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During glass-batch melting, solid silica (quartz) usually dissolves last. The measured rate of dissolution, while the temperature was increasing at a constant rate, was compared with the hypothetical diffusion-controlled volume flux from regularly distributed particles through concentration layers of a uniform thickness. The actual rate was up to two orders of magnitude lower than that of the “ideal” case, revealing a progressive inhibition of silica dissolution. As a measure of this retarded dissolution, we introduced a retardation factor defined as a ratio of the actual and “ideal” dissolution rates measured as the volume flux at the melt-particle interface. The severe inhibition of silica dissolution has been attributed to the irregular spatial distribution of silica particles that is associated with the formation of nearly saturated melt on a portion of their surfaces. Irregular shapes and unequal sizes of particles also contribute to their extended lifetime. 相似文献
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G. Andrehs R. Blossfeld O. Bhme W. Sachsenrder P. Schreiter 《Crystal Research and Technology》1978,13(10):1157-1161
In comparison with the solid state synthesis from the oxide components melilites are formed from vitreous materials with added oxides at lower temperatures and with higher velocity of reaction resulting in larger and more idiomorphic crystals. The crystallization mechanism of the applied vitreous materials is important for the course of the formation of the texture during sintering depending on their chemical composition. In copper slag with polyphase composition the melilite forming reaction starts during the crystallization of pyroxene using the newly relased relictic glass phase in statu nascendi. Lateron, at higher temperatures the pyroxene also reacts with other components to form melilite. In phosphor furnace slag with stoichiometric composition the polymorphic transformation of calcium silicate (CS) is important for the formation of melilite. The change of the glass structure during the nucleation of wollastonite is the process initiating the reaction. Later at higher temperatures the polymorphic transformation of metastable pseudowollastonite accelerates the formation of melilite. It could be proved that the excess of free energy in glasses speeds up crystal forming reactions with admixture components in sintering processes. The mechanism of structural and phase change processes is reflected in the temperature dependence of reaction velocity. 相似文献
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Pavel Hrma José Marcial Kevin J. Swearingen Samuel H. Henager Michael J. Schweiger Nathan E. TeGrotenhuis 《Journal of Non》2011,357(3):820-828
Quartz dissolution during the batch-to-glass conversion influences the melt viscosity and ultimately the temperature at which the glass forms. Batches to make a high-alumina borosilicate glass (formulated for the vitrification of nuclear waste) were heated at 5 K min− 1 and quenched from temperatures of 400 to 1200 °C at 100 K intervals. The batches contained quartz as a silica source, with particles ranging from 5 to 195 μm in diameter. The content of unreacted quartz in the samples was determined with X-ray diffraction. Most of the fine quartz dissolved during the early batch reactions (at temperatures < 800 °C), whereas coarser quartz dissolved mostly in a continuous glass phase via diffusion. The mass-transfer coefficients were assessed from the data as functions of the initial particle sizes and the temperature. A series of batches were also tested that contained nitrated components and additions of sucrose, known to accelerate melting. While sucrose addition had no discernible impact on quartz dissolution, nitrate batches melted somewhat more slowly than batches containing carbonates and hydroxides in addition to nitrates. 相似文献
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We have measured the distribution of insoluble particles within a few millimeters of the tube wall in a downward flowing liquid in order to verify that the velocity gradient mechanism for particle pushing works for particles less than a few hundred microns in diameter. The experimental results show that particles that are denser than the liquid concentrate near the tube wall whereas particles less dense than the liquid migrate away from the wall, leaving a particle deficient layer near the surface, confirming the velocity gradient mechanism. The experimental results have been modelled using the velocity gradient mechanism and the model is in good agreement with experimental results. Calculation shows that the concentration of particles near a solid surface depends on the density and size of the particles, the flow speed and the distance from the inlet of the flow. 相似文献
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This article deals with results of the invsetigations connected with limitation of oxygen dissolution in niobium by means of creating of the carbide protective coating on the niobium surface or forming of the solid solution of carbon in niobium. It is shown that the carbide coating increases essentially the life time of the niobium parts for vacuum equipment and is consumed as a result of interaction with oxygen. The carbon dissolution in niobium does not hinder redox reaction at the Nb(C)/Nb2O5 boundary and causes the conductivity increase of the dielectric film Nb2O5. 相似文献
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Generally well-annealed alkali halide crystals exhibit three lifetime components. τ1 arises due to annihilation with anion and τ2 due to Bloch Ps. The origin of τ3 is not well understood. Positron lifetime and etching techniques have been performed on solid solutions of KCl100−xBrx with mole % of x = 0.0, 15, 30, 37, 50, 70, and 100. The positron lifetime spectra have been analysed to resolve into three components. The positron capture rate K3 for the longest lived component and the dislocation density shows a non-linear variation with composition. The positron capture rate varies linearly with the dislocation density of the samples. The origin of τ3 is identified as due to positronium formation in the dislocations present in the samples. The trapping probability per dislocation has been estimated to be 2.2 × 10−12/s. 相似文献
