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.     

Electronic Structure and Chemical Bond of Ti3SiC2 and Adding Al Element

The relation among electronic structure, chemical bond and property of Ti3SiC2 and Al-doped was studied by density function and discrete variation （ DFT- DVM） method. When Al element is added into Ti3 SiC2 , there is a less difference of ionic bond, which does not play a leading role to influent the properties. After adding Al, the covalent bond of Al and the near Ti becomes somewhat weaker, but the covalent bond of Al and the Si in the same layer is obviously stronger than that of Si and Si before adding. Therefore, in preparation of Ti3 SiC2 , adding a proper quantity of Al can promote the formation of Ti3 SiC2 . The density of stnte shows that there is a mixed conductor character in both of Ti3 SiC2 and adding Al element. Ti3 SiC2 is with more tendencies to form a semiconductor. The total density of state near Fermi lever after adding Al is larger than that before adding, so the electric conductivity may increase after adding Al.     

The Quantum Chemistry Calculation and Thermoelectrics of Bi-Sb-Te Series

The density function theory and discrete variation method ( DFT- DVM ) was used to study correlation between composition, structure, chemical bond, and property of thermoelectrics of Bi-Sb-Te series. 8 models of Bi2o-xSbxTe32( x = 0,2,6,8,12, 14,18 and 20) were ca/zulated. The resu/ts show that there is less difference in the ionic bonds between Te(I)-Bi( Sb ) and Te(Ⅱ)-Bi( Sb ) , but the covalent bond of Te( I )-Bi( Sb ) is stronger than that of Te(Ⅱ)-Bi( Sb ). The interaction between Te( I ) and Te( I ) in different layers is the weakest and the interaction should be Van Der Waals power. The charge of Sb is lower than that of Bi , and the ionic bond of Te- Sb is weaker than that of Te- Bi . 7he covalent bond of Te- Sb is also weaker than that of Te-Bi. Therefore, the thermoelectric property may be improved by adjusting the electrical conduaidty and thermal conductivity through changing the composition in the compounds of Bi-Sb- Te. The calculated results are consistent with the experiments.     

First Principle Calculation of Electromagnetic Mechanism for Fe_2Si Bulk Material

The electronic structure and the magnetic properties of Fe_2Si bulk have been calculated by the first-principle density function theory method. The band structure shows that the hexagonal Fe~2 Si bulk is ferromagnetic which is a metal structure under spin-up, and a semiconductor with the band gap of 0.518 eV under spin-down. The density of states shows the Fe~1 3d–spin and Fe~3 3d–spin in the electronic system are the main factors that is the source of the ferromagnetic properties of Fe_2Si bulk. The electronic structure Si-ions is 3s23p6 and that of Fe-ions is e_g~2↑e_g~(*1)↓t_(2g)~3↑. The molecular magnetic moment of Fe_2Si is 2.00 μB. The potential diagram of Fe_2Si bulk shows the formation of covalent and ionic bonds between the Fe atom and the Si atom, it leads to the center charge of Fe is polarized and off center position. These special properties of Fe_2Si bulk are mainly caused by d-d exchange and p-d hybridization. The results offer a certain reference for the magnetic semiconductor Fe_2Si material.     

First-principles Study on the Electronic Structure and Stability of Mo2-xCrxC

This paper deals with the electronic structure and stability of a series of carbides Mo2-xCrxC based on the pseudopotential plane-waves approach of density functional theory and using the generalized gradient approximation(GGA) for the exchange and correlation potential.The calculation results of formation energies demonstrate that the structure Mo2-xCrxC in range of 0≤x≤1.75 is stable under ambient conditions.The formation energies/stabilities of the structures Mo2-xCrxC increase /reduce with enhancing the content of Cr in the structure.Calculated density of state(DOS) show that an increase in the content of Cr dissolving in Mo2-xCrxC crystal cell can lead to the crystal cell volume slightly to shrinkage,TDOS to be elevated at the Fermi level and the peak value of DOS to decrease at the lower energy region from-13.78 to-10.16 eV.Mulliken population analysis explains that the Mo2-xCrxC phase possess the metallic,covalent and ionic bonds.     

ON STRUCTURES,PROPERTIES AND CHEMICAL BOND OF Te-Se-I GLASS AND OF ADDING As,Ge ELEMENTS ONE

Te-Se glass and adding As,Ge elements to itare studied with Selt-Consistent-Field Discrete Variatioal X_α(SEF-DV-X_α),one of the molecule orbital calculating meth-ods in quantum chemistry.The chemical bonding is used todiscuss the relations between structures and properties with thevarations of compositions of the glasses.The calculated re-sults show that the strength of covalent and ionic bonds areboth in the order of Ge-Se>As>Se>Te-Se,which is consis-tent with the experimental result of the glass-transition tem-perature(T_g) of the corresponding glasses.The Te-I bond inwhich Ⅰ atom is one-coordinate is sstronger than that in whichⅠ atom is two-coordinate,As-Ⅰ and As-As bonds are bothstronger than the two types of Te-Ⅰ bonds.The waek Te-Ⅰbonds have been replaced by the stronger As-Ⅰ and As-Asbonds,which is just the reason why As addition in TeX glass-es can obviously improve the thermal and chemical properties.     

