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Silicon nitride ceramics with Y2O3 and Al2O3 as sintering additives were brazed with aluminium, and the brazed strength and the interfacial structure of the joints were compared with those of the joints made of additive-free silicon nitride ceramics. It is concluded that the additives in silicon nitride ceramics take part in the interfacial reaction, make the reaction layer thicker, and hence increase the brazed strength greatly.

The phase diagrams of Al-Si system under pressure of 0 and 2.1GPa were obtained experimentally and theoretically. The solubility of silicon in aluminum at 2.1GPa is larger than that at 0GPa. The maximum-solubility of Si at 2.1GPa is 4.9at.% and this value is about three times large as one at 0GPa. The solidus and liquidus lines shift with pressure toward the higher temperatures and higher concentration ranges. The calculated results agree with the experimental data.

An investigation on amorphous silicon fibers grown by thermal decomposition of mono-silane (SiH4) at around 550°C in argon atmosphere was made. The fibers can be nucleated on the surface of a silicon single crystal stained with small traces of finger prints. Diameter of the fibers ranged from a few thousand angstroms to 50 microns and length was about 1 millimeter for the longest fiber. The as-grown fibers were amorphous as revealed by electron diffraction,whereas annealing at 900°C caused crystallization accompanied with volume reduction by 30 %.

The combustion reaction characteristics in the nitridation of Si was investigated by measuring the combustion velocity (in the propagation process) and the combustion temperature. Silicon powder mixed with proper amount of Si{sub 3}N{sub 4}, which was added to produce a uniform and steady-state combustion reaction without residual Si, was packed into a porous refractory container and burned under high nitrogen pressures (up to 50 MPa). The combustion velocities and the maximum combustion temperatures were independent of the starting compositions despite large differences in the adiabatic combustion temperatures depending on the starting compositions. The combustion reaction is considered to be strongly controlled by the decomposition reaction of Si{sub 3}N{sub 4}. The primary structure of the combustion wave in the nitridation of Si is proposed.

Grain size dependences of Vickers microhardness and fracture toughness in fully dense Al2O3 and Y2O3 ceramics without additives were examined. No grain size dependence of Vickers microhardness in Al2O3 and Y2O3 was observed, but the fracture toughness of Al2O3 ceramics decreased sharply with increasing grain size at about 20 μm. The grain size dependence of the fracture toughness in Y2O3 ceramics was almost constant from 0·2 to 214 μm. The difference in grain size dependence of fracture toughness for Al2O3 and Y2O3 ceramics was caused by the formation of microcracks due to the local microstresses produced by thermal expansion anisotropy, the crystal structure of Y2O3 being cubic and that of Al2O3 being hexagonal.

This paper reports on fine SiC powders synthesized by burning the mixed reactants Si and C in a nitrogen atmosphere of 3 to 10 MPa. The exothermic synthesis reaction propagated spontaneously after igniting the reactants at room temperature. The SiC powders obtained had a uniform size distribution of about 0.2 {mu}m. The combustion velocity was 0.8 to 1.5 mm/s. The maximum temperature measured at the reaction was 2500 K, which was higher than the adiabatic combustion temperature of SiC, but slightly lower than the decomposition temperature of Si{sub 3}N{sub 4} under nitrogen pressure.

An investigation has been made on the synthesis of superconducting NbN and (Nb, Mo)N powders by a combustion reaction of nitriding their metal elements under high nitrogen pressures up to 10MPa. Nearly stoichiometric B1-NbN powders were prepared by the combustion reaction of Nb powders with a proper amount of NbN addition under 1-10MPa nitrogen pressures. The characteristics of this combustion reaction were investigated by measuring the propagating velocity of combustion for the packed reactants with various amounts of diluent. The superconducting transition temperature of NbN powders obtained was 16.9K, which is very close to the highest value of 17.3K reported for NbN up to now. On the other hand, Nb powders with 5, 10, and 20wt% Mo were burned under 10MPa nitrogen pressure in order to synthesize (Nb, Mo)N solid solutions. The combustion product of Nb with 5wt% Mo consisted of B1 phase only, while those with 10 and 20wt% Mo contained a small amount of residual Mo. The transition temperatures in the Nb-Mo-N system were 12.7-15.9K, which were lower than that of pure NbN obtained in this study.

