Phase Transitions of Urea Adducts with n-Paraffins under High Pressure

The temperatures of the phase transition between orthorhombic and hexagonal structures of the urea adducts with n-paraffins, n-C_nH_2n+2 (n = 14–44), have been measured by differential thermal analysis for the pressure range up to 250MPa. The transition temperatures increase both with pressure and with the number of carbon atoms of the guest paraffins. The increasing rate of the transition temperatures with pressure is about half of that of pure paraffin crystals. The pressure dependences of the lattice constants have also been measured by X-ray diffraction. The specific volume changes discontinuously at the transition pressure; the result suggests that the phase transition of the urea adducts with n-paraffins is a first-order one. Furthermore, the specific volume change at the transition pressure decreases with the number of carbon atoms of the guest paraffins; 3.4 × 10-⁶ m³/Kg for n = 16 and 2.1 × 10-⁶ m³/Kg for n = 44.     

Crystal Structure and Phase Transition of Diglycine Perchlorate

Abstract  

Diglycine perchlorate (DGPCl), a new 2:1 adduct formed between glycine and perchloric acid has been obtained and studied using differential scanning calorimetry and single crystal X-ray diffraction. DGPCl undergoes a reversible first-order phase transition at 261.5 K. The crystal structures at 150 and 293 K have been determined to be triclinic, space group P‐1, Z = 2, suggesting the first-order phase transition to be an isostructural phase transition. The DGPCl crystal consists of five glycinium-monoprotonated glycinium dimers and five perchlorate anions in an asymmetric unit. The glycine moieties in the glycinium-monoprotonated glycinium dimers are non-planar. Two types of hydrogen bonds are present in the crystal, strong O–H···O hydrogen bonds and a weak N–H···O hydrogen bonds. The short, strong O–H···O hydrogen bond connects the glycinium ion and mono protonated glycinium ion. In four of the dimers, the O–H and H···O bond lengths are different, indicating the hydrogen atom to be located more close to the monoprotonated glycinium ion. However, in one of the glycinium-monoprotonated glycinium dimer the O··H and H··O bond lengths are nearly equal, suggesting the hydrogen atom (O···H···O) to be attached to the oxygen atoms of both glycine moieties. On thermal transition some of these hydrogen bonds are weakened and in all dimers the hydrogen atom seems to be located more close to the mono protonated glycinium ion.     

四方相区NBT-KBT无铅铁电介电弛豫与铁电相变研究

本文研究了具有四方结构的(Na1-xKx)0.5Bi0.5TiO3体系中x＝0.22,0.26和0.30陶瓷材料不同温度下的电滞回线,结合变温XRD和介电温谱分析,发现该体系四方结构的组成为反铁电体,但紧靠准同型相界的四方结构由于场诱导下引起的反铁电-铁电相变,表现出铁电体特性,材料在升温过程中由于反铁电宏畴向微畴的转变导致了介电峰的产生,且在介电温谱上表现出强烈的介电常数-频率依赖性,为弛豫铁电体特征.     

纤锌矿和岩盐结构ZnO的相变、弹性性质和电子结构研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势(USP)方法,结合广义梯度近似(GGA)计算了岩盐结构(B1)和纤锌矿结构(B4)ZnO的相变、弹性性质,并分析了B1和B4相ZnO在相变点处的电子结构特征.计算结果表明:ZnO在12.72 GPa时发生了由B4相向B1相的转变.B1和B4相ZnO的体弹性模量分别为171.5 GPa和132.8 GPa.能带结构的结果表明B1相是间接带隙半导体,带隙值为1.404 eV,而B4相是直接带隙半导体,带隙值为1.107 eV.     

相变存储材料及其相变机制研究进展

介绍了相变存储材料的工作原理与其工作性能之间的对应关系,相变存储材料的开发与材料性能要求密不可分,在分析当前主流相变存储材料Ge2 Sb2 Te5的优点与不足的基础上,指出无Te富Sb为未来相变存储材料体系的研究方向.控制相变行为是实现相变存储技术商业化的关键,材料的微观结构和相变机制决定了相变存储材料的相变行为,而计算材料学是研究相变存储材料微观结构与相变机制之间联系的必要手段.运用第一性原理动态模拟相变存储材料的相变过程,可以弥补静态结构分析的不足,推动对快速可逆相变机理动态过程的理解运用,同时为进一步研究相变存储材料提供借鉴.     