Electronic Structure and Thermoelectric Properties of Ca3 Co4O9

The relation among electronic structure, chemical bond and thermoelectric property of Ca3 Co4 O9 was studied using density function and discrete variation method （DFT-DVM）. The gap between the highest valence band （HVB） and the lowest conduction band （LCB） shows a semiconducting property. Ca3 Co4 O9 consists of CoO2 and Ca2 CoO3 two layers. The HVB and LCB near Fermi level are only mainly from O（ 2 ） 2p and Co （2） 3d in Ca2 CoO3 layer. Therefore, the semiconducting or thermoelectric property of Ca3 Co4 O9 should be mainly from Ca2 CoO3 layer, but it seems to have no direct relation to the CoO2 layer, which is consistent with that binary oxides hardly have a thermoelectric property, but trinary oxide compounds have quite a good thermoelectric property. The covalent and ionic bonds of Ca2 CoO3 layer are both weaker than those of CoO2 layer. Ca plays the role of connections between CoO2 and Ca2 CoO3 layers in Ca3 Co4 O9, decrease the ionic and covalent bond strength, and improve the thermoelectric property.     

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.     

Structure and Chemical Bond of Thermoelectric Ce-Co-Sb Skutterudites

The correlations among composition, structure, chemical bond and thermoelectric property of skutterudites CoSb3 and CeCo5Fe3Sb12 have been studied by using density function and discrete variation (DFT-DVM) method. Three models for this study were proposed and calculated by which the "rattling" pattern was described. Model 1 is with Ce in the center, model 2 is with Ce away the center and near to Sb, and model 3 is also with Ce away the center but near to Fe. The calculated results show that in model 3, the ionic bond is the strongest, but the covalent bond is the weakest. Due to the different changes between ionic and covalent bond, there is less difference in the stability among the models 1, 2 and 3. Therefore, these different models can exist at the same time, or can translate from one to another more easily. In other words, the "rattling" pattern has taken place. Unfilled model of CoSb3, without Ce and Fe, is called model 4. The covalent bond of Co-Sb or Fe-Sb in models 1, 2 and 3 is weaker than that of     

Gradient Structure of Ti-Al-C Ternary Carbide Prepared by Hot-pressing Sintering

X-ray diffraction ( XRD ) analysis on different polished surfaces normal to the hot pressing direction reveals that the phase compositions of the polished surfaces from the outside to the inside are pure TiC,Ti3 AlC2 TiC, pure Ti3 AlC2 and Ti2 AlC Ti3 AlC2 , no matter elemental powder or TiC is used as raw materials, It is found that ternary-la2ered carbide Ti2 AlC samples synthesized at 1500℃ by hot-pressing sintering are inhomogeneous and have a gradient structure. Electron probe X-ray micro-analysis ( EPMA ) indicates that the Al content along the hot pressing axis is parabolic, it is the highest in the center and the lowest at the both ends,while the Ti content is constant along the axis. The experimental result reveals that the evaporation of Al in samples in an open ,system during hot pressing sintering results in a gradient structure.     

机械诱发自蔓延反应合成Ti3AlC2的机理研究

以3Ti／Al／2C粉体为原料,采用机械合金化的方法以合成Ti3AlC2材料。研究结果表明,在机械合金化过程中诱发自蔓延反应,反应会产生大量坚硬的小块体颗粒,大小约为0．2-11mm。粉体的组成相为TiC、Ti3AlC2、Ti2AlC,而块体仅含有TiC和Ti3AlC2。获得的粉体和块体产物中Ti3AlC2含量分别约为63wt％和84．8wt％。提出了一个机械诱发自蔓延反应合成Ti3AlC2的反应机制,即Ti3AlC2是从固相TiC与Ti-Al液相中形核并长大。     

可加工Ti2AlC陶瓷的研究进展

综述了可加工性Ti2AlC陶瓷的研究进展，三元碳化物Ti2AlC属于六方晶体结构，空间群为P63／mmc。它具有许多优良的性能，有较高的强度和弹性模量，在室浊下有抗损伤能力，它还有高的导热和导电系数，在高温下有良好的抗氧化性及显著的塑性变形。应用SHS、HP／HIP及SPS（等离子放电烧结）可制备该化合物。用HIP及SPS可制备高纯、致贩Ti2AlC陶瓷。     

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.     

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.     

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.     

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.     

Al2O3对3Ti/Si/2C体系自蔓延高温合成Ti3SiC2的影响

以3Ti/Si/2C粉体为原料,通过自蔓延高温合成技术合成了Ti3SiC2材料。研究了Al2O3助剂对自蔓延高温合成Ti3SiC2的影响。研究结果表明,3Ti/Si/2C粉体会发生自蔓延反应,产物的组成相为TiC、Ti3SiC2和Ti5Si3,产物中Ti3SiC2含量约为23%。添加适量的细粒度Al2O3可显著促进反应合成Ti3SiC2,3Ti/Si/2C/0.1Al2O3原料反应后得到的产物中Ti3SiC2含量达64%。