Effect of boron on the crystallization of amorphous Si–(B–)C–N polymer-derived ceramics

Amorphous Si–(B–)C–N polymer-derived ceramics (PDCs) with a boron content ranging from 0 to 8.3 at.%, were synthesized by thermolysis of boron-modified poly(methylvinylsilazane). Correlation of the boron content and the thermal stability of these materials in the course of annealing were investigated using high temperature thermal gravimetric analysis (HT-TGA). Furthermore, the initial crystallization of the as-thermolyzed amorphous ceramics was studied by X-ray diffraction (XRD) measurements. The increase of boron content promotes the crystallization of SiC, and inhibits the crystallization of Si₃N₄. Moreover, the ratio of α-Si₃N₄/β-Si₃N₄ in crystalline ceramic decreases with increasing boron content. Thermodynamic modeling proves the influence of boron content on driving force for crystallization. The available thermodynamic model of amorphous Si–C–N domains, nano-crystalline silicon nitride, and nano-crystalline silicon carbide treated as the separated phases has been used to interpret these results.     

Magnetic properties of nanoparticles glass–ceramic rich with copper ions

The current study aimed at the assessment of CuO gradual addition on the crystallization behavior and magnetic properties of ferromagnetic glass ceramic in the system Fe₂O₃·CaO·ZnO·SiO₂. Ferromagnetic glass–ceramics with a high quantity of magnetite were designed to be crystallized in the system Fe₂O₃·CaO·ZnO·SiO₂. The influences of gradual addition of CuO and melting temperature on the sequence of crystallization and magnetic properties were studied. The X-ray diffraction patterns showed the presence of nanometric magnetite crystals in a glassy matrix after direct cooling from the melt without any additional heat treatments. Increasing the melting temperature resulted in an increase in the magnetite crystallization. The addition of up to 20 g CuO/100 g batch composition revealed a decreasing effects in both endo- and exothermic values, whereas, the same values were inversely increased with increasing the CuO addition to 30 g. In general, increasing the CuO amounts greatly enhanced the crystallization of magnetite. A significant amount of delaffosite (CuFeO₂) was unexpectedly detected and increased by increasing both the added amount of CuO and the treatment temperature. The XRD results detected some traces of cuprite (Cu₂O) in the samples of high CuO content. The TEM results reflected the precipitation of nano-magnetite crystals in the 2–10 nm size range. Magnetic hysteresis cycles were analyzed using a vibrating sample magnetometer with a maximum applied field of 20 kOe at room temperature in quasi-static conditions. From the obtained hysteresis loops, the saturation magnetization (Ms), remanence magnetization (Mr) and coercivity (Hc) were determined.     

Effect of neodymium oxide on the solubility of MoO3 in an aluminoborosilicate glass

High molybdenum and rare earth concentrations in soda-lime aluminoborosilicate glasses may lead to the crystallization of molybdenum-rich phases such as alkali and alkaline-earth molybdates (Na₂MoO₄, CaMoO₄) and also rare earth (RE)-rich phases such as apatite (Ca₂RE₈(SiO₄)₆O₂) during melt cooling that must be controlled particularly during the preparation of highly radioactive nuclear glassy wasteforms. To understand the effect of neodymium addition (from 0 to 16 wt.% Nd₂O₃) on the phase separation and the crystallization tendency of molydbate phases and on the structure of a Mo-bearing nuclear glass belonging to the SiO₂-B₂O₃-Al₂O₃-Na₂O-CaO-MoO₃ system, crystallization and structural studies have been performed by X-ray diffraction, scanning electron microscopy, electron microprobe analysis, Raman and optical absorption spectroscopies. The results obtained show that the addition of an increasing amount of Nd₂O₃ induces a significant increase of the solubility of molybdenum in the glass, characterized by a decrease of the phase separation and of the crystallization tendency of molybdate phases. The increase of chemical disorder in the structure of Mo-bearing glasses when Nd₂O₃ is added - and more precisely in the depolymerized regions where Nd³⁺ cations and [MoO₄]²⁻ entities are located - could be at the origin of the evolution of the molybdenum solubility in the glass.     

Complex crystallization mode of amorphous/nanocrystalline composite Al86Ni2Co5.8Gd5.7Si0.5

Kinetics of the first crystallization stage of Al₈₆Ni₂Co_5.8Gd_5.7Si_0.5 amorphous alloy and structure of the partially crystallized specimens were investigated by measurements of electrical resistance, X-ray diffraction and transmission electron microscopy. The presence of Al nanocrystals and eutectic colonies consisted of mutually oriented crystals of Al and metastable phase was found. The transient nucleation and slowing-down diffusion-limited growth and the interface-controlled growth of the quenched-in nuclei were identified as mechanisms of formation of the Al nanocrystals and eutectic colonies, respectively.     

