Reaction-bonded Si3N4 Research Articles

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 m1/2, 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.

Fabrication and properties of pressure-sintered reaction-bonded Si3N4 ceramics with addition of Eu2O3–MgO–Y2O3

Dense pressure-sintered reaction-bonded Si3N4 (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu2O3–MgO–Y2O3 sintering additive. The addition of Eu2O3–MgO–Y2O3 was shown to promote full nitridation of the Si powder. The nitrided Si3N4 particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu2O3–MgO–Y2O3 were nitrided at 1400 °C for 2 h. After hot pressing, the relative density, Vickers hardness, flexural strength, and fracture toughness of the PSRBSN ceramics, with 5 wt% Eu2O3 doping, were 98.3 ± 0.2%, 17.8 ± 0.8 GPa, 697.0 ± 67.0 MPa, and 7.3 ± 0.3 MPa m1/2, respectively. The thermal conductivity was 73.6 ± 0.2 W m−1 K−1, significantly higher than the counterpart without Eu2O3 doping, or with ZrO2 doping by conventional methods.

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.

Preparation of a Porous, Sintered and Reaction-Bonded Si₃N₄ (SRBSN) Planar Membrane for Filtration of an Oil-in-Water Emulsion with High Flux Performance.

A porous, sintered, and reaction-bonded Si3N4 (SRBSN) planar membrane was prepared by phase-inversion tape-casting, nitridation (at 1350 °C), and sintering (at 1650 °C) of silicon slurry. The membrane was comprised of uniform rod-like β-Si3N4 crystals with a large length/diameter ratio and had high porosity and bending strength. The prepared membrane features a typical asymmetric structure with a skin layer, a sponge layer, and finger-like voids and an average pore size of 0.61 μm. A high permeation flux of 367 L m−2 h−1 and an oil rejection of 88.6% were recorded in oil-in-water emulsion separation experiments. These results suggest that SRBSN membranes have excellent potential for the treatment of oily wastewater.

High temperature mechanical properties of porous Si3N4 prepared via SRBSN

The high temperature strength and fracture behavior of porous Si3N4 ceramics prepared via reaction bonded Si3N4 (RBSN) and sintered reaction bonded Si3N4 (SRBSN) were investigated at 800–1400 °C. The weight gain after oxidation for 15 min and the microstructure of the edge and center of the fracture surface clearly show that the internal oxidation of porous SRBSN is unavoidable with porosity of ~ 50% and mean pore size of 700 nm. The oxidation of Si3N4 and intergranular Y2Si3O3N4 phase may responsible for the high temperature strength degradation of SRBSN. Porous Si3N4 ceramics prepared with addition of 1 wt% C showed low strength degradation at temperature > 1200 °C.

Temperature dependent first matrix cracking stress model for the unidirectional fiber reinforced ceramic composites

Based on a kind of equivalence between heat energy and fracture energy, assuming that there is a constant maximum storage of energy that includes both heat energy and fracture energy, a new temperature dependent fracture surface energy model is developed. Using the new model and the classical ACK theory, a temperature dependent first matrix cracking stress model is obtained for the fiber reinforced ceramic composites. According to the model, the temperature dependent first matrix cracking stress of materials can be easily predicted using some basic material parameters such as matrix fracture surface energy and Young’s modulus. The model is verified by comparison with experimental data of SiC fiber reinforced reaction-bonded Si3N4 composites at different temperatures. Good agreement is obtained between predicted and experimental data of first matrix cracking stress. The dependency of first matrix cracking stress on fracture surface energy and interfacial shear strength is systematically analyzed.