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The effect of temperature and concentration of a variety of inorganic and organic acids, on etch rates is investigated. It is observed that in HCl, CH3COOH and C2H5COOH the value of activation energy of dissolution and that of the corresponding pre-exponential factor are sensitive to acid concentration. In H3PO4 and HCOOH, while the value of activation energy remains unchanged, only the pre-exponential factor changes. The concentration dependence of etch rates in HCl, CH3COOH and C2H5COOH is different from that of H3PO4 and HCOOH. In the former acids maximum and minimum values are obtained on the etch rate versus concentration curves, but in the letter ones etch rate slowly increases with concentration. The influence of temperature on the maximum value of etch rate on the etch rate-concentration plots of HCl, and that of the degree of dissociation of the acids on etch rate are also studied. The observations are discussed, and important conclusions are enumerated. 相似文献
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Growth rate of potassium sulfate crystals was studied in a fluidized bed crystallizer. Higher growth rates were observed at larger bed heights. Larger crystals grow faster than smaller size crystals due to the changed fluid dynamics. The growth rate was found to be very sensitive to the pH value of the solution. The growth rate increases with increasing the pH value and the increase of growth in the acidic medium is faster than in the basic medium. The presence of small amounts of Cr3+ ions reduces the growth rate of potassium sulfate dramatically. The presence of Cr3+ ions lowers the saturation temperature and increases the width of the metastable zone, i.e. shifts the metastability of the solution to be at lower level of supersaturation. 相似文献
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The rate of dissolution and growth of settling crystals of spinel was measured optically in a borosilicate melt that was pre-heated at a temperature above liquidus to erase the effects of previous history. The Hixson-Crowell equation, which is based on Fick's first law, was used to determine mass-transfer coefficients (kH) for dissolution and growth; both were found to fit the same Arrhenius function of temperature (T). An attempt was made to estimate the diffusion coefficient (D) and the concentration-boundary-layer thickness (δ). The calculated values of δ compared well with experimental results and observations. The D vs. T function was similar to a literature function obtained for the dissolution of magnetite in sodium disilicate glass. 相似文献
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单晶MgO基片超精密加工技术研究 总被引:1,自引:0,他引:1
单晶MgO具有良好的物理化学性能及光学性能,是性能优异的薄膜基片及光学零件材料,广泛应用于高温超导、航空航天、光电技术等领域.用作薄膜生长的基片必须具有高精度超光滑无损伤的表面,而单晶MgO是典型的硬脆难加工材料,这对MgO晶体的超精密加工技术提出了很高的要求.本文介绍了单晶MgO的特性及其应用领域,针对高温超导薄膜制备对MgO基片的要求,讨论了现有的MgO基片加工工艺存在的问题,分析了几种可用于MgO基片超精密加工的先进工艺技术的特点和应用研究现状. 相似文献
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Feng Gu Shu Fen Wang Meng Kai Lü Wen Guo Zou Guang Jun Zhou Dong Xu Duo Rong Yuan 《Journal of Crystal Growth》2004,260(3-4):507-510
MgO nanocrystals doped with Dy3+ have been synthesized by a combustion method. The synthesized sample is characterized by X-ray diffraction, transmission electron micrograph, Fourier transform infrared, and photoluminescence spectroscopy. The as-prepared MgO nanocrystals appear to be single cubic crystalline phase and the diameter is in the range of 20–25 nm. The hypersensitive transition (4F9/2→6H13/2 of Dy3+) emission is prominent in the emission spectra resulting from the low-symmetry local site at which Dy3+ ions locate. In addition, the dependence of the luminescence intensity on Dy3+ concentration is also discussed. 相似文献
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Dislocation etching and dissolution of gel-grown cadmium oxalate trihydrate crystals are carried out using different etchants. The study revealed the existence of dislocation network in the body of the crystal and shallow dislocation loops in the substructure. Faster dissolution of the crystal at elevated temperatures revealed that the rate of dissolution in the initial part of the dissolution time is much lesser, which increases fast to attain a steady value. This observation is related to the available specific surface area of the crystal. 相似文献
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The etching of MgO smoke in wet conditions – an archetype for aqueous dissolution of oxide crystals – is explored on the basis of transmission electron microscope (TEM) analyses. In pure water with neutral pH, in acid solutions and in moist air, the MgO cubic crystallites show first (1 1 0) truncations of the 〈1 1 0〉 edges and then (1 1 1) cuts at the corners. Criteria are given to characterize (1 1 0) and (1 1 1) cuts on the TEM images. It is shown that (1 1 0) truncations appear independently of the formation of the (1 1 1) cuts and are associated with a stationary state. The picture is fully consistent with the previous suggestion of a “constrained Wulff profile” [9] which explains the formation of (1 1 0) cuts by excluding (1 1 1) facets. 相似文献
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Y. Moriyoshi T. Ikegami Sh. Matsuda Y. Sekikawa Sh. Shirasaki 《Crystal Research and Technology》1980,15(7):803-808
The dislocation structures of MgO single crystals annealed at 1900 °C for 3 hrs and at 2000°C for 1 hr were observed electron microscopically, and it was found that the following reactions took place at high temperatures: a/2 〈11 0〉 + a/2 [110], = a [100], a/2 [110] + a/2 〈11 01〉 = a/2 [011]. The resultant dislocations were sessile. Their interaction with impurities would make the subgrain boundaries stable. 相似文献
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以六水氯化镁和六次甲基四胺为原料,采用水热法合成四方体MgO,考察其对有机染料甲基橙和亚甲基蓝的吸附行为.通过TGA-DTA、SEM、XRD、N2-sorption和FT-IR等手段表征样品.结果表明,原料浓度、温度和表面活性剂对四方体MgO结构的形成影响较小,而反应时间的延长有助于有序结构的组装.温度170℃、时间24h、MgCl2·6H2O与C6H12N4浓度比为1∶2和表面活性剂PVP是制备四方体MgO的最佳条件.在溶液浓度10mg · L-1的单一吸附实验过程中,四方体MgO对甲基橙和亚甲基蓝的去除率分别为91.3;和22.3;,吸附过程均为单层吸附且符合Langmuir等温吸附模型和伪二级吸附动力学方程.在溶液浓度40 mg·L-1、甲基橙和亚甲基蓝浓度比3∶1的混合溶液吸附过程中,四方体MgO对甲基橙和亚甲基蓝的去除率分别为80.1;和97.9;. 相似文献