Combustion synthesis of titanium carbide: Theory and experiment

The combustion synthesis of titanium carbide from elemental powders has been theoretically and experimentally studied as a model system for self -propagating high temperature synthesis (SHS) of refractory compounds. Calculations of the adiabatic temperature of combustion of graphite and titanium powders to form TiC _x have been made to show the effects of stoichiometry, dilution and the initial temperature of the reactants. Experimental observations on the stability of the combustion front, combined with theoretical predictions, lead to an estimated activation energy of 117 kJ mol^–1 for the process. This value is at least a factor of about four too low to correspond to a diffusion-controlled process. The combustion of graphite and titanium powders was accompanied by the evolution of gases whose primary constituent was found to be hydrogen. This observation was attributed to the reaction of adsorbed moisture with titanium powder. The titanium carbide phase resulting from the combustion of compacted mixed powders of the elements was highly porous ( 50% porosity). It can be obtained in high density (5% porosity) when pressure is applied during the combustion process.     

Surface Modification of Sericite Using TiO2 Powders Prepared by Alkoxide Hydrolysis: Whiteness and SPF Indices of TiO2-Adsorbed Sericite

Titania (TiO₂) powders have been prepared from the 0.025-M titanium isopropoxide/ethanol solution and the 0.5-M distilled water/ethanol solution. The prepared TiO₂ powders showed an anatase phase and a rutile phase after heat treatment at 500°C for 2 h and 1000°C for 2 h, respectively. The heterocoagulation adsorption between TiO₂ powder and sericite surface in water was achieved in the range of pH 3.63.7 (where this pH range shows a maximum Zeta-potential difference for two powders). On the other hand, an anomalous transformation behavior appeared in the TiO₂-adsorbed sericite after heat treatment at 1000°C. The surface modification of sericite through the TiO₂-adsorption improved the whiteness as well as the SPF (Sun Protection Factor) indices.     

Synthesis of titanium nitride by a spark-discharge method in liquid ammonia

Titanium nitride powders were synthesized by the spark-discharge method in liquid ammonia at — 78 to 130 °C and 3.5–10.5 kV discharge voltage using titanium pellets as the starting materials. Titanium nitride possessing nitrogen defect, TiN_1–x (x0.5), was obtained as the main product, together with small amounts of -Ti alloyed with nitrogen. The increase in temperature of the liquid ammonia resulted in an increase in the titanium nitride content in the product but a decrease in the powder production rate. By calcining the mixed powders of TiN_1–x and -Ti in a nitrogen atmosphere around 1200 °C, stoichiometric TiN was obtained as single phase.     

Laser-induced vapour-phase syntheses of boron and titanium diboride powders

Small diameter boron and titanium diboride powders were synthesized from vapour phase reactants heated with infrared radiation from a CO₂ laser. Boron powders were synthesized from BCI₃ + H₂ gas mixtures undfrom B₂H₆. TiCl₄ + B₂H₆ gas mixtures yielded TiB₂ powder. BCI₃ + H₂ + TiCl₄ gas mixtures yielded TiCl₂ powder but no TiB₂. Novel equipment designed to vapourize TiCI₄ liquid is described, Detailed characterizations of the product powders are presented.     

Production of flaky amorphous powders in Fe-Zr-B system by a supercooled liquid quenching method,and their magnetic properties

By using a supercooled liquid (two-stage) quenching method, in which supercooled liquid droplets produced by high-pressure gas atomization are flattened at high kinetic energies on a rapidly rotating wheel, flaky amorphous powders with a thickness of 1–3 m and a large aspect ratio of 20–300 were produced for an Fe₈₅Zr₈B₆Cu₁ alloy. This is in contrast to the result that spherical powders produced only by high-pressure gas atomization consist of amorphous, bcc and compound, even in the particle size range below 25 m. The amorphous powder changes to a mostly single bcc phase by annealing for 3.6 ks at 873 K, and the further rise in annealing temperature causes the mixed structure of -Fe + Fe₃(Zr,B). The annealed flaky powders with the bcc phase exhibit soft magnetic properties of 140 emu g^–1 for saturation magnetization _10k and 0.6 Oe for coercivity, H _c. The composite made from the bcc flaky powders and phenol resin at a weight ratio of 91 has a high degree of laminated structure. The composite also exhibits soft magnetic properties of 5.3 kG for D₁₀₀, 150 for _max and 1.1 Oe for H _c. The B ₁₀₀ value is 2.1 times as high as that for the composite of amorphous Co-Fe-Si-B flaky powders and resin. Thus the present composite is expected to be used in applications which require both high saturation magnetization and soft magnetic properties, which cannot be obtained for the composites made from amorphous Co- and Fe-based flaky powders.     

