Electric conductivity of Fe2SiO4–Fe3O4 spinel solid solutions     

T. Yamanaka  H. Shimazu  K. Ota 《Physics and Chemistry of Minerals》2001,28(2):110-118

 The spinel solid solution was found to exist in the whole range between Fe₃O₄ and γ-Fe₂SiO₄ at over 10 GPa. The resistivity of Fe_{3− x} Si _x O₄ (0.0<x<0.288) was measured in the temperature range of 80∼300 K by the AC impedance method. Electron hopping between Fe³⁺ and Fe²⁺ in the octahedral site of iron-rich phases gives a large electric conductivity at room temperature. The activation energy of the electron hopping becomes larger with increasing γ-Fe₂SiO₄ component. A nonlinear change in electric conductivity is not simply caused by the statistical probability of Fe³⁺–Fe²⁺ electron hopping with increasing the total Si content. This is probably because a large number of Si⁴⁺ ions occupies the octahedral site and the adjacent Fe²⁺ keeping the local electric neutrality around Si⁴⁺ makes a cluster, which generates a local deformation by Si substitution. The temperature dependence of the conductivity of solid solutions indicates the Verwey transition temperature, which decreases from 124(±2) K at x=0 (Fe₃O₄) to 102(±5) K at x=0.288, and the electric conductivity gap at the transition temperature decreases with Si⁴⁺ substitution. Received: 15 March 2000 / Accepted: 4 September 2000  相似文献   

Electronic structure and electric field gradient calculations of Al2SiO5 polymorphs     

M. Iglesias  K. Schwarz  P. Blaha  D. Baldomir 《Physics and Chemistry of Minerals》2001,28(1):67-75

 The electronic structure of the three polymorphs of Al₂SiO₅, andalusite, sillimanite, and kyanite, is studied by linearized-augmented-plane-wave (LAPW) calculations using the WIEN code. Total energy calculations verify, in agreement with recent pseudopotential calculations, that andalusite is the most stable phase, followed by sillimanite and kyanite.We determine the electronic charge density distribution and find strong polarizations on all oxygen ions. We identify different polarizations due to Al or Si neighbors which depend on their respective distances to the oxygen atom. The chemical bonding is not purely ionic in nature but has important covalent contributions. Electric field gradients (EFGs) at all sites are calculated and agree well (within 10%) with available experimental data on Al. We identify the origin of the EFGs and demonstrate its relation to the nearest-neighbor coordination and the resulting anisotropy of the electronic charge distribution. Received: 22 March 2000 / Accepted: 3 August 2000  相似文献   

福建龙卷风的活动特点   总被引：11，自引：1，他引：11  

鹿世瑾 《气象》1996,22(7):36-39

分析福建千余年２５次的龙卷风记载，得出如下几点认识：（１）福建的龙卷均在春夏两季，且几率相近；１９４９年以来的１２次龙卷有１１次出现于１３－１８时（占９２％），另一次是上午９时（占８％）。空间分布为：沿海地区有２０次，占８０％，内陆５次，占２０％，且有多发挥地段，（２）龙卷风多出现于低压，高温，高湿点附近，天空以积雨云占多数，地面多有强风相伴，（３）福建１２次龙卷风的诱发系统为：台风（６次，均在７  相似文献   

雨滴尺度谱谱参数和形状因子的分析     

周文贤  陈金荣  齐久成 《气象》1996,22(4):13-16

作者计算了３次降不过程的雨滴尺度谱参数，并进行Ｍ－Ｐ分布拟合和Г分布拟合。同时还计算了雨滴谱形状因子Ｓ（Ｚ，Ｒ），分析了形状因子与雨强之间的关系。  相似文献   

西沙群岛大气中海盐粒子的分布特征   总被引：9，自引：1，他引：9  

吴兑  游积平  关越坚 《热带气象学报》1996,12(2):122-129

分析了在西沙永兴岛观测的Ｃｌ＾－海盐核资料，主要结果有：（１）东北季风期干直径大于２μｍ的Ｃｌ＾－核平均浓度８７８个／Ｌ，平均含盐量１０５．４μｇ／ｍ＾３，其中干直径大于４μｍ的特大核平均为１９０个／Ｌ，最大间观测到干直径５７μｍ的Ｃｌ＾－粒子，均较该地在１９８７年５月西南季风盛行期的观测值高。谱型呈准单调下降的幂函数递减谱。（２）随距海面高度增加海盐粒子浓度与含盐量均有所减小。（３）Ｃｌ＾－粒子  相似文献   

