Nucleation and Crystallization of a Lead Halide Phosphate Glass by Differential Thermal Analysis

The nucleation and crystallization mechanisms of a lead halide phosphate glass [40P₂O₅·30PbBr₂·30PbF₂ (mol%)] were investigated by differential thermal analysis (DTA) and X-ray diffraction analysis. There were two crystalline phases in the crystallized samples: the major phase was PbP₂O₄, and the minor phase was PbP₂O₆. The average activation energy for crystallization, E , for two different particle sizes of this glass was determined to be 119 ± 4 kJ/mol by the Kissinger method and 124 ± 4 kJ/mol by the Augis–Bennett method. The Avrami constants were determined to be 1.6 and 2.5 for particle sizes of 203 and 1040 μm, respectively, by the Ozawa equation, and 1.7 and 2.4 for particle sizes of 203 and 1040 μm, respectively, by the Augis–Bennett equation. The decrease in the crystallization peak height in the DTA curve with increasing particle size suggested that the particles crystallize primarily by surface crystallization. A nucleation-rate type curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T _p, or the height of the crystallization peak, (δ T )_p, as a function of nucleation temperature, T _n. The temperature where nucleation can occur for this glass ranges from 360°–450°C and the maximum nucleation rate is at 420°± 10°C.     

Nucleation in Lithium Disilicate Glass: A Test of Classical Theory by Quantitative Modeling

Published data on the kinetics of crystal nucleation in lithium disilicate glass are analyzed using a numerical simulation based on classical nucleation theory. With a reasonable set of materials parameters, a quantitative fit is possible to all measured data, including those showing transient behavior in single-stage or multistage anneals. The fitting constitutes a comprehensive test of classical nucleation theory, which, even in the simple form used here, is found to be capable of quantitative prediction. The numerical simulation will be of use in developing heat treatments for glassceramics.     

Nucleation and Crystallization of Na2O · 2CaO · 3SiO2 Glass by Differential Thermal Analysis

The nucleation and crystallization of the Na₂O · 2CaO · 3SiO₂ (NC₂S₃) glass were studied by differential thermal analysis (DTA), and a (nucleation rate—temperature)-like curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T_p , or the height of the crystallization peak, (δ T ) _p , as a function of nucleation temperature, T_n. The temperature where nucleation can occur for this glass ranges from 550° to 650°C and the temperature for maximum nucleation is 600°± 5°C. Both temperatures are in excellent agreement with those determined by the classical technique of nucleation followed by isothermal crystallization. The activation energy for crystallization, E_c , for this glass is the same for surface and/or bulk crystallization, and is 370 ± 15 kJ/mol. The analysis of the crystallization data with the Kissinger equation yelds the correct value for E_c only when crystal growth occurs on a fixed number of nuclei. When a majority of the nucleation occurs during the DTA measurements, a modified Kissinger equation must be used to calculate E_c . E_c is also independent of the heating rate when determined using a single-crystallization-peak analysis technique. The single-peak analysis technique is useful for a rapid determination of E_c or when only a small amount of sample is available.     

Interpretation of DTA Experiments Used for Crystal Nucleation Rate Determinations

An analysis is presented of two schemes which have been proposed for the determination of the temperature dependence of homogeneous crystal nucleation rates in glasses via DTA measurements. The first method is based upon the postulate that the inverse of the temperature at which the DTA crystallization rate is maximum, T ¹_p, will increase monotonically as the number density of nucleated particles increases. The second method is based on the observation that the intensity at T _p (peak height) increases as T _p grows. The validity of both of these methods is assessed for inorganic glasses for two specific crystal growth models.     

Crystallization Kinetics of a Lithia–Silica Glass: Effect of Sample Characteristics and Thermal Analysis Measurement Techniques

The crystallization kinetics of a 40Li₂O · 60SiO₂ (mol%) glass were studied using DTA and both isothermal and nonisothermal DSC techniques as a function of (1) particle size of the glass powder, (2) crucible material, e.g., alumina or platinum, (3) furnace atmosphere, e.g., nitrogen, oxygen, or argon, and (4) surface pretreatment of the glass powder with deionized water, HCl, or HF. The kinetic parameters for crystallization, namely, the activation energy, E , frequency factor, v , and Avrami exponent, n , for a particular sample particle size are in close agreement for all three techniques. E and v both decrease while n increases (from 0.9 to 3.0) as the particle size of the glass powder increases from 40 to ∼250 μm, whereupon these parameters become fairly constant for larger particles. Neither the furnace atmosphere nor crucible material had a significant effect on E , v , or n. Washing the glass with deionized water, HCl, or HF before the measurements decreased the crystallization temperature by 25° to 30°C. E and v remain unchanged by this treatment, but n decreased from 3 to 1.6 for a sample of particle size ∼250 μm.     

