Polycarbosilane derived Ti3SiC2

Titanium silicon carbide (Ti₃SiC₂) ceramic was synthesized by in-situ reaction of metal titanium and polycarbosilane. Reaction mechanisms which lead to the formation of Ti₃SiC₂ were suggested on the basis of XRD analysis. The content of Ti₃SiC₂ reached 93% in products obtained from heating the Ti/polycarbosilane green compact at 1400 °C in Ar. The morphology and compositions of the products were examined by SEM equipped with EDX. The typical laminate structure of Ti₃SiC₂ particles with 1-4 μm in thickness and 4-15 μm in length was observed. EDX results showed that the atomic ratio of Ti:Si:C of grains is close to 3:1:2, which agrees with Ti₃SiC₂ composition.     

Synthesis and phases evolution of Si–C–Ti from polycarbosilane (PCS) and metal Ti

Si–C–Ti ceramics were synthesized by reactive pyrolysis of polycarbosilane (PCS) precursor filled with metal Ti powder. Pyrolysis of mixture with atomic ratio of Ti:Si through 3:1–3:2 was carried out in argon atmosphere at given temperature up to 1500 °C. The metal–precursor reactions, and phase evolution were studied using X-ray diffraction and scanning electron microscopy with EDX. The Ti₃SiC₂ phase was obtained firstly from reaction of PCS and Ti. Ti₃SiC₂ formation starts at 1300 °C and its amount increases significantly in a narrow temperature range between 1400 °C and 1500 °C. In addition, addition of CaF₂ can promote the formation of Ti₃SiC₂ phase.     

Formation of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> from Ti-Si-TiC powders by pulse discharge sintering (PDS) technique

Ti/Si/TiC powder mixture with molar ratios of 2:2:3 were sintered at various temperatures from 700–1300 °C for 15 min by PDS technique. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the evaluation of phase composition in different samples for the understanding of the sintering mechanism for this system. Results showed that Ti₅Si₃ formed as the intermediate phase during sintering. The reaction between Ti₅Si₃ and TiC as well as Si induces the formation of Ti₃SiC₂, and TiSi₂ appears as the byproduct in this process. At temperature above 1000 °C, TiSi₂ reacts with TiC to form Ti₃SiC₂. High Ti₃SiC₂ phase content bulk material can be synthesized at 1300 °C for 15 min.     

Fabrication of Monolithic Ti3SiC2 Ceramic Through Reactive Sintering of Ti/Si/2TiC

Fabrication of monolithic Ti₃SiC₂ has been investigated through the route of reactive sintering of Ti/Si/2TiC mixtures. Significant phase differences existed between the surface and the interior of as-synthesized products due to the evaporation of Si during the reaction process. The use of a 3Ti/SiC/C mixture as a powder bed could control the evaporation of Si and develop monolithic Ti₃SiC₂. A reaction model for the formation of Ti₃SiC₂ in the Ti/Si/2TiC system is discussed.On leave from     

Synthesis of Ti3Si0.8Al0.4C1.8 solid solution from Ti-Si-Al-C powder mixture

In this study, free Ti/Si/Al/C powder mixtures with molar ratios of 3:0.8:0.4:1.8 were heated in argon with various schedules, in order to reveal the possibility for the synthesis of high Ti₃Si_0.8Al_0.4C_1.8 content powder. X-ray diffraction (XRD) was used for the evaluation of phase identities of the powder after different treatments. Scanning electron microscopy (SEM) was used to observe the morphology of the Ti₃Si_0.8Al_0.4C_1.8 solid solution. XRD results showed that predominantly single phase samples of Ti₃Si_0.8Al_0.4C_1.8 were prepared after heating at 1450 °C for 5 min in argon and the lattice parameters of Ti₃Si_0.8Al_0.4C_1.8 lay between those of Ti₃SiC₂ and Ti₃AlC₂. SEM observation showed that the grains of Ti₃Si_0.8Al_0.4C_1.8 solid solution exhibited a lamellar shape, which is a characteristic feature of Ti₃SiC₂ and Ti₃AlC₂.     

