共查询到20条相似文献,搜索用时 15 毫秒
1.
Houda Kharrat Nizar Elfaleh Slaheddine Kamoun 《Journal of Physical Organic Chemistry》2016,29(10):532-543
The result of the X‐ray diffraction, differential scanning calorimetry and dielectric studies on a new crystal material C6H18N2SbCl5 is presented. The new organic–inorganic compound has been synthesized and characterized by the X‐ray diffraction method at 296(2) K. It crystallizes in the monoclinic P21/n space group. The cell dimensions are: a = 5.8617(1) Å, b = 15.7069(2) Å, c = 16.6693(2) Å, β = 97.627(1)° and Z = 4. The crystal structure consists of a discrete ionic layer of (C6H18N2)2+ cations and [SbCl5]2? anions linked via simple and bifurcated N―H · · · Cl hydrogen bonds. DSC analysis shows that this compound undergoes a phase transition at about (384 ± 2) K. AC and DC conductivities, complex dielectric permittivity ε*(ω) and complex electrical modulus M*(ω) were respectively studied as temperature and frequency functions. The combined data support each other and confirm the existence of a structural phase transition at about 384 K. Moreover, the temperature dependence of the DC conductivity and relaxation frequency followed the Arrhenius relation. The frequency dependence of the real part of the AC conductivity in both phases follows the Jonscher's universal dynamic law: . The behavior of s(T) with temperature suggests that the hopping over barrier model (CBH) and the small polaron tunneling mechanism (SPTM) prevail in phases I and II, respectively. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
2.
The molybdenum purple bronze KMo6O17 is quasi two-dimensional (2D) metallic oxide that shows a Peierls transition towards a metallic charge density wave state. Since this specific transition is directly related to the electron properties of the normal state, we have investigated the electronic structure of this bronze at room temperature. The shape of the MoK1s absorption edge reveals the presence of distorted MoO6 octahedra in the crystallographic structure. Photoemission experiments evidence a large conduction band, with a bandwidth of 800 meV and confirm the metallic character of this bronze. A wide depleted zone separates the conduction band from the valence band that exhibits a fourfold structure, directly connected to the octahedral symmetry of the Mo sites. The band structure is determined by ARUPS in two main directions of the (0 0 1) Brillouin zone. It exhibits some unpredicted features but corroborates the earlier theoretical band structure and Fermi surface. It confirms the hidden one-dimensionality of KMo6O17 that has been proposed to explain the origin of the Peierls transition in this 2D compound. 相似文献
3.
GK Johri Akhilesh Tiwari Saumya Saxena Rajesh Sharma Kuldeep Srivastava Manoj Johri 《Pramana》2002,58(3):563-567
Calculations for the relative width (Δω/ω0) as a function of refractive index and relative radius of the photonic band gap for the fcc closed packed 3-D dielectric
microstructure are reported and comparison of experimental observations and theoretical predictions are given. This work is
useful for the understanding of photonic crystals and occurrence of the photonic band gap. 相似文献
4.
A crystal chemical study has allowed us to identify new phosphates with the eulytine structure and the formulae A3-x Cd x Bi(PO 4)3 and A3-2x Na x Bi1+x (PO4)3 (A = Cd, Ca, Sr, Pb, Ba). Second harmonic generation tests carried out on powder samples have confirmed the non-centro-symmetrical character of their space group I43d. Bismuth cations were found necessary for the stability of the eulytine Cd3Bi(PO4)3. The study has also confirmed that the eulytine structure is easily stabilized with large cations. 相似文献
5.
Dielectric and ferroelectric properties of Sr_4CaSmTi_3Nb_7O_(30) with tetragonal tungsten bronze structure 下载免费PDF全文
Sr4CaSmTi3Nb7O30ceramics are synthesized and indexed as tetragonal tungsten bronze structure. The dielectric behavior and ferroelectric nature are investigated. Three dielectric anomalies are observed. The phase transition is a displacive phase transition with some diffusive characteristics, which indicates possible compositional variations within the materials on the microscopic scale. The weak distortion disappears in cooling process for differential scanning calorimetry measurement, and the large depression of Curie–Weiss temperature T0 indicates the difficulty in forming macroferroelectric domain. The ferroelectric nature in these filled tungsten bronze niobates originates from the off-center displacement of Bsite cations, but they are primarily dominated by A-site cation occupation. Both the radius and the valence of A1-site cations play an important role on ferroelectric properties of the filled tungsten bronze compounds. Existence of spontaneous polarization with a remanent polarization of 0.16 μC/cm2 a coercive field of Ec = 11.74 kV/cm confirms the room-temperature ferroelectric nature of Sr4CaSmTi3Nb7O30 ceramics. 相似文献
6.
