Leucite crystallization kinetics with kalsilite as a transition phase

The leucite crystallization kinetics via kalsilite from the hydrothermally-derived precursor was investigated by XRD and non-isothermal DTA. The leucite precursor was prepared by hydrothermal method from silica sol, aluminum nitrate and potassium nitrate. The crystallization of the precursor experienced two steps. Kalsilite crystallized at first, and then it reacted with the redundant SiO₂ in the precursor and finally the leucite formed. On the DTA curve, the two exothermic peaks were corresponding to the two times of crystallization. Determined by the Kissinger equation, the activation energy of kalsilite crystallization and leucite crystallization via kalsilite was 103(SD 8) kJ/mol and 125(SD 4) kJ/mol respectively and both the crystallizations were nucleation and three-dimensional growth, which follows the diffusion-controlled mechanism with increasing nucleation rate.     

Nucleation kinetics of paramagnetic and diamagnetic metal melts under a high magnetic field

The influence of a high magnetic field (HMF) on the nucleation kinetics of paramagnetic aluminum and diamagnetic zinc melts has been investigated by differential thermal analysis (DTA).It is found that the application of an HMF increases the undercooling of pure aluminum and pure zinc at the same heatingcooling rates.Moreover,the quantitative analysis of activation energy calculated from the DTA results using the Kissinger method manifests that the HMF reduces the activation energy of pure aluminum and pure zinc.Regardless of magnetism,the nucleation frequency under an HMF is higher than that without an HMF.Furthermore,the increase in undercooling under the HMF is mainly attributed to the increase of the contact angle,calculated by the functional relationship between the cooling rate and undercooling.This result is consistent with the increase of the calculated nucleation work for pure aluminum and pure zinc.Additionally,the increase in undercooling caused by the HMF is partly ascribed to the magnetic field-induced suppression of thermal convection in the undercooled melt.     

钠镁铝硅系统微晶玻璃的分相成核行为

本文以20Na2O、4CaO、12MgO、22Al2O3、4TiO2、38SiO2(重量百分比)组成玻璃为基础,制备了具有快速微晶化特性的钠镁铝硅系统微晶玻璃。分别采用差热分析技术(DTA)、X射线衍射技术(XRD)研究了不同热处理条件下玻璃分相成核的动力学、热力学和它们之间的依存关系。玻璃分相成核最大速率时的温度接近于923K。在成核温度的热处理对分相的放热行为有影响,成核导致系统自由能的降低与分相峰面积和峰值温度变化有关。用差热分析仪测量的玻璃分相的放热效应,是快速微晶化玻璃的重要特征,用XRD证实分相放热过程中没有微晶相产生。玻璃的分相过程为成核生长机理,采用修正kissinger方程求得玻璃分相活化能为310.445KJ/mol。     

Electromagnetic vibration process for producing bulk metallic glasses

It is known that the cooling rate from the liquid state is an important factor in the production of bulk metallic glasses. However, the effects of other factors such as electric and magnetic fields have not been thoroughly investigated. Here, we present a new method for producing bulk metallic glasses by using electromagnetic vibrations with simultaneous imposition of an alternating electric current and a magnetic field. This method was found to be effective in enhancing apparent glass-forming ability in Mg65-Cu25-Y10 (atomic percent) alloys. Indeed, larger bulk metallic glasses could be obtained by the electromagnetic vibration process under the same cooling conditions. We presume that disappearance or decrement of clusters by the electromagnetic vibrations applied to the liquid state cause suppression of crystal nucleation. This electromagnetic vibration process should be effective in other bulk metallic glass systems if the clusters in the liquid state cause the crystal nucleation.     

Electrodeposited hydroxyapatite coatings in static magnetic field

Uniform hydroxyapatite (HA) coatings were deposited electrochemically on titanium in magnetic fields. The structure and morphology of the deposited films were investigated by scanning electron microscopy, X-ray diffraction and transmission electron microscopy (SEM, XRD and TEM). It was found that the morphology of HA deposits could be altered by direction and intensity of applied magnetic field. Needle-like crystals formed when magnetic field was applied perpendicularly to electric field (B⊥ J), whereas spherical nanocrystals formed when magnetic and electric fields were in parallel (B||J). In addition, the nucleation rate of the HA crystals was proportional to the magnetic field intensity. Therefore, the resultant crystal size decreased with increasing magnetic field intensity.     

