Magnetic susceptibility study on Li-Mn spinel oxides

Li-Mn spinel oxide particles were prepared by the solid state reaction using Mn₃O₄ particles and Li-hydroxide monohydrate, and the effects of the reaction temperature on the produced phase were studied with the powder X-ray diffraction and the magnetic susceptibility measurement. From the lattice constant variation versus the reaction temperature, it was found that the spinel oxides prepared by low temperature reaction were cation deficient type, and that the cation deficient degree decreased with an increase in the reaction temperature. The asymptotic Curie temperature (Weiss constant), derived from the temperature dependent magnetic susceptibility, was always negative, and it implied that the antiferromagnetic coupling among Mn ions is dominant. The absolute value of the Weiss constant increased with the reaction temperature, took a maximum at the reaction temperature around 750°C, and then decreased with a further increase in the reaction temperature. These were attributed to the variation of the average valence of Mn ions. At the reaction temperature, where the absolute value of the Weiss constant takes a maximum, the non-stoichiometry of Li-Mn spinel oxide becomes minimum. In the specimen prepared above the temperature, it was likely that there existed oxygen vacancy, and the phase transformation between cubic and orthorhombic structures was found in the DSC curves.     

In situ synthesis of TiC composite for structural application

In situ TiC reinforced Al composite has been synthesized from the reaction between Ti and activated carbon. The size of in situ TiC is in the range from sub-micron to a maximum of 1.7 μm. Degree of reaction between Ti and activated carbon increases with increasing reaction temperature and amount of carbon. At low reaction temperature, the tensile strength of the composite is low as a result of the presence of unreacted activated carbon retained in the material. The retained carbon can be reduced by increasing reaction temperature. It has been observed that at low reaction temperature formation of Al₃Ti is a dominant phenomenon while at high reaction temperature Al₃Ti is not stable. Along with its disappearance, formation of TiC becomes favorable.     

聚酯切片结晶熔融粘结与固相缩聚工艺优化

差示扫描量热(DSC)实验研究了聚酯切片在不同的结晶温度等温结晶样品的升温熔融行为。发现结晶温度升高,结晶熔融温度升高,切片的抗粘结温度升高;固相缩聚过程是结晶与反应同时发生的过程,反应时间延长,结晶层分布变窄,结晶熔融温度升高,且逐渐与升温熔融峰重合在一起;多段升温反应的方式,可逐步提高熔融粘结温度,避免粘结,提高反应速度。利用这种优化的反应方式,可缩短反应时间,特别适用于固相缩聚法生产高分子量(η1.0dL/g)聚酯。     

氰酸酯/环氧树脂CE-40体系的固化反应

采用差示扫描量热(DSC)方法计算得到氰酸酯/环氧树脂CE-40体系反应动力学常数,分析了温度和浓度对反应速率的影响,同时结合红外光谱对各温度段发生的反应进行了推断。研究表明,CE-40体系有两个反应温度区——低温区(150℃-180℃)和高温区(200℃-250℃)。低温区是环氧与氰酸酯直接反应,产物为三聚氰酸酯和口恶唑啉,反应速率对浓度敏感,随反应物浓度下降而降低;高温区是低温区的生成物与环氧的进一步反应,主要产物为口恶唑烷酮和异氰酸脲,反应温度决定了反应速率。     

Effect of compact density and preheating temperature of the Al–Ti–C preform on the fabrication of in situ Mg–TiC composites

Magnesium reinforced in situ TiC particulates was successfully synthesized by utilizing the self-propagating high temperature synthesis (SHS) process. The result showed that preform temperature and compact density have effects on the SHS reaction. It is observed that when the compact density was below 68% of the theoretical density, no SHS reaction occurred. However, with an increase in density from 68 to 72%, the successful thermal explosion reaction was observed in the Mg melt. Besides this, the effect of preheat temperature on the fabrication of Mg/TiC composite was extensively studied and found that the preheat temperature below 300 °C failed to give rise to SHS reaction. However, the preheat temperature of 450, 500, and 550 °C favors the reaction inside the liquid melt, but the temperature of 600 °C leads to the ignition reaction in the preheating furnace itself. SEM and EDX study confirms fine distribution of TiC in the matrix.     