冲击压缩对TiO2粉末结构和热处理相变的影响

为研究动高压加载下TiO2的冲击改性特点,利用平面冲击波加载技术对TiO2粉体进行冲击压缩实验,以XRD和Raman光谱为主要手段,对样品的结构和热处理相变行为进行了表征和比较.结果表明:冲击压缩作用使TiO2的衍射峰宽化,在样品中形成较大的晶格畸变和应力;飞片速度对TiO2晶粒的粉碎细化效果影响较大.经飞片2258 m/s高速冲击后,微结构中的晶格畸变和应力使TiO2在热处理时的相变温度降低约100℃.TiO2在高速冲击后生成了一种具有锆钛矿结构的同素异构体.     

Phase Diagrams of New Compounds with Nematic Reentrant Mesophases

Two new series of compounds exhibiting nematic and smectic reentrant mesophases are presented here; the first one is 4-n-alkoxy benzoyloxybenzylidene 4'-cyanoaniline series the second one the 1-(4-n-alkylbenzoyloxyphenyl) 2-(4'-cyanophenyl) ethane series. Isobaric binary phase diagrams are plotted for each compound by means of the contact method and the mesophases so identified.     

Phase Transition of 1,16-Hexadecanediol

It was found that 1,16-hexadecanediol exhibited a solid-solid phase transition. X-ray diffraction experiments and dielectric constant measurements were carried out in order to investigate the structure of the high-temperature phase. A model of molecular arrangement in the high-temperature phase was presented. The high-dielectric constant values were observed in the high-temperature phase. It is considered that the high-temperature phase of 1,16-hexadecanediol is a rotator phase in analogy with n-higher alcohols.     

Phase Transition Studies in Mixed Polymesomorphic Liquid Crystals with Cholesteric and Smectic A Phase

Cholesteric mesomorphism was induced in a polymesomorphic liquid crystal with nematic and smectic A phases by mixing it with a low pitch cholesteric. A variety of mixtures for a wide range of concentrations were studied. It was observed that the addition of cholesteric liquid crystal disrupts the smectic phase of the parent polymesomorphic liquid crystal. The measured valuses of ultrasound velocity and specific volume and their related parameters were found to exhibit anomalies near each of the phase transitions. The mean interaction parameter $ \bar \alpha $ was estimated for different mixtures and was found to decrease with increasing concentration of cholesteric liquid crystal in the mixture. It was observed that smectic A-cholesteric transition becomes second-order at a particular concentration in agreement with the theoretical predictions.     

高低温偏硼酸钡晶体的相变和生长

本文探讨了BBO晶体α相和β相之间的相变问题,同时简要地介绍了国内外α-BBO晶体和β-BBO晶体的生长方法,并展望了α-BBO晶体和β-BBO晶体研究的发展趋势×-β相变的深入研究;α-BBO和β-BBO亚稳态生长条件的研究;生长工艺的优化以及新的生长工艺的发展;β-BBO薄膜的研究.     

Internal Friction in Copolymer Vinylidenefluoride with Trifluoroethylene at Ferroelectric Phase Transition

Crystallography Reports - Temperature dependences of internal friction Q–1, obtained at different heating rates of a copolymer VDF/TrFE (75/25) sample, are studied in the temperature range of...     

高相变温度弛豫铁电单晶PMN-PT-PZ的生长与性能表征

按照0.624Pb(Mg1/3Nb2/3)O3-0.336PbTiO3-0.04PbZrO3化学式所示组分比例,采用分步高温固相反应合成出PMN-PT-PZ多晶,通过熔体坩埚下降法生长出尺寸φ25 mm×90 mm的PMN-PT-PZ单晶.应用X射线衍射对所获多晶和单晶试样进行了物相分析,测试了PMN-PT-PZ晶片的介电温谱、电滞回线和压电常数.结果表明,所得三元固溶体单晶PMN-PT-PZ为不含焦绿石相的纯钙钛矿相结构,其三方-四方相变温度Trt达130℃,居里温度Tc为165～ 170℃,取自单晶原坯三方相区段的(001)取向晶片的压电常数d33在1300～ 1800 pC/N之间;其矫顽电场Ec为4～ 4.5 kV/cm,剩余极化强度Pr为20 ～ 31.5μC/cm2.跟PMN-PT单晶比较,PMN-PT-PZ单晶仍具有较大压电常数d33,而三方-四方相变温度明显提高,其矫顽电场有所增大.     

