MPa Nitrogen Atmosphere Research Articles

Thermodynamic and Dielectric Properties of MgSiN<sub>2</sub> Ceramics

Reaction bonded MgSiN2 (RBMSN) was prepared by direct nitridation of a Si/Mg2Si/Mg/Si3N4 powder compact in a temperature range of 1350-1550°C. The oxygen content of MgSiN2 was in the range of 0.4 – 0.6 wt%. A thermal stability examination showed that MgSiN2 is stable up to 1400°C at 0.1 MPa N2 pressure. The activation energy of decomposition calculated from the temperature dependence of weight loss is H = 383 kJ⋅mol-1. The time dependence and nitrogen pressure dependence of MgSiN2 decomposition was also investigated at constant temperature. MgSiN2 is stable at 1560°C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs free energy of formation of MgSiN2 was calculated in a temperature range 300-2500 K. Dense MgSiN2 ceramics or MgSiN2/Si3N4 composites with fluorine-based additives were prepared by hot pressing. The composite materials had a 4-point bending strength of 427 MPa and Vickers hardness (HV1) of 20.8 GPa, respectively. The indentation fracture toughness was 5.3 MPa.m1/2, due to the presence of elongated β-Si3N4 grains. The dielectric constant of dense reaction bonded MgSiN2 at 100 kHz was 9.5-10, while that of MgSiN2/Si3N4 composite in a wide range 50 – 6000, depending on composition and heat treatment.

Chemistry of the co-pyrolysis of an aromatic petroleum residue with a pyridine–borane complex

Synthesis of boron-doped mesophase by controlled co-pyrolysis of an aromatic petroleum residue and a pyridine–borane complex, PB, as a boron source, was carried out. Pyrolysis was performed at temperatures ranging from 400 to 440 °C, under 1 MPa nitrogen atmosphere. Soaking time was varied between 1.5 and 6 h, and the boron concentration in the pyrolysis mixtures ranged from 0 to 1 wt.%. The effect of the presence of boron upon solid yield, insoluble content, mesophase development and microstructure was studied. PB reacts with molecules of the petroleum residue and creates B–C bonds, B substituting C in the polyaromatic molecules, with sp 2 hybridisation, and probably also creating B crosslinking between polyaromatic units. The enhanced reactivity caused by B addition is reflected in an increment of viscosity; as a result, the development of mesophase is strongly increased, but the size of the mesophase structures is decreased to a mosaic texture; B also causes an increase of the interlayer spacing and a noticeable enhancement of insoluble material.

Study on gelcasting of silicon nitride-bonded silicon carbide refractories

Gelcasting is a very useful technique for producing ceramic green parts with near-net-shapes. Although it has been successfully applied to shape fine ceramic systems where micrometer- and submicrometer-size particles are used as starting powders, there is little report on gelcasting of refractories manufactured by coarse raw materials. In the present paper, dispersion and rheological properties of concentrated suspensions of SiC/Si mixtures with coarse particles (∼350 μm) and their gelcasting processes were discussed. A stable SiC/Si suspension with solids loading up to 70 vol.% was prepared, from which uniform green bodies with complex shapes were successfully gelcast. The performances of green bodies and sintered samples were measured and their microstructures were studied. After nitridation of the gelcast components at 1450 °C for 10 h in 0.1 MPa nitrogen atmosphere, the bulk density of 2.61 g cm −3 and flexural strength of 55.4 MPa were obtained.

Co-pyrolysis of an aromatic petroleum residue with triphenylsilane

The effect of silicon on the pyrolysis of a petroleum residue has been studied. In this work, controlled co-pyrolysis of an aromatic petroleum residue and triphenylsilane, TPS, as a silicon source, under 1 MPa nitrogen atmosphere and at 440°C is performed. Soaking time was varied between 1.5 and 6 h, and the silicon concentration added to the petroleum residue ranged from 0 to 2 wt%. Silicon content in the final solid product increases with soaking time. The effect of silicon upon the solid yield, insoluble content and mesophase development is appreciable only when the concentration of silicon in the residue is above 0.4 wt%. Initially TPS acts as a diluting agent and a mild proton donor, inhibiting reactivity of the radicals formed in the system and molecular growth. As reaction progresses TPS reacts, creating tetrahedral bonds within macromolecules and, as a consequence, after a period, molecular growth is enhanced. However, the planar stacking leading to mesophase formation and growth is inhibited by silicon and consequently the development of mesophase structures is limited to small spheres and fine mosaics. Optically isotropic material is obtained when silicon concentration is above 1 wt%.

