High temperature phase transition in KH2PO4 crystal

DTA, weight loss, infrared and dielectric measurements have been performed on KH₂PO₄ (KDP) single crystal as well as on different particle size specimens. DTA result reveals two endothermic peaks. The lower peak at 180°C is particle size dependent and vanishes in specimen of particle size ≤ 0·1 mm. Dielectric measurements also show similar behaviour. No significant weight loss of the crystal was noticed when kept at 180°C. We are inclined to believe that fragmentation of crystal is likely to be responsible for the transition rather than PO₄-group rotation or decomposition of KDP.     

VOC2O4·H2O热分解制备纳米VO2粉体及相变特性

提出了一种采用草酸氧钒(VOC2O4·H2O)热分解制备纳米VO2的方法.通过热分析(TG-DSC)、XRD和TEM手段,分析了草酸氧钒前驱体的热分解过程,纳米VO2的结构和形貌,测定了纳米VO2粉体的电阻-温度曲线.实验结果表明VOC2O4·H2O热分解开始温度为343℃,在380℃,真空度为20～50Pa的条件下,热分解获得VO2的平均尺寸为22nm,相变温度为69℃.     

Effect of magnetic fields on solid-melt phase transformation in pure bismuth

The differential thermal analysis (DTA) apparatus has been designed in order to investigate the effect of magnetic fields on solid-melt phase transformation in pure bismuth. The endothermic peaks of DTA curves show that melting is insensitive to magnetic fields, which can be verified from thermodynamics. However, the exothermic peak obviously shifts to higher temperature as the magnetic field strength increases, from which the magnetic field does not affect the crystal growth but nucleation. On the basis of the assumption that there is an intermediate state between a crystal nucleus and a liquid atom, one possible reason for the shift of exothermic peaks is that kinetic barrier of nucleation is lowered and nucleation is activated by magnetic fields.     

Thermomechanical studies on phase transition of KNO2-H3BO3 binary

Thermomechanical properties like thermal expansion (α_c) and electrostriction along with ΔH and transition temperature are reported for the KNO₂-H₃BO₃ binary phase. The transition is classified as second order phase transition. Some of these properties were compared with those of its pure compound. On the basis of thermodynamic properties it is predicted that binary phase is more stable rather than its pure phase.     

Analysis of the precipitation process of secondary phase MgNi2.5B2 and superconducting properties in Ni-doped MgB2 bulk

A polycrystalline MgB₂ sample with 4 at.% Ni powder addition has been synthesized by a solid-state reaction at 750 °C. Two different Ni powders, raw (40 μm) and ball-milled (10 μm) particles were selected to clarify the influence of the Ni particle size on the secondary phase MgNi_2.5B₂ formation and the critical current density J_c of MgB₂ bulks. According to the investigation of the sintering, it was found that the smaller Ni particles may significantly decrease the formation temperature of the MgNi_2.5B₂ ternary compound to 550 °C. Combined with scanning electron microscopy (SEM) analysis and magnetic measurements, it was suggested that the addition of fine Ni particles led to MgNi_2.5B₂ inclusion precipitating along the spiral terraces of MgB₂ grains and with small particles sizes. The unique microstructure is responsible for the smaller reduction in J_c, which partly eliminates the effect of weak links compared with the coarse Ni particles. Besides, the formation mechanism of the microstructures and the relative location of MgNi_2.5B₂ varying from cluster at the MgB₂ grain boundaries to a screw arrangement around the MgB₂ grains with the decrease of the Ni grain sizes were also discussed in detail.     

(14-x)SrCO3-xCaCO3-24CuO系统低于1000℃固态反应过程中物相的演变

详细报告了采用粉末x射线衍射(XRD)和差热分析(DTA)技术研究(14-x)SrCO3-xCaCO3-24CuO系统在环境压力下低于1000 ℃温度范围内固态反应过程中物相的演变的结果.根据实验,当钙的掺入量x=8.4,11.2,13.6和14时,该系统的固态反应过程比较复杂,1000 ℃下得到的样品都是多相物;在945 ℃附近新观测到了一个尚未鉴定的相,它与其DTA曲线上第三个吸热峰相吻合.对于x=0,0.4,2.8和5.6的样品,其固态反应的规律非常相似:     

色氨酸配位聚合物的热重和差热分析

氨基酸类聚合物材料具有良好的生物相容性,可通过生物降解来调控药物释放.研究其性质可为色氨酸配位聚合物材料的制备和应用提供重要参考.对L-色氨酸和铁(Ⅱ)、钴(Ⅱ)、镍(Ⅱ)、锌(Ⅱ)、镉(Ⅱ)、锰(Ⅱ)等过渡金属离子制备的6种M (Ⅱ) -DL-Trp混配型色氨酸配位聚合物进行了热稳定性研究.其热稳定性次序符合埃文-威...     

Excellent Jc in the low-temperature sintered MgB2 superconductors consisted of uncompleted MgB2 phase and residual Mg

Though low-temperature sintering of MgB₂ superconductors below Mg melting point can effectively depress volatility of Mg and increase sintering density, its development was limited in recent years for the reason that it usually took very long time to form complete MgB₂ phase with excellent J_c at low temperature. In present work, significantly improved J_c was surprisingly obtained in the MgB₂ samples sintered at 575 °C for only 5 h after short-time ball milling, even though formation of MgB₂ phase is not completed and lots of residual Mg is still present in these samples. The grain connectivity in prepared samples is obviously improved compared to referred MgB₂ sample sintered at high temperature, which is responsible for the improvement of J_c. The method developed in present work seems bring a new opportunity for the development of low-cost practical MgB₂ superconductors with improved J_c without using expensive nanometer-size dopants or high-temperature sintering.     