Effects of Debinding Atmosphere on Properties of Sintered Reaction-bonded Si3N4 Prepared by Tape Casting Method

Effects of Debinding Atmosphere on Properties of Sintered Reaction-bonded Si3N4 Prepared by Tape Casting Method

Rapid fabrication of Si3N4 ceramics by reaction-bonding and pressureless sintering

Phase compositions, densification, microstructures, thermal conductivities, and mechanical properties of sintered reaction-bonded Si3N4 (SRBSN) and sintered Si3N4 (SSN) using the same additives and procedure were compared. The present results showed that the density of SRBSN with MgO-Y2O3 additives was much lower than that of SSN with the same additives. The Eu2O3-MgO-Y2O3 additives inhibited the densification, while the ZrO2-MgO-Y2O3 additives promoted the densification. The SRBSN with relative density of 99.5% and thermal conductivity of 66.5Wm−1K−1 was rapidly prepared using ZrO2-MgO-Y2O3 as aids. The SSN with relative density of 99.0% and thermal conductivity of 56.8Wm−1K−1 was fabricated using ZrO2-MgO-Y2O3 additives. The comparable density and higher thermal conductivity made the SRBSN prepared rapidly from low cost Si starting powder superior over the SSN. This study suggested that the SRBSN route with a combination of nitridation catalyst and densification aids provided a promising route to fabricate cost-effective Si3N4 ceramics.

Effect of TiO2 additives on nitridation of Si powders

Effect of TiO2 additives on nitridation of Si powders at 1300°C and 1400°C was studied by combining X-ray diffraction, thermogravimetric analysis, and thermodynamic calculations. Weak Si3N4 phase was detected in pure Si powders after nitridation at 1300°C, whereas no Si3N4 phase was found in TiO2-doped Si powders and only secondary TiN phase was observed in addition to main Si phase. The TiO2 demonstrated an inhibiting effect on nitridation of Si powders at 1300°C. However, the TiO2 led to the rapid nitridation of Si powders at 1400°C by circulating reaction mechanism between TiO2 and TiN, and promoted α-to β-Si3N4 phase transformation. In addition to TiN phase, secondary Si2N2O phase was observed in the TiO2-doped Si powders. The present study suggested that TiO2 additives exhibited effectively nitridation enhancing effect on the formation of reaction-bonded Si3N4 at 1400°C.

규소 고분자 복합체를 이용한 반응소결 질화규소

Reaction-bonded Si₃N₄ has cost-reduction merit because inexpensive silicon powder was used as a start material. But its density was not so high enough to be used for structural materials. So the sintered reaction-bonded Si₃N₄ techniques were developed to solve the low density problem. In this study the sintered reaction-bonded Si3N4 manufacturing method by using polymer precursor which recently attained significant interest owing to the good shaping and processing ability was proposed. The formations, properties of reaction-bonded Si₃N₄ from silicon and polysilazane mixture were investigated. High density reaction-bonded Si₃N₄ was manufactured from silicon and silicon-containing preceramic polymers and post-sintering technique. The mixtures of silicon powder and polysilazane were prepared and reaction sintered in N₂ atmosphere at 1350℃ and post-sintered at 1600~1950℃. Density and phase were analyzed and correlated to the resulting material properties.

ChemInform Abstract: Development of Reaction-Bonded Electroconductive Silicon Nitride- Titanium Nitride and Resistive Silicon Nitride-Aluminum Oxide Composites.

Reaction-bonded Si3N4· TiN and Si3N4· Al2O3 composites were successfully fabricated by heating mixed powder compacts of Si and TiN or Si and Al2O3 in a nitrogen atmosphere. The former showed electrical conductivity, owing to the presence of TiN. An electrical resistivity of 2.6 × 10−5Ω· m was obtained for the Si3N4· TiN composite with 70 vol% TiN. The composite with 20 vol% TiN showed an electrical resistivity of 0.22 Ω· m and a bending strength of 460 MPa. On the other hand, the Si3N4· Al2O3 composite had insulating properties. The use of an appropriate amount of resin binder resulted in a higher green density and, consequently, a higher bending strength. Moreover, electroconductive Si3N4· TiN/resistive Si3N4· Al2O3 complex ceramics could be fabricated by heating green compacts composed of two different portions, one composed of mixed powders of Si and TiN and the other of Si and Al2O3. Attainment of such complex ceramics was attributed to the small dimensional change at the nitriding stage, under 0.3% and the similarity of the thermal expansion coefficients of the two composites.