The self-propagating high-temperature synthesis (SHS) method can be used to fabricate numerous ceramics in a very short time by taking advantage of exothermic reaction. In case of SiC, the heat released from exothermic reaction is not sufficient to convert the mixed powders of Si and C into SiC completely in the actual non-adiabatic system. This disadvantage could be overcome by a new ignition process, which we call "direct passing method of electric current". By using this method, stoichiometric β-SiC could be obtained efficiently with low electric power. The obtained β-SiC powder was sintered by using highpressure hot-pressing without additives and the properties of the compact were examined. The relative density, Vickers microhardness and fracture toughness of the β-SiC compact at room temperature were 98.0-98.6%, 35-36 GN/m2 and 5.6 MN/m3/2, respectively.

Metallization of Al2O3 ceramic by low pressure plasma-spraying (LPPS) was studied for ceramic/metal bonding applications. Metal coating by LPPS had the same phase as the starting powder, but was porous and 80-85% dense. The adhesion strength of the coating depended upon the surface roughness of the ceramic substrates and the strength of 50MPa was attained by blasting at 3.5kg/cm2 for 1min. Metallized substrates were brazed and the bonding strength was evaluated using a 4-point bending test. Bending strength of the test piece was about 30MPa. EPMA detected no reaction layer between the coating and substrate.

Single crystals of a new polymorph of silver pyrophosphate, Ag4P2O7, were synthesized under high temperature and pressure conditions in the range of 400–550°C, 1.0–3.0GPa. Lattice parameters of the new phase are a=10.88, b=18.39, c=13.96A with space group C2221D52 (orthorhombic). The cell volume is contracted by a factor of 3.2% against the normal phase. The results of differential thermal analysis also proved the existence of a high-pressure polymorph.

In order to examine the effect of cold isostatic pressing (CIP) in powder compacting process on sintering of Y-TZP with 3 mol% Y2O3, the press formed powders by uniaxial loading were isostatically compressed under water pressures from 25 MPa to 500 MPa and fired at temperatures from 1300°C to 1500°C in air. The densification was achieved at lower temperatures with increasing the magnitude of pressure in CIP treatment. The relative density reached over 99% of theoretical when the powders were sintered at 1300°C after CIP treatment.

Powders of Fe-2.20wt.%C-0.85wt.%Si and Fe-4.16wt.%C-0.87wt.%Si alloys were plasma sprayed on a steel substrate in a low pressure chamber. The deposite layers formed on non-water-cooled substrate are composed of stable phases of ferrite and cementite. When the substrate is water cooled, the main constituents of the deposit layers are retained austenite and a metastable h.c.p. ϵ phase on the free-surface side for the respective alloys, and self-annealing occurs to some extent on the substrate side. On spraying, molten powders solidify at a cooling rate higher than 10 6 K s −1 in the case of a water-cooled substrate, and the cooling rate is larger than that attained in flame spraying. Such a high cooling rate in low pressure plasma spraying is attributed to the fact that the flattened particles which come from the molten powders to produce a deposit layer are thin and that the heat resistance at the interface between the flattenned particle and the underlying deposit layer is small because of the reduced amounts of pores and oxides at the interface.

Studies were made on the synthesis of transition metal nitrides (TiN, ZrN, HfN, NbN and TaN) by the combustion reaction of metal powder compacts under 0.1-25MPa nitrogen pressures. Under low nitrogen pressures (≅0.1MPa), stoichiometric nitrides were not obtained in all cases due to incomplete supply of nitrogen gas into the powder compacts. Under high nitrogen pressures (≅10MPa), it was found that the nitriding ratio of product depends on the adiabatic combustion temperature and melting point of nitrides. In the cases of Ti, Zr, Hf and Nb, which have quite high adiabatic temperatures compared with the melting points of the corresponding nitrides, the nitriding ratio was low (

Magnetic measurements under hydrostatic pressure up to 8 kbar were made on f.c.c. Fe 65 (Ni 1- x Mn x ) 35 alloys in magnetic fields up to 12 kOe and at temperatures down to 4.2°K. The results indicate that in the ferromagnetic alloys ( x