Preparation of silicon carbide powders by chemical vapour deposition of the (CH3)2SiCl2-H2 system

Silicon carbide (SiC) powders were prepared by chemical vapour deposition (CVD) using (CH₃)₂SiCl₂ and H₂ as source gases at temperatures of 1273 to 1673 K. Various kinds of SiC powders such as amorphous powder, -type single-phase powder and composite powder were obtained. The composite powders contained free silicon and/or free carbon phases of about a few nanometres in diameter. All the particles observed were spherical in shape and uniform in size. The particle size increased from 45 to 130 nm with decreasing reaction temperature and gas flow rate, as well as with increasing reactant concentration. The lattice parameter of the -SiC particles decreased with increasing reaction temperature. All the lattice parameters were larger than those of bulk -SiC.     

RF plasma synthesis of YBa2Cu3O7?x powders

Fine powders of YBa₂Cu₃O_7–x have been synthesized by injecting mixed nitrate solutions of yttrium, barium, and copper into an argon rf thermal plasma. In general, the as-produced powders were dark brown and nonconducting. To obtain superconductivity, the as-produced powders were annealed either in a flowing oxygen tube furnace (at 900C) or in a lowpressure oxygen rf plasma. X-ray powder diffraction, scanning electron microscopy, and centrifugal sedimentation were used for powder characterization. For resistance measurements, bulk samples were prepared by isostatic pressing and tube furnace sintering of the annealed powders. The superconducting transition temperature (at 50% drop of resistivity) was 86 K.     

Formation mechanism of titanium silicide by mechanical alloying

The syntheses of five titanium silicides (Ti₃Si, TiSi₂, Ti₅Si₄, Ti₅Si₃, and TiSi) by mechanical alloying (MA) have been investigated. Rapid, self-propagating high temperature synthesis (SHS) reactions were involved in producing the last three materials during room temperature high-energy ball-milling of elemental powders. Such reactions appeared to occur through ignition by mechanical impact in the fine powder mixture formed after a critical milling period. From in-situ thermal analyses, each critical milling period for the formation of Ti₅Si₄, Ti₅Si₃, and TiSi was observed to be 22, 35.5 and 53.5 minutes, respectively. However, the formation of Ti₃Si and TiSi₂ did not occur even after 360 minutes of milling of as-received Ti and Si powder mixture, due to the lack of homogeneity of the powder mixture. Other ball-milling procedures were employed for the syntheses of Ti₃Si and TiSi₂ using different sizes of Si powder and milling medium materials. Ti₃Si was synthesized by milling a Ti and 60 minutes premilled Si powder mixture for 240 minutes. -TiSi₂ and TiSi₂ were produced by high energy partially stabilized zirconia (PSZ) ball-milling for 360 minutes in a steel vial followed by jar-milling of a Ti and 60 min premilled Si powder mixture for 48 hr. The formation of Ti₃Si and TiSi₂ occurs through a slow solid state diffusion reaction, and the product(s) and reactants coexist for a certain period of time. The formation of titanium silicides by MA and the reaction rate appeared to depend on the homogeneity of the powder mixture, milling medium materials, and heat of formation of the product involved.     