ENDOR Study of the Phase Transition in Benzil

ENDOR spectroscopy was used to elucidate the domain structure of benzil accompanying its phase transition at 84°K. Spectral twinning was observed on a single-crystalline sample as a result of multidomain formation below the phase transition temperature. This twinning was explained by applying the Landau theory of phase transitions. With knowledge of the anisotropy of the proton hyperfine interaction, we extracted the angle between the domains and the morphologic two-fold axis to be 1.55° at 2°K.     

C12H9SN晶体铁弹相变的研究

采用慢度曲面法研究C12H9SN晶体的铁弹相变.在现有实验数据的基础上,模拟计算C 12 H9 SN晶体高温正交相的弹性劲度系数对慢度曲面的影响.结果表明,非对角元c12,c13,c23的变化对慢度曲面最大值的取向没有影响,同时也确定出与该相变直接相关的弹性劲度系数为c55.该晶体相变的对称性变化是由高温正交相mmm点群变成低温单斜相2/m点群.     

Observation of Intermediate Smectic A Structure at the Phase Transition Nematic-Smectic C in 4,4′-di-n-Heptyloxyazoxybenzene

Using different boundary conditions in thin film of 4,4′-di-n-heptyloxyazoxy benzene and studying the microscopic textures as well as the depolarized Rayleigh light scattering, an intermediate smectic A-like state during the nematic-smectic C phase transition was observed.     

Synthesis,Characterisation and Phase Transition Studies of 50.9 and 90.5

The synthesis, characterisation of phases of the compounds N(p-n-pentyloxy benzylidene) p-n-nonylaniline, 50.9 and N(p-n-nonyloxy benzylidene) p-n-pentylaniline, 90.5 and phase transition studies using different experimental techniques are presented. The compound 50.9 exhibits a phase sequence NAB whereas 90.5 shows a phase variant ABG. A large density jump across IN transition is observed in 50.9 similar to that observed in 50.10 compound.     

不同表面层铁电薄膜的相变理论研究

基于横场伊辛模型,利用平均场近似理论推导久期方程的解,研究了单、双表面层铁电薄膜在不同总层数时,系统表面交换相互作用和内、外横场参量对铁电-顺电相变的影响,讨论了铁电薄膜各交换相互作用和横场参数以及薄膜层数对单、双表面铁电薄膜相图的影响,并计算了各个相互作用参数的过渡值特性.研究结果表明:薄膜总层数n、表面层数、内外部横场、表面交换相互作用都会改变铁电薄膜的相图.利用薄膜的尺寸效应和表面效应,增大系统总层数、表面层数和表面交换相互作用,可以提高薄膜相变温度,扩大铁电相区域,从而有利于改善铁电功能器件的环境温度.     

过渡金属轻元素化合物的高温高压制备

过渡金属轻元素化合物（TMLEs）由于具备高硬度,高熔点,优异电学、磁学、超导等性质受到广泛关注,是一类 具有优异力学性能的功能性材料。优异力学性能与功能性的结合使TMLEs成为极端环境下使用的特种材料。然而, TMLEs的制备往往需要高温高压（HPHT）极端实验条件来克服能垒。目前,已经有了大量HPHT制备TMLEs的报道, 然而,多数只关注产物的性质,对在HPHT下TMLEs的生长机制报道较少。因此,总结HPHT制备的TMLEs,分析TMLEs的晶体生长过程,对理解TMLEs的晶体生长机理、探究新型 TMLEs的制备具有重要意义。结合本课题组研究 经验及其他相关文献,总结了HPHT方法制备的过渡金属硼化物（TMBs）、碳化物（TMCs）和氮化物（TMNs）的晶体生 长情况,分别从起始原料、温压条件、晶体形貌等方面分析了TMLEs的生长机制。总结如下：通过原料配比和温度控 制是制备TMBs单一相的关键,提出硼亚结构单元是使TMBs形成台阶式生长模式的本质因素,碳源和氮源的选择决 定了 TMCs和TMNs的生长机制。同时提出,缺少利用HPHT制备TMLEs毫米级单晶的报道,限制了TMLEs部分本 征的性质探究;并且,新型高轻元素含量的TMLEs结构依然有待开发。随着人类对材料的要求越来越苛刻,以及TMLEs的不断发展,TMLEs将在未来特种材料领域具有不可替代的地位。