聚N-异丙基丙烯酰胺水凝胶的微波合成与性能

以水为溶剂,N-异丙基丙烯酰胺(NIPAM)为单体,N,N-亚甲基双丙烯酰胺(Bis)为交联剂,分别通过微波合成法和水浴加热法制备了聚N-异丙基丙烯酰胺(PNIPAM)水凝胶,研究了交联剂浓度、反应时间、反应温度等对反应的影响,及PNIPAM水凝胶在不同温度和pH值下的溶胀性变化。研究结果表明,与水浴加热法相比,微波合成法缩短了反应时间,从10h缩减至1h左右;适当延长反应时间和提升反应温度有利于提高单体转化率。微波法合成PNIPAM的最佳条件为:H2O为溶剂,100℃~110℃反应40 min~60 min,交联剂m(Bis)/m(NIPAM)=5/100,单体转化率97%~98%。而且,微波合成法制备的水凝胶具有更显著的温度和pH敏感性能。     

Effects of chemical reaction on free convection flow through a porous medium bounded by a vertical surface

The effect of a chemical reaction on a free convection flow through a porous medium bounded by a vertical infinite surface has been studied. Velocity, temperature, and concentration profiles have been obtained for different values of parameters like the Grashof number, Prandtl number, and the chemical reaction parameter in the presence of homogeneous chemical reaction of first order. It is observed that the velocity and concentration increase during a generative reaction and decrease in a destructive reaction. The same is true for the behavior of the fluid temperature. The presence of the porous media diminishes the temperature.     

DSC法研究不同邻对位比值酚醛树脂的固化行为

利用差示扫描量热(DSC)分析技术研究了不同邻对位比值(O/P值)的酚醛树脂的固化活化能及固化进程。采用Ozawa模型求解了不同O/P值酚醛树脂的固化活化能及固化动力学参数,结果表明,酚醛树脂的固化活化能随其O/P值的增加而降低,固化进程变得更加容易。固化反应过程中的Ozawa反应指数n0随着固化反应温度的升高而下降,并且随着树脂O/P值的增加,n0出现在更低的温度区间内。Ozawa反应速度常数k0在固化反应初期随温度的升高而增加,在反应后期则随着温度的升高而下降,并且随着树脂O/P值的增加,k0出现峰值对应的温度降低,表明固化先后经历了微凝胶增长与扩散反应控制两个阶段,树脂O/P值的增加加快了微凝胶的增长进程。     

Evaluation on thermal explosion induced by slightly exothermic interface reaction

An asphalt-salt mixture (ASM), which once caused a fire and explosion in a reprocessing plant, was prepared by imitating the real bituminization process of waste on a lab scale to evaluate its actual thermal hazards. Heat flux reaction calorimeters were used to measure the release of heat for the simulated ASM at a constant heating rate and at a constant temperature, respectively. Experimental results show that the reaction in the ASM below about 250 degrees C is a slightly exothermic interface reaction between the asphalt and the salt particles contained in the asphalt, and that the heat release rate increases sharply above about 250 degrees C due to melting of the salt particles. The reaction rates were formulated on the basis of an assumed reaction model, and the kinetic parameters were determined. Using the model with the kinetic parameters, temperature changes with time and drum-radius axes for the ASM-filled drum were numerically simulated assuming a one-dimensional infinite cylinder system, where the drum was being cooled at an ambient temperature of 50 degrees C. The minimum filling temperature, at which the runaway reaction (MFTRR) can occur for the simulated ASM in the drum is about 194 degrees C. Furthermore, a very good linear correlation exists between this MFTRR and the initial radius of salt particles formed in the bituminization product. The critical filling temperature to the runaway reaction is about 162 degrees C for the asphalt-salt mixture, containing zero-size salt particles, filled in the same drum at an ambient temperature of 50 degrees C. Thus, the runaway reaction will never occur in the drum filled with the asphalt-salt mixture under the conditions of the filling temperature below 162 degrees C and a constant ambient temperature of 50 degrees C. As a consequence, the ASM explosion occurred in the reprocessing plant likely was due to a slightly exothermically reaction and self heating.     