Formation of amorphous Ni-Zr alloy powder by mechanical alloying of intermetallic powder mixtures and mixtures of nickel or zirconium with intermetallics

Mechanical alloying was used to synthesize Ni_xZr_1–x alloys from mixtures of intermetallic compound powders, and also from mixtures of intermetallic compound powders and pure elemental powders. The mechanically alloyed powders were amorphous in the range 0.24 x 0.85. This range is larger than amorphous alloys produced by the melt-spinning technique and mechanical alloying of elemental crystalline powders. Two-phase mixtures of the amorphous phase and the corresponding crystalline terminal solid solution were formed in the range 0.10 x 0.22, and x=0.90. It is found that the morphological development during mechanical alloying of these powders is different from mechanical alloying using only pure ductile crystalline elemental powders. The thermal stability has been investigated. The enthalpy and activation energy of crystallization for Ni-Zr amorphous powders prepared by mechanical alloying are lower than those for melt-spun samples of the same composition. The crystallization temperature of the mechanically alloyed Ni-Zr amorphous powders is higher than that of meltspun samples in the composition range Ni₂₀Zr₈₀ to Ni₃₃Zr₆₇ and Ni₄₀Zr₆₀ to Ni₆₀Zr₄₀. The presence of tiny crystallites as nucleation centres and high oxygen levels in the mechanically alloyed amorphous alloys might be responsible for the differences in crystallization behaviour. A new crystalline metastable phase was observed during crystallization studies of Ni₂₄Zr₇₆ amorphous powder.     

Compositional modification of titanium carbide powders by induction plasma treatment

Non-stoichiometric titanium carbide powders were treated in an r.f. induction plasma. The composition of plasma gas, reactor pressure and powder feed rate were changed as experimental parameters, but plate power was kept constant. As the titanium carbide powders passed through the plasma, they melted, partially evaporated, and finally solidified. During the in-flight process, compositional modification was noted involving lattice modification and a change of the non-stoichiometry of titanium carbide depending on the plasma and powder feeding conditions. These were mostly due to the removal of carbon and oxygen impurity in titanium carbide while melting. The -AES analysis indicated that the removal of carbon occurred in the plasma treatment. The deposits formed from the vapour phase consisted mainly of very fine cubic crystals, some tens of nanometres in size, with an appreciable number of vacancies at carbon sites.     

Effect of selected impurities on the high temperature mechanical properties of hot-pressed silicon nitride

The effect of impurities on the high temperature mechanical properties of hot-pressed silicon nitride has been determined. Selected impurity additions were made to both relatively pure -phase and -phase silicon nitride starting powders. These powders were hot-pressed to full density using 5 wt % MgO as the pressive additive. The silicon nitride hot-pressed from the -phase powder exhibited higher strength at both 25 and 1400 C than that fabricated from the -phase powder. The impurity additions had no effect on the room temperature mechanical properties. The CaO additions had the most significant effect on the high temperature mechanical properties. In both the material hot-pressed from the -phase and -phase powders, increasing CaO additions severely reduced the high temperature strength and increased the amount of non-elastic deformation observed prior to fracture. Although alkali additions (Na₂CO₃, Li₂CO₃, K₂CO₃) also tended to have the same effects as the CaO, the high volatility of these compounds resulted in a much reduced concentration in the hot-pressed material, thus minimizing somewhat their tendency to enhance the high temperature strength degradation. The Fe₂O₃ and Al₂O₃ had no apparent effect on the high temperature mechanical properties.     

Influence of powder characteristics on gas pressure sintering of Si3N4

The sintering behaviours of four kinds of Si₃N₄ powders were investigated by dilatometry in 10 atm N₂ at 1890, 1930 and 2050° C. The sinterabilities of powders were compared and discussed in relation to the powder characteristics. A large size distribution in the powder accelerated grain and pore growth at <1800° C, which resulted in the inhibition of further densification at >1800° C. The presence of carbon in a powder prevented densification. A powder with a uniform grain size kept the microstructure of the sintered material uniform during sintering at <1800° C and gave a high degree of shrinkage at >1800° C. Densification at >1800° C was accompanied by the dissolution of equi-axial -Si₃N₄ grains and reprecipitation as elongated -Si₃N₄ grains from the oxynitride liquid. The relation between the densification and microstructure is discussed in terms of the relative rates of densification and grain growth.     