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     

Low temperature synthesis of Ti3 SiC2 from Ti/SiC/C powders

Abstract

Ternary carbide Ti₃ SiC₂ was first synthesised through a pulse discharge sintering (PDS) technique from mixtures of Ti, SiC, and C with different molar ratios. Sintering processes were conducted at 1200 – 1400°C for 15 – 60 min at a pressure of 50 MPa. The phase constituents and microstructures of the synthesised samples were analysed by X-ray diffraction (XRD) technique and observed by scanning electron microscopy (SEM). The results showed that, for samples sintered from 3Ti/SiC/C powder at 1200 – 1400°C, TiC is always the main phase and only little Ti₃ SiC₂ phase is formed. When the molar ratios Ti : SiC : C were adjusted to 3 : 1.1 : 2 and 5 : 2 : 1, the purity of Ti₃ SiC₂ in the synthesised samples was improved to about 93 wt-%. The optimum sintering temperature for Ti₃ SiC₂ samples was found to be in the range 1250 – 1300°C and all the synthesised samples contain platelike grains. The relative density of Ti₃ SiC₂ samples was measured to be higher than 99% at sintering temperatures above 1300?C. It is suggested that the PDS technique can rapidly synthesise ternary carbide Ti₃ SiC₂ with good densification at lower sintering temperature.     

Characterisation of amorphous silica in air-oxidised Ti3SiC2 at 500–1000 °C using secondary-ion mass spectrometry,nuclear magnetic resonance and transmission electron microscopy

In this paper we have described the use of secondary-ion mass spectrometry (SIMS), solid state ²⁹Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM) to detect the existence of amorphous silica in Ti₃SiC₂ oxidised at 500–1000 °C. The formation of amorphous SiO₂ and growth of crystalline TiO₂ with temperature was monitored using dynamic SIMS and synchrotron radiation diffraction. A duplex structure with an outer TiO₂-rich layer and an inner mixed layer of SiO₂ and TiO₂ was observed. Results of NMR and TEM verified for the first time the direct evidence of amorphous silica formation during the oxidation of Ti₃SiC₂ at the temperature range 500–1000 °C.     

Joining of SiC ceramic-based materials with ternary carbide Ti3SiC2

The joining of two pieces of SiC-based ceramic materials (SiC or C_f/SiC composite) was conducted using Ti₃SiC₂ as filler in vacuum in the joining temperatures range from 1200 °C to 1600 °C. The similar chemical reactions took place at the interface between Ti₃SiC₂ and SiC or C_f/SiC, and became more complete with joining temperature increases, and with the consequent increased joining strengths of the SiC and C_f/SiC joints. Based on the XRD and SEM analyses, it turns out that two reasons are most important for the high joining strengths of the SiC and C_f/SiC joints. One is the development of layered Ti₃SiC₂ ceramic, which has plasticity in nature and can contribute to thermal stress relaxation of the joints; the other is the chemical reactions between Ti₃SiC₂ and the base materials which result in good interface bonding.     

Improvement of intermediate-temperature oxidation resistance of Ti3AlC2 by pre-oxidation at high temperatures

Ti₃AlC₂ has good oxidation resistance at high temperatures owing to the formation of protective Al₂O₃ layer. However, as it is directly exposed to air at 500 °C and especially 600 °C, the scales formed on Ti₃AlC₂ exhibits poor protectivity because of the oxidation-induced microcracks, which limits the potential application of this technologically important material. In this work, the oxidation resistance of Ti₃AlC₂ at 500 °C and 600 °C was significantly improved by pre-oxidation at 1000–1300 °C for a short time of 2 h in air. The oxidation of the pre-oxidized Ti₃AlC₂ at 600 °C generally obeyed a parabolic rate law instead of the speed-up linear relation in the case without pre-oxidation. As revealed by X-ray diffraction, Raman spectroscopy results and SEM observations, the remarkable improvement of oxidation resistance after pre-oxidation was attributed to the protective layers formed on Ti₃AlC₂ during pre-oxidation, which inhibited the formation of anatase TiO₂ that resulted in the oxidation-induced microcracks. Moreover, this work confirmed that the mechanism proposed previously for the anomalous oxidation of Ti₃AlC₂ in air at 500–600 °C was correct.     

Fluctuation synthesis and characterization of Ti3SiC2 powders

 A novel fluctuation method for the synthesis of Ti₃SiC₂ powders was developed. The raw materials used in this process are Ti, Si, and graphite powders. Fluctuation synthesis utilized Si as in-situ liquid forming phase (additive), which was formed by heating the powder mixtures to 1300°C and using the heat released from the exothermic reaction for Ti₃SiC₂ formation. The result demonstrated that the reaction time for the formation of Ti₃SiC₂ was dramatically shortened using fluctuation method and the powders produced using this method contained more than twice amount of Ti₃SiC₂ compared to the solid reaction synthesized powders. The powders prepared by fluctuation method are fiber-like in morphology with dimensions of 0.8–2 μm in width and 5–10 μm in length. The growth direction of the fiber-like Ti₃SiC₂ particulate is {101^–1}*. The lattice parameters for Ti₃SiC₂ were determined by a trial-and-error method and are a=3.067 ? and c=17.645 ?. Received: 28 September 1998 / Reviewed and accepted: 1 October 1998     