固相缩聚与PET的熔融性能

应用差示扫描量热法（ＤＳＣ）和广角Ｘ射线衍射（ＷＡＸＤ）研究了低温干燥高温固相缩聚ＰＥＴ的熔融行为和结晶结构。根据ＤＳＣ结果分析了固相缩聚反应温度和时间对熔融双峰的影响,得到熔点与固相缩聚反应温度有线性关系,熔点和结晶度与相缩聚反应时间的对数呈线性关系。利用ＷＡＸＤ研究了固相缩聚反应温度和时间对表观晶粒尺寸的影响。结果说明低温结晶时形成的不完善晶体在固体缩聚过程中发生部分熔融、重结晶,结晶结构趋于     

玻璃纤维/环氧树脂泡沫夹层板的热解动力学

使用DTG-60(AH)热重分析仪分析了玻璃纤维/环氧树脂泡沫夹层板在不同升温速率和不同氧含量条件下的热分解特性。结果表明,在空气中玻璃纤维/环氧树脂泡沫夹层板的热分解反应可分为三个阶段。随着升温速率的提高,热分解反应的初始反应温度、终止反应温度以及最大质量损失速率温度均向高温方向移动。氧含量的降低对热分解的第三阶段有较大的影响。采用Flynn-Wall-Ozawa法和Starink法进行热解动力学分析,得到玻璃纤维/环氧树脂泡沫夹层板的表观活化能。     

固相缩聚与PET的熔融性能

应用差示扫描量热法（ＤＳＣ）和广角Ｘ射结衍射（ＷＡＸＤ）研究了低温干燥高温固相缩聚ＰＥＴ的熔融行为和结晶结构。根据ＤＳＣ结果分析了固相缩聚反应温度和时间对熔融双峰的影响,得到熔点与固相缩聚反应温度有红性关系,熔点和结晶度与固相缩聚反应时间的对数呈线性关系。利用ＷＡＸＤ研究了固相缩聚反应温度和时间对表观晶粒尺寸的影响。结果说明低温结晶时形成的不完善晶体在固相缩聚过程中发生部分熔融、重结晶,结晶结构趋     

聚乙烯醇缩间硝基苯甲醛的合成与热稳定性

以聚乙烯醇和间硝基苯甲醛为原料,对甲苯磺酸为催化剂,合成了聚乙烯醇缩间硝基苯甲醛(PVMNB),探讨了反应物物料比、反应温度、反应时间对PVMNB缩醛度的影响,获得适宜的工艺条件为:PVA(-OH)和间硝基苯甲醛的物质的量比为2:1,反应温度100℃,反应时间9 h,此时PVMNB为86%.通过衰减全反射-傅里叶红外光...     

碱熔法从硼精矿中提取硅的研究

以硼镁铁矿磁选得到的硼精矿为原料,通过单因素实验和正交实验,采用碱熔法研究硼精矿在提取硅过程中焙烧温度、焙烧时间、碱矿比对硅提取率的影响。结果表明,实验过程中各个因素对硅提取率的影响由大到小的顺序依次为碱矿比、焙烧温度、焙烧时间;最佳的实验条件为:焙烧温度550℃,焙烧时间30min,碱矿比4∶1。在最佳条件下,硅提取率可达到92%以上,硼和硅元素以可溶盐的形式富集在提取液中,而铁和镁等元素则富集在渣中,实现了硼、硅与铁、镁的分离。     

空心玻璃微球表面接枝聚苯乙烯及其性能研究

空心玻璃微球表面改性有利于改善其与基体间的相容性,提高复合泡沫性能.通过接枝聚苯乙烯对空心玻璃微球进行表面处理,采用SEM、IR、热失重等方法分析了反应条件对接枝的影响.结果表明,接枝反应过程中提高反应温度、延长反应时间有利于接枝率的提高,但温度过高会加速单体自聚,降低接枝率.接枝处理后,随着聚合物层的增厚,空心玻璃微球表面缺陷减小,破损率降低,抗压强度得到一定程度提高.     

