Reaction Bonded Silicon Nitride Ceramics Research Articles

Thermal conductivity prediction of sintered reaction bonded silicon nitride ceramics using a machine learning approach based on process conditions

Thermal conductivity (TC) of sintered reaction bonded silicon nitride (SRBSN) ceramics was predicted from process conditions using support vector regression (SVR) as a machine learning algorithm. A total of ten explanatory variables were selected from the process conditions for building a SVR model. The determination coefficient (R2) in the testing results of the trained SVR model reached a satisfactory level (R2 = 0.8 on average) when more than 100 data points were used as the training data. The relative importance scores of the explanatory variables revealed that sintering conditions (temperature and time) had the largest contribution to the TC prediction while those of oxygen impurity content in the main starting powder, types and concentrations of sintering additives and nitriding conditions were also influential to a certain extent.

Sintered reaction-bonded silicon nitride ceramics for power-device substrates -review-

This review addressed the recent progress of sintered reaction‐bonded Si3N4 (SRBSN) ceramics which are expected to be used as ceramic substrates of the next-generation power devices. Following an overview of thermal conductivities of Si3N4 ceramics and their improvement strategies, we described the advantages of the SRBSN routes. Because of the low content of lattice oxygen, the SRBSN ceramics possess significantly high thermal conductivities, compared to Si3N4 ceramics prepared by conventional methods. This material also showed exceptionally high fracture toughness and the strong rising R-curve behavior. The properties of the SRBSN ceramics are largely affected by the nitridation conditions, including the temperature, pressure and heating rate, and the sintering additive ratios. The thermal fatigue tests revealed the excellent thermal fatigue resistance of the SRBSN substrates, compared with those of other ceramic substrates. The strength retention ratio after thermal fatigue was clearly proportional to the fracture toughness. The dielectric breakdown strength (DBS) of the SRBSN ceramics with large fibrous grains showed a thickness dependency reverse to the normal (lower DBS in thinner substrates).

Investigation of the Effect of Sintering Additive on the Microstructure and the Antibacterial Behavior of Reaction Bonded Silicon Nitride Ceramics

Silisyum Nitrür (Si3N4) genellikle yüksek sıcaklık ve yapısal uygulamalarda tercih edilen, üstün özellikli bir oksit dışı seramiktir. Çok sert bir malzeme olmasının yanı sıra, çubuksu beta () tanelerin gelişimi ile seramik malzemeler içinde yüksek kabul edilebilecek tokluk değerleri elde etmek mümkün olabilmektedir. Bu özelliklerine ilave olarak biyo uyumluluğunun da anlaşılması ile birlikte özellikle, bu malzemenin biyoseramik olarak kullanım potansiyelini arttırmak amacıyla son on yılda yapılan çalışmaların sayısı da artmıştır. Bu çalışmada da reaksiyon bağlama tekniği ile, çevre biyolojik dokularla entegrasyon açısından avantajlı olan gözenekli Si3N4 seramiklerinin üretilmesi amaçlanmıştır. Sinterleme ilavesi olarak sisteme yapılan CaO, Y2O3 ve CeO2 katkılarının, elde edilen Si3N4 seramiklerin fiziksel ve mikroyapısal özelliklerine ve bunların sonucu olarak antibakteriyel davranışlarına etkileri incelenmiştir. Elde edilen sonuçlar CeO2’in hem E. coli hem de S. aureus açısından dezavantajlı bir sinterleme ilavesi olduğunu, Y2O3’in ise en avantajlı ilave olduğunu görülmüştür. CaO ilavesi yapılan sistem ise E. coli için düşük, S. aureus için yüksek oranda bakteri üremesi ile sonuçlanmıştır.

