The Effect of Surface Oxides During Hot Pressing of TiB2

Hot pressing of TiB₂ has been investigated with particular emphasis on the evolution of secondary phases originating from the initial surface oxide layer on the TiB₂ powders. Carbothermal reduction of the surface oxides during sintering was also investigated by adding carbon to the TiB₂ powder. TiO_{1− x} C _x was shown to be the main secondary phase in hot-pressed TiB₂, and carbon was shown to strongly influence on sintering process and the amount, composition and distribution of the secondary phase TiO_{1− x} C _x . The formation of TiO_{1− x} C _x is discussed in relation to volatile boron oxide, which reacts with the graphite die to produce CO gas, which further may cause transport of carbon into TiB₂ during sintering before pore closure. Finally it was demonstrated that the density could be controlled by addition of carbon to the TiB₂ powder.     

New Route for the Extraction of Crude Zirconia from Zircon

A commercial grade of zircon (ZrSiO₄) concentrate was mechanically milled with MgO for up to 100 h in a laboratory-scale mill. The resultant powders were subjected to thermal processing, chemical leaching, and X-ray diffraction (XRD). There was no direct evidence of reaction during the milling step, with no new phases evident from XRD. Leaching of the powder showed that a reaction had occurred, and increased solubility with milling time was attributed to the formation of a nanostructured Mg-Zr-Si oxide, which dissolved congruently. Heating the powders resulted in a number of thermal events, including the formation/crystallization of ZrO₂ and Mg₂SiO₄. Thermal treatment of the milled powders allowed selective chemical leaching of the magnesium and silicon, leaving a powder containing ∼90% ZrO₂.     

Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2

Compacts of TiB₂ with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB₂. The oxygen content of the raw TiB₂ powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO₂ and B₂O₃. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB₂ samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO₂+ SiC → SiO₂+ TiC. Therefore, the improved sinterability of TiB₂ resulted from the SiO₂ liquid phase that was formed during sintering when the raw TiB₂ powder had 1.5 wt% oxygen.     

Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid

Titanium diboride (TiB₂) was hot-pressed at a temperature of 1800°C, and silicon nitride (Si₃N₄) was added as a sintering aid. The amount of Si₃N₄ that was added had a significant influence on the sinterability and mechanical properties of the TiB₂. When a small amount (2.5 wt%) of Si₃N₄ was added, the Si₃N₄ reacted with titania (TiO₂) that was present on the surface of the TiB₂ powder to form titanium nitride (TiN), boron nitride (BN), and amorphous silica (SiO₂). The elimination of TiO₂ suppressed the grain growth effectively, which led to an improvement in the densification of TiB₂. The formation of SiO₂ also was deemed beneficial for densification. The mechanical properties-especially, the flexural strength-were enhanced remarkably through these improvements in the sinterability and microstructure. On the other hand, when a large amount (greaterthan equal to5 wt%) of Si₃N₄ was added, the mechanical properties were not improved much, presumably because of the extensive formation of a glassy Si-Ti-O-N phase at the grain boundaries.     

Ablation-Resistance of Combustion Synthesized TiB2–Cu Cermet

TiB₂–Cu ceramic–metal composites were prepared by combustion synthesis of elemental titanium, boron, and copper powders. The synthesized product consisted of two phases: TiB₂ and copper. The addition of copper improved the strength and fracture toughness, thermal expansion coefficient, and thermal conductivity of TiB₂. Thermal shock and ablation resistances of TiB₂–Cu composites were studied using a plasma torch arc heater. Monolithic TiB₂ failed catastrophically when the plasma arc flow reached the specimen surface. However, no cracks were found on the ablation surface of the TiB₂–Cu ceramic–metal composites. The fractional mass loss was 4.09% for a TiB₂–40Cu composite, which was close to the traditional W/Cu alloys. Volatilization of metal binder and mechanical erosion of TiB₂ were observed to be the major ablation mechanisms. An ablation process model is proposed for the TiB₂–Cu composites.     

Removal of Oxide Contamination from TiB2 Powders

Surface oxide contamination, which commonly occurs in fine TiB₂ powders and afects their properties, can be removed by treatment with BCl₃(g) at 650°C .     

