaction-bonding step. One question to be answered was how the sintering aids affect the nitridation behaviour of a silicon green body. For the processing of micro-components it was of special interest to study, how a decreasing sample size and wall thickness would influence the rate of Si₃N₄ formation. By varying the added amounts of the sintering aids, it was found that increasing the Y₂O₃ and MgO contents both improved the nitridation rate, whereas an increase of Al₂O₃ content resulted in reduced nitridation rates. Within the investigated range of sample dimensions (0.2–4.0 g) the unexpected observation was made, that with decreasing sample weight the nitridation rate also decreased. This was explained by the exothermic nature of the reaction between Si and N₂ and the fact that small samples with a large surface-to-volume ratio attain thermal equilibrium with their environment better than large samples which may be subject to local overheating.     

Fabrication of fine-grained α/β Si3N4 by Hot Pressing Flowing Sintering at 1550 °C

In this paper, Hot Pressing Flowing Sintering (HPFS) was successfully applied to the consolidation of silicon nitride ceramics at low sintering temperature. Fine-grained α/β-Si₃N₄ with high density was fabricated by HPFS at 1550 °C. The low temperature densification process was based on the hot pressing flow enhanced particle rearrangement mechanism. The shear stress which generated in the flowing process helped enhance the particle rearrangement with the lubrication of the liquid phase, which lead to collapse of the pores. The densification process was finished with minimum grain growth. Meanwhile, the rod-like β-Si₃N₄ nuclei was textured during the flowing process. The (101)/(210) ratio of β-Si₃N₄ was 0.09 on the plane perpendicular to the hot-pressing direction. The Vickers hardness and flexural strength of the sintered sample was 19.59 ± 0.36 GPa and 1022 ± 138 MPa, respectively.     

Spark Plasma Sintering of fine alpha-silicon nitride ceramics with LAS for spatial applications

Many space systems such as satellite mirrors and their supporting structures require to be made from very low-thermal expansion materials combining both high hydrostability and relatively high mechanical properties. In this study, we have applied the “composite concept” in order to explore the possibility of fabricating near zero thermal expansion silicon nitride based ceramics. Consequently, a negative thermal expansion material belonged to the lithium aluminosilicate family (LAS powder crystallized under de β-eucryptite structure) was introduced in an alpha-silicon nitride fine powder (5 and 20 vol% of LAS) and the resulting composite system was sintered by Spark Plasma Sintering (SPS) at 1400 and 1500 °C. In the case of 20 vol% LAS compositions, relatively well-densified ceramics (94.4% of the theoretical density) were produced without adding any further sintering additive. The addition of yttria and alumina oxides allowed enhancing the densification level up to 98.2% (20 vol% LAS compositions) or from 62.3% up to 96.7% of the theoretical density in 5 vol% LAS materials. Nevertheless, it was impossible to full consolidate silicon nitride/LAS composite ceramics at temperatures lower than the temperature at which β-eucryptite melts, even by using SPS technology. Moreover, because of the relatively low temperatures involved in SPS, the α to β-Si₃N₄ transformation was avoided, resulting in microstructures composed of fine equiaxed α-Si₃N₄ grains (<200 nm) and of a glassy phase. Even if the effect of having a very large negative thermal expansion material was lost during the sintering step (because of the β-eucryptite melting), ceramics containing only 20 vol% of LAS-based phase exhibited very interesting values as regards of mechanical properties (strength, hardness, toughness, and Young's modulus), thermal conductivity and thermal expansion coefficient. We discuss in this work why we are so interested in developing dense silicon nitride/LAS ceramics sintered without any further additive addition, even though β-eucryptite is melted during the process and the transformation to the β phase is avoided.     