The structure of plasma-prepared Al2O3-TiO2 powders

The morphology and phase constitution of sub-micron Al₂O₃-TiO₂ powders prepared by oxidation of mixtures of Al₂Br₆ and TiCl₄ in an oxygen-argon high-frequency plasma have been studied. The particle size and distribution were consistent with formation of liquid particles by rapid nucleation and surface reaction followed by growth by coalescence of droplets. The particle size of the powders is related to the concentration of reactants in the gas stream and the temperature difference between condensation and solidification. A metastable solution of TiO₂ in -Al₂O₃ was formed in the range 0 to 7 wt% TiO₂, at higher TiO₂ concentrations particles consisted of a dispersion of rutile particles (~ 10 nm) within single crystals of -Al₂O₃. A metastable phase identified as 3Al₂O₃·TiO₂ was also formed in powder with compositions in the range 14 to 40 wt% TiO₂. Over the composition range 40 to 80 wt% TiO₂ the powder consisted predominantly of crystals with a two-phase Al₂TiO₅-rutile structure. Pure TiO₂ consisted largely of anatase and the addition of Al₂O₃ resulted in the formation of rutile as the major phase. The phase constitution of the powders is interpreted in terms of the nucleation kinetics of the various phases.     

Fabrication of spherical magnetite particles by the flame fusion method

Fabrication of spherical magnetite powders was investigated in the propane-oxygen flame using sponge iron powders as a starting powder. Spherical particles produced by fusion, sphering and oxidation of iron powder were composed of residual Fe, FeO, Fe₃O₄ and -Fe₂O₃ in the case of particles collected by a cyclone. The amount of Fe₃O₄ in the products was strongly dependent on the propane/oxygen ration and the flow rate of carrier air, but weakly on the feed rate of iron powder. Injection of quenching gas was found to be effective to improve the yield of Fe₃O₄. Particle size of products reflected directly that of starting powders, indicating fairly easy control of particle size of products. The saturation magnetization of the produced powder under the optimum condition was 88 emu/g. These facts suggest that the fusion and oxidation treatment of iron powders in the propane-oxygen flame is a suitable process for the manufacture of the magnetic carrier for plain paper copy (PPC) on an industrial scale. Powders collected by a bag filter were found to be fine -Fe₂O₃ particles with a diameter of about 100 nm.     

Preparation and crystallization of Fe-Co-B amorphous powder

Ultrafine (Fe_0.7Co_0.3)_100–x B _x amorphous powders with boron contentx in the range 27–40 at % have been prepared by borohydride reduction. The preparation process shows that the powders have a catalysis property. The results of X-ray and electron diffractions show that all of the samples were spherical amorphous particles of dimension <0.2 m. The difference in the ease of composition between Fe-Co-B powder and ribbon indicates that the local environment of Fe-Co-B powder may be different from that of Fe-Co-B ribbon. The study of crystallization behaviour using Mössbauer, X-ray and differential scanning calorimetry measurements indicates that there are two steps in the crystallization process. These steps represent the formation of -Fe₇₀Co₃₀ phase and Fe₂B-like phase, respectively.     

Ultrasonic spray pyrolysis of a chelated precursor into spherical YBa2Cu3O7−x high temperature superconductor powders

YBa₂Cu₃O_7–x powders have been prepared directly by ultrasonic spray pyrolysis using nitrate salts as precursors and citric acid and ethylene glycol as chelating agents. This method consists of ultrasonically atomizing a precursor solution into droplets, thermally chelating, drying, decomposing and solid state reacting these droplets in a carrier gas flowing through a tube furnace, forming a well characterized powder. The chelated precursor adjusted to pH 8 forms bidentate bonding between the cations and the chelating agents. Thermal analysis and infrared spectroscopy identify the decomposition steps of the precursor. The dry gel of the chelated precursor is nearly amorphous indicating intimate mixing on the atomic level. X-ray diffraction suggests the mechanism of forming the 123 crystalline phase. Spherical powders are produced with diameters ranging from 0.2 to 0.8 m depending on the ultrasonic frequency and the solution concentration. The spherical particles are hollow or solid depending on the precursor type and the furnace temperature. The primary crystallite size is about 10–50 nm. X-ray diffraction data and infrared spectra show that the spray pyrolysed powder from the chelating precursor forms the YBa₂Cu₃O_7–x phase at 800 °C, which is 100 °C lower than that formed from unchelated precursors.     