Porous silicon nitride-based ceramics fabricated from silicon and carbon powders

Porous ceramic materials are widely used as fluid flow filters, separation membranes, lightweight structural components, crystal supports, electrodes, bioreactors, catalysts, and so on, since they show superior properties such as high temperature stability, lower Young’s modulus, enhanced thermal shock resistance, erosion/corrosion resistance, catalytic activity etc. [1, 2]. Porous silicon nitride-based ceramics have also been studied. Recently, Kawai et al. successfully fabricated high strength porous silicon nitride with rod-shaped grains, which formed a three-dimensional network structure [3]. The silicon nitride with the relative density of 61.7% exhibited a high bending strength of 455 MPa. This high strength was achieved due to the microstructure of elongated rod-shaped grains. Based on their strategies, Yang et al. studied optimum fabrication conditions to achieve higher porosity and higher strength [4]. It is needless to say that a low-cost process is essentially important when production of such silicon nitride-based porous ceramics are put into commercial application. In this study, low-cost silicon and carbon powders are used as the starting material to fabricate silicon nitride-based porous ceramics through their reactions in a nitrogen atmosphere. The advantages of using silicon-carbon mixed powder are as follows. I. The nitridation reaction of silicon is difficult to control, since it is an exothermic reaction [5]. On the other hand, the reaction can be well controlled by adding SiC or carbon powder [6, 7]. II. A substantial amount of silicon is consumed for SiC formation from silicon and carbon, which is advantageous to control the porosity. Combining the following two stages can be considered appropriate to fabricate porous silicon nitridebased ceramics [5, 8]. The first stage is the reaction one. The silicon-carbon mixed raw powder with sintering additives is reacted to form silicon nitride and silicon carbide through reactions 3Si+ 2N2→ Si3N4 and Si+ C→ SiC at 1200–1400 ◦C in nitrogen atmosphere. The second stage is the microstructure control stage. The reacted body is heat-treated at 1600–1900 ◦C to form rod-shaped grains. Using this procedure, porous silicon nitride-based ceramics are fabricated under several conditions, and the mechanical properties are evaluated in relation to their microstructures. Silicon powder (High Purity Chemetals Laboratory, 99.9%, 1 μm), carbon black (Mitsubishi Chemical, #750, 18 nm), Y2O3 powder (Shinetsu, UF) and Al2O3 powder (Sumitomo, AKP 30) were first mixed. The weight ratio of Si : C : Y2O3 : Al2O3 was 0.8084 : 0.0782 : 0.0709 : 0.0425. This mixture would ideally form 73.6 wt.% Si3N4− 18.4 wt.% SiC− 5 wt.% Y2O3− 3 wt.% Al2O3 (the weight ratio of Si3N4 : SiC was 0.8 : 0.2), if reactions 3Si+ 2N2→ Si3N4 and Si+C→ SiC were completed. First, in order to evaluate the reaction condition, the powder compacts were heated at 1250 and 1350 ◦C for various periods of time in 0.1 MPa nitrogen atmosphere. The mixed powder was cold isostatically pressed (CIP) under a pressure of 490 MPa to form columnar green bodies, 14 mm in diameter and 10 mm in height. The degree of nitridation was calculated from the weight change of the specimen. Then, post-heat-treatment condition was studied for platelet-shaped specimens. The mixed powder of 25 g was compacted with a uniaxial pressure of 2.5 MPa in a carbon die with a base of 45 × 45 mm. A reaction was conducted at 1250 ◦C for 16 h in 0.1 MPa nitrogen atmosphere, subsequently followed by a heat treatment at 1650 ◦C for 3 h in 0.1 MPa nitrogen atmosphere, or by that at 1850 ◦C for 3 h in 1 MPa nitrogen atmosphere. Hereafter, the former and the latter are designated as specimens A and B, respectively. Note that 1650 ◦C is the highest temperature for heat treatment in atmospheric nitrogen pressure, while 1850 ◦C is presumably high enough to enhance grain growth of β-silicon nitride and form elongated grains. X-ray diffraction (XRD) analysis was performed to examine the phases of the specimens. The microstructures of the specimens were examined by scanning electron microscopy (SEM). The porosity and the pore-size distribution of the porous composites were determined by the mercury porosimetry. The three-point bending strength was measured using specimens of 3 × 4 × 40 mm cut from the sintered bodies. The measurement was conducted under the condition with a span of 30 mm at a cross-head speed of 0.5 mm ·min−1 at room temperature. A reaction study was conducted using the CIP columnar powder compact. Its initial density was 1.36× 103 kg ·m−3. The degree of nitridation is shown in Fig. 1. Note that the calculation was conducted assuming that the SiC formation occupied about 70% of the total reaction, which was estimated from our previous study [8]. Melting behavior was not observed at either temperature. A very short reaction time was actualized by adding carbon powder (and formation of SiC [6, 7]);

Fabrication and morphology of different color NbC x whiskers

Different color NbC x whiskers with NaCl crystal structure were fabricated at temperatures between 970°C and 1720°C in 0.1 MPa nitrogen atmosphere. Nb 2O 5 and reactive carbon powders as raw materials and NaF and sucrose powders as additives were used. The composition, crystal structure, morphology and the growth mechanism of NbC x whiskers have been investigated.

