共查询到19条相似文献,搜索用时 202 毫秒
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以[001]c和[011]c极化的铌锌酸铅-钛酸铅晶体为研究对象, 利用子波理论对其无限大自由薄层中传播的Lamb波的色散及模式交叉特性进行了研究. 发现只有[001]c极化的晶体中的对称与反对称模式Lamb波之间出现了多次交叉, 并且变化规律与铌镁酸铅-钛酸铅的情形相同. Lamb波的A0和S0模式的交叉是由准纵向剪切波慢度曲线的多值关系引起的, 此时其x3方向的波数在一定范围内存在一对非纯虚数的复共轭根. 利用此结论推导出A0和S0模式交叉时弹性常数需要满足的条件, 为判断正交、四方对称性晶体中Lamb波的A0和S0模式是否交叉提供了一种直观、简便的方法. 相似文献
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本文报导了对以Ni3Al为基的r′单晶体所做的蠕变及持久试验的结果。结果表明,r′晶体的稳态蠕变速εs遵从 εs=Cσmexp(-Qc′/RT) 的关系。指数m仅和温度有关。m(850℃)=3.95,m(950℃)=3.62。对Ni3Al,Qc′=88kcal/g-at。合金元素Ti,Nb,W都使Qc′升高,其中Nb的作用最大,然而W在提高蠕变强度上最有效。晶体取向对εs有显著影响。<111>取向的Ni3Al晶体εs最低,持久寿命tr最长。在1050℃,σ=5kg/mm2时,tr可≥100小时,Ni3Al单晶体也同样符合Monkman-Grant关系tr(εs)p=常数,其中p=0.82。r′晶体蠕变中的滑移系统是{111}<211>。 相似文献
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动态断裂过程的数值分析及LY-12铝的层裂 总被引:9,自引:4,他引:5
本文从文献[1]中用于分析柱壳动态膨胀断裂过程的损伤度函数出发,将它推广到对一维应变下层裂过程的数值模拟研究。试件材料为LY-12铝,其特性方程取为含粘性的本构方程形式。数值计算结果很好地再现了实测自由面速度ufs随时间t的变化过程,并表现出层裂强度σc及层裂面上的临界损伤度αc都分别是应变率εc'的单调递增函数关系。σc~εc'的这种变化规律在许多文献中已屡见报道,例如可见文献[2-3]。在105 s-1~106 s-1应变率范围内,σc~εc'关系可以表示为εc'exp(-11.4αc)=2 100 s-1,这个式子可以作为一种层裂判据使用。数值计算还给出了层裂片的损伤度剖面,其形状特征与Barbee等对回收试件的细观测量结果在定性上一致。 相似文献
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利用PVP/SDS聚集体作为探针研究了水溶液中β-环糊精与SDS之间的包合作用,结果表明在含有PVP/SDS聚集体溶液的相对粘度对β-环糊精浓度作图中存在着特征浓度cs,当β-环糊精的浓度低于cs时,随着β-环糊精浓度的增加,溶液的相对粘度迅速下降;与此相反,当β-环糊精浓度高于cs时,随着β-环糊精浓度的增加,溶液的相对粘度逐渐增加. 含有PVP/SDS聚集体溶液相对粘度随着β-环糊精浓度增加而迅速下降是由于β-环糊精包合了客体分子SDS,该包合作用将导致SDS分子从高分子链中脱落. β-环糊精和SDS包合比例可以由cs计算得出,实验结果是1比1. 进一步的实验结果表明,cs与PVP/SDS聚集体中SDS的含量有关、和PVP的含量无关,但是β-环糊精和SDS的包合比与PVP/SDS聚集体中SDS和PVP的含量皆无关. 相似文献
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利用超声“脉冲回波重合法”技术,测定了立方晶体镱镓石榴石(GGG)各对称方向的超声波声速,及它们在流体静压力和单轴应力条件下,随压力的变化,计算了GGG的二阶和三阶弹性常数。它们分别是:c11=3.04,c12=1.31,c44=0.94和c111=-17.97,c112=-12.86,c123=-1.06,c144=-2.11,c166=-3.29,c456=-0.94,单位是1011 N/m2。根据这些数据,我们还计算了GGG晶体的德拜温度以及沿它的[001]→[111]→[110]→[100]传播的三个声学波分支(一个纵波,两个横波)的Gruneisen参数γ(p,N)的分布。 相似文献
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Vacancies effect on the mechanical properties in B2 FeAl intermetallic by the first-principles study
ABSTRACTThe geometric structures, electronic and mechanical properties of the high vacancy concentration intermetallic FeAl (experimental value: 3.3 at.% at 1451?K) were investigated by first-principles calculations based on density functional theory. The FeAl structures of different vacancy concentration with minimum energy were addressed, which shows that vacancies of iron (VFe) are more favourable and tend to gather together. For mechanical properties, both Young's modulus and elastic constants show an overall downward trend as vacancy concentration increases, but increase abnormally with the vacancy concentration ranging from 3.7 at.% to 5.6 at.%. All can be explained by the strength of Al–Fe bond, in other words, the Al–Fe interaction. Interestingly enough, intermetallic FeAl shows a transfer from the brittle manner to ductile manner, which also behaves as an important feature of FeAl in experiments. All the mechanical properties agree well with experimental data, indicating the reasonable vacancy model of FeAl intermetallic. 相似文献
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M. Heidari Saani H. Hashemi A. Ranjbar M. A. Vesaghi A. Shafiekhani 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,65(2):219-223
