Temperature induced structural changes in sapphire whiskers

The effect of high temperature annealing (800 to 1500° C) on the structure of individual sapphire whiskers has been determined with an electron microscope technique. Two types of whiskers, grown by a similar process, but containing different levels of silicon impurity (6 and 0.2% respectively) were studied. Discrete second phase particles were observed within and at the surface of many of the whiskers with 6% silicon. After heat treatments at 1000 to 1300° C in high purity argon, these particles coarsened and coalesced in the larger whiskers and spheroidised on the surface of the smaller whiskers. In addition, a dispersion of fine particles was formed in some whiskers free from grown in particles.Some melting of the second phase occurred between 1000 and 1400° C, with an attendant disintegration of the whiskers. Although the sapphire whiskers with 0.2 silicon % did not contain second particles, some breakdown of the whiskers also occurred at about 1300° C, a process which is attributed to the presence, and melting, of a surface coating.Surface pits were formed at temperatures above 1000° C, and became extensive at 1400 to 1500° C, particularly in the 6% silicon whiskers. It is considered that the surface pitting is a consequence of impurity diffusion and internal stress in the sapphire whiskers.     

Microstructural study of SnO2 thin layers deposited on sapphire by sol-gel dip-coating

Tin oxide (SnO₂) films have been grown onto (006) sapphire substrates by sol-gel dip-coating using tin alkoxide solutions. It is shown, using grazing-incidence X-ray diffraction, reciprocal space mapping and atomic force microscopy, that thermal annealing at 500 °C induces the crystallization of SnO₂ in the rutile-type phase. Further annealing treatments at temperatures lower than 1100 °C give rise to slow grain growth controlled by surface diffusion, whereas rapid grain growth (controlled by an evaporation-condensation mechanism) takes place at temperatures higher than 1100 °C. Concomitantly, the film splits into isolated islands and a fibre texture occurs at higher temperatures.     

Microstructure and mechanical properties of biocompatible high density Ti–6Al–4V/W produced by high frequency induction heating sintering

High frequency induction heating sintering method is used for sintering of the metal and ceramics powder. This technique has been used to produce high density compacts, containing as small grains as possible of powders. The alloy of Ti–6Al–4V was modified by addition of 2.5, 5, and 10 wt.% tungsten through powder metallurgy. Ti–6Al–4V/W was prepared by high-energy mechanical milling. The use of the high frequency induction heating sintering technique allows sintering to nearly full density at comparatively low temperatures and short holding times, and therefore suppressing grain growth. Different process parameters such as sintering temperature, and applied pressure have been investigated. The obtained compacts are characterized with respect to their densities, grain morphologies and pore distributions as well as hardness. Ti–6Al–4V/W powder precursors have been successfully compacted and consolidated to densities exceeding 98.8%. The maximum compressive strengths were obtained at sintering temperature 1000 °C for the samples containing 5% W, and at 1100 °C for the samples with 10% W. Maximum hardness was obtained 45 HRC at 1100 °C for 10% W.     

Fracture strength of melt-infiltrated SiC-mullite composite

The fracture strength of a melt-infiltrated SiC-mullite composite was measured from room temperature to 1500°C using a three-point bending test. The strength under argon at atmospheric pressure was not high. Mullite decomposition was found to be severe even at 1100°C in a reducing atmosphere, thus significantly degrading its strength. The strength in air, where the decomposition was suppressed, was moderately high and retained up to 1100°C. The composite revealed typical brittle failure up to the highest investigated temperature of 1500°C, with an indication of failure by slow crack growth at high temperature.     

