On the chemical vapour deposition of Ti3SiC2 from TiCl4-SiCl4-CH4-H2 gas mixtures

An experimental study of the deposition of Ti₃SiC₂-based ceramics from TiCI₄-SiCI₄-CH₄-H₂ gaseous precursors is carried out under conditions chosen on the basis of a previous thermodynamic approach, i.e. a temperature of 1100°C, a total pressure of 1 7 kPa, various initial compositions and substrates of silicon or carbon. Ti₃SiC₂ is deposited within a narrow composition range and never as a pure phase. A two-step deposition process is observed, in agreement with the thermodynamic calculations. For a silicon substrate, TiSi₂ is formed as an intermediate phase from consumption of Si by TiCI₄ and then is carburized by CH₄ into Ti₃SiC₂. For a carbon substrate, the first step is the formation of TiC_x either from consumption of carbon by TiCI₄ or by reaction between TiCI₄ and CH₄ and then TiC_x reacts with the gaseous mixture to give rise to Ti₃SiC₂. In most cases, Ti₃SiC₂ is obtained as small hexagonal plates oriented perpendicular to the substrate surface. These nano- or micro-crystals are usually co-deposited with TiC_x and to a lesser extent SiC, and their size is increased by increasing the dilution of the gaseous mixture in hydrogen.     

Dense Ti3SiC2 prepared by reactive HIP

The dense polycrystalline Ti₃SiC₂ has been synthesized by reactive HIPing of Ti, SiC and C powders. The bulk material with the highest Ti₃SiC₂ content about 97 vol % was obtained when treated at 1500°C, 40 MPa for 30 min. The density was 99% of the theoretical value. The Ti₃SiC₂ grains had the columnar and plate-like shapes. The grains were well boned to form a network structure. Many stacking faults were observed along the (001) plane of Ti₃SiC₂. The Vickers hardness, Young's modulus, flexural strength and fracture toughness were 4 GPa, 283 GPa, 410 MPa and 11.2 MPa m^1/2, respectively. The Ti₃SiC₂ was stable up to 1100°C in air. The electrical resistivity was 2.7×10^–7 ·m at room temperature. The resistivity increased linearly with the increasing temperature. It may be attributed to a second order phase transition. The Seebeck coefficient was from 4 to 20 V/K in the temperature range 300–1200 K. It seems that Ti₃SiC₂ is semi-metallic with hole carriers from this small positive value.     

Fabrication of highly dense Ti3SiC2 ceramics by pressureless sintering of mechanically alloyed elemental powders

Pressureless sintering of Ti₃SiC₂ ceramics has been investigated by using mechanically alloyed elemental Ti, Si and C powder mixture as the starting materials. It has been found that mechanical alloying enhanced both the formation of Ti₃SiC₂ phase and the densification during sintering process. Highly dense Ti₃SiC₂ ceramics with a relative density up to 99% and a phase purity of 80% Ti₃SiC₂ (TiC_x as the secondary phase) were obtained by sintering the mechanically alloyed powders at relatively low temperatures near 1773 K in an argon atmosphere of 0.1 MPa. The physical properties of the present pressureless-sintered Ti₃SiC₂-based ceramics are comparable to those of nearly single phase Ti₃SiC₂ ceramics fabricated by the reactive hot-isostatic pressing (HIP) that had been used so far.     

Reactive chemical vapor deposition of Ti3SiC2 with and without pressure pulses: Effect on the ternary carbide texture

Ti₃SiC₂ layers were grown by reactive chemical vapor deposition (RCVD) of a H₂/TiCl₄ gaseous mixture on previously deposited SiC layers. A comparison was made between classical RCVD in which the gases continuously flow at a constant low pressure during several minutes in the reactor and pressure-pulsed RCVD (P-RCVD) in which the reactor is (periodically) (re)filled with the H₂/TiCl₄ gas and (re)emptied every few seconds. Long duration single treatments resulted in similar thick multi-phased coatings growing by solid state diffusion with both RCVD and P-RCVD methods. Conversely, in relation with the steps of nucleation and growth by surface reaction, the repetition of short duration SiC deposition/RCVD sequences with or without pressure pulses gave rise to Ti₃SiC₂ coatings with different textures.     