A comparative study on nitridation mechanism and microstructural development of porous reaction bonded silicon nitride in the presence of CaO, MgO and Al2O3

ABSTRACT During the in-situ nitridation of Si in the presence of CaO, MgO and Al2O3 formation of reaction bonded silicon nitride ceramics (RBSN) having various microstructures indicated the possibility of different nitridation mechanisms operating. Experimental evidence suggested that, whereas the morphology of pores was controlled by nitridation of Si(g) in CaO-RBSN and SiO(g) in Al2O3-RBSN, a combination of these two reactions occurred during the nitridation of the MgO-RBSN. By inhibiting the growth of whiskers and maximizing the α/β ratio, the CaO addition led to the formation of matte grains creating clean spherical cavities with d < 40 µm and flexural strength of 0.8 MPa. In contrast, when using Al2O3 additions, a microstructure with a very low α/β ratio, fine inter-particle pores and 2.4 MPa flexural strength, reinforced with interlocking whiskers, was produced. The highest porosity (85%) and the lowest strength (0.3 MPa) occurred in the MgO-RBSN, which was composed of both matte grains and fine whiskers. Local supersaturation and low content of β-nuclei led to the formation of anisotropic β-grains with a bimodal microstructure in heat-treated CaO-RBSN while a unimodal microstructure was observed in heat-treated MgO-RBSN. No porosity loss or β-grain growth occurred in the heat-treated Al2O3-RBSN.

Pore structure, porosity and compressive strength of highly porous reaction-bonded silicon nitride ceramics with various grain morphologies

Complex characteristics of the pores and properties of porous reaction-bonded Si3N4 have been investigated and correlated with the microstructure of Si3N4 grains. Porous ceramics with porosities of ≤ 75 vol% and α-Si3N4 matte grains (α/β phase ratio of 1.5) or α-Si3N4 whiskers (α/β phase ratio of 0.36) were prepared by in situ nitridation of silicon powder. To obtain various microstructures by α → β-phase transformation and grain morphology modification, samples were heat-treated at 1700 °C while embedded in a Si3N4 powder bed containing MgO. By the growth of α-matte or β-Si3N4 grains on the pore walls, highly interconnected structures with spherical cavities and unimodal pore size distributions resulted with d50 ≈ 8.8 µm and ≈ 6.5 µm, respectively. In contrast, α-whiskers grew inside the pore cavities; thus, complex and irregular inter-particle pores appeared which generated an extra peak near d50 ≈ 1 µm forming a bimodal pore size distribution. Compared to the α-matte grains, α-whiskers densified upon heat treatment and produced a large drop in porosity, which resulted in a structure with less interconnectivity. As a consequence of growth of fine β-rods, pore walls became relatively smooth and whisker free; thus, inter-cluster channels were modified to spherical cavities with d50 ≈ 3.7 µm. Samples exhibiting networked whiskers and fine pores or low porosity demonstrated higher compressive strength than the interconnected structures with spherical cavities.

Microstructural evolution mechanism of porous reaction bonded silicon nitride ceramics heat-treated in two powder beds

Porous Si3N4 with various grain morphologies was prepared by direct nitriding of silicon powder and subjected to heat-treatment while embedded in a Si3N4 powder bed. The influence of MgO, added either to the starting silicon or to the powder bed, on the microstructural transformation and morphology of pores at temperatures between 1425 °C and 1700 °C are discussed. In the presence of MgO, α-Si3N4 grains with equiaxed morphology resulted in an interconnected microstructure with spherical pores. By contrast, in the absence of MgO, an α-whisker-dominant microstructure resulted in pores of various morphologies. During the post heat-treatment process, the α-whiskers gradually vanished, and grains recrystallized as very fine β-Si3N4 rods. Consequently, pores became spherical, large and whisker-free. In agreement with the results of SEM-EDX along with XRD analysis, the observed morphology transition and full phase transformation occurred by vapor phase transport of MgO from the powder bed and a subsequent solution-precipitation mechanism. The presence of volatile MgO in the powder bed caused a substantial decrease in weight losses, while enhancing β phase formation, grain coarsening and linear shrinkage. The development of coarse β-rods from α-matte grains in a Si–Mg–O–N glassy phase was related to the presence of substantial liquid phase during the growth mechanism. Compared to the granular morphology, whiskers with high aspect ratio gave rise to a high sintering driving force and led to a maximum value of ≈2% linear shrinkage and porosity of ≈30 vol%. Consequently, this ceramic exhibited the highest compressive strength of ≈10 MPa.