The effect of the angular dependence of the particle nucleation field on the magnetic recording processes

The magnetic recording process is examined by using a model which easily clarifies the role of the particle nucleation field. An inverse cosine and constant angular dependence of the nucleation field are utilized to encompass measured single particle switching behavior. The model neglects demagnetization fields and is applied to contact recording on thick media. The primary conclusion is that the angular dependence affects the output of longitudinally well-oriented particles by raising the voltage maximum approximately 5 dB above that for a constant nucleation field. Vertically oriented particles exhibit large maximum voltages independent of the nucleation field angular dependence.     

Crystallization Behaviour and Nanostructuring in Alkali Niobiosilicate Glasses

23K2O·27Nb2O5·50SiO 2(KNS),13K2O·10Na2O·27Nb2O5·50SiO 2(KNaNS) and 15K2O·12Li2O·27Nb2O5·46SiO2(KLiNS) transparent glasses were synthesized by melt-quenching technique,and studied by differential thermal analysis(DTA),X-ray diffraction(XRD) and high-resolution transmission electron microscopy(HRTEM) to reveal the effect of the devitrification behaviour on transparent nanostructure.Just above the glass transition temperature T g in the KNS glass,an unidentified phase was formed,while in KNaNS and KLiNS,mixed-alkali niobate phases with tungsten bronze structure were obtained by bulk crystallization.Heat treatments at T g performed on the KNS glass resulted in the transparent nanostructure with second order harmonic generation(SHG) activity.Heat treatment for 10 h on KNaNS and KLiNS decreased the first DTA exothermic peaks(at least 24C),indicating the bulk nucleation,which was confirmed by the DTA in comparison with the powdered as-quenched samples.KNaNS and KLiNS showed similar XRD profiles as the K3Li2Nb5O15 crystal with the five most intense peaks at 22.7,29.4,32.3,46.3 and 52.0 deg.HRTEM micrograph showed clear-cut nano-sized circular domains and spherical nanocrystals dispersed into the amorphous matrix.     

Magnetic Quantum Tunneling in the Single-Molecule Magnet Mn12-Acetate

The symmetry of magnetic quantum tunneling (MQT) in the single molecule magnet Mn₂-acetate has been determined by sensitive low-temperature magnetic measurements in the pure quantum tunneling regime and high frequency EPR spectroscopy in the presence of large transverse magnetic fields. The combined data set definitely establishes the transverse anisotropy terms responsible for the low temperature quantum dynamics. MQT is due to a disorder induced locally varying quadratic transverse anisotropy associated with rhombic distortions in the molecular environment (2nd order in the spin-operators). This is superimposed on a 4th order transverse magnetic anisotropy consistent with the global (average) S₄ molecule site symmetry. These forms of the transverse anisotropy are incommensurate, leading to a complex interplay between local and global symmetries, the consequences of which are analyzed in detail. The resulting model explains: (1) the observation of a twofold symmetry of MQT as a function of the angle of the transverse magnetic field when a subset of molecules in a single crystal are studied; (2) the non-monotonic dependence of the tunneling probability on the magnitude of the transverse magnetic field, which is ascribed to an interference (Berry phase)effect; and (3) the angular dependence of EPR absorption peaks, including the fine structure in the peaks, among many other phenomena. This work also establishes the magnitude of the 2nd and 4th order transverse anisotropy terms for Mn₁₂-acetate single crystals and the angle between the hard magnetic anisotropy axes of these terms. EPR as a function of the angle of the field with respect to the easy axes (close to the hard-medium plane) confirms that there are discrete tilts of the molecular magnetic easy axis from the global (average) easy axis of a crystal, also associated with solvent disorder. The latter observation provides a very plausible explanation for the lack of MQT selection rules, which has been a puzzle for many years.     

The effect of a magnetic field on the kinetics of melting of ionic crystals

The kinetics of melting of an ionic crystal treated in a pulsed magnetic field (H ∼ 10⁶ A/m) was studied by a computer-aided DTA technique. Significant changes in the nonequilibrium thermodynamic parameters of transient processes during melting and in the enthalpy of melting were observed for KCl crystals, which is evidence that the magnetic field induces a special nonequilibrium state in these nonmagnetic crystals.     