Preparation of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> through reduction of titanium dioxide with silicon carbide

This paper describes the reduction of titanium dioxide with a mixture of silicon carbide and silicon powders at a temperature of 1550°C under vacuum. It has been shown that the use of the combined reductant enables the preparation of the ternary phase Ti₃SiC₂ through concurrent carboand silicothermic processes. The optimal compositions for Ti₃SiC₂ formation are TiO₂ + (1.5–x)SiC + 2xSi with x = 0.4–0.5. The Ti₃SiC₂ yield then reaches 96 wt %.     

Thermodynamics and kinetics of Ti/CdTe contact metallurgy

The ternary phase diagram Ti-Cd-Te was investigated by using approximate thermodynamic calculations as well as experimental methods (XRD, SEM, EDX, DTA). Isothermal sections at 500 C and 600 C and a tentative liquidus surface are given. Massive Ti/CdTe diffusion couples have been used to study the reaction mechanism and the kinetics of interface reactions in cross-sections with optical microscopy and SEM/EDX and also by qualitative X-ray depth profiling. Both the phase equilibrium and diffusion study reveal that Ti forms a thermodynamically unstable contact on CdTe. The diffusion path with the reaction layer sequence Ti/Ti₂Cd/Ti₅Te₄/Ti₃Te₄/CdTe indicates that local equilibria prevail in the reaction diffusion. The thermal stability of metallization structures alternative to Ti/CdTe and based on the intermediate phases in the ternary Ti-Cd-Te system is discussed.     

Optimization of the Carbosilicothermic Synthesis of the Ti<Subscript>4</Subscript>SiC<Subscript>3</Subscript> MAX Phase

We have studied the formation of the Ti4SiC3 MAX phase during the vacuum carbosilicothermic reduction of TiO₂ with a combined reducing agent consisting of SiC and elemental Si and analyzed the effects of the synthesis temperature, heat treatment time, and percentage of elemental silicon in the starting mixture on the Ti₄SiC₃ yield. Optimal Ti₄SiC₃ synthesis conditions are as follows: temperature from 1550 to 1650°C, isothermal holding time of 360 min, and the starting-mixture composition TiO₂ + 1.2SiC + 0.6Si. The Ti₄SiC₃ yield then reaches 92 wt %.     

Investigation of structural transport and magnetic properties of the system Li0.25Cu0.5Fe2.25- x Al x O4

Experimental data on X-ray electrical conductivity, thermoelectric coefficient, magnetic hysteresis and infrared absorption spectra of the system Li_0.25Cu_0.5Fe_2.25–x Al _x O₄ are presented. All the compounds, 0x2.25, showed cubic symmetry. Lattice constant values progressively decreased on increasing the Al³⁺ content. X-ray intensity calculations, magnetic hysteresis and infrared spectroscopy studies indicated the presence of Li⁺, Al³⁺ and Fe³⁺ at tetrahedral and octahedral sites, while Cu²⁺ is present only at the octahedral site. The activation energy and threshold frequency increased with increasing values ofx. The compounds withx1.50 aren-type, and those withx2.0 arep-type semiconductors. Magnetic hysteresis indicated that compounds withx1.50 are ferrimagnetic, and those withx2.0 are antiferrimagnetic. High coercive force,H _c, values and remanence ratios (J _R/J_S) showed that all the compounds exceptx2.0 exhibit single-domain behaviour. The probable ionic configuration for the system is suggested as Li _0.15 ⁺ Al _0.5 ³⁺ Fe _0.35 ³⁺ [Li _0.1 ⁺ Cu _0.5 ²⁺ Fe _0.4 ³⁺ Al³⁺]O ₄ ^2– .     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

Multidimensional singular diffusion

We consider the nonlinear diffusion equation u _t = (u ^–n u) for dimension N 2 in the cases n>1, N = 2 and n 1, N 3 in which there are no finite mass solutions. We concentrate on the physically motivated case of a small but non-zero background concentration, using asymptotic methods to analyse the limit in which this background concentration goes to zero.     

Mechanical and Electromagnetic Interference Shielding Behavior of C/SiC Composite Containing Ti3SiC2

In order to obtain the composites with the integration of structural and functional properties, Ti₃SiC₂ is introduced into C/SiC due to its excellent damage tolerance and electromagnetic interference (EMI) shielding properties. C/SiC–Ti₃SiC₂ has the lower tensile strength, while the higher compressive strength than C/SiC. The penetration energy of C/SiC–Ti₃SiC₂ in the impact experiment is improved at least three times than that of C/SiC, resulting from the improved damage tolerance. With the introduction of Ti₃SiC₂, the EMI shielding effectiveness increases from 31 to 41 dB in X‐band (8.2 to 12.8 GHz) due to the increase of electrical conductivity. C/SiC–Ti₃SiC₂ reveals the great potential as structural and functional materials based on the multi‐functional properties.