Estimation of Hourly and Daily Global Solar Radiation at Clear Days Using an Approach Based on Modified Version of Gaussian Distribution     

S. M. El shazly 《大气科学进展》1996,13(3):349-358

The performance of two models,Jam and Baig,based on the modified version of Gaussian distribution function in estimating the daily total of global solar radiation and its distribution through the hours of the day from sunrise to sunset al any clear day is evaluated with our own measured data in the period from June 1992 to May 1993 in Qena Egypt The results show a high relative deviation of calculated values from measured ones,especially for Jain model,in the most hours of the day,except for those near to local noon.This misfit behavior is quite obvious in the early morning and late afternoon A new approach has been proposed in this paper to estimate the daily and hourly global solar radiation This model performs with very high accuracy on the recorded data in our region.The validity of this approach was verified with new measurements in some clear days in June and August 1994.The resultant very low relative deviation of the calculated values of global solar radiation from the measured ones confirms the  相似文献   

上行离子密度分布特性的理论研究          下载免费PDF全文

史建魁  刘振兴 《地球物理学报》1996,39(2):159-168

采用动力学方程，求解了定态情况下磁层中上行离子沿磁力线的分布函数，针对不同的Ｋ_ｐ指数，分别对北半球极光带区起源的上行离子Ｏ_＋、Ｈ_＋和Ｈｅ_＋在子午面内沿磁力线的密度分布及其特性进行了研究．结果表明：１．沿磁力线向外，上行离子密度在近地空间呈急剧下降趋势，在远地空间呈缓慢下降趋势；２．重离子或初始能量较小的离子，其密度沿磁力线向外下降较快；３．Ｋ_ｐ指数越大，离子进入磁层的空间范围越大；４．离子的投掷角分布对密度分布的影响甚小；５．离子密度沿磁力线的下降程度随到Ｘ轴距离的增大而呈增大趋势；６．在典型参数条件下，求得上行离子Ｏ_＋在等离子体片边界附近的密度为１０^－３－１０^－２个ｃｍ^－３，这与观测结果相一致．  相似文献   

潜在震源区内部空间非均匀分布地震区划方法在攀西地区和晋中南地区的应用   总被引：2，自引：1，他引：2  

高孟潭  胥广银 《中国地震》1996,(Z1)

本文依据潜在震源区内部地震非均匀分布模型研究了攀西地区和晋中南地区地震区划问题。研究结果表明，一般使用的均匀分布模型会低估地震危险性结果，导致高烈度分区的面积缩小。本文讨论了不同的确定非均匀分布概率的方法对地震区划结果的影响。  相似文献   

Power-laws and applications to earthquake populations     

A. Rahulji Parsa  Gurajada Suryanarayana Murty 《Pure and Applied Geophysics》1996,147(3):455-466

In this paper we study the rooted tree model applying the concepts of probability to obtain results of importance in understanding power-law distributions in pure populations and also in an ensemble of pure populations. The well-known Gutenberg-Richter relation, which is an empirical relation providing the number of earthquakes whose magnitude exceeds a given value, is shown to be an asymptotic form of survivor function of earthquake magnitudes. The implications of this model are briefly discussed in relation to other branches of sciences where power-law distributions are encountered.  相似文献   

Estimation of quantiles and confidence intervals for the log-Gumbel distribution     

J. -H. Heo  J. D. Salas 《Stochastic Environmental Research and Risk Assessment (SERRA)》1996,10(3):187-207

The log-Gumbel distribution is one of the extreme value distributions which has been widely used in flood frequency analysis. This distribution has been examined in this paper regarding quantile estimation and confidence intervals of quantiles. Specific estimation algorithms based on the methods of moments (MOM), probability weighted moments (PWM) and maximum likelihood (ML) are presented. The applicability of the estimation procedures and comparison among the methods have been illustrated based on an application example considering the flood data of the St. Mary's River.  相似文献