Sintering Shrinkage Behavior of Si<sub>3</sub>N<sub>4</sub> Ceramics Prepared by a Post-Reaction Sintering Technique

Post-reaction sintering is one of the fabrication processes of Si3N4 ceramics, which has received considerable attention as a cost-effective process due to the use inexpensive Si powder as a raw material. So far, many researches on the development of this method have been performed in order to improve their properties; however, the sintering shrinkage behavior, which is valuable for the optimization of the firing conditions, has not been well clarified. In this study, we focus on the post-reaction sintering of the Si-Y2O3-Al2O3 system, and investigate its sintering shrinkage behavior by dilatometery. It was found that there is no shrinkage from 1400 to 1600 °C due to grain rearrangements in the green body of the reaction-bonded Si3N4. Furthermore, the shrinkage of the reaction-bonded Si3N4 commenced at approximately 1750 °C, which is higher than the shrinkage temperature of the green body of conventional Si3N4 powder. The restriction of the shrinkage appears to result from the neck growth and strong aggregation among the reacted Si3N4 particles.

Bridging stresses in sintered reaction-bonded Si3N4 from COD measurements

Crack opening displacement measurements on a sintered reaction-bonded Si3N4 ceramic obtained in bending tests with edge-V-notched specimens were evaluated. A procedure is proposed which allows a straight-forward evaluation of the experimental results in order to obtain the bridging stresses as a function of crack opening displacements. Bridging stresses up to 160 MPa were found acting over a distance of about 0.4 μm. In addition crack profile measurements were performed for Vickers indentation cracks which could approximately be described by use of the bridging relation obtained.

Development of Sintered Reaction-Bonded Si3N4 Materials with High Thermal Conductivity

Development of Sintered Reaction-Bonded Si3N4 Materials with High Thermal Conductivity

Development of Sintered Reaction-Bonded Si3N4 Materials with High Thermal Conductivity

Development of Sintered Reaction-Bonded Si3N4 Materials with High Thermal Conductivity

Fabrication and characterization of SiC rod-particulate-reinforced reaction-bonded Si3N4 composites

Reaction-bonded Si3N4 composites reinforced by whisker-like SiC rod particulates have been fabricated by nitriding SiCrod-containing Si powder compacts, which were presintered to a controlled density by spark plasma sintering (SPS). The effects of processing parameters, including presintered density, SiCrod content, nitriding temperature, atmosphere pressure and heating rate on the nitridation reaction were investigated. It was found that the addition of SiCrod to Si powders led to easier control of complete nitridation compared to the case of pure Si powder compacts, probably because the SiCrod not only acted as diluting filler to suppress the rapid rise in temperature resulting from an SHS-like reaction between Si and N2, but also provided paths for diffusion of reactant gases. The density of the reaction-bonded Si3N4–50 vol% SiCrod composites was increased up to near 90% which is quite a lot higher than that of conventional reaction-bonded Si3N4 ceramics, by using SPS to improve the density prior to nitridation. The relative density increased from ∼70 to 90% after the nitridation without appreciable dimensional change, being characteristic of reaction-sintering. As the reinforcement in the reaction products, the SiCrod improved Young's modulus and bending strength both at ambient and elevated temperatures, as well as thermal and electric conductivity.