The relationship between unpaired electron delocalization and nearest-neighbor atomic relaxations in the vacancies of diamond
has been determined in order to understand the microscopic reason behind the neighboring atomic relaxation. The Density Functional
Theory (DFT) cluster method is applied to calculate the single-electron wavefunction of the vacancy in different charge states.
Depending on the charge and spin state of the vacancies, at outward relaxations, 84-90% of the unpaired electron densities
are localized on the first neighboring atoms. The calculated spin localizations on the first neighboring atoms in the ground
state of the negatively charged vacancy and in the spin quintet excited state of the neutral vacancy are in good agreement
with Electron Paramagnetic Resonance (EPR) measurements. The calculated spin localization of the positively charged vacancy
contrasts with the tentative assignment of the NIRIM-3 EPR signal to this center in (p-type) semiconductor diamond. The sign
of the lattice relaxation in the diamond vacancy is explained based on the effect of electron delocalization on nearest-neighbor
ion-ion screening, and also its effect on the bond length of neighboring atoms. 相似文献
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Various dispersions of the dielectric permittivity of Rochelle Salt, which are caused by crystal imperfections, are known to occur at low frequencies. On the other hand some properties of Rochelle Salt point out that at room temperature rather high concentrations of crystal water defects are present. Therefore the dependence of the relaxation strengths on the content of crystal water is investigated in the case of two different dispersions. One of them shows no connection with the stoichiometry as far as the water of crystallization is concerned, whereas the relaxation strength of the other decreases by about two decades, if the relative humidity, at which the crystals are stored, is increased from 9 to 54 per cent. This dependence is explained quantitatively by concentration changes of water vacancies in the lattice. The energy of activation of the corresponding dielectric relaxation and the energy necessary to create a water vacancy are determined as 0.5 eV. 相似文献
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Tsuyoshi Maeda 《Journal of Physics and Chemistry of Solids》2005,66(11):1924-1927
In order to quantitatively evaluate the formation energies of Cu, In/Ga/Al and Se vacancies in chalcopyrite-type CuInSe2 (CIS), CuGaSe2 (CGS) and CuAlSe2 (CAS), first-principles pseudopotential calculations using plane-wave basis functions were performed. All calculations were performed using a supercell with 64 atoms, which was eight times greater than the number of atoms in a primitive cell with eight atoms. The formation energies of point defects were calculated as a function of the atomic chemical potentials of constituent elements. Atomic arrangements around the vacancy were optimized allowing relaxation of the first- and second-nearest-neighbor atoms of the vacancy. The obtained results were as follows: (1) the formation energy of Cu vacancy was smaller than those of the other vacancies in CIS, CGS and CAS. Under the Cu-poor condition, the formation energy of Cu vacancy in CIS was lowest among those in them; (2) the formation energy of Se vacancy