     

Ti3SiC2-formation during Ti–C–Si multilayer deposition by magnetron sputtering at 650 °C

Titanium Silicon Carbide films were deposited from three separate magnetrons with elemental targets onto Si wafer substrates. The substrate was moved in a circular motion such that the substrate faces each magnetron in turn and only one atomic species (Ti, Si or C) is deposited at a time. This allows layer-by-layer film deposition. Material average composition was determined to Ti_0.47Si_0.14C_0.39 by energy-dispersive X-ray spectroscopy. High-resolution transmission electron microscopy and Raman spectroscopy were used to gain insights into thin film atomic structure arrangements. Using this new deposition technique formation of Ti₃SiC₂ MAX phase was obtained at a deposition temperature of 650 °C, while at lower temperatures only silicides and carbides are formed. Significant sharpening of Raman E_2g and A_g peaks associated with Ti₃SiC₂ formation was observed.     

TiCl3-doped Ba0.92Ca0.08TiO3 PTCR ceramics with low r.t. resistivity

The influence of TiCl₃ solution on the room temperature (r.t.) resistivity and electrical properties of Ba_0.92Ca_0.08TiO₃ PTCR ceramics was studied. The results indicate that the PTC effect can be improved significantly when an appropriate amount of TiCl₃ in solution is added to the original materials. Some of the doped Ti³⁺ ions segregate at grain boundaries behaving as acceptors by substituting for Ti site or valence varying (from Ti³⁺ to Ti⁴⁺). As a result, the surface charge density N _s) increases and the barrier height at grain boundaries () is enhanced.     

Preparation and some properties of chemically vapour-deposited Si3N4-TiN composite

Chemically vapour-deposited (CVD) Si₃N₄-TiN composite (a plate with the maximum thickness of 1.9 mm) has been prepared on a graphite substrate using a mixture of SiCl₄, TiCl₄, NH₃ and H₂ gases. The CVD was carried out at deposition temperatures,T _dep, in the range of 1050 to 1450 ° C, total gas pressures,P _tot, from 1.33 to 10.7 kPa and gas flow rates of 136 (SiCl₄), 18 (TiCl₄), 120 (NH₃) and 2720 (H₂) cm³ min^–1. The deposits thus obtained appeared black. The Ti content in the composites ranged from 2.1 to 24.8 wt % and was found in the form of Tin. The structure of the Si₃N₄ matrices varied from amorphous (initially) to the- and-type, with increasingT _dep. Most of the- and-type deposits had a preferred orientation (001) parallel to the deposition surface. While the deposition surface of the amorphous deposits showed a pebble structure, the surfaces of the- and-type deposits were composed of various kinds of facets. The heat-treating experiment suggested that-Si₃N₄ obtained in the present work was formed directly via a vapour phase, and not from crystallization of amorphous Si₃N₄ or from transformation of-Si₃N₄.     

Crystallization and properties of CaO-P2O2-B2O3 glasses

Glasses were prepared with compositions (50–0.5 x) CaO.(50–0.5 x) P₂O₅ · x B₂O₃ with B₂O₃ contents (x) from 0 to 45 mol%. The glass transformation temperature (T _g), dilatational softening temperature (T _D) and Vickers hardness (H _V) initially increased with x, but showed maxima at about x=20 for T _g and T _D and at about x=35 for H _V. The thermal expansion coefficient decreased with x, levelling off at about 35 mol% B₂O₃. The maximum tendency to crystallize occurred at around 25 mol% B₂O₃. Volume nucleation (and hence glass-ceramic formation) and surface nucleation were obtained for x between 15 and 25 mol%. The first phase to appear was BPO₄, which was probably homogeneously nucleated. Subsequently the 4CaO · P₂O₅ phase was heterogeneously nucleated on the BPO₄. For 10 x 35 only surface nucleation was observed. The kinetics of nucleation were investigated in the 20 mol% B₂O₃ glass. The changes in properties and crystallisation behaviour with B₂O₃ content were related to short-range structural information. Infrared spectra and literature data indicated a threedimensional network of B-O-B and B-O-P linkages in the glasses.     