A new method of ceramic-to-metal welding has been developed using a pressurized combustion reaction to form Mo—TiB2—Mo and Mo—TiC—Mo couples. Mixtures of Ti powder with B or C between Mo disks were electrically ignited under 3 GPa pressure. The exothermic reaction of the powders resulted in the formation of TiB2 and TiC and reaction with Mo at the metal interface. Weld strengths of the Mo—TiB2—Mo and Mo—TiC—Mo couples were 20—40 MPa and ∼ 10 MPa, respectively.

A high-pressure self-combusiton sintering process has been applied to the synthesis and simultaneous sintering of titanium carbide. A dense titanium carbide compact was easily fabricated from the constituent elements without sintering aids under 3 GPa. The compact was composed of well-crystallized grains with a mean diameter of 20 μm at a molar ratio of C/Ti =0.95. The maximum values of the relative density and the Vickers microhardness were 96.5% and 31 GN/m2, respectively, at room temperature.

Hirota et al. [1] have indicated that some rare earth sulfides, ALn2S4 (A=Mg, Mn; Ln=Tm, Yb), transform from the spinel-type to the Th3P4-type phase by compression. Another kind of Compound exists whose composition is also denoted as ALn2S4 but whose structure is classified as the CaFe2O4-type [2]. The packing of constituent ions in the CaFe2O4-type is denser than that in spinel but looser than in the Th3P4-type. It is expected, therefore, that the CaFe2O4-type Compounds transfrom to the Th3P4-type phase under pressure more easily than in the case of the spinel-type. This report provides some results of this kind of pressure-induced phase transformation observed in EuLn2S4 (Ln=Y, Er, Yb, Lu) and SrLn2S4 (Ln=Y, Er, Yb). A thermodynamical consideration of the phase transformation of EUY2S4 between the CaFe2O4- and the Th3P4- types is also given.

Dense, ZrO2-dispersed Si3N4 composites without additives were fabricated at 180 MPa and ∼1850° to 1900°C for l h by hot isostatic pressing using a glass-encapsulation method; the densities reached >96% of theoretical. The dispersion of 20 wt% of 2.5YZrO2 (2.5 mol% Y2O3) in Si3N4 was advantageous to increase the room-temperature fracture toughness (∼7.5 MPa˙m1/2) without degradation of hardness (∼15 GPa) because of the high retention of tetragonal ZrO2. The dependence of fracture toughness of Si3N4–2.5YZrO2 on ZrO2 content can be related to the formation of zirconium oxynitride because of the reaction between ZrO2 and Si3N4 matrix in hot isostatic pressing.

Titanium carbide ceramics have been fabricated from the reactants titanium powder and carbon fiber by the highpressure self-combustion sintering method (HPCS) under two pressure conditions (65 MPa and 3 GPa). Porous TiC with a density of about 50% of theoretical was obtained under a pressure of 65 MPa. A possible model accounting for the formation mechanism of TiC is proposed on the basis of observation of the microstructure of the products formed from the incomplete reaction. On the other hand, dense TiC (>95% of theoretical) was fabricated under 3 GPa. The mean grain size varied, depending on the mixing molar ratio of the reactants. The influence of differences in the carbon (powder and fiber forms) on the structure of TiC compacts is discussed.

On peut utiliser le processus de frittage haute pression a auto-combustion pour synthetiser et fritter simultanement les ceramiques dans un tres court laps de temps en tirant avantage des reactions exothermiques sous pression. Un compact dense de TiB 2 a ete fabrique en quelques secondes par allumage electrique a une extremite d'un melange de poudres de Ti et B sous 3 GPa

The role of high-purity silica in the fracture of Si3N4 at high temperatures has been investigated. The flexural strength at 1400°C was found to be greater than that at room temperature. Little plastic deformation was observed even when 10 wt% SiO2 was added and the strain rate was decreased 2 orders of magnitude from that for a standard bend test. Microstructural observations revealed that the glassy phase was localized at intergranular pockets when SiO2 additions were ≤ 10 wt%. High strength at 1400°C despite the presence of a fairly large amount of glassy phase is attributed to a high cavitation threshold in such glassy pockets consisting of high-purity SiO2. However, the deformation behavior changed abruptly for SiO2 additions of 10 and 20 wt%, which is explained by the morphological change of the glassy phase to thicker intergranular layers which allow macroscopic viscous flow.