Preparation of PLZT powders from several aqueous solutions

Fine, soft agglomerate and chemically homogeneous PLZT powders were prepared from nitrate, chloride and alkoxide precursors. The preparation is based on a coprecipitation method in which the aqueous clear solution of multicomponent systems is reacted with ammonia gas at the liquid surface. As-dried powder characteristics — the microstructure of agglomerates, colour and X-ray diffraction structure — were similar in all cases; however, the properties of calcined powders and of two-stage sintered pellet were affected not only by precursors, but also by postprecipitation processing: (a) washing with organic solvent, (b) drying and ageing, (c) calcining. A novel lattice behaviour, i.e. decomposition and recovery of perovskite structure of calcined powders regardless of precursors, was found over a wide temperature range from room temperature to 1150° C. Transparent ceramics were fabricated by sintering the acetone washed-aged powder derived from TiO(NO₃)₂ precursor.     

Preparation of silicon carbide powders by chemical vapour deposition of the SiH4-CH4-H2 system

Chemical vapour deposition (CVD) of the SiH₄ + CH₄ + H₂ system was applied to synthesize-silicon carbide powders in the temperature range 1523 to 1673 K. The powders obtained at 1673 K were single-phase-SiC containing neither free silicon nor free carbon. The powders obtained below 1623 K were composite powders containing free silicon. The carburization ratio (SiC/(SiC + Si)) increased with increasing reaction temperature and total gas flow rate, and with decreasing reactant concentration. The average particle sizes measured by TEM ranged from 46 to 114nm, The particle size increased with the reaction temperature and gas concentration but decreased with gas flow rate. The-SiC particles obtained below 1623 K consisted of a silicon core and a-SiC shell, as opposed to the-SiC particles obtained at 1673 K which were hollow. Infrared absorption peaks were observed at 940 and 810 cm^–1 for particles containing a silicon core; whereas a single peak at about 830 cm^–1 with a shoulder at about 930 cm^–1 was observed for the-SiC hollow particles. The lattice parameter of-SiC having a carburization ratio lower than 70 wt%, was larger than that of bulk-SiC and decreased with the increasing carburization ratio. However, when the carburization ratio exceeded 70 wt%, the lattice parameter became approximately equal to that of bulk-SiC.     

Sinterability of β-SiAlON powder prepared by carbothermal reduction and simultaneous nitridation of ultrafine powder in the Al2O3-SiO2 system

Three types of -SiAlON (Si_{6 – z} Al _z O _z N_{8 – z} ) powder were prepared by the carbothermal reduction and simultaneous nitridation of ultrafine powders in the Al₂O₃-SiO₂ system. The ultrafine starting oxide powders, prepared using the vapour-phase reaction technique, were mixed with carbon powder and heated at 1400 °C for 1 h under flowing nitrogen to form -SiAlON and followed by heating at 570 °C for 1 h in air to remove residual carbon. The resulting powders contained only -SiAlON with z values of 1.63, 2.05, and 2.99. The relative density (bulk density/true density) of -SiAlON compacts pressureless sintered at 1800 °C for 1 h under flowing nitrogen increased with z and reached 89.9% at z = 2.99. When the -SiAlON compact with z = 2.99 was hot pressed at 1800 °C for 1 h under flowing nitrogen, a maximum relative density of 93.6% was achieved. Although this hot pressed compact contained a small amount of 15R-SiAlON in addition to -SiAlON, it possessed a small average grain size (typically 0.5 m diameter) and high Vickers hardness (19.2 GPa).     

Dependence of oxidation rate of WC powder on particle size

Isothermal oxidation experiments on WC powders revealed a systematic dependence of oxidation rate on powder particle size. Oxidation was followed by measuring the change in mass of the WC powder as WC is converted to WO₃. Fine powders oxidized more quickly than coarse powders because for the same initial mass the fine powder had a larger surface area. Measurement of the change in mass with time were shown to resolve differences in mean size of 0.1 m, and possibly less, between separate batches of powder. A theoretical expression for the change in mass with time of spherical particles has been derived which compares well with experimental measurements and which can also be used with appropriate assumptions to calculate the initial powder-size distribution.