Bonding mechanism between silicon carbide and thin foils of reactive metals

Pressureless-sintered SiC pieces and SiC single crystals were joined with foils of reactive metals at 1500° C (1773 K) for titanium and zirconium foils or at 1000° C (1273 K) for Al/Ti/Al foils. Bend testing at various temperatures up to 1400° C (1673 K), optical and electron microscopy, and electron-probe X-ray microanalysis studies were carried out on the specimens. From the results, it was concluded that the fairly high bond strength of titanium-foil joined SiC specimens might be attributed to the formation of a Ti₃SiC₂ compound, since good lattice matching between SiC and Ti₃SiC₂ was obtained in the SiC single crystals. Also in the Al/Ti/Al-foil joined SiC, high bond strength was obtained, but it decreased steeply at 600° C (873 K) because of a retained aluminium phase. The bond strength in the zirconium-foil joined SiC was low.     

Effect of Aluminum on Synthesis of Ti3SiC2 by Spark Plasma Sintering (SPS) from Elemental Powders

The effect of aluminum on synthesis of Ti₃SiC₂ by spark plasma sintering (SPS) from elemental powders was investigated in this paper. X-ray diffraction patterns and scanning electron microscopy photographs of samples with different content of aluminum indicated that proper addition of aluminum both favored the formation and accelerated the crystal growth of Ti₃SiC₂. The process parameters in the sintering course revealed that addition of aluminum decreased the temperature for the synthesis reaction of Ti₃SiC₂. Polycrystalline bulk Ti₃SiC₂ material with high purity and density could be fabricated by spark plasma sintering from the elemental powder mixture with starting composition of Ti₃Si_1–xAl_xC₂, where x = 0.05–0.2. SEM photographs showed Ti₃SiC₂ synthesized from elemental powders was in plane-shape with a size of about 10–20 µm in the elongated dimension. Solid solution of aluminum decreased the thermal stability of Ti₃SiC₂ and made the temperature at which Ti₃SiC₂ decomposed be as low as 1300°C.     

The low temperature synthesis and characterization of Bi3.25La0.75Ti3O12 powder by hydroxide coprecipitation in aqueous medium

The fine powders of Bi_3.25La_0.75Ti₃O₁₂ (BLT) were prepared by coprecipitaton method in aqueous medium at low temperature. The differential thermal analysis (DTA), thermo-gravimetric analysis (TG) and X-ray diffraction (XRD) were employed to evaluate the phase formation of BLT and TEM was used to characterize and observe the particle size and morphology of BLT powder obtained. The results show that the bismuth layer perovskite phase of BLT can begin to form at as low as 500 °C by the coprecipitation method. When the precipitates obtained were calcined at 600 °C for 2 h, the mono-phase and perfect BLT powder was synthesized. The BLT powder obtained consists of irregular or plate-like particles which are less than about 100 nm and is nearly aggregate free.     

Fabrication of Ti2AIC by hot pressing of Ti, TiC, Al and active carbon powder mixtures

Polycrystalline Ti₂AlC samples were synthesized by hot pressing of Ti, Al, TiC and active carbon powder mixtures. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase identification and microstructure evaluation. No other phase except Ti₂AlC was detected in samples synthesized by hot pressing of the 0.5TiC/1.5Ti/1.0Al/0.5C powder mixtures at 1400°C for 1 and 3 h under a pressure of 30 MPa. The densities of these two samples were 96.1 and 98% of the theoretical value of pure Ti₂AlC, respectively. The reason that the densities of these two samples were lower than the theoretical density of pure Ti₂AlC is that pore existed in these two samples. At lower temperature of 1300°C, the speed of the reaction forming Ti₂AlC was slow. While at higher temperature of 1500°C, Ti₂AlC transformed to Ti₃AlC₂. So these two temperatures are not suitable for the fabrication of Ti₂AlC.     