The thermal stability of AlN

The thermal stability of AlN powders and thin films has been investigated using reflection high-energy electron diffraction (RHEED) and X-ray diffraction. AlN powder was treated thermally and chemically to assess the oxidation resistance of this compound and to identify the phases formed. The results show that AlN is stable up to 1000° C in air and remains stable up to 1400° Cin vacuo. -AIOOH is formed when AlN is treated with water at 100° C but AlN does not react readily with atmospheric moisture at room temperature. The thermal stability of thin films of AlN on GaAs has been evaluated at temperatures between 900 and 1100° C in a nitrogen atomosphere. It was found that AlN did not oxidize under these conditions. Pure AlN is a suitable encapsulant for GaAs at high annealing temperatures in an inert atmosphere.     

Aluminide coating formation on nickel-base superalloys by pack cementation process

A detailed study was carried out to investigate the effects of pack powder compositions, coating temperature and time on the aluminide coating formation process on a superalloy CMSX-4 by pack cementation. With the aid of recently developed thermodynamic analytical tools, powder mixtures that are activated by a series of fluoride and chloride salts were analysed and the effectiveness of these activators in transferring and depositing Al was evaluated at a range of coating temperatures. The Al chloride vapours formed at coating temperatures from 900°C to 1100°C were also analysed thermodynamically as a function of Al concentration in the original pack for the powder mixtures activated by 4 wt% CrCl₃·6H₂O. Based on the thermochemical calculations, a series of coating experiments was carried out. Aluminide coatings were formed at temperatures from 850°C to 1100°C for periods varying from 4 hours to 8 hours using powder mixtures activated by NH₄Cl, NaCl and CrCl₃·6H₂O and AlF₃. The effects of changing Al concentration as well as adding small quantities of Cr in the powder mixtures on the coating formation process were also investigated. The aluminide coatings were analysed using a range of techniques including SEM, EDX and XRD. The relationships between the mass gain and coating thickness and structure were investigated. The experimental results were compared with the predictions from thermochemical calculations. Based on the understandings established, an effective approach to control the aluminide coating parameters and structures was identified, which made it possible to optimise powder mixture compositions and coating conditions for different coating requirements.     

Low pressure chemical vapor deposition of niobium coatings on graphite

Niobium coatings were prepared on graphite by low pressure chemical vapor deposition using niobium chloride and hydrogen as the reactant gases. The effects of deposition temperature on the morphology, phase, and deposition rate of niobium coatings were studied. The as-deposited niobium coatings were characterized by scanning electron microscopy and X-ray diffraction. The results indicate that the niobium coatings exhibit a granular hillock structure at 850-900 °C while a laminar structure at 950-1100 °C. The deposition is dominated by surface chemical kinetics with an apparent activation energy of 93.2 kJ/mol at 850-950 °C, while it is dominated by mass transport with an apparent activation energy of 7.9 kJ/mol at 950-1050 °C. At temperatures below 1100 °C, the deposited coatings mainly contain niobium. At temperatures above 1100 °C, the deposited coatings mainly contain niobium carbides. Considering the deposition kinetics and interfacial reactions, the deposition temperature should be controlled below 950 °C.     

Effect of grain size and orientation on the initial permeability of 36 wt % Ni-Fe alloys

The effect of grain size and grain orientation on the initial permeability of a 36 wt % Ni-Fe alloy with additions of molybdenum, chromium and copper is reported. The initial permeability was found to increase with annealing temperature between 600° C and approximately 900° C due to the formation of a (1 2 3) [4 1 ¯2] primary recrystallization texture. Increasing the annealing temperature in the range 900 to 1100° C led to progressively lower permeabilities due to the growth of randomly oriented abnormal grains within the textured matrix. It is suggested that an increase in the misorientation between adjacent grains gives rise to an increase in the local magnetostatic energy, leading to much stronger pinning of magnetic domain walls, with a consequent decrease in permeability. Annealing at temperatures above 1100° C tends to increase the permeability, because of the increase in grain size.     

