Hideki Hyuga

A Tough Silicon Nitride Ceramic with High Thermal Conductivity

IO N The world is shifting energy sources from fossil fuel to electric power in order to cope with the energy and environmental problems. Driven by the demand for effi cient control and conversion of electric power, power electronic device technology is advancing toward higher voltage, larger current, greater power density, and smaller size, and this trend is poised to be accelerated with the replacement of Si by the wide-bandgap semiconductors (SiC and GaN) in the near future. [ 1 , 2 ] However, the high power will induce large thermal stresses in the devices, which pose great challenges for the assembly of the devices and the packaging materials, in particular the brittle ceramic substrates that provide functions of electrical insulation and heat dissipation. In many occasions, even the two high-grade ceramic substrate materials, AlN and Si 3 N 4 , become cracked due to low mechanical strength and fracture toughness (for AlN) or insuffi cient thermal conductivity (for Si 3 N 4 ). [ 3 , 4 ] The reliability problem caused by the ceramic substrates has become a bottleneck hindering the advancement of power device technology. Search for new ceramic materials with better thermomechanical properties is an urgent issue. Here, we show a new Si 3 N 4 ceramic that possesses a very high thermal conductivity (177 W m − 1 K − 1 ) along with a high fracture toughness (11.2 MPa m 1/2 ) and a high fracture strength (460 MPa). We expect this Si 3 N 4 will be used as the next-generation insulating substrate material for high-power electronic devices. Silicon nitride mainly exists in two hexagonal polymorphs, namely α and β -Si 3 N 4 , which are generally regarded as lowand high-temperature crystal forms, respectively. [ 5 , 6 ] As a highly covalent compound, Si 3 N 4 transports heat primarily by phonons at room temperature and below. In 1995, Haggerty and Lightfoot predicted that the intrinsic thermal conductivity of Si 3 N 4 might be 200 to 320 W m − 1 K − 1 at room temperature. [ 7 ] Later, Hirosaki et al. estimated that the intrinsic thermal conductivities of a β -Si 3 N 4 crystal were 170 and 450 W m − 1 K − 1 along the a -axis and c -axis, respectively. [ 8 ] However, the thermal conductivity of Si 3 N 4 ceramics is much lower than the intrinsic values. Si 3 N 4 ceramics are polycrystalline materials consolidated by liquid-phase sintering. During sintering, Si 3 N 4 raw powder, which is usually α phase, converts to the more stable β phase. In the microstructure of Si 3 N 4 ceramics,

Wear properties of Y–α/β composite sialon ceramics

Abstract The tribological properties of yttrium containing α/β composite sialon ceramics have been studied under non-lubricated conditions by means of block-on-ring and ball-on-disk type experiments against a commercial silicon nitride material. The sialon ceramics were produced by hot pressing powder mixtures of Si3N4, AlN, Al2O3 and Y2O3, resulting in composite ceramics containing different amounts of the α/β phases. The effects of microstructural differences on the mechanical properties of the ceramics, and their wear characteristics under a range of testing conditions have been assessed. It was found that Vickers hardness decreased whilst both fracture toughness and bending strength increased with increasing amount of β phase in the composite. Under mild testing conditions, material removal was considered to occur by polishing of the surface, and in this case the high α-sialon composites exhibited the highest wear resistance, reflecting their higher hardness. Under severe testing conditions, the wear behaviour was characterised as microcracking caused by the higher Hertzian stress levels, and resulted in grain removal or “dropping” from the surface of the materials. Under these conditions, the elongated microstructure and higher fracture toughness of the low α-sialon composites hinder the crack propagation and result in better wear characteristics when compared to the fine equiaxed α-sialon materials.

Development of high-thermal-conductivity silicon nitride ceramics

Abstract Silicon nitride (Si3N4) with high thermal conductivity has emerged as one of the most promising substrate materials for the next-generation power devices. This paper gives an overview on recent developments in preparing high-thermal-conductivity Si3N4 by a sintering of reaction-bonded silicon nitride (SRBSN) method. Due to the reduction of lattice oxygen content, the SRBSN ceramics could attain substantially higher thermal conductivities than the Si3N4 ceramics prepared by the conventional gas-pressure sintering of silicon nitride (SSN) method. Thermal conductivity could further be improved through increasing the β/α phase ratio during nitridation and enhancing grain growth during post-sintering. Studies on fracture resistance behaviors of the SRBSN ceramics revealed that they possessed high fracture toughness and exhibited obvious R-curve behaviors. Using the SRBSN method, a Si3N4 with a record-high thermal conductivity of 177 Wm−1K−1 and a fracture toughness of 11.2 MPa m1/2 was developed. Studies on the influences of two typical metallic impurity elements, Fe and Al, on thermal conductivities of the SRBSN ceramics revealed that the tolerable content limits for the two impurities were different. While 1 wt% of impurity Fe hardly degraded thermal conductivity, only 0.01 wt% of Al caused large decrease in thermal conductivity.

Influence of carbon fibre content on the processing and tribological properties of silicon nitride/carbon fibre composites

Abstract Si 3 N 4 /carbon fibre composites have been fabricated, and the effect of fibre content on the tribological properties was investigated under dry sliding conditions. The friction coefficient of the composites was around 30% of that of a monolithic Si 3 N 4 composite. A fibre content of 5 vol.% was sufficient to maintain a graphite interface during the sliding tests, such that the friction coefficient did not decrease further with increasing graphite fibre content above this level. The carbon fibre content was effective for maintaining a low friction coefficient throughout the duration of the experiment.