Hydrothermal Synthesis of Spherical Perovskite Oxide Powders Using Spherical Gel Powders

Spherical perovskite oxide powders, composed of fine particulates, were prepared by using spherical gel powders under hydrothermal conditions. Spherical PbTiO₃, BaTiO₃, and SrTiO₃ powders were synthesized from spherical TiO₂ gel powders, and spherical PbZrO₃ powder from spherical ZrO₂ gel powder. Spherical Pb(Zr_0.5, Ti_0.5)O₃ and Ba(Zr_0.5,Ti_0.5)O₃ powders were prepared from spherical ZrTiO₄ gel powders. Lead acetate trihydrate, barium hydroxide octahydrate, and strontium hydroxide octahydrate were used as the sources of A-site ions in each perovskite oxide (ABO₃). The spherical TiO₂ and ZrO₂ gel powders were prepared by thermal hydrolysis of titanium tetrachloride and zirconium oxychloride, respectively, and spherical ZrTiO₄ gel powder by thermal hydrolysis of a mixture of them in alcohol-water mixed solvent. During the hydrothermal treatment, the spherical gel powders retained their spherical shape to produce spherical perovskite oxide powders, composed of nanometer-sized particulates.     

Formation of Titanium Carbide/Aluminum Oxide Nanocomposite Powder by High-Energy Ball Milling and Subsequent Heat Treatment

The preparation of titanium carbide/aluminum oxide (TiC/Al₂O₃) nanocomposite powders from a mixture of titanium, carbon, and Al₂O₃ powders via a high-energy ball milling process and subsequent heat treatment was investigated. The microstructure development of the powder mixtures was monitored by X-ray diffraction and transmission electron microscopy. The ball milling of an elemental carbon, titanium, and Al₂O₃ powder mixture at ambient temperature resulted in the formation of TiC within 15 h of milling. With further milling of up to 25 h, the resulting powder mixture was composed of nanosized TiC particles and nanocrystalline carbon, titanium, and Al₂O₃. The nanocrystalline titanium and carbon were transformed into nanosized TiC particles after subsequent heat treatment. The final product was composed of nanosized TiC and microcrystalline Al₂O_3. Most of the nanosized TiC particles were located within Al₂O₃ grains.     

Densification, Sintering Reactions, and Properties of Titanium Diboride With Titanium Disilicide as a Sintering Aid

In the present investigation, we explore the feasibility of using TiSi₂ as a sintering aid to densify titanium diboride (TiB₂) at a lower sintering temperature (<1700°C). The hot-pressing experiments were conducted in the temperature range of 1400°–1650°C for 1 h in an argon atmosphere and TiSi₂ addition to TiB₂ was restricted up to 10 wt%, with an overall objective to densify the materials with a fine microstructure as well as to assess the feasibility of enhancing the mechanical and electrical properties. When all the materials were hot pressed at 1650°C, the hot-pressed TiB₂– X % TiSi₂ ( X =0, 2.5, 5, 10 wt%) composites were found to be densified to more than 98%ρ_th (theoretical density), except monolithic TiB₂ (∼94%ρ_th). An interesting observation is the formation of a Ti₅Si₃ phase and this phase formation is described by thermodynamically feasible sintering reactions. Our experimental results suggest that the optimal TiB₂–5 wt% TiSi₂ composite can exhibit an excellent combination of properties, including a high hardness of 25 GPa, an elastic modulus of 518 GPa, an indentation toughness of ∼6 MPa·m^1/2, a four-point flexural strength of more than 400 MPa, and an electrical resistivity of 10 μΩ·cm.     

Homogeneous TiB2 Ceramics Achieved by Electric Current-Assisted Self-Propagating Reaction Sintering

Using spark plasma sintering techniques, homogeneous microstructures of titanium diboride (TiB₂) ceramics were obtained by sintering of boron and titanium powder mixtures. The results show that an additional electric current is essential for achieving a large number of evenly distributed ignition points that ensure that the self-propagating reaction simultaneously takes place within the entire volume. The effects of the electric current, the use of Mg additions, and the heating rates on the resulting TiB₂ ceramic densities and microstructures are discussed.     