Effect of temperature on the strength and conductivity of a deformation processed Cu-20%Fe composite

The high temperature (22–600 °C) properties were evaluated for a Cu-20%Fe composite deformation processed from a powder metallurgy compact. The ultimate tensile strengths decreased with increasing temperature but were appreciably better than those of similarly processed Cu at temperatures up to 450 °C. At 600 °C, the strength of Cu-20%Fe was only slightly better than that of Cu as a result of the pronounced coarsening of the Fe filaments. However, at temperatures of 300 and 450 °C, the strength of Cu-20%Fe is about seven and six times greater, respectively, than that of Cu, as compared to about a two fold advantage at room temperature. Therefore, Cu-20%Fe composites made by deformation processing of powder metallurgy compacts have mechanical properties much superior to those of similarly processed Cu at room temperature and at temperatures up to 450 °C. The pronounced decrease in electrical conductivity of deformation processed Cu-20%Fe as compared to Cu is attributed to the appreciable dissolution of Fe into the Cu matrix which occurred during the fabrication of the starting compacts where temperatures up to 675 °C were used. While the powder metallurgy compacts used for the starting material for deformation processing in this study did not lead to a high conductivity composite, the powder metallurgy approach should still be a viable one if processing temperatures can be reduced further to prevent the dissolution of Fe into the Cu matrix.     

Effect of the fluoride additives on the oxidation of AlN

The oxidation of AlN powder added by the fluorides at temperatures below 700°C in air was discovered in this study. The obvious onset of oxidation of AlN with cryolite and YF₃ additions is below 700°C with the product of α-Al₂O₃ phase, which usually occurs in single AlN powder above 1100°C. The changes on the weight and the FTIR spectra of the AlN powder fired at temperatures lower than 700°C show that cryolite and YF₃ greatly promote the oxidation of AlN powder at these temperatures. Different from the action of cryolite and YF₃ powder, CaF₂ has no obvious effect on the oxidation of AlN. A possible oxidation process, in part corroborated by FTIR and XRF, was proposed to explain the results in the experiments. The oxidation kinetics of AlN in the presence of cryolite were also discussed at the temperatures ranging from 550 to 700°C from the data of the weight gains in this region. The result shows that the oxidation follows a linear law, which implies a reaction rate-controlled process. The considerably low activation energy of 67 kJ mol⁻¹, which is associated with the quick oxidation and the formation of α-Al₂O₃ at temperatures below 700°C, was determined from the slope of the line fit.     

Hot tensile properties and microstructural evolution of as cast NiTi and NiTiCu shape memory alloys

Hot tensile properties of as cast NiTi and NiTiCu shape memory alloys were investigated by hot tensile test at temperature range of 700–1100 °C using the strain rate of 0.1 s⁻¹. The NiTi alloy exhibited a maximum hot ductility at temperature range of 750–1000 °C, while the NiTiCu alloy showed it at temperature range of 800–1000 °C. It was found that at temperatures less than 750 °C, diffusion-assisted deformation mechanism was inactive leading to semi-brittle type of failure and limited ductility in both alloys. Also it was found that at temperature range of 800–1000 °C, dynamic recrystallization is dominant leading to high ductility. Likewise, the fracture surface of the specimens presenting the maximum hot ductility showed an ideal type of ductile rupture in which they gradually pulled out to a fine point. On the other hand, the decline in ductility occurred at the temperatures above 1000 °C was attributed to the liquid phase formation leading to interdendritic and intergranular type of fracture.     

Processing of porous Ti and Ti5Mn foams by spark plasma sintering

Titanium and its alloys are one of the best metallic biomaterials to be used for implant application. In this study, porous Ti and Ti5Mn alloy with different porosities were successfully synthesized by powder metallurgy process with the addition of NH₄HCO₃ as space holder and TiH₂ as foaming agent. The consolidation of powder was achieved by spark plasma sintering process (SPS) at 16 MPa and pressureless conditions. The morphology of porous structure was investigated by using scanning electron microscopy (SEM) and X-ray micro-tomography (μ-CT). Nano-indentation tester was used to evaluate Young’s modulus of the porous Ti and Ti5Mn alloy. Experimental results showed that pure Ti sample, which sintered under pressure of 16 MPa, full relative density was achieved even at a relative low sintering temperature 750 °C; however, in the case of pressureless condition at sintering temperature 1000 °C the porosity was 53% and Young’s modulus was 40 GPa. The Ti5Mn alloy indicated a good pore distribution, and the porosity decreased from 56% to 21% by increasing the sintering temperature from 950 °C to 1100 °C. Young’s modulus was increased from 35 GPa to 51.83 GPa with increasing of the sintering temperatures from 950 °C to 1100 °C.     

