Fabrication of High-purity Ternary Carbide Ti3AlC2 by Spark Plasma Sintering （SPS） Technique

The effect of silicon on synthesis of Ti3AlC2 by spark plasma sintering （SPS） from TiC/Ti/Al powders was investigated. X-ray diffraction （XRD） and scanning electron microscope （SEM） were used for phase identification and microstructure evaluation. The results show that addition of silicon can considerably accelerate the synthesis reaction of Ti3AlC2 and fully dense, essentially single-phase （purity 〉98%） polycrystalline Ti3AlC2 could be successfully obtained by sintering 2TiC/lTi/lAl/0.2Si powders at 1 200- 1 250 ℃ under a pressure of 30 MPa. SEM photographs show that the obtained Ti3AlC2 samples from mixtures powders are in plane-shape with a size of about 2-5 μm and 10-25 μm in the thickness dimension and elongated dimension, respectively.     

Fabrication of Ti2AlC by Spark Plasma Sintcring from Elemental Powders and Thermodynamics Analysis of Ti-M-C System

A ternary-layered carbide Ti2AlC material could be synthesized by spark plasma sintering（SPS） technology using elemental powder mixture of Ti, Al and active carbon. By means of XRD and SEM, phases were identified and microscopically evaluated. The experimental results show that the main phase in the product was fully crystallized Ti2AlC with small particle size when sintered at 1200℃. The synthesis temperature of SPS was 200-400℃ lower than that of hot pressing （HP） or hot isostatic pressing （HIP）. Through thermodynamics calculations, the mechanism of Ti2AlC was studied by calculating changes of Gibbs free energy of reactions.     

Fabrication of Ti_2AlC by Spark Plasma Sintering from Elemental Powders and Thermodynamics Analysis of Ti-Al-C System

A ternary-layered carbide Ti 2 AlC material could be synthesized by spark plasma sintering(SPS)technology using elemental powder mixture of Ti,Al and active carbon.By means of XRD and SEM,phases were identified and microscopically evaluated.The experimental results show that the main phase in the product was fully crystallized Ti 2 AlC with small particle size when sintered at 1200 C.The syn-thesis temperature of SPS was 200-400 C lower than that of hot pressing(HP)or hot isostatic pressing(HIP). Through thermodynamics calculations,the mechanism of Ti 2 AlC was studied by calculating changes of Gibbs free energy of reactions.     

Preparation of TiAl/Ti2AlC Composites with Ti/Al/C Powders by In-situ Hot Pressing

TiAl/Ti2AlC composites were prepared by in-situ hot pressing of TilAl/C powders mixtures and sintered at different temperatures were investigated by X- ray diffraction （ XRD ） of samples. The reaction procedure of Ti-Al-C system could be divided into three stnges. Below 900℃ , Ti reacts with Al to form TiAl intermetallics ; above 900 ℃ , C reacts with remain Ti to form TiC triggered by the exothermal reaction of Ti and Al ; TiAl reacts with TiC to produce dense TiAl/Ti2AlC compasites.In the holding stage, ternary Ti2AlC develops to layered polycrystal and composites pyknosis in the meanwhile. The mechanism of synthesis and microstructure was especially discussed.     

Electronic Structures and Bonding Properties of Ti_2AlC and Ti_3AlC_2

The relation among electronic structure, chemical bond and property of Ti2AlC, Ti3AlC2 and doping Si into Ti2AlC was studied by density function and the discrete variation (DFT-DVM) method. After adding Si into Ti2AlC, the interaction between Si and Ti is weaker than that between Al and Ti, and the strengths of ionic and covalent bonds decrease both. The ionic and covalent bonds in Ti3AlC2, especially in Ti-Al, are stronger than those in Ti2AlC. Therefore, in synthesis of Ti2AlC, the addition of Si enhances the Ti3AlC2 content instead of Ti2AlC. The density of state (DOS) shows that there is mixed conductor characteristic in Ti2AlC and Ti3AlC2. The DOS of Ti3AlC2 is much like that of Ti2AlC. Ti2SixAl1-xC has more obvious tendency to form a semiconductor than Ti2AlC, which is seen from the obvious difference of partial DOS between Si and Al 3p.     