Factors Affecting the Porosity and Bending Strength of Ti(CN)-TiB2 Materials

This paper describes the effects that the particle size of Ti(CN) and TiB₂ powder, oxygen content of the TiB₂ powder, and Co impurity content in the starting raw materials have on the porosity and bending strength ofTi(CN)-30% TiB₂ materials obtained by ordinary sintering.     

Combustion Synthesis of the Titanium-Aluminum-Boron System

TiB₂ and Al base composite powders, which will offer a weight-saving improvement in stiffness, were produced by combustion synthesis of Ti, Al, and B ternary powder mixtures. Finely dispersed TiB₂ was synthesized by reacting a mixture of Ti, Al, and B in the molar ratio of 1:1:4. The grain size of the TiB₂ formed was <0.5 μm, which was much smaller than that obtained from the reaction of a mixture of Ti, Al, and B in the molar ratio 1:1:2. These results are discussed in light of the reaction propagating velocity and heat removal during the combustion synthesis process.     

Oxidation of TiB2 Powders below 900°C

Oxidation studies have been performed on titanium dibor-ide (TiB₂) powders, from room temperature up to 900°C. The studies were performed at low partial pressures of oxygen, at 10 and 0. 05 ppm of O₂ in argon, simulating furnace atmosphere with flowing neutral gas of medium and high purity, and in air. It has been found that titanium borate (TiBO₃) is formed in these processes. It has also been revealed that the oxidation process starts below 400°C and is reversible in this low temperature range from 100° to 400°C.     

Titanium Diboride–Tungsten Diboride Solid Solutions Formed by Induction-Field-Activated Combustion Synthesis

Solid solutions of titanium diboride–tungsten diboride (TiB₂–WB₂) were synthesized by induction-field-activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ∼2 min. Solutions with compositions ranging from 40–60 mol% WB₂ were synthesized with a stoichiometric ratio (Ti + W)/B =½; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The "a" parameter decreased only slightly with an increase in the WB₂ content, whereas the "c" parameter exhibited a significant decrease over the range 40–60 mol% WB₂. Solid-solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid-solution phase.     

Y2O2S:Eu Red Phosphor Powders Coated with Silica

Y₂O₂S:Eu red phosphor powders were coated with silica (SiO₂), using sol–gel and heterocoagulation techniques. Phosphor powders were dispersed in ethanol with tetraethyl orthosilicate and water. Hydrochloric acid was used to catalyze the sol–gel reaction, and an amorphous film 10–20 nm thick was observed via transmission electron microscopy (TEM). Colloidal SiO₂ powders 10–70 nm in size were used, and the SiO₂ powder coating was made by controlling pH values in the range of 4.5–8, in which a negatively charged surface of SiO₂ powder and a positively charged surface of red phosphor powder were formed. Then, SiO₂ powders were adsorbed electrically onto the phosphor powder surface, as evidenced by TEM, dissolution, and zeta potential measurements. Chemical bonding in the coating was studied using electron spectroscopy for chemical analysis and Fourier transform infrared spectroscopy.     

Low-Temperature Transient Glass-Phase Processing of Monoclinic SrAl2Si2O8

The use of transient glass-phase processing to lower the glass-melting temperature and subsequent heat-treatment temperature of stoichiometric SrAl₂Si₂O₈ to produce the stable monoclinic form has been described. Two nonstoichiometric, low-melting, alumina-deficient, strontium aluminosilicate compositions were melted, quenched, and milled into glass powders. B₂O₃ was dissolved into one of the glass compositions to control the crystallization behavior. The glass powders were then wet-mixed with enough alpha-Al₂O₃ powder so that the overall composition was that of stoichiometric SrAl₂Si₂O₈ (B₂O₃ neglected). The four compositions were dry-pressed into pellets and sintered in three processes. Glass-alumina pellets with dissolved B₂O₃ were densified via viscous-phase sintering at 1100°C, followed by complete dissolution of the alumina and crystallization to ~100% monoclinic SrAl₂Si₂O₈. Pellets without dissolved B₂O₃ required considerably higher temperatures to form ~100% monoclinic SrAl₂Si₂O₈ in a modified process.     