Kinetic study of the reaction between copper(I) chloride and commercial silicon or silicides

The reaction of CuCl with silicon containing as impurities Al, Fe, Ca and Ti or with some silicides (Si₂Ca, Si₂Fe, Si₂Ti) has been investigated in the temperature range 250–310 °C. For the reaction between CuCl and commercial Si, it has been found that at 282 °C, the aluminium promotes the reaction between Cu₃Si and CuCl while its rate of consumption is greatly decreased by the presence of iron impurity. The combined action of these two impurities improves the quantity of the copper-silicon alloy formed. In the presence of silicides, the reaction with CuCl leads to copper formation with a high degree of dispersion.     

The break point in the pressure-density curve of magnesia prepared by the sea water magnesia process

Uniaxial compacting behaviour of MgO powders calcined at a temperature between 900 and 1200°C in air was investigated; the break point which appeared in the pressure-density curve of MgO powder was effective in elucidating the agglomeration state in the powder. The packing density of MgO powder as a function of calcination temperature was measured before and after milling operations. The plots of relative density versus logarithmic pressure also exhibited a break point indicating the pressure at which the contact points in porous agglomerates began to be destroyed. The agglomerate strength of MgO powders calcined at low and high temperatures (900 and 1200°C) only were measured. The microstructural differences between agglomerates in MgO powder and the surface of a compressed powder (applied pressure 150 MPa) were examined by scanning electron microscopy.     

No hope for ceramic whiskers or fibres as reinforcement of metal matrices at high temperature

A study has been made of the incorporation of aligned alumina whiskers (from both USA and French commercial sources) and aligned alumina fibres (0.01 in. diameter) into nickel and the resultant tensile properties at 20 and 1100° C studied. Conditions have been established which preclude significant whisker/fibre break-up, densify the matrix and promote chemical bonding between the whisker/fibre and the matrix. The factors which influence these aspects are described, viz. impurities on the whisker surface, classification of the whiskers, deposition of nickel on aligned whiskers or fibres from pure nickel carbonyl, hot-pressing conditions to promote bonding and effect densification.High strengthening efficiency (up to 75%) can be ascribed to the use of whiskers or fibres in tests at 20° C but at high temperatures (1100° C) this efficiency is poor, 10% or less. This poor efficiency is ascribed to the rupture of the chemical bond between whisker or fibre and matrix during the first reheat after composite manufacture. Strong chemical bonding is vital for a successful composite since in any practical ceramic whisker/fibre-metal matrix composite the inherent disparity in thermal expansion coefficients acts to disrupt the chemical bond. Moreover, even if the chemical bond was sufficiently strong not to rupture as a result of the thermal expansion disparity, the implication of increased chemical affinity to produce the strong bond would lead to complete erosion of a whisker in times of about 100 h. Even in the case of fibres with a large diameter (0.010 in.) there is sufficient degradation of mechanical properties although without apparent loss of the cylindrical form to markedly reduce composite properties.Apart from any economic consideration, therefore, it is concluded on technical grounds that the reinforcement of metals by ceramic whiskers or fibres for high temperature (>700° C) use is impossible.     