A new synthesis reaction of Ti3SiC2 from Ti/TiSi2/TiC powder mixtures through pulse discharge sintering (PDS) technique

Ti/TiSi₂/TiC powder mixtures with molar ratios of 1:1:4 (M1) and 1:1:3 (M2) were first employed for the synthesis of Ti₃SiC₂ through pulse discharge sintering (PDS) technique in a temperature range of 1100–1325 °C. It was found that Ti₃SiC₂ phase began to form at the temperature above 1200 °C and its purity did not show obvious dependence on the sintering temperature at 1225–1325 °C. The TiC contents in M2 samples is always lower than that of the M1 samples, and the lowest TiC contents in the M1 and M2 samples were calculated to be about 7 wt% and 5 wt% when the sintering was conducted at the temperature near 1300 °C for 15 minutes. The relative density of the M1 samples is always higher than 99% at sintering temperature above 1225 °C, indicating a good densification effect produced by the PDS technique. A solid-liquid reaction mechanism between Ti-Si liquid phase and TiC particles was proposed to explain the rapid formation of Ti₃SiC₂. Furthermore, it is suggested that Ti/TiSi₂/TiC powder can be regarded as a new mixture to fabricate ternary carbide Ti₃SiC₂. Received: 5 September 2001 / Accepted: 11 September 2001     

Rheological properties of Al2O3-SiC whisker composite suspensions

The rheological properties of both aqueous and non-aqueous composite suspensions were investigated together with a characterization of the colloidal stability of the separate components. The colloidally stable, concentrated aqueous SiC_w and Al₂O₃-SiC_w composite suspensions displayed strong shear thinning followed by a severe, sometimes discontinuous, shear thickening at a critical shear rate. The non-aqueous composite suspensions, containing weakly flocculated SiC_w, only showed a continuous shear thinning behaviour. The variations in the steady shear behaviour were related to the differences in the colloidal stability and possible shear induced effects on the suspension structure. It was possible to fit the volume fraction dependence of both the SiC_w and the Al₂O₃ suspensions to a modified Krieger-Dougherty model which yields values of the maximum volume fraction; _m(Al₂O₃)=0.61, _m(SiC_w)=0.28. The viscosity of the composite suspensions were successfully predicted from the Farris theory using the rheological data for the separate components.     

Pressureless sintering of boron carbide with an addition of polycarbosilane

Boron carbide B _x C (x 4) is pressureless sintered with an addition of both polycarbosilane and a small amount of phenolic resin. After testing many green mixtures, it was found that it was necessary to have a minimal addition of carbon which could denxydize the B _x C and also a second phase (SiC in this work) which could act in inhibiting grain coarsening. The resulting B _x C ceramics have high relative density ( 92% theoretical density) and contain no free carbon and a small amount of SiC (about 5 wt%).     

SiC-nanoparticle-reinforced Si3N4 matrix composites

Hot-pressed nanosized SiC-particle (SiC_p-reinforced Si₃N₄ composites have been studied with respect to their microstructures, room temperature mechanical properties and thermal shock resistance. The experimental results indicate that the flexural strength, fracture toughness and thermal shock resistance are all increased by the addition of 5 vol% SiC_p. Further additions of SiC_p, however, have a detrimental effect on these properties. These changes are closely related to the effects of SiC_p on the matrix grain morphology. The mechanisms of nucleation and growth of -Si₃N₄ grains in the presence of SiC nano-particles are discussed.     

Direct diffusion bonding of Ti3SiC2 and Ti3AlC2

Two typical layered ternary compounds, Ti₃SiC₂ and Ti₃AlC₂, were joined directly by solid-state diffusion bonding method. By various bonding tests at 1100-1300 °C for 30-120 min under 10-30 MPa, and characterizing the microstructure and diffusion reactive phases of the joints by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD), the optimal condition for direct joining of Ti₃SiC₂ and Ti₃AlC₂ was obtained. Strong joints of Ti₃SiC₂/Ti₃AlC₂ can be achieved via diffusion bonding, which is attributed to remarkable interdiffusion of Si and Al at the joint interface. The shear strength of the Ti₃SiC₂/Ti₃AlC₂ joints was determined.