改性多孔氧化硅材料的差热性能分析

采用溶胶凝胶+超临界干燥法制备了纳米氧化硅气凝胶块体,用KH550和氟硅烷对其进行了表面改性,利用DTA分析了两种改性剂对多孔氧化硅的接枝特性.结果表明,KH550与氟硅烷相比,可与气凝胶表面接枝反应良好,降低表面羟基含量以及对空气中水分的吸附,从而在超临界干燥之后形成结构强度最高的块体.     

采用有限元分析法量化不同中空玻璃边缘密封的热位移

间隔条和玻璃板片之间的相对热位移是中空玻璃边缘密封乃至中空玻璃(IGU)本身老化的主要原因之一.作者采用有限元分析法(FEA),建立了发生在大尺寸(1.5 m×2.1 m)的中空玻璃边缘密封的热位移与温差变化之间的模型,这是三种商业化的、不同材质的、不同设计的间隔条,在温度变化为-30～ 60℃范围内的模型.此模型是基于铝间隔条和镀锌钢间隔条与尼龙插角的搭配,不锈钢间隔条和弯曲插角的搭配.尼龙插角假定为实心的并稳固地固定于间隔条上;弯曲插角被建模成实心弯曲的金属插角,并同样稳固地固定于间隔条上.由于实际的弯曲插角是空心的,模型可能高估了这种转角设计的应力.正如预期那样,在低温时,插角会被向内拉,导致弯曲角度大于90°;在高温下,插角会被向外推,导致弯曲角度小于90°.通过检测发生在中空玻璃周边第一道密封的聚异丁烯密封胶厚度的变化,作者发现,到目前为止,不锈钢间隔条对于截面积的变化影响最小;铝制间隔条的影响最大.这个发现与基于间隔条材料与浮法玻璃之间热膨胀系数差异而预期的性能一致.因此,热位移引起的第一道密封的有效扩散横截面变化,可以解释为何具有不同间隔条材料的中空玻璃之间的性能差异.     

Influence of Dopants on Nucleation and Growth of High-Quartz Solid Solution in Lithium Aluminosilicate Glass

The kinetic parameters of nucleation and crystal growth of high-quartz solid solution in multicomponent lithium aluminosilicate glasses doped with various transition-metalions were studied by nonisothermal DTA. The crystallization of glasses nucleated at different temperatures was carried out, and plots of the DTA peak versus the nucleation temperatures were used to determine the maximum nucleation rate temperature. Peak temperature data of nucleated samples at varying heating rates (5–20 K/min) were used to determine the activation energy for crystallization via the JMA equation. The temperature of maximum nucleation rate depends greatly on the doped transition- metal ions present. The activation energy for crystallization obtained for undoped glass or glasses doped with Fe₂O₃ is of the same order as that already published, and the Avrami exponent is consistent with predominantly three-dimensional crystal growth. The much higher activation energy values for glasses doped with CoO could be a consequence of two crystallization processes proceeding simultaneously.     

隔热玻璃涂料中隔热材料之浅析

本文叙述了隔热玻璃涂料的隔热原理和隔热材料的选择.     

Determining the Kinetic Parameters for Isothermal Crystallization in a Lithium Disilicate (Ls2) Glass by OM and DSC

It is well known that the variation in the crystallized fraction, x(t), can be determined directly by optical microscopy (OM) or indirectly by differential scanning calorimetry (DSC) under isothermal conditions. In this work, the kinetic crystallization parameters E (activation energy) and n (Avrami index) of a quasi‐stoichiometric lithium disilicate glass (Li₂O·2SiO₂–LS₂) were studied. First, bulk samples were heat treated in a vertical tube furnace between 500°C and 530°C for different periods of time, and were carefully prepared. Furthermore, the same samples were then heated in a calorimeter from room temperature to 850°C at 10°C/min. The following E and n values were obtained: (i) OM: 462.2 ± 1.23 kJ/mol and 4; and (ii) DSC: 187.5 ± 0.45 kJ/mol and 2.4. The results indicated that the activation energy was reduced due to the formation of new crystals and growing of the “preexisting crystals” during the DSC experiments, and that the predominant crystallization mechanism in the samples changed according to the chosen thermal history.     

Linking Cure Process to Adhesive Bulk Strength by Differential Thermal Analysis

A first order kinetic reaction model is utilized to model the cure process of epoxy adhesive Metlbond 1113. Actual states of full cure corresponding to a spectrum of cure temperature-time schedules are identified using the DTA method. Comparison of these experimental results with the corresponding theoretical predictions reveals that the first order kinetic reaction model provides accurate prediction of full cure when low temperature-long time schedules in close proximity to T_g are used. These cure schedules also result in the highest adhesive bulk tensile strength.     

共聚酰胺对聚甲醛热稳定作用和结晶成核作用研究

用220℃等温热失重法通过对热失重率和热失重速率的测试研究了共聚酰胺(COPA)作为甲醛吸收剂对聚甲醛(POM)热稳定性的影响同时用HAAKE转矩流变仪对平衡扭矩进行测试．其熔体粘度随COPA加人量的增加而增大，表明POM体系热稳定性提高。热重分析(TGA)所得的特，征温度的增大也表明COPA能提高POM的热稳定性.差示扫描量热分析(DSC)和偏光显做镜(PLM)观察的结果表明COPA能提高POM的结晶性能，即COPA对POM具有显著的热稳定作用和一定的结晶成核作用.     