Crystallization kinetics of the Zr61Al7.5Cu17.5Ni10Si4 alloy using isothermal DSC and TEM observation

The crystallization behavior and microstructure development of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy during annealing were investigated by isothermal differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. During isothermal annealing of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy at 703 K, Zr₂Cu crystals with an average size of about 5 nm were first observed during the early stages (30% crystallization) of crystallization by TEM. The Zr₂Cu crystal size increased with annealing time and attained an average size of 20 nm corresponding to the stage of 80% crystallization. In addition, the change in particle size with increasing annealing time exhibited a linear relationship between grain growth time and the cube of the particle size for the Zr₂Cu type crystalline phase. This indicates that the crystal growth of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy belongs to a thermal activated process of the Arrhenius type. The activation energy for the grain growth of Zr₂Cu is 155 ± 20 kJ/mol in the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ amorphous alloy. The lower activation energy for grain growth in compared to that for crystallization in Zr₆₅Cu₃₅ 440 kJ/mol crystal corresponds to the rearrangement of smaller atoms in the metallic glass, Al or Si (compare to Zr).     

Effects of functionalized graphenes on the isothermal crystallization of poly(L-lactide) nanocomposites

The effects of graphene oxide(GO) with polar groups and functionalized GO(f GO) with nonpolar groups on the isothermal crystallization of poly(L-lactide)(PLLA) were compared. Functionalized GO was obtained by grafting octadecylamine and characterized by FTIR, WAXD and TGA. Isothermal crystallization kinetics of PLLA/GO and PLLA/f GO nanocomposites were investigated by combining DSC data and Avrami equation. The results showed that f GO could improve PLLA crystallization rate more obviously than GO. By analyzing the morphology obtained from POM, SEM and TEM, it was found f GO with large layer space dispersed better in PLLA and supplied more nucleation sites than GO. Therefore, for the multilayer graphene, increasing the layer spaces is important to improve its dispersion in polymers, which will cause the crystal kinetics changing of polymers.     

Effects of nanosilica on crystallization and thermal ageing behaviors of polyethylene terephthalate

The effects of nanosilica(Si O2) on crystallization and thermal aging behaviors of polyethylene terephthalate(PET) have been studied using differential scanning calorimetry(DSC) and polarized optical microscopy(POM), viscometry, tensile testing and scanning electron microscopy(SEM). For non-isothermal and isothermal crystallizations, the crystallization rate of PET increases considerably with increasing content of Si O2 providing a large number of nucleation sites, but the relative crystallinity of the nanocomposites has little differences with that of neat PET. According to POM observation, the nucleation of PET becomes faster and the nucleation density increases significantly with increasing Si O2 content. For PET and its nanocomposites thermally aged at 190 °C, the results of intrinsic viscosity, carboxyl content and tensile test show that the degradation rate of PET is reduced with the addition of a small content of Si O2, but the degradation rate increases with further addition of Si O2, owing to the dual effect of Si O2 on PET degradation.     

流场作用对聚丙烯结晶成核过程和晶体生长过程动力学的影响

通过改进基于构象张量构建的流动诱导结晶理论模型,考虑了流场对结晶生长的影响,对比研究了流场作用对聚丙烯(PP)结晶过程中成核与晶体生长两阶段速率的不同影响程度.理论研究表明,虽然相比成核过程,流场诱导作用对PP晶体生长速率的加速程度有限,但要更为精确地描述流动引起的整个结晶动力学过程,特别是在剪切速率较高且熔体结晶温度较低的情况下,考虑流场对结晶生长速率的影响是十分必要的.研究结果还显示,由于分子链的松弛作用,PP在较低温度下的晶体生长速率受剪切流场的影响要大于较高温度下的影响.本文理论预测结果与实验现象和实验数据的很好吻合亦说明模型改进的合理性和可行性.     

Pressure versus pH phase diagrams of two lysozymes crystallized in agarose gel

The effect of hydrostatic pressure and pH on the crystallization of tetragonal hen lysozyme crystals (HeL, M _r 14,300) and hexagonal turkey lysozyme crystals (TeL, M _r 14,200) in agarose gel was studied. Samples (adjusted to a pH in the range 4–6) were pressurized at 0.1–100?MPa for nine days at 293?K. The morphology and number of crystals as well as protein solubility were analyzed after depressurization. For both proteins and whatever the pH, a higher pressure resulted in greater numbers of crystals and greater solubility values. At any pressure, the solubility and number of crystals were lower at the highest pH. Thus the physical chemical parameters and the outcome of the crystallization process are more affected by pressure at pH 4.0 than at pH 6.0. In the case of HeL, either high pressure or high pH induces a transition from the initial tetragonal form to an orthorhombic one. The observed effects are related to minor differences in the macromolecular structures.