Nucleation and crystal growth in a fly ash derived glass

The devitrification behaviour of a fly ash derived glass, examined by differential thermal analysis (DTA), X-ray diffraction and scanning electron microscopy (SEM), is reported and discussed. The crystallized phases were identified as mullite (3Al₂O₃·2SiO₂) and anorthite (CaO·Al₂O₃·2SiO₂). Kinetic parameters for nucleation and crystal growth were estimated from the DTA curves. The temperature of maximum nucleation rate was 790°C and the activation energy for crystal growth E=370 kJ mol^–1. The crystal morphology was investigated by SEM and the crystal shape found to be consistent with the morphological index n calculated by DTA. The glass-ceramic obtained from a previously nucleated glass showed a fine-grained texture.     

Crystallization kinetics of neodymium disilicate obtained by polymeric xerogels

The kinetics of tetragonal Nd₂Si₂O₇ crystallization from polymeric xerogels was investigated under non-isothermal conditions via differential thermal analysis (DTA). The polymeric xerogels were prepared by the Pechini sol–gel method from tetraethoxysilane and neodymium nitrate. The crystallization fraction and rate of tetragonal Nd₂Si₂O₇ were determined based on the exothermic DTA data. The activation energy of tetragonal Nd₂Si₂O₇ crystallization, which is calculated by the Kissinger equation, is 673 kJ/mol. The crystallization process of tetragonal Nd₂Si₂O₇ could involve the nucleation and three-dimensional growth that follows the diffusion-controlled mechanism when the nucleation rate increased.     

The effects of amorphous phase separation on crystal nucleation kinetics in BaO-SiO2 glasses

The nucleation kinetics of the barium disilicate crystal phase were determined in BaO-SiO₂ glasses containing 25.3 to 33.1 mol % BaO at temperatures from 673 to 807°C, using quantitative optical microscopy. The highest nucleation rates were found in the 33.1 mol % BaO glass close to the stoichiometric BaO·2SiO₂ composition, which was just outside the immiscibility region. Much lower rates were observed in the glasses with lower BaO contents including those exhibiting amorphous phase separation. However phase separation had a marked indirect effect on crystal nucleation. Crystal nucleation was strongly dependent on the composition of the baria-rich matrix phase in the phase separated glasses, so that after amorphous phase separation had occurred at a given temperature the crystal nucleation rates of different glasses tended to converge to similar values. There was no evidence from the present results for a significant enhancement of crystal nucleation rates at the interfaces between the amorphous phases. However, nucleation was significantly influenced by certain impurities, particularly alumina, in the glasses. The early stages of crystallization in the glasses were studied by TEM. Immiscibility temperature measurements and results of DTA and X-ray diffraction are also reported.     

Fluxon Pinning by Artificial Magnetic Arrays

The field dependent transport properties of superconducting niobium films, which are modulated by a regular array of non-magnetic and magnetic normal-metal inclusions (dots), have been investigated. Strong peaks in the critical current are seen for fields at which the vortex density in the superconductor is some rational or sub-rational multiple of the dot density. This commensurate peak effect is enhanced for the magnetic dot arrays when the dots are magnetized in a direction parallel to the applied magnetic field and suppressed when they are anti-aligned. Qualitative information on the strength of this dot-vortex interaction is inferred from the commensurate peaks present and missing for different regular lattices.     

Coping with Brown's paradox: The pinning and nucleation of magnetic domain walls at antiphase boundaries

The critical field for unpinning or nucleation of a domain wall in a ferromagnetic crystal at an antiphase boundary (APB) is calculated as a function of the magnetic coupling constant η across the APB. It turns out that for positive values of η pinning occurs and that for negative values there is very strong pinning and also nucleation.     