Effect of doping on microstructure and technical usage property of tantalum wire

Effect of different dopants and various dopants quantity at different annealing temperatures on microstructure of tantalum wire, bending of tantalum wire after sintering and bending after pressing into tantalum powder and sintering were investigated through observation of microstructure and testing of bending of tantalum wire after sintering and bending after pressing into tantalum powder and sintering. The results show that the recrystallization temperature of tantalum wire increases and the grain of microstructure can be reduced with the increase of dopants quantity. At the same time, the effect of dopant Ce on reduction of the grain is more obvious than that of dopant Ge. The bending time of tantalum wire after sintering increases with the increase of dopant Ge or Ce quantity. Under the same condition, the bending time of tantalum wire after pressing into tantalum powder and sintering worsens with the increase of oxygen content in tantalum powder. The bending time of tantalum wire doped with Ge and Ce after pressing into tantalum powder and sintering is better than that of tantalum wire doped with Ge, while that of tantalum wire doped with Ge is better than that of pure one when oxygen content in tantalum powder is not too high.     

Eleetronie Structures and Bonding Properties of Ti2AlC and Ti3AlC2

The relation among electronic structure, chemical bond and property of Ti2AlC, Ti3AlC2 and doping Si into Ti2AlC was studied by density function and the discrete variation （DFT-DVM） method. After adding Si into Ti2AlC, the interaction between Si and Ti is weaker than that between Al and Ti, and the strengths of ionic and covalent bonds decrease both. The ionic and covalent bonds in Ti3AlC2, especially in Ti-Al, are stronger than those in Ti2AlC. Therefore, in synthesis of Ti2AlC, the addition of Si enhances the Ti3AlC2 content instead of Ti2AlC. The density of state （DOS） shows that there is mixed conductor characteristic in Ti2AlC and Ti3AlC2. The DOS of Ti3AlC2 is much like that of Ti2AlC. Ti2SiAl1-xC has more obvious tendency to form a semiconductor than Ti2AlC, which is seen from the obvious difference of partial DOS between Si and Al 3/7.     

Thermoelectric properties of BiSb_x (x=0.6-0.8) thermoelectric materials fabricated by different processing

In order to improve the thermoelectric properties, hot-pressing sintering and ultra high pressure sintering methods were adopted to fabricate BiSb.,. The phase and crystal structures were determined by X-ray diffraction analysis (XRD). The thermoelectric properties were measured at 303 K along the direction parallel to the pressing direction. The electric conductivity of the samples was measured at 303 K by the four-probe technique. To measure the Seebeck coefficient, heat was applied to the samples placed between two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature differences ( △T < 2℃) between the both ends of the samples. The Seebeck coefficient of the samples was determined from the value of E/△T. The results indicate that the thermoelectric properties of the samples fabricated by UHPS (ultra high pressure sintering) method are much higher than that by HPS (hot pressing sintering) method and have the highest values at x=0.7.     

Study on the isothermal oxidation behavior in air of Ti_3AlC_2 sintered by hot pressing

The isothermal oxidation behavior at 900―1300℃ for 20 h in air of bulk Ti3AlC2 with 2.8 wt% TiC sintered by means of hot pressing was investigated in the work. The isothermal oxidation behavior generally followed a parabolic rate law. The parabolic rate constants increased from 1.39×10-10 kg2·m-4·s-1 at 900℃ to 5.56×10-9 kg2·m-4·s-1 at 1300℃. The calculated activation energy was 136.45 kJ/mol. It was demonstrated that Ti3AlC2 had excellent oxidation resistance due to the continuous, dense and adhesive protect scales consisted of a mass of α-Al2O3 and a little of TiO2 and/or Al2TiO5. In principle, the oxide scale was grown by the inward diffusion of O2- and the outward diffusion of Ti4 and Al3 . The rapid outward diffusion of cations usually resulted in the formation of cracks, gaps, and holes.     

Microstructures of FeSi2 based thermoelectric materials prepared by rapid solidification and hot pressing

FeSi2 based thermoelectric materials have been prepared by melt spinning and vacuum hot pressing. Most of the rapidly solidified (melt spinning) powders are thin flakes with a thickness less than 0.1 mm. Scanning electron microscope (SEM) surface profiles show there are further finer grain structures with the characteristic size of about 100 nm in a flake. The samples obtained by hot uniaxial pressing (HUP) in vacuum have densities higher than 90% the theoretical density of the materials. It was found by SEM observations that the microstructures are very different for vertical and parallel sections of the HUP samples. X-ray diffraction (XRD)analyses show there are some texture features in the samples. It is considered that the textures of the samples are originated from the orientation of the flakes that tended to align perpendicular to the hot press axis. WSi2 was introduced into the powders unexpectedly during melting process before the rapid solidification, but it makes the microstrucmres more easily to be explained.     