Enhancement of Seebeck coefficient for SrO(SrTiO3)2 by Sm substitution: Crystal symmetry restoration of distorted TiO6 octahedra

We found that Sm3+ substitution in SrO(SrTiO3)2 is effective in improving the Seebeck coefficient (S). The ∣S∣ value increases notably with temperature, benefiting from an enhancement of the density of states (DOS) effective mass md* from ∼3m0 (300K)to∼7.5m0 (1000K), due to an improvement of the local symmetry of TiO6 octahedra, enhancing the degeneracy in the Ti 3d orbitals, which form the conduction band (CB), and also to an accompanying lattice expansion, which gives rise to a higher DOS at the bottom of the CB and, consequently, a larger md*.

Measurement of Indentation Fracture Toughness of Silicon Nitride Ceramics: II, Effect of the Experimental Conditions

The influence of two measuring conditions, the elapsed time after indentation and the condition of edge of an indenter, on the indentation fracture toughness of silicon nitrides was assessed. No slow crack-growth after unloading was confirmed by optical microscopic observation of a crack tip induced by the indentation, which led to the negligible difference in fracture toughness measured at 1 and 30 min after the indentation. Measurements with relatively new and used indenters gave almost the same fracture toughness data, indicating that the crack lengths were hardly affected by the slight damage of the corner of the indenter. It was suggested that the large scattering of the indentation fracture toughness reported by the round-robin tests such as VAMAS was not originated from these factors.

Effect of rare-earth species on the wear properties of a sialon and β silicon nitride ceramics under tribochemical type conditions

The wear properties under low loads of β Si 3 N 4 and α sialon materials sintered with different rare-earth oxide sintering additives have been studied under dry sliding conditions using block-on-ring wear tests. All the worn surfaces showed an absence of fracture and smooth surfaces with the presence of an oxygen-rich filmlike debris indicating tribochemically induced oxidation of the surfaces. Extensive grain boundary removal was observed on the worn surfaces thought to be due to adhesion between this silicate phase and the tribochemically oxidized surfaces. The resistance to such oxidation and the properties of the residual grain boundary phase are thought to be important parameters affecting the wear behavior under the present testing conditions. For both the β Si 3 N 4 and α sialon materials, there was an increase in wear resistance with decreasing cationic radius of the rare earth, thought to be due to improved oxidation resistance, and this was more remarkable in the case of the sialon materials where the incorporation of the sintering additives into the Si 3 N 4 structure results in a lower amount of residual boundary phase.

Reaction Bonded Silicon Nitride - Silicon Carbide and SiAlON - Silicon Carbide Refractories for Aluminium Smelting

The widely used Si3N4-SiC sidewall refractories for aluminum smelting cells, and β SiAlON-SiC composites that can be potentially used for this purpose, have been produced by reaction bonding and their corrosion performance assessed in simulated aluminum electrochemical cell conditions. The formation of the Si3N4 and SiAlON phases were studied by reaction bonding of silicon powders in a nitrogen atmosphere at low temperatures to promote the formation of silicon nitride, followed by a higher heating step to produce β SiAlON composites of different composition. The corrosion performance was studied in a laboratory scale aluminum electrolysis cell where samples were exposed to both liquid attack from molten salt bath and corrosive gas attack. The corrosion resistance of the samples was shown to be dependent on the composition but more importantly on the environment during corrosion, with samples in the gas phase showing higher corrosion.

Measurement of Indentation Fracture Toughness of Silicon Nitride Ceramics: I, Effect of Microstructure of Materials

Effect of microstructure of silicon nitride on the fracture toughness, KIc evaluated by the IF method was studied with various indentation loads ranging from 49 N to 490 N, since practical assessment of fracture toughness of small Si3N4 parts is needed in the ceramic ball bearing market. The plot of KIc against the as-indented crack length revealed the rising R-curve behavior for the coarse Si3N4 and slight R-curve for the fine Si3N4. By comparing KIc estimated from the SEPB and IF methods using 4 different equations, it was revealed that the IF equation which gave the nearest value to KIc from SEPB was different depending on the microstructures. These results were discussed in conjunction with their R-curve behavior and the effective crack length in the SEPB specimens.

Microstructural characteristics in silicon nitride/tungsten composites by different in-situ processing

Abstract The effect of processing route on the microstructure, phase structure and mechanical properties of silicon nitride/tungsten composites produced by hot pressing has been studied. It was shown that silicon nitride/metallic tungsten composites could be formed by both the reduction of W(NH 4 ) 6 (H 2 W 12 O 40 )·4H 2 O in a H 2 atmosphere, and by the more simple decomposition of tungsten boride powders in a nitrogen atmosphere during sintering. Both processing routes resulted in elongated β-Si 3 N 4 grains similar to conventional silicon nitride, but the dispersion of W particles in the composite produced from WB was more homogeneous than that of the one produced from tungsten ammonate. The sample produced from WB had a fracture strength approximately 33% higher than the one produced from tungsten ammonate, attributed to the more uniform particle dispersion in the former, and the formation of the deleterious Si 2 N 2 O phase in the latter.