Kinetic Study of Amorphous Yttria-Alumina Fibers Using Differential Thermal Analysis

Differential thermal analysis (DTA) was used to study the crystallization phenomena in Y₂O₃-Al₂O₃ fibers formed via a melt extraction process. Such fibers are X-ray amorphous and optically transparent regardless of composition in the as-extracted state but can be crystallized to produce various equilibrium phases that depend on the original composition of the ceramic rods used in the extraction process. The activation energy values were calculated using Kissinger and Augis-Bennett equations by measuring the variation of the peak temperature in the differential thermal patterns with heating rate. X-ray diffractometry analysis was used to determine the phase formation in the fibers crystallized after the DTA scan.     

防腐型HPDTA的建立及在催化剂器外预硫化上的应用

自建了一套防腐型高压差热装置（反应）系统（HPDTA）,得到的数据重复性好,已应用于测定器外预硫化催化剂上放热情况的研究。结果发现,单一硫化物或不同硫化物作为硫化剂简单添加时,器外预硫化催化剂上放热明显;而经过对硫化剂添加方式或制备方案的改进,则可以大幅度地降低催化剂上的局部集中放热。该装置还可改造为适合于加氢等催化剂的HPDTA-硫化联用装置、HPDTA-微反联用装置、HPDTA-再生联用装置等,具有一定的推广价值。     

New Method for Determining the Nucleation and Crystal-Growth Rates in Glasses

The rates for nucleation ( I ) and crystal growth ( U ) for a lithium disilicate (Li₂O·2SiO₂, LS₂) glass were determined, as a function of temperature, using a new differential thermal analysis (DTA) technique. This technique requires in situ nucleation and crystal-growth heat treatment of a small amount of powdered sample inside the DTA apparatus, which then are followed by a DTA scan at a constant heating rate. The I and U values that have been determined at selected temperatures for the LS₂ glass are in excellent agreement with those reported in the literature. The technique also has been used to determine the concentration of quenched-in nuclei in LS₂ glasses prepared from melts that have been quenched at different rates, which are in reasonable agreement with those estimated from theoretical considerations. This new DTA technique is less tedious, requires a smaller amount of sample, and is at least 10 times faster than the conventional methods that have been used to measure I and U . Also, no special sample preparation, other than simply grinding and screening the glass to a particle size that is suitable for use, is required in this technique, whereas grinding, polishing, or etching is required in conventional methods. The excellent agreement in the I or U values that have been determined for the LS₂ glass via the present and conventional methods demonstrates the accuracy, validity, and usefulness of this DTA method for rapid determination of the nucleation and crystal-growth rates in glasses.     

差热分析在物理化学二元相图实验中的应用

物理化学相平衡的知识内容比较抽象,学生很难从直观方面去理解,通过实验的方法可使学生加深对相图知识的认识。为了提高学生的实验积极性,开阔学生的眼界和思路,有必要将燕热分析技术引入物理化学二元相图实验教学。在此,以Zn38206-ZnW04二元相图为例,介绍差热分析在物理化学实验中的应用。     

聚酯的差示扫描量热法热性能分析

使用DSC差示扫描量热仪分析了三种聚酯产品的热性能。同等试样量时,从测得试样的熔点、结晶度及曲线图形可以分析判断出,PET-2聚酯样品产品质量最好。     

The Heterogeneity of Glass Surfaces Revealed by Temperature Programmed Desorption

Adsorption of butanol and pyridine on E‐glass fibers with three different compositions, as well as on powders of silica and the crushed fibers, was studied by temperature programmed desorption (TPD) with a mass‐sensitive detector. In the case of butanol, there are two types of desorbing molecules: at lower temperatures butanol desorbs, but in the range 450°C–600°C, 1‐butene desorption is also observed. It is shown that 1‐butene desorption is due to thermal decomposition of butanol chemisorbed to OH groups on both the glass and silica surfaces. The binding energy distributions of adsorption sites for butanol and pyridine are similar on all three glass compositions, but they are much more heterogeneous compared to silica; this difference is most evident for pyridine and is attributed to the presence Al and B in the glasses. The decomposition temperature of chemisorbed butanol is highest for silica and depends on glass composition for the fibers and powders. Interestingly, the glass which does not contain boron shows a well‐defined peak for the decomposition of chemisorbed butanol, suggestive of unique adsorption sites on this boron‐free surface; but they are much less temperature stable than the chemisorption sites on silica. In situ exposure to water vapor increased the number of active sites for chemisorption.     

差热分析在测定松香软化点、酸值研究上的应用

用实际例子论述了差热分析 (DTA)在测定松香软化点、酸值研究上的应用