Synthesis of high pure Ti3AlC2 and Ti2AlC powders from TiH2 powders as Ti source by tube furnace

Titanium aluminum carbide (Ti₃AlC₂ and Ti₂AlC) powders were synthesized from TiH₂ powders instead of Ti powders as Ti source by a tube furnace under argon atmosphere without preliminary dehydrogenation. 95 wt% pure Ti₃AlC₂ powders were synthesized from TiH₂/1.1Al/2TiC at 1 450 °C for 120 min. High-purity Ti₂AlC powders were also prepared from 3TiH₂/1.5Al/C and 2TiH₂/1.5Al/TiC powders at 1 400 °C for 120 min. The as-synthesized samples were porous and easy to be ground into powders. Sn or Si additives in starting materials increased the purity of synthesized Ti₃AlC₂ obviously and expanded the temperature range for the synthesis of Ti₃AlC₂. With Si or Sn as additives, high pure Ti₃AlC₂ was synthesized at 1 200 °C for 60 min from TiH₂/x Si/Al/2TiC and TiH₂/x Sn/Al/2TiC (x = 0.1, 0.2), respectively.     

Effect of tin on the reaction synthesis of ternary carbide Ti3AlC2

The effect of tin on synthesis of Ti₃AlC₂ by spark plasma sintering (SPS) from TiC/Ti/Al powders was investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for phase identification and microstructure evaluation. The experimental results show that addition of tin can considerably accelerate the synthesis reaction of Ti₃AlC₂ and fully dense, essentially single-phase polycrystalline Ti₃AlC₂ could be successfully obtained by sintering 2TiC/1Ti/1Al/0.2Sn powders at 1200–1250 °C under a pressure of 30 MPa. SEM images show that Ti₃AlC₂ samples in about 2–5 μm thick and 10–25 μm long platelets can be obtained. The fracture toughness and flexural strength of Ti₃AlC₂ were 6.5±0.2 MPa·m^1/2 and 560±10 MPa, respectively. Funded by the National Natural Science Foundation of China (No.20771088, No.50572080) and Doctoral Foundation of Wuhan University of Technology (No.471-38650142)     

Fabrication of in situ Ti2AlN/TiAl composites by reaction hot pressing and their properties

Ti2AIN/TiAI composites with different volume fractions of reinforcement were successfully fabricated by hot-pressing sintering method （reaction hot pressing） using Ti, Al and TiN powders as starting materials. The synthesis process includes four stages： first, the reactions between Al and Ti powers and between Al and TiN powders respectively occur and result in TiAl3 phase; secondly, AI powders in the sample are exhausted; the remaining Ti cores react with TiAl3 layer to form Ti-Al intermetallics; moreover, a few Ti2AlN particles precipitate from the TiAl3 phase; thirdly, Ti-Al intermetallics react with the remaining Ti cores to form Ti3Al and TiAl phases. TiAl phase and original TiN powers are in direct contact each other; finally, the residual TiN powers react with TiAl phase and result in a plenty of TizAIN phase. Compared with TiAl matrix, the hardness, elastic modulus and high-temperature compressive strength of Ti2AlN/TiAl composite are improved obviously and they are all enhanced with increasing the volume fraction of Ti2AlN phase.     

Study on the isothermal oxidation behavior in air of Ti3AlC2 sintered by hot pressing

The isothermal oxidation behavior at 900–1300°C for 20 h in air of bulk Ti₃AlC₂ with 2.8 wt% TiC sintered by means of hot pressing was investigated in the work. The isothermal oxidation behavior generally followed a parabolic rate law. The parabolic rate constants increased from 1.39×10⁻¹⁰ kg²·m⁻⁴·s⁻¹ at 900°C to 5.56×10⁻⁹ kg²·m⁻⁴·s⁻¹ at 1300°C. The calculated activation energy was 136.45 kJ/mol. It was demonstrated that Ti₃AlC₂ had excellent oxidation resistance due to the continuous, dense and adhesive protect scales consisted of a mass of α-Al₂O₃ and a little of TiO₂ and/or Al₂TiO₅. In principle, the oxide scale was grown by the inward diffusion of O²⁻ and the outward diffusion of Ti⁴⁺ and Al³⁺. The rapid outward diffusion of cations usually resulted in the formation of cracks, gaps, and holes.     

Effects of HIPing pressure on microstructures and properties of TiAl alloy

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