in CIS was relatively lower than those in CGS and CAS; (3) the formation energy of (2VCu+InCu) pair in CIS was greatly dependent on the chemical potential of the constituent elements, i.e. Cu, In and Se. On the other hand, the formation energies of (2VCu+GaCu) in CGS and (2VCu+AlCu) in CAS were not largely dependent on the chemical potential of the constituent elements. Under the Cu-poor condition, the formation energy of (2VCu+InCu) pair in CIS was much lower than those of (2VCu+GaCu) in CGS and (2VCu+AlCu) in CAS. 相似文献
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I.R. Shein 《Journal of Physics and Chemistry of Solids》2006,67(7):1436-1439
The electronic, magnetic properties and lattice relaxations of oxygen-deficient cubic strontium ferrite, SrFeO2.875, in ferromagnetic configuration are studied by means of the density functional theory using LCAO basis (SIESTA code) calculations. It is shown that Fe and Sr atoms are displaced from oxygen vacancies while oxygen anions are attracted to the vacancies. The DOS distributions, magnetic moments and atomic effective charges are analyzed in comparison with vacancy free SrFeO3; these parameters are found to change weakly with appearance of oxygen vacancies, in contrast to conventional ionic picture. Some strengthening of Fe-O covalent bonds in the vicinity of the oxygen vacancy is found. The formation energy of oxygen vacancies and divacancies are evaluated. 相似文献
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采用平面波展开和基于密度泛函理论框架下的第一性原理赝势法,计算了102GPa下LiF化合物中Li空位和F空位的形成能及空间周围的原子弛豫,讨论了空位形成时电荷密度的重新分布,相应的电子态密度以及能带结构等性质.结果表明:LiF晶体中F空位的形成能比大于Li空位的形成能;F空位对LiF晶体的电子结构等性质的影响要比Li空位的大. 相似文献
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采用平面波展开和基于密度泛函理论框架下的第一性原理赝势法,计算了102GPa下LiF化合物中Li空位和F空位的形成能及空间周围的原子弛豫,讨论了空位形成时电荷密度的重新分布,相应的电子态密度以及能带结构等性质.结果表明:LiF晶体中F空位的形成能比大于Li空位的形成能;F空位对LiF晶体的电子结构等性质的影响要比Li空位的大. 相似文献
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K. Kishio J.R. Owers-Bradley W.P. Halperin J.O. Brittain 《Journal of Physics and Chemistry of Solids》1981,42(12):1031-1041
The NMR spin-lattice relaxation time, TI, has been measured as a function of temperature for both 7Li and 27Al in pure and doped β-LiAl alloys. Compositions with 7Li concentration in the range 48.3–54.5% and doping in the form Li50Al50?xMx, where M = Ag or In, were studied. The relaxation rates T1?1 for the 27Li and the 27Al resonances were found to be peaked functions of temperature with the maxima for 7Li appearing at composition dependent temperatures. The 27Al maxima always appeared at a lower temperature, independent of composition, and the 27Al maximum relaxation rate was a strong function of composition in contrast with 7Li where the maximum rate was only weakly dependent on composition. The principle relaxation mechanisms are identified as dipole-dipole coupling in the 7Li and coupling of the 27Al quadrupole moment to electric field gradients. The temperature dependence of these rates is attributed to the thermally activated diffusion of vacancies of a non-thermal origin in the Li sub-lattice. These vacancies are also responsible for the fluctuating electric field gradients. The results have been analyzed to give the Li diffusion coefficients with associated activation energies and estimates of the vacancy concentration as functions of alloy composition. 相似文献