Synthesis of CaCu3Ti4O12 ceramic via a sol-gel method

The novel nano-ultrafine powders for the preparation of CaCu₃Ti₄O₁₂ ceramic were prepared by the sol-gel method and citrate auto-ignition method. The obtained precursor powders were pressed, sintered at 1000 °C to fabricate microcrystal CaCu₃Ti₄O₁₂ ceramic. The microcrystalline phase of CaCu₃Ti₄O₁₂ was confirmed by X-ray powder diffraction (XRD). The morphology and size of the grains of the powders and ceramics under different heat treatments were observed using scanning electron microscopy (SEM). The relative dielectric constant of the ceramic sintered at 1000 °C was measured with a magnitude of more than 10⁴ at room temperature, which was approaching to those of Pb-containing complex perovskite ceramics, and the loss tangent was less than 0.20 in a broad frequency region. The relative dielectric constant and loss tangent were also compared with that of CaCu₃Ti₄O₁₂ ceramic prepared by other reported methods.     

Effects of Bi2O3 and Cr2Ti3O9 Co-doping on dielectric properties in BaTiO3-based ceramics

A microwave ceramic with general composition (1-x-y) BaTiO₃ + x Cr₂Ti₃O₉ + y Bi₂O₃ has been prepared by solid state synthesis at 1300-1400 °C. The phase composition, perovskite structural parameters and dielectric properties have been obtained by X-ray diffraction and dielectric measurements as a function of chemical composition and temperature. At low doping levels the formation of BaTiO₃-based solid solution has been found. The precipitation of BaCrO₃ has been detected at x = y = 2.0 mol%. A model of the incorporation of Cr³⁺ and Bi³⁺ ions into BaTiO₃-based crystal lattice has been proposed. Diffused phase transition in the temperature range 100-140 °C have been revealed by dielectric measurements for different ceramic composition. As high dielectric constant as 7311 and as low dielectric loss as 0.02 have been found for the composition of 0.98BaTiO₃-0.01Cr₂Ti₃O₉-0.01Bi₂O₃.     

Solid-state synthesis and characterization of the ternary phase Ti3SiC2

Ti₃SiC₂ is the only true ternary compound in the Ti-Si-C system. It seems to exhibit promising thermal and mechanical behaviour. With the exception of its layered crystal structure, most of its properties are unknown, owing to the great difficulty of synthesis. A new procedure of solid-state synthesis with several steps is proposed, which results in Ti₃SiC₂ with less than 5 at % of TiC. Ti₃SiC₂ is stable at least up to 1300 °C. Beyond this temperature, it can decompose with formation of non-stoichiometric titanium carbide and gaseous silicon, with kinetics highly dependent on the nature of the surroundings. As an example, graphite can initiate this process by reacting with silicon, while alumina does not favour the decomposition which remains very slow. The oxidation of Ti₃SiC₂ under flowing oxygen starts at 400 °C with formation of anatase-type TiO₂ film, as studied by TGA, XRD, SEM and AES. Between 650 and 850 °C both rutile and anatase are formed, rapidly becoming protecting films and giving rise to slow formation of SiO₂ and more TiO₂. The oxidation kinetics is slower than for TiC, owing to a protecting effect of silica. By increasing the temperature, both oxidation processes (i.e. direct reaction and diffusion through oxide layers) are activated and an almost total oxidation is achieved between 1050 and 1250 °C resulting in titania (rutile) and silica (cristobalite).     

Synthesis of Ti2SnC from Ti/Sn/TiC powder mixtures by pressureless sintering technique

Ti/Sn/TiC powder mixtures were first employed to synthesize Ti₂SnC powder by pressureless sintering in the temperature range of 950–1250 °C at vacuum atmosphere. Ti₂SnC began to form at 950 °C, its content increased with increasing temperature. High purity of Ti₂SnC was obtained by sintering the mixtures with deficient Sn and TiC at 1200 °C for 15 min. A reaction mechanism was proposed to explain the formation of Ti₂SnC. The Ti₂SnC powder was characterized by scan electron microscopy (SEM) and X-ray diffraction (XRD). Using the above mixtures and process, the Ti₂SnC ceramic powder can be obtained on a larger scale.     

New compound ScTiNbO6 synthesized via phase transformation from anatase to srilankite-like structure

A new srilankite-like compound ScTiNbO₆ was synthesized as a single phase at 900 °C via phase transformation using anatase-type nanoparticles that were directly formed from solution mixture of Sc(NO₃)₃, TiOSO₄, and NbCl₅ under mild and weakly basic hydrothermal condition at 180 °C for 5 h. Anatase-type Sc_0.5Ti₂Nb_0.5O₆ nanoparticles transformed into rutile phase at 1000 °C. The new phase that was appeared via phase transformation was identified as srilankite-like structure with a trace of symptomatic wolframite, α-PbO₂ related structure. The optical band gap of ScTiNbO₆ was 3.58 eV. This titania-base material has potential for application as catalyst and photoluminescence, etc.