Ceramic KBiScNbO6 compound was synthesized for the first time. X‐ray diffraction analysis revealed that at room temperature the samples consist of a mixture of the perovskite and pyrochlore phases. The dominant phase is the perovskite one. This phase can be described as distorted pseudocubic Pm3m structure with unit‐cell parameter ac = 4.069 Å. Rhombohedral distortion leading the polar R3m phase is assumed to be one of possible structural distortions. Dielectric measurements of the compound under study demonstrated anomalies associated with relaxor ferroelectric behavior (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
E. Castel F. Roulland D. Michau L. Raison M. Maglione 《Journal of magnetism and magnetic materials》2009,321(11):1773-1777
Recent studies of ceramics of formula Ba2LnFeNb4O15 (Ln=rare earth) with the “tetragonal tungsten bronze” (TTB) structure have correlated their room temperature multiferroics properties to the occurrence of barium ferrite parasitic phases. This work presents the elaboration of Ba2LaFeNb4O15 and Ba2EuFeNb4O15 composite samples with an excess of hematite in the TTB nominal composition. The influence of crystal-chemistry on the phase content and properties of Ba2LnFeNb4O15 TTB composites is discussed. A particular focus on the mechanisms related to the in-situ formation of barium ferrite is given. We show that we can control the spurious ferrite phase in TTB multiferroic composites and thus modulate their magnetic response. 相似文献
8.
采用中频感应加热烧结方法制备了W-1.5%La2O3-0.1%Y2O3-0.1%ZrO2和W-1.5%La2O3-0.1%Y2O3-0.08%ZrH2电子发射材料,烧结样品的致密度约为95.5%。热电子发射测试结果表明,添加氢化锆的热电子发射材料样品的零场发射电流密度大于添加氧化锆的样品,分析认为是添加的氢化锆在烧结时,发生分解,生成活性的Zr可以捕获钨晶界中的杂质氧,净化晶界,从而提高了电子发射;维氏显微硬度表明添加氢化锆样品的硬度高于添加氧化锆的样品,分析表明是氢化锆的添加有效改善了钨晶粒之间的结合性,提升了钨电子发射材料的硬度。利用SEM,EDS,XRD、金相显微镜等表面分析设备对样品进行了表征,样品结构显示添加氢化锆与添加氧化锆相比,不仅钨晶粒尺寸由13.63 μm降至11.63 μm,而且稀土相尺寸由1.87 μm降至1.66 μm,这种组织结构的变化有利于电子发射。 相似文献
9.
介电弥散和介电隔离率的温度非线性关系是弛豫铁电体的主要特征. 通过对掺杂成分以线性梯度递减的核壳结构进行热力学函数分析, 认为核壳结构能够在低温区保持较高的介电常数, 但不能导致介电隔离率与温度的非线性关系. 通过对不同浓度掺杂的铁电体扩散相变的比较, 认为掺杂浓度会影响晶粒掺杂成分的不均匀性, 在较宽的分布条件下会导致介电隔离率与温度的非线性关系. 因而在介电常数的峰值温度区域, 顺电相与铁电相的晶粒共存. 温度变化会影响两相比例及铁电畴的变化, 从而导致弛豫铁电体的介电弥散性. 核壳结构会增大介电弥散性. 铁电陶瓷的掺杂物种类、掺杂物浓度和烧结温度均会影响核壳结构的成分不均匀性和介电弥散性. 相似文献
10.
Tungsten oxide nanowires of diameters ranging from 7 to 200~nm are
prepared on a tungsten rod substrate by using the chemical vapour
deposition (CVD) method with vapour--solid (VS) mechanism. Tin
powders are used to control oxygen concentration in the furnace,
thereby assisting the growth of the tungsten oxide nanowires. The
grown tungsten oxide nanowires are determined to be of crystalline
W18O49. I--V curves are measured by an \textit{in
situ} transmission electron microscope (TEM) to investigate the
electrical properties of the nanowires. All of the I--V curves
observed are symmetric, which reveals that the tungsten oxide
nanowires are semiconducting. Quantitative analyses of the
experimental I--V curves by using a metal--semiconductor--metal
(MSM) model give some intrinsic parameters of the tungsten oxide
nanowires, such as the carrier concentration, the carrier mobility
and the conductivity. 相似文献
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在氩气和空气混合气体介质阻挡放电中,研究了放电丝结构随外加电压及气体压强的变化,并从二维体系相变的角度进行了分析.随着电压的增加,放电丝结构的演变过程为:稀疏的随机放电丝—稠密的随机放电丝—六边形结构—超六边形结构—混沌态,此过程相应于二维体系的气相—液相—简单晶体—超点阵晶体—液相的相变过程.实验还研究了相变过程中超六边形形成中晶格常数及相邻格点间距离的变化、超六边形结构中大点的形成过程以及超六边形结构的Penta-Hepta缺陷. 相似文献
13.