Improved thermal conductivity of β-Si3N4 ceramics by lowering SiO2/Y2O3 ratio using YH2 as sintering additive

A two-step sintering process was conducted to produce β-Si₃N₄ ceramics with high thermal conductivity. During the first step, native SiO₂ was eliminated, and Y₂O₃ was in situ generated by a metal hydride reduction process, resulting in a high Y₂O₃/SiO₂ ratio. The substitution YH₂ for Y₂O₃ endow Si₃N₄ ceramics with an increase of 29% in thermal conductivity from 95.3 to 123 W m⁻¹ K⁻¹ after sintered at 1900°C for 12 hours despite an inferior sinterability. This was primarily attributed to the purified enlarged grains, devitrified grain boundary phase, and reduced lattice oxygen content in the YH₂-MgO-doped material.     

ZrSi2–MgO as novel additives for high thermal conductivity of β-Si3N4 ceramics

A novel ZrSi₂–MgO system was used as sintering additive for fabricating high thermal conductivity silicon nitride ceramics by gas pressure sintering at 1900°C for 12 hours. By keeping the total amount of additives at 7 mol% and adjusting the amount of ZrSi₂ in the range of 0-7 mol%, the effect of ZrSi₂ addition on sintering behaviors and thermal conductivity of silicon nitride were investigated. It was found that binary additives ZrSi₂–MgO were effective for the densification of Si₃N₄ ceramics. XRD observations demonstrated that ZrSi₂ reacted with native silica on the Si₃N₄ surface to generate ZrO₂ and β-Si₃N₄ grains. TEM and in situ dilatometry confirmed that the as formed ZrO₂ collaborated with MgO and Si₃N₄ to form Si–Zr–Mg–O–N liquid phase promoting the densification of Si₃N₄. Abnormal grain growth was promoted by in situ generated β-Si₃N₄ grains. Consequently, compared to ZrO₂-doped materials, the addition of ZrSi₂ led to enlarged grains, extremely thin grain boundary film and high contiguity of Si₃N₄–Si₃N₄ grains. Ultimately, the thermal conductivity increased by 34.6% from 84.58 to 113.91 W·(m·K)⁻¹ when ZrO₂ was substituted by ZrSi₂.     

Effect of a new nonoxide additive,Y3Si2C2, on the thermal conductivity and mechanical properties of Si3N4 ceramics

Enhancement of the thermal conductivity of silicon nitride is usually achieved by sacrificing its mechanical properties (bending strength). In this study, β-Si₃N₄ ceramics were prepared using self-synthesized Y₃Si₂C₂ and MgO as sintering additives. It was found that the thermal conductivity of the Si₃N₄ ceramics was remarkably improved without sacrificing their mechanical properties. The microstructure and properties of the Si₃N₄ ceramics were analyzed and compared with those of the Y₂O₃-MgO additives. The addition of Y₃Si₂C₂ eliminated the inherent SiO₂ and introduced nitrogen to increase the N/O ratio of the grain-boundary phase, inducing Si₃N₄ grain growth, increasing Si₃N₄ grain contiguity, and reducing lattice oxygen content in Si₃N₄. Therefore, by replacing Y₂O₃ with Y₃Si₂C₂, the thermal conductivity of the Si₃N₄ ceramics was significantly increased by 31.5% from 85 to 111.8Wm⁻¹K⁻¹, but the bending strength only slightly decreased from 704 ± 63MPa to 669 ± 33MPa.     

Effect of β-Si3N4 starting powder size on elongated grain growth in β-Si3N4 ceramics

The microstructural evolution of pressureless sintered silicon nitride ceramics prepared from different particle sizes of β-Si₃N₄ as starting powders, has been investigated. When the specimen prepared from as-received β-powder of 0.66 μm in average size, was sintered at 1850°C, equiaxed β-Si₃N₄ grains were observed. As the size of the initial β-powder went down to 0.26 μm, however, the growth of elongated grains was enhanced, which resulted in a whisker-like microstructure similar to that made from α-starting powder. When the sintering temperature was increased to 2000°C, the elongated grains were also developed even in the specimen made from 0.66 μm β-powder. The observed results were discussed with relation to the two dimensional nucleation and growth theory for faceted crystals. In addition, fracture toughness of the specimen consisting of elongated grains, which was prepared from finer powders, increased.