Synthesis and densification of lutetium pyrosilicate from lutetia and silica

Cerium-doped lutetium pyrosilicate (Lu₂Si₂O₇:Ce) powder was synthesized by solid state reaction of Lu₂O₃ and SiO₂. Stoichiometric mixtures of the starting materials were heat treated at various different temperatures and their phase contents were measured by XRD technique. It was found that the first step in the formation of Lu₂Si₂O₇ (LPS) is the appearance of Lu₂SiO₅ (LSO). This takes place at 1100 °C, fully 300 °C below the first appearance of LPS. Between 1400 and 1500 °C both LSO and LPS coexist in the calcined batch, but by 1550 °C all LSO is completely converted to LPS. LPS formation temperature does not have appreciable effect on the density of the hot pressed samples. Hot pressed samples obtained from powder synthesized at 1650 °C are nearly transparent, although the particle size of the starting powder is higher than that of the powder formed at lower temperatures.     

Control of the morphology and surface properties of nickel ferrite

Nickel ferrite powders with particle sizes in the 3–5 m range have been prepared from coprecipitated nickel–iron oxalate precursors. Firing the nickel–iron oxalate precursor in the range 300–1100°C produced samples of high chemical purity, while introducing significant variations in the distribution of crystallite sizes and surface morphologies. An increase in the powder density from 4.2 to 5.2 g cm-3 and a decrease in the surface area of the nickel ferrite from 120 to 0.2 m2g-1 were effected by increasing the firing temperature to 1100°C.     

Fabrication of YBa2Cu3Ox superconductor using Y2BaCuO5, BaCuO2 and CuO

YBa₂Cu₃O _x superconductor was synthesized using Y₂BaCuO₅, BaCuO₂, and CuO powder mixture. Reaction temperatures were identified using differential thermal analysis (DTA) and thermogravimetry (TG) for syntheses of precursor powders and the powder mixtures. Appropriate reaction temperatures for Y₂BaCuO₅ and BaCuO₂ precursor powders were 950 and 930°C, respectively. Two endothermic reactions involving melt formations were identified on the DTA and TG curves of the powder mixture, and the liquid increased the reactivity of the YBa₂Cu₃O _x formation. Powder mixture samples were sintered at various temperatures ranging from 880 to 1000°C. Microstructural and X-ray powder diffraction studies showed YBa₂Cu₃O _x and impurities to be formed in the samples sintered at various temperatures. The samples sintered at 990 and 1000°C showed dense microstructures. The critical temperature was 84 K for the sample sintered at 880°C and rose to 92 K as the sintering temperature increased.     

Influence of preparation and storage of polycrystalline specimens on low-temperature transport property measurements in tin of various purities

During room-temperature storage over periods of the order of 1 yr, pure tin releases the stored energy produced by cold-working. In high-purity (99.999–99.99997%) extruded tin wires, the helium temperature resistivity decreases with increasing storage time because of room-temperature annealing. A high-temperature heat treatment (36 hr at 210°C in vacuum) stabilizes the low-temperature mean free path. In cold-worked, less pure tin (99.9%) with Cu and Pb impurities, droplets with high impurity content are formed during room-temperature storage. This leads to strong deviations from Ohm's law in the neighborhood of the superconducting transition temperature.Work supported by the Fonds National Suisse de la Recherche Scientifique.     

Sol-gel/co-precipitation method for the preparation and characterization of zirconia-tungsten composite powders

Sol-gel and co-precipitation are interesting processes in view to prepare ceramic-metal composite powder. Using metal salts as starting compounds it is possible in a first step to synthetise a mixed precipitate and in a second reduction step, in a H₂/Ar atmosphere, to obtain such a composite powder. Stabilized zirconia-tungsten composite powder have been prepared by this way without mixing and milling steps. Each step of the synthesis has been characterized. Stability of the mixed precipitate suspension and surface area of the dried powder are largely influenced by the tungsten salts content. The temperature of formation of zirconia changes from 510 °C without tungsten precursor up to 695 °C for a 60% tungsten molar ratio composition. Composite zirconia-tungsten powders, in a large range of composition, are obtained at 1100 °C with morphologies depending on humidity and thermal treatment conditions.