The crystal structure of a new form of L-isoleucine hydrochloride monohydrate C6H13O2NHClH2O (termed form II) has been determined using three-dimensional photographic data. This differs conformationally from the hydrochloride
derivative (termed form I, Trommel and Bijvoet 1954) reported earlier. The crystal belongs to the orthorhombic space group
P212121 with cell dimensions,a=5.87±0.01,b=24.77±0.02 andc=6.85±0.01 ? and four molecules per cell,ρ
obs=1.240 g/cm3,ρ
cal=1.238 g/cm3,μ for CuKa=32.6 cm−1.
Contribution No. 420 from the Department of Crystallography and Biophysics, University of Madras, Madras 600025. 相似文献
14.
卫星上某些介质结构会遭遇较大范围的温度变化, 其电导率会随之出现数量级的变化, 这将显著影响内带电结果. 受限于电导率-温度模型和内带电三维仿真工具, 该温度效应远没有得到深入研究. 为此, 在真空变温(253-353 K)和强电场(MV/m量级)条件下测试了某种星用改性聚酰亚胺介质的电导率, 借鉴Arrhenius电导率-温度模型并考虑强电场下电导率的增强效应, 发现电导活化能取值为0.40 eV时, 可得到良好的拟合结果. 在此基础上, 同时考虑辐射诱导电导率, 采用地球同步轨道恶劣电子辐射能谱, 对该类介质盘环结构进行内带电三维仿真, 发现其内带电程度随温度降低而显著增加, 带电最严重的区域位于靠近辐射源的接地面边线. 温度低于250 K时, 2 mm屏蔽铝板下该区域的场强可达到107 V/m量级, 发生介质击穿放电的可能性较大. 所讨论的电导率-温度模型与内带电三维建模方法对进一步评估卫星介质结构内带电程度和做好防护设计具有重要参考意义. 相似文献
15.
Correlation of magnetic anisotropy with dielectric anisotropy in fluorinated phenyl bicyclohexane liquid crystal 下载免费PDF全文
Temperature dependence of magnetic anisotropy of homologous nematic fluorinated phenyl bicyclohexane liquid crystals is measured by a magneto-electric method.The result shows that the diamagnetic property is slightly influenced by the positions and the numbers of fluorine atoms substituted at the phenyl ring.By investigating the correlation of the dielectric anisotropy with the magnetic anisotropy,a novel explanation is proposed for the behaviour of the molecular dipole-dipole dimerization in the polar liquid crystal compounds. 相似文献
16.
In this paper, a large number of the micro/nano-sized tungsten single-crystalline whiskers were fabricated via a vapour deposition method. The morphology and structure of the whiskers were investigated using scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The factors affecting the growth of tungsten whiskers were systematically studied, including temperature, holding time and the position of Si substrates. Results indicated that temperature, holding time and position of the Si substrates have significant effects on the growth of tungsten whiskers. 相似文献
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18.
为了在介质壁加速器中增大轴向加速电场, 提高加速梯度的同时抑制径向电场对束包络的扩张, 提出了在每个加速电极上添加金属栅网结构。采用基于粒子云网格方法的电磁粒子模拟软件对不加栅网与添加栅网的电极结构进行了数值仿真, 分析了不同结构下加速管道中的电场分布和束包络变化。通过实验对比了两种不同结构下经过相同的加速长度获得的粒子能量。结果表明:添加金属栅网结构相对于不加栅网的金属小孔式结构, 轴向加速电场强度提高20%, 同时径向电场得到有效抑制;栅网结构下, 被加速的粒子束在自由漂移空间中的径向发散基本得到抑制;在相同的加速长度下加速H3+粒子, 栅网结构得到的能量增益提高了一倍。 相似文献
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20.
Ken-ichi Miyazono Yoriko Sawano Masaru Tanokura 《Proceedings of the Japan Academy. Series B, Physical and biological sciences》2004,80(9):439
Analysis of the hyperthermophilic archaeon Pyrococcus horikoshii OT3 genome database led to the discovery and cloning of acylphosphatase (ORF PH0305a). To elucidate the first structure of archaeal acylphosphatase, we determined the crystal structure of P. horikoshii acylphosphatase at 1.72 Å resolution. The space group of the crystals was P3221, with unit-cell parameters a = b = 86.6 Å and c = 75.4 Å. The overall fold of P. horikoshii acylphosphatase was very similar to the structures of the eukaryotic enzymes. The conformation of putative active site was highly conserved. 相似文献