Elastic Properties of Titanium Monoboride Measured by Nanoindentation

In order to measure the elastic properties of titanium monoboride (TiB), which are not well known, a Ti/TiB composite with 95% volume fraction of TiB was fabricated by the powder metallurgy route. After compacting mixed Ti+TiB₂ powders by a hot unidirectional pressure (1200°C, 25 MPa, 60 min, <10⁻² Pa), annealing at 1200°C was performed for 48 h, after which the main phase in the material was TiB. The elastic properties of TiB were directly measured by using the nanoindentation technique. The polycrystalline modulus and hardness of the TiB were 450 and 27.5 GPa, respectively.     

Effects of Titanium Oxide on Equilibria Among Refractory Phases in the System CaO-MgO-Iron Oxide

The role of titanium oxide in some important refractory systems was elucidated by studying selected equilibria in the system CaO-MgO-iron oxide-titanium oxide at O₂ pressures of 0.21 atm (air) and 10⁻⁹ atm and under the extreme reducing conditions imposed by the presence of metallic Ti in contact with the oxide phases. Solidus relations were determined for the system CaO-MgO-TiO₂ in air; 6 composition triangles were delineated, within each of which 3 crystalline phases coexist in equilibrium with liquid at a constant solidus temperature. The solidus temperatures range from 1407° to 1670°C. There is also a composition area within which MgO coexists with a Ca₄Ti₃O₁₀-Ca₃Ti₂O₇ solid solution, with solidus temperatures varying continuously from 1659° to 1670°C. Studies of reactions between MgO and titanium oxide in contact with metallic Ti in a closed system indicate that the mutual solubility between MgO and TiO at 1400°C is very small. Addition of 5 wt% TiO₂ to the system CaO-MgO-iron oxide at 1500°C in air and in 10⁻⁹ atm O₂ decreases the amount of iron oxide which can be absorbed by a CaO-MgO body without formation of a liquid phase; hence, titanium oxide has a strong deleterious effect on the refractoriness of such bodies.     

Microstructure and Mechanical Properties of In Situ Produced TiC/TiB2/MoSi2 Composites

A novel microstructure of in situ produced TiC/TiB₂/MoSi₂ composite and its mechanical properties were investigated. The results indicate that TiC/TiB₂/MoSi₂ composites can be fabricated by reactive hot pressing the mixed powders of MoSi₂, B₄C, and Ti. A novel microstructure consisting of hollow particles of TiC and TiB₂ grains in an MoSi₂ matrix was obtained. Grains of in situ produced TiC and TiB₂ were much finer, from 100 to 400 nm. During the fracture process, hollow particles relieved crack tip stress, encouraging crack branching and changing the original direction of the main crack. The highest bending strength of this composite achieved was 480 MPa, twice that of monolithic MoSi₂, and the greatest fracture toughness of the composite reached 5.2 MPa·m^1/2.     

Microstructural Evolution during Transient Plastic Phase Processing of Titanium Carbide-Titanium Boride Composites

Transient plastic phase processing is a form of reactive hot pressing for fabricating fully dense ceramic-ceramic composites at relative low homologous temperatures. In this study, this technique has been used on two powder mixtures—4:1 Ti/B₄C and 1:1 TiC_0.5/TiB₂, which are equivalent in terms of elemental compositions—to produce fully dense titanium carbide-titanium boride composites. The composites formed in each case are comprised of the same final phases—TiC_x, TiB₂, and Ti₃B₄, in roughly the same volume fractions—but exhibit distinctly different grain morphologies. Ti₃B₄ phase nucleates and grows as platelets for the 4:1 Ti/B₄C starting composition but as equiaxed grains for the 1:1 TiC_0.5/TiB₂ composition. TiB has been identified as an intermediate phase in the "platelet" composition and appears to be important to the development of the Ti₃B₄ platelets. X-ray diffractometry and scanning electron microscopy results indicate that the evolution of the microstructure is governed by the diffusion of boron and carbon, rather than titanium. In addition, the faster diffusion of carbon, relative to boron, is instrumental in the microstructural evolution of the platelet composite. The produced composites possess >99% density and good mechanical properties. The higher strength and toughness of the platelet composite are believed to be due to the platelet morphology of the Ti₃B₄ phase.