Reaction bonded silicon nitride foams: The influence of iron disilicide on microstructure and mechanical strength

In this study open-cell reaction bonded silicon nitride (RBSN) foams with high and controlled level of porosity have been produced based on the wet processing of Si-PMMA suspensions in the presence of iron disilicide (FeSi2) as an additive. The investigation focuses on the effect of FeSi2 on the mechanical strength, nitriding mechanisms, and microstructure of the RBSN foam. The nitriding reactions and the foam microstructures and properties were investigated under various nitriding conditions and for different FeSi2 contents.It was observed that 1wt% FeSi2 significantly increases the strength of the RBSN foams regardless of the nitriding temperature and atmosphere. The nitriding conditions have a strong influence on RBSN foam strength. The maximum strength was achieved under a N2 atmosphere at 1390°C with a foam porosity of 71vol%. The high porosity level of the foam significantly affects the nitriding mechanisms and consequently the foam microstructure, compared to “dense” RBSN ceramics. FeSi2 promotes the formation of both α- and β-Si3N4 phases depending on its concentration and its addition also increases the whisker content of the foam microstructure due to an increase in SiO and Si vapor formation.

Structural and Electrical Properties of Reaction Bonded Silicon Nitride Ceramics

The effects of structural properties on the d.c. and a.c. electrical properties of different weight gain reaction bonded silicon nitride (RBSN) have been studied in this work. The degree of nitridation is assessed by the ‘weight gain’ of the ceramic, the percentage by which the weight is increased in the nitriding reaction. From X-ray diffraction (XRD) patterns, it is observed that a higher degree of nitriadation sample has strong α-silicon nitride peaks. Intensity of α-silicon nitride peaks decreases with decreasing weight gain. The higher degrees of nitridation, the samples have less significant Si peak. XRD patterns were recorded to calculate the lattice parameters of RBSN. The lattice parameters for three weight gain RBSN samples are found to be a =b = 7.7727 Å, c= 5.6565 Å (26% weight gain), a=b= 7.6272 Å, c= 5.6374 Å (42% weight gain) and a=b=7.6158 Å, c= 5.7732 Å (58.27% weight gain) and are in good agreement with the reported values from XRD patterns. Porosity (%) and surface morphology was observed by SEM.

Effect of Pore-forming Agent on Porous Reaction-bonded Silicon Nitride Ceramics

Porous reaction-bonded silicon nitride(RBSN) ceramics were fabricated by using potassium chloride(KCl) and urea(CO(NH2)2) as pore-forming agent, respectively. Green bodies with 30% in mass KCl were subjected to presinter in Ar atmosphere at 1200°C and then reaction-sintered. The properties of porous silicon nitride ceramics adding different pore-forming agent were explored. And the influence of presintering on apparent porosity, bulk density and bending strength of porous ceramics were investigated. The results indicated that the porosity of Si3N4 ceramics showed a nearly linear increase as the content of KCl increasing, but urea was not. And after presintering, the porosity had small decrease, but bending strength increased obviously. Low bulk density with about 58.6% porosity Si3N4 ceramic was prepared by adding 50wt% KCl, and the main phase composition of porous ceramic was α-Si3N4. Lots of needle-liked α-Si3N4, especially in the pores, could be observed.

The Characterization of Porous Sintered Reaction-Bonded Silicon Nitride Ceramics Fabricated by Si-Additive Mixture Granules

Porous sintered reaction-bonded silicon nitride ceramics with dual-sized pore channels were fabricated using granules consisting of a Si-additive. A pressureless tapping method was used to maintain the spherical morphology of the weak spray-dried granules. Emphasis was placed on the formation of wide pore channels (ϕ≥10 μm). In addition to the successful formation of wide pore channels between the granules, narrow channels formed inside the granules. The size of the wide channels (ϕ=5–20 μm) depended on the granule. On the other hand, the narrow channel size (ϕ<1 μm) remained the same regardless of the granule size.