Deformation Conditions of β-SiAlON to Achieve Large Superplastic Elongation

Superplastic elongation of β-SiAlON was investigated in tensile deformation condition. The test was conducted under 0.1MPa nitrogen atmosphere at 1580°C, and under a constant crosshead speed of 0.078mm/min (the initial strain rate was 1.3×10-4s-1). The β-SiAlON exhibited 470% superplastic elongation, and this value was the largest in Superplasticity of silicon nitride based ceramics reported previously. It was demonstrated that a suitable condition was necessary to achieve larger superplastic elongation.

Effect of rare-earth oxide addition on the thermal conductivity of sintered aluminium nitride

The densification of A1N ceramics and the improvement of its thermal conductivity are typically achieved by the addition of a small amount of oxides. The effects of oxide addition on the density and thermal conductivity of sintered A1N were investigated by Tsuge and co-workers [1, 2]. They found that the alkaliand rare-earth oxides (Re203) were useful additives for the fabrication of fully densified and highly thermally conductive A1N; Y203 is an especially good sintering additive. In the A1N sintering process, Y203 additive reacts with the surface oxide layer of A1N particles, forms the liquid phase of the Y A I O N system, causes the precipitation of yttrium-aluminium-oxide phases such as Y4A1209 (2Y203.A1203) in grain boundaries and lowers the oxygen content dissolved in A1N grains, and then the thermal conductivity of sintered A1N increases [2]. However, the effects of Re203 addition other than Y203 on the thermal conductivity of A1N have not been studied in depth. Recently A1N sintering with Y203 additive under reducing nitrogen gas atmosphere was reported to be a useful method to obtain sintered bodies with high thermal conductivity (>150 W m -a K -1) [3-7]. In this study the effects of Re203 addition on the thermal conductivity of A1N specimens sintered in reducing nitrogen atmosphere were investigated. A commercial A1N powder (Tokuyama Soda Co., Tokyo) with an average particle size of 0.6/xm and a specific surface area of 3.2 m 2 g-1 was used in this research. According to the manufacturer, this powder contains 1 wt % oxygen, 220 p.p.m, carbon and trace amounts of iron, calcium and silicon. In this work ND203, Sm203, Gd203, Dy203, Er203 and Yb20 3 powders (Hokko Chemicals Co., Tokyo) were used as the Re203 additives. I mol % of each of the rare-earth oxide powders as sintering aids were added, respectively, to the raw A1N powder. These powders were mixed in ethanol in a ball mill. After drying, the mixed powder was formed into pellets 14 mm in diameter and 8 mm high using a stainless steel die, and then cold isostatically pressed (CIPed) under 200 MPa pressure for 60 s. The CIPed pellets were placed in a graphite crucible, and then sintered using a graphite heater furnance at temperatures of 1773, 1873, 1973, 2073 and 2173 K for 1 and 3 h under 0.1 MPa nitrogen atmosphere. Furthermore, the CIPed samples were sintered in a graphite crucible with A1N packing powder in order to investigate the effect of the sintering atmosphere. The heating rate was 0.17 Ks Gd203 > Er203 > Yb203 >Nd203 >Sm203 addition. On the other hand, in sintering with packing powder, the thermal conductivity of A1N specimens with Re203 were about 95 W m -1K -1. The maximum thermal conductivity of A1N ceramics obtained in this work was 195 W m -1K -a for Dy203 addition. The results for the oxygen content of sintered A1N with the Re203 are shown in Fig. 2. The oxygen content of A1N specimens sintered without packing

The role of interfaces in the creep-deformation of a Sic whisker-reinforced Si3N4 composite

The high temperature mechanical properties of a SiC whisker-reinforced composite are currently being investigated. The deformation characteristics of the composite are controlled not only by the distribution of the constituent phases, but also by the interfaces between phases. The material is a hot pressed 30 vol.% SiC whisker-reinforced Si3N4 containing sintering aids of 1.5 wt.% A12O3 and 6.0 wt.% Y2O3. Constant compressive stress creep experiments were conducted in a continuously purified 0.11 MPa nitrogen atmosphere in a system described elsewhere. This work focussed on two creep experiments conducted at temperatures of 1470 K and 1620 K, both in the stress range 50-350 MPa. Figure 1 shows the stress exponent, n, for creep of the composite at both 1470 K and 1620 K. At 1620 K, n exhibited a bi-linear behavior. However, this behavior was not observed for creep at 1470 K.In the as-hot pressed state, the composite had an amorphous grain boundary phase containing N, O, Al, Si, and Y as determined by energy dispersive spectroscopy. Prior to loading, the specimens were annealed for 4 hr at the creep temperature. During the anneal, the grain boundary phase crystallized and underwent a change in composition as determined by X-ray diffraction and transmission electron microscopy (TEM).