Effect of Mechanical Activation on the Packed-Bed, High-Temperature Behavior of Hematite and Graphite Mixture in Air

The present study reports the effect of mechanical activation on the reaction behavior of the Fe₂O₃/C powder mixture at high temperature under air atmosphere. Hematite and graphite were ground up to 150 hours using a ball mill with an alumina vial. The mixture was heated isothermally in the temperature range of 1173-1373°K using an electric furnace. The degree of reaction was determined by weight-loss measurement using a high accurate balance. It was found that low-energy mechanical milling at room temperature increases (the degree and) the rate of reaction at constant temperature. However, this effect was more significant at temperatures above 1273°K. At temperatures below 1273°K, the main reaction is oxidation of graphite and the total reaction process is controlled by diffusion of gases, whereas above 1273°K both chemical reaction (gasification reaction) and diffusion were controlling mechanisms. However, increasing milling time would shift the controlling mechanism from diffusion toward pure chemical reaction above 1273°K. It was observed that the mechanical milling might cause the mechanism to be changed at lower temperatures. This could be attributed to the increase of the rate of reaction due to mechanical milling. It was also observed that milling of powder mixture would decrease the difference in the average reaction rates at various degrees of reaction.     

高温多元醇法制备超顺磁CoFe2O4纳米颗粒磁共振造影剂

以FeCl₃·6H₂O、CoCl₂·6H₂O和HOOC-PEG-COOH为反应物, 利用高温多元醇法制备了核心粒径为5~10nm的超顺磁CoFe₂O₄纳米颗粒, 样品在水溶液中具有良好分散性. 通过改变修饰剂的种类和用量、反应温度及反应时间可以对纳米颗粒的尺寸、水中分散性及磁性能产生影响. 研究表明：选用带有强极性基团的修饰剂, 增加修饰剂的用量, 提高反应温度和延长反应时间, 可以增大颗粒的尺寸, 改善颗粒的分散性, 窄化粒径分布. 实验获得的最佳生长条件为：金属盐总量与修饰剂质量比为1∶10, 在210~220℃之间反应2h. 磁性能研究表明所得样品在室温下具有超顺磁性, 其饱和磁化强度与尺寸有关.     

Effects of reaction temperature on size and optical properties of CdSe nanocrystals

We report experimental results on the reaction temperature dependence of luminescence properties in size-controlled CdSe nanocrystals. Such reaction temperature dependent property is also sizedependent. The diameter of the CdSe nanocrystals is tuned from 4–11.0 nm by varying the reaction temperatures. The growth process and characterization of CdSe nanocrystals are determined by photoluminescence (PL) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, X-ray photoelectron spectrometry (XPS), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The influence of reaction conditions on the growth of CdSe nanocrystals demonstrates that low reaction temperature is favourable for the formation of high quality CdSe nanocrystals.     

制备工艺对原位合成Al3Ti/Al复合材料显微组织的影响

以工业钛剂和纯铝为反应物,采用熔体反应法原位合成了Al3Ti/Al复合材料,并采用XRD、SEM、EDS等手段研究了反应温度、稀土元素Ce、浇注温度等制备工艺因数对增强相Al3Ti形貌及尺寸的影响,探讨了相应的影响机制.结果表明,随着反应温度升高,Ti原子溶解扩散越快,Al3Ti分布更均匀;提高浇注温度和加入Ce后,A...     

不同温度下硒化镉(CdSe)量子点的生长及荧光性研究

研究了以氧化镉（CdO）和硒（Se）粉为前驱体,在三辛基膦（TOP）和油酸中合成无机半导体量子点（quantum dots, QDs）CdSe.研究了在不同的反应温度下粒子的生长,通过紫外吸收光谱（UV-Vis）、荧光发射光谱（PL）、透射电子显微镜（TEM）等手段跟踪反应过程并对样品性能进行了表征.实验结果表明,反应温度和反应时间对量子点的生长和荧光性能有很大的影响.