玻璃形核与析晶的研究进展

研究玻璃的形核和析晶具有十分重要的科学意义，综述了近二十年来国内外采用热分析法研究玻璃形核与析晶的最新进展，重点分析了玻璃在非等温过程中的析晶动力学，同时也评述了研究玻璃形核与晶体长大速率比较科学的理论方法。     

Studies on the formation of γ-Fe2O3 (maghemite) by thermal decomposition of ferrous oxalate dihydrate

The thermal decomposition of ferrous oxalate dihydrate FeC₂O₄.2H₂O, has been investigated using thermogravimetric (TG), differential thermogravimetric (DTG), differential thermal analysis (DTA), X-ray diffraction and infra-red spectroscopic techniques. The DTA data obtained in air consist of an endothermic dehydration peak at ～ 190? C and two exothermic peaks at ～ 245 and ～ 360? C. The first exotherm corresponds to the primary oxidative decomposition while the second one seems to be due to a structural transformation from a disordered or non-crystalline to a crystalline state ofα-Fe₂O₃. Similar experiments carried out using a nitrogen atmosphere show an endothermic peak ～ 190? C followed immediately by an exothermic peak at 240? C. The final product of this decomposition has been identified as Fe₃O₄. The oxalate dihydrate containing traces of moisture decomposes in air under the ambient of its own gaseous products at ～ 300? C to giveγ-Fe₂O₃. This compound has been characterized by X-ray diffraction and magnetic hysteresis measurements.     

静磁场在材料生产过程中的应用研究评述

在材料电磁过程研究中，静磁场尤其是强磁场材料科学是当今世界的研究热点。本文从静磁场作用下生成的洛仑兹力和磁化力两个角度系统地归纳总结了静磁场技术在材料生产领域的应用原理和实践。对静磁场下的洛仑兹力，主要介绍了流体流动、波动和对流控制、电磁振动及电磁超声波等方面的研究现状；对强磁场下的磁化力，主要介绍了其在相变、结晶配向、磁悬浮、磁对流等方面的研究进展。最后对强磁场材料科学的研究趋势和发展前景做了展望。     

球磨参数对MgNi非晶热稳定性能的影响

采用带高温附件的X射线衍射仪(XRD)、差热分析仪(DTA)等研究了磨球配比、球料比、球磨速度、球磨时间等参数对MgNi样品的结构、热稳定性能的影响及其在加热过程中的相变规律。结果表明MgNi非晶的热稳定性能随着样品中球磨能量的增加而降低。当样品的球磨能量较低时(球磨时间短、球磨速度低、球料比小、磨球配比中大球质量大而小球数量少),非晶样品只有一个放热峰,对应着Mg2Ni和Mg Ni2晶体相的生成;当样品的球磨能量较高时则出现二个放热峰,分别对应着Mg2Ni、MgNi2晶体相的生成。     

Control over nanocrystalization in turbulent flow in the presence of magnetic fields

The influence of the magnetic field and the water flow on the crystal form of calcium carbonate precipitated from low-concentration water solution was followed systematically. By changing the strength of the field, the calcite/aragonite/vaterite ratio varied. The crystal form and the particle size distribution of the precipitated calcium carbonate were determined by using X-ray analyses and transmission electron microscopy (TEM). A simple hydrodynamical model, using the Navier–Stoke's and Maxwell's equations, predicts that there is a strong energy coupling and transfer between turbulent flow and the magnetic fields, which can be amplified to high values. Since the formation of aragonite is enhanced in the presence of magnetic field, scaling is prevented in turbulent flow.     

Spherical particles of phenolic resin treated with iron oxide

This paper describes the preparation and characterization of phenolic resins’ thermospheres covered by a magnetic phase of iron oxide. The thermospheres were prepared by allowing phenol and formaldehyde to react under dispersion polymerization conditions and the iron oxide phase was incorporated in situ onto the phenolic resin particles by adding concentrated NH₃ to FeCl₂ in DMSO. This reaction was conducted at 70 °C under nitrogen atmosphere in a controlled temperature vessel, and the modified resin was isolated and dried in vacuo. Both pure and modified resins were characterized by DRX, TG-DTA, and MEV/EDX. The modified particles were attracted by a magnetic field, indicating the fixation of magnetic iron oxide. No diffraction peaks were observed in DRX analysis; thermal analysis (DTA) of both pure and modified resins presented exothermic events between 300 and 680 °C, and 300 and 570 °C, respectively, indicating the microstructure of the resin was modified after the treatment. Thermogravimetric analysis (TGA) of the pure resin registered a 2.0% residue, compared to 8.0% for the modified resin. These residues correspond to about 7.0% of fixed iron oxide. MEV/EDX analyses confirm the modification of the resins by the process of fixing iron oxide.