Raman spectra of highly-oriented pyrolytic graphite irradiated with He+ are systematically investigated in terms of irradiation dose and temperature. The irradiation produces a new peak at 1355 cm−1 in addition to the original peak at 1580 cm−1. The effect of irradiation on the Raman spectra becomes less appreciable with increasing irradiation temperature, probably because of dynamic annealing during irradiation. The reduction of the crystalline size, which is calculated from the intensity ratio of I1355/I1580, does not coincide with the amorphization process investigated in our previous TEM study, whereas the peakwidth, especially that of the 1355 cm−1 corresponds with amorphization. Therefore, the amorphization can not be simply attributed to the reduction of the crystalline size. The Raman spectra seem to represent some other effect, probably the change of bonding nature of the carbon atoms.

The influence of SiO2 addition on the densification and microstructural development of high-purity Si3N4 during hot isostatic pressing (HIP) was studied. During HIP, densification was promoted, but the phase transformation from α-Si3N4 to β-Si3N4 was impeded by SiO2. Analysis using a simple model shows that the enhanced densification was mainly due to the viscous flow of SiO2. The microstructure changed remarkably at between 10 and 20 wt% SiO2 additions. Analysis of the phase transformation kinetics suggests that the diffusion of Si3N4 through SiO2 glass is the ratecontrolling step for the transformation.

Dense Si3N4 ceramics containing 2.5 and 20 wt% SiO2 were hot isostatically pressed by a glass-encapsulation method. Their fracture strengths showed no degradation up to 1400°C, despite the presence of a large amount of SiO2 glass. Moreover, in the case of Si3N4-20 wt% SiO2, plastic deformation of as much as 2.5% was observed during bending tests at the strain rate e˙= 1.5 × 10−4/s. The α- to β-phase-transformation rate decreased with increasing SiO2 content.

On peut utiliser le processus de frittage haute pression a auto-combustion pour synthetiser et fritter simultanement les ceramiques dans un tres court laps de temps en tirant avantage des reactions exothermiques sous pression. Un compact dense de TiB 2 a ete fabrique en quelques secondes par allumage electrique a une extremite d'un melange de poudres de Ti et B sous 3 GPa

Dense, ZrO2-dispersed Si3N4 composites without additives were fabricated at 180 MPa and ∼1850° to 1900°C for l h by hot isostatic pressing using a glass-encapsulation method; the densities reached >96% of theoretical. The dispersion of 20 wt% of 2.5YZrO2 (2.5 mol% Y2O3) in Si3N4 was advantageous to increase the room-temperature fracture toughness (∼7.5 MPa˙m1/2) without degradation of hardness (∼15 GPa) because of the high retention of tetragonal ZrO2. The dependence of fracture toughness of Si3N4–2.5YZrO2 on ZrO2 content can be related to the formation of zirconium oxynitride because of the reaction between ZrO2 and Si3N4 matrix in hot isostatic pressing.

A high-pressure self-combusiton sintering process has been applied to the synthesis and simultaneous sintering of titanium carbide. A dense titanium carbide compact was easily fabricated from the constituent elements without sintering aids under 3 GPa. The compact was composed of well-crystallized grains with a mean diameter of 20 μm at a molar ratio of C/Ti =0.95. The maximum values of the relative density and the Vickers microhardness were 96.5% and 31 GN/m2, respectively, at room temperature.

Fine Si3N4 powders were prepared by the combustion reaction of an Si powder compact under 10 MPa nitrogen pressure. Addition of Si3N4 powder to the starting Si promoted conversion of the reactants to homogeneous Si3N4 particles. Submicrometer Si3N4 powders with a uniform size distribution around 0.5 microns were obtained from a 1.8 Si-0.4 Si3N4 mixture (molar ratio); they were free of residual Si. 8 references.