α/β Phase Transformation in Porous Reaction-Bonded Silicon Nitride Ceramics

Porous reaction-bonded silicon nitride ceramics with different values of α/β ratio were obtained by setting the nitriding temperature and time. The influence of α/β phase transformation on the dielectric properties of porous silicon nitride ceramics has been investigated. The results show that the α/β transformation occurs when the nitriding temperature is higher than 1400°C . The values of α/β ratio decrease with increase of nitriding temperature and time. The dielectric constant ε′ and the dielectric loss tan δ of the samples increase because of the α/β transformation, and the change of the dielectric loss is more obvious. The increase of concentrations of point defects due to the α/β transformation leads to the significant increase of the dielectric loss.

Influence of zirconia addition on reaction bonded silicon nitride produced from various silicon particle sizes

Reaction bonded silicon nitride ceramics have been produced from Si powders of different particle size and both with and without zirconia as a nitride catalyzing element. The effect of the ZrO 2 on the nitridation rate of the specimens was studied as a function of temperature, and the mechanism of enhanced nitridation was discussed. At all temperatures, the degree of nitridation of specimens sintered with ZrO 2 was higher than those without additions for all particle sizes. In the case of nitridation of specimens produced without ZrO 2 , the degree of nitridation was dependent on particle size, with larger sizes resulting in less nitridation. For specimens with ZrO 2 the finest powder also showed highest transformation, but there was no difference in the degree of nitridation of medium and coarse powders. It was found that during heating of the specimens the ZrO 2 was transformed to ZrN at low temperature and enhanced nitridation is considered to be due to this phase re-converting to ZrO 2 at higher temperature and thereby supplying nitrogen to the interior of the specimens. At lower temperatures, the strength of the specimens with ZrO 2 were higher than those without, attributed to the enhanced nitridation. However, comparing strengths of fully nitrided specimens, those with ZrO 2 were lower than those without, thought to be due to the presence of ZrO 2 particles disrupting the connectivity of adjacent silicon nitride grains.

Effect of β-Si 3N 4 seed particles on the property of sintered reaction-bonded silicon nitride

The effect of seeding on the microstructural development and mechanical properties of sintered reaction-bonded silicon nitride was investigated by the use of β-Si 3N 4 particles. Seeding of an appropriate amount of the β-Si 3N 4 particles gave rise to the promotion of density and the resulting increase in fracture strength and hardness. The fracture toughness was also increased due to the development of elongated grains. On the other hand, addition of a large amount of seed particles showed lower strength and fracture toughness owing to the coalescence of large elongated grains. By seeding of 2 wt.% β-Si 3N 4 particles on Si powders of 7 μm, high fracture strength of 1100 MPa and fracture toughness of 7.2 MPa·m 1/2 were obtained in sintered reaction-bonded silicon nitride ceramics.

Nitridation of whisker-reinforced reaction bonded silicon nitride ceramics

Ceramic matrix composites were fabricated from silicon carbide whisker-reinforced reaction bonded silicon nitride. Optimal dispersion of the SiC whiskers in the silicon powder slip was achieved by milling and pH control; a pH range of 4–5 giving the best results. Only a slight drop in green density was observed for a 30 wt% addition of SiC whiskers. The effects of the whisker additions on the nitridation kinetics of reaction bonding were investigated and the additions were found to increase the induction period before nitridation and to slightly decrease the nitridation rate but green density and temperature were still found to be the main factors controlling nitridation. Modulus of Rupture measurements for the composites showed a decrease in strength compared to the monolithic material.

Low pressure injection moulding of SiC platelet reinforced reaction bonded silicon nitride

Reaction-bonded Si3N4 toughened by oriented SiC platelets was fabricated via low pressure injection moulding (LPIM). Initially, the rheology of ceramic suspensions was optimized with respect to solid content, SiC platelet loading, particle surface properties and binder composition. Surface active additives were used to modify the particle–polymeric binder interphase in order to prevent particle reagglomeration, to reduce the viscosity and/or to increase the solid content. The relationship between LPIM processing variables and platelet orientation in injection moulded reaction bonded silicon nitride ceramics was studied and the resultant mechanical properties were compared to composites containing randomly dispersed platelets.