Ferroelectric phase transition temperatures of self-flux-grown RbTiOPO4 crystals

A series of RTP crystals have been grown by the top-seeded solution growth method from self-fluxes containing different [Rb]/[P] atomic ratios (1.25–2) and solute concentrations (0.55–1.1 g-RTP/g-flux) covering a large portion of the RTP crystallization field in the Rb₂O–P₂O₅–TiO₂ ternary system. The Curie points of as-grown crystals have been determined using the standard dielectric (capacitance) technique, and they show a spread from 770 to 800 °C. This indicates a clear correlation between the crystal stoichiometry and the flux chemical composition in similarity with KTP crystals. The major difference from the KTP case is that each growth sector (solidified simultaneously) in nominally pure RTP crystals exhibits its specific Curie temperature. Chemical analysis and intentional doping experiments show that trace impurities may have a very limited influence on this effect. The abrupt changes of the Curie temperatures along simultaneously grown surfaces are discussed in terms of the diverse incorporation mechanisms of the stoichiometric components or native defects into specific crystallographic faces during growth.     

Comparative study of dielectric properties of K2ZnCl4 and K2Zn1−xFexCl4 in the vicinity of incommensurate-commensurate phase transition

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Tetragonal to cubic phase transition in silver chlorate

Silver chlorate crystallizes in the tetragonal system at room temperature. The crystal undergoes a reversible first order phase tronsition to a cubic phase at 139°C. The transition temperature along with the enthalpy change was determined by thedsc technique and the change of enthalpy was found to be 3·12 cal/g. The high temperature form as determined by x-ray powder diffractometric method was found to be primitive cubic with a lattice constant of 6·681 Å at 145°C.     

Phase transition in copper(II) pyrovanadate

A new phase Cu₂V₂O₇ synthesized, exhibits phase transitions between 475°C and 500°C. These phase transitions are reversible with ease in contrast toα →β phase transition at 712°C of Cu₂V₂O₇ phase reported earlier. These phase transitions are identified by DTA technique and characterized by detailed XRD investigations at different temperatures. The crystal structures of these Cu₂V₂O₇ phases are related to either thortveitite (Sc₂Si₂O₇) type or a modification of it.     

A simple differential thermal analysis technique for determining the pressure dependence of solid-liquid and solid-solid transition temperatures

A simple differential thermal analysis technique for determining solid-liquid and solid-solid transition temperatures as functions of pressure is described. The important feature of the technique is the use of a thin-walled ptfe tube both to contain the sample and to transmit pressure to it. The method has been tested at temperatures in the range 200–310 K at pressures of up to 350 MPa. Transition temperatures as functions of pressure are reported for benzene, carbon tetrachloride, and acetonitrile and compared with literature data.     

Growth and electro-optic studies in mixed (NH4) x K1−x H2PO4 single crystals

Single crystals and mixed crystals of KH₂PO₄ (KDP) and NH₄H₂PO₄ (ADP) were grown with different dopant concentrations of NH₄H₂PO₄ in KH₂PO₄ in solution by Holden’s rotary crystallizer technique. The effect of additives like Borax (Na₂B₄O₇·10H₂O), seed crystal rotation rate and qualities of the crystals were studied. The half-wave voltages (in longitudinal mode) for KDP mixed with 1% ADP (by weight) were found and hence the unclamped (low frequency) electro-optic coefficients (r ₆₃) were calculated for various wavelengths in the visible region of the spectrum. It was noted that the half-wave voltage increases with increase in wavelength and temperature.     

纳米SiO2制备时相转变研究

以稻壳为原料,采用两步酸性预处理法制备纳米SiO2粉末,利用SEM、HRTEM和EDS等检测手段对纳米SiO2进行表征分析,结果表明,焙烧温度<570℃时,制备试样为原生的非晶态纳米SiO2,随着焙烧温度的升高,出现微晶,SiO2晶化程度越来越大,焙烧温度为1080℃时,非晶态纳米SiO2转化为柱形晶态纳米SiO2,并对其相转变机理作了探讨。     

GaAs(001)薄膜的表面形貌相变和表面重构

从原子级平坦的 GaAs(001)-β2(2×4)重构表面出发,结合 Reflection High Energy Electron Diffrac-tion(RHEED)衍射图像演变和不同尺度的Scanning tunneling microscope(STM)实空间扫描图像,获取GaAs(001)薄膜表面形貌相变和...     

Ti离子辐照诱发焦绿石Gd2Zr2O7相变的电子显微研究

通过高分辨和剖面电子显微技术研究了离子注入在单晶焦绿石Gd2Zr2O7中的产生的结构相变.在单晶Gd2Zr2O7中注入200keV Ti+离子,注量为1×1017ions/cm2,注入温度为常温.电子显微研究表明,1×1017ions/cm2的Ti+离子注入没有使样品非晶化,而是诱发了单晶Gd2Zr2O7从焦绿石结构转变成萤石结构.从高分辨像和选区电子衍射花样发现Gd3+和Zr3+发生了阳离子无序.重离子入射产生的级联碰撞致使阳离子无序被认为是相变发生的物理机制.