Microstructural Characterization of GPSed-RBSN and GPSed-Si<SUB>3</SUB>N<SUB>4</SUB> Ceramics

Microstructural characterization of gas-pressure-sintered reaction-bonded Si3N4 (GPSed RBSN) and gas pressure-sintered Si3N4 (GPSed Si3N4) ceramics at 2223 K for 12.6 ks has been investigated using by back-scattered SEM, TEM and XRD. No residual Si phase was observed in reaction bonded body nitrided at 1673 K for 72 ks, but many residual pores with about 3 μ was observed. Most of Si3N4 grain boundaries and triple points in both GPSed bodies were covered with amorphous phases. The aspect ratio of large rod-like Si3N4 grains in GPSed Si3N4 was about 10, which was larger than that in GPSed RBSN body. The main fracture mode in both sintered bodies was intergranular fracture with rough surfaces. The values of fracture toughness and fracture strength in both samples were 7.8 MPa·m1⁄2, 731 MPa and 8.0 MPa·m1⁄2, 632 MPa, respectively.

Effect of β–Si3N4 seed crystal on the microstructure and mechanical properties of sintered reaction-bonded silicon nitride

β–Si3N4 seed crystal has been synthesized from α–Si3N4 powder. Reaction-bonded Si3N4/SiC composite has been fabricated with β–Si3N4 seed crystals. The nitridation behavior and the changes in mechanical properties resulting from the addition of seed crystals were studied before and after gas pressure sintering. Addition of seeds showed a considerable improvement in the nitridation, resulting in increase in fracture strength of the composite. Highly nitrided reaction-bonded Si3N4 (RBSN) as a result of the addition of seed crystals gave rise to high strength of composite after postsintering. Fracture toughness of the seeded Si3N4 was also improved up to 35% compared to the baseline Si3N4. Micrographs showed that the seeded Si3N4 developed a bimodal microstructure which resulted in an improvement in fracture toughness.

In situ formation and high-temperature reinforcing effect of MoSi2 and WSi2 on reaction-bonded Si3N4

Abstract A three-step procedure consisting of presintering, reaction-bonding and post-hot-pressing was employed to prepare Si3N4/MoSi2 and Si3N4/WSi2 composites, using as starting materials powder mixtures of Si–Mo and Si–W, respectively. A presintering step in an Ar-base atmosphere was performed for the in situ formation of intermetallic compounds—MoSi2 and WSi2; the nitridation in an N2-base atmosphere with a total cycle of 1350°C×20 h+1400°C×20 h+1450°C×2 h in order to realize the complete conversion of Si into Si3N4; post-hot-pressing was employed for the further densification of the reaction-bonded specimens. The 3- and 4-point bend tests were performed at 25, 1000, 1200 and 1400°C. Si3N4/MoSi2 composite shows a maximum value of bend strength at about 1200°C, whereas the bend strength of Si3N4/WSi2 composite slightly increases from room temperature up to 1000°C.

Effects of thermal cycling on thermal expansion and mechanical properties of SiC fiber-reinforced reaction-bonded Si3N4 composites

Thermal expansion curves for SiC fibre-reinforced reaction-bonded Si3N4 matrix composites (SiC/RBSN) and unreinforced RBSN were measured from 25 to 1400 °C in nitrogen and in oxygen. The effects of fibre/matrix bonding and cycling on the thermal expansion curves and room-temperature tensile properties of unidirectional composites were determined. The measured thermal expansion curves were compared with those predicted from composite theory. Predicted thermal expansion curves parallel to the fibre direction were between the measured curves for the strongly- and weakly-bonded composites, but those normal to the fibre direction for both bonding cases were similar to that of the unreinforced RBSN. Thermal cycling in nitrogen for both bonding cases resulted in no net dimensional changes at room temperature and no loss in tensile properties from the as-fabricated condition. In contrast, thermal cycling in oxygen for both composites caused volume expansion primarily due to internal oxidation of RBSN. Cyclic oxidation affected the mechanical properties of the weakly-bonded SiC/RBSN composites the most, resulting in loss of strain capability beyond matrix fracture and catastrophic, brittle fracture. Increased bonding between the SiC fibre and RBSN matrix due to oxidation of the carbon-rich fibre surface coating and an altered residual stress pattern in the composite due to internal oxidation of the matrix are the main reasons for the poor mechanical performance of these composites.