Studies on nucleation kinetics of InxGa1−xN/GaN heterostructures

Nucleation kinetics during the growth of In_xGa_1−xN on a GaN substrate have been studied. The behavior of nonequilibrium between the In_xGa_1−xN and the GaN substrate has been analyzed, and hence, the expression derived for the stress-induced supercooling/superheating has been numerically evaluated. The maximum amount of stress-induced supercooling is found to be 1.017 K at x=0.12. These values are incorporated in the classical heterogeneous nucleation theory. Using the regular solution model, the interfacial tension between the nucleus and substrate and, hence, the interfacial tension between nucleus and mother phase and thermodynamical potential of the compounds have been calculated. The amount of driving force available for the nucleation has been determined for different compositions and degrees of supercooling. It has been shown that the value of the interaction parameter of InN-GaN plays a dominant role in nucleation and growth kinetics of In_xGa_1−xN on a GaN substrate. These values have been used to evaluate the nucleation parameters. It is shown that the nucleation barrier for the formation of a In_xGa_1−xN nucleus on a GaN substrate is minimum in the range of x=0.12 to x=0.17, and it has been qualitatively proved that good quality In_xGa_1−xN on GaN can be grown only in the range 0<x≤0.2.     

共晶自生复合材料的晶形转变

自生复合材料的晶形转变受到成分过冷与曲面过冷两个因素的控制.非金属相具有较大的熔化熵且易形成小平面是改变领先相的因素,而领先相的改变则可能是晶形转变的根本原因.     

Development and numerical validation of a new model for walls with phase change materials implemented in TRNSYS

This paper presents a model of a wall with variable properties dedicated to modelling phase change materials (PCMs) in building envelopes. The model is implemented in the TRNSYS simulation tool and referred to as Type 3258. The 1-D conduction heat transfer equation is solved using an explicit finite-difference method coupled with an enthalpy method to consider the variable PCM thermal capacity. This model includes temperature-dependent thermal conductivity and PCM-specific effects like hysteresis and supercooling. The stability conditions are discussed and the algorithm implemented in TRNSYS is described. A numerical validation performed on wall test cases proposed by the International Energy Agency is presented, showing that the developed model is in agreement with reference models. The paper also discusses the impact of temporal and spatial discretization on the model performance. Modelling problems encountered when using an effective heat capacity method (compared to an enthalpy method) and when representing supercooling are also discussed.     

真空闪蒸制冰技术研究进展

冰浆作为一种性能优良的相变蓄冷材料广泛应用于生活及工业中,真空闪蒸制冰技术作为一种最具潜力的冰浆制备方法,有广泛的发展前景。介绍了真空闪蒸制冰的基本原理及物理过程,分别根据冰浆生成方式、真空维持系统设计方式对真空闪蒸制冰系统进行分类,详细分析了解决真空闪蒸制冰技术问题（维持真空度、强化传热、过冷）的研究,并对真空闪蒸制冰技术的进一步研究做出展望。     

渗碳体的结构和一次渗碳体形态的研究

对钢铁中的基本相渗碳体的晶格结构作了进一步的描述:一个碳原子位于由6个铁原子组成的三棱柱体中,每个铁原子为2个三棱柱体所共有;每个渗碳体晶胞含4个这种上下分层、反向、错位排列的三棱柱体,相应的化学计量比为12:4(即3:1)。应用计算机可以形象地揭示渗碳体的结构。根据成分过冷原理对从过共晶铸铁液相中结晶出树枝状一次渗碳体进行了分析。     

Na2HPO4?12H2O相变储能复合材料制备及热物性

十二水磷酸氢二钠的过冷度、相分离以及热导率低等问题影响了其在低温蓄热场合的应用,因此需要对其进行相关的改性研究。本文通过成核剂和增稠剂的筛选实验及添加导热增强剂纳米氧化铁（α-Fe₂O₃）,制备了质量分数为Na₂HPO₄·12H₂O+2% Na₄P₂O₇·10H₂O+1%黄原胶（GX）+0.2%α-Fe₂O₃复合相变储能材料,并对其进行了凝固放热测试、热物性测试及循环稳定性测试。结果表明：2%的Na₄P₂O₇·10H₂O抑制过冷效果最好,成核效果不随循环次数的增加而减小,过冷度维持在2℃左右;GX可以有效抑制Na₂HPO₄·12H₂O的相分离现象,且质量分数为0.75%～1.25%是较合适的剂量;α-Fe₂O₃可以有效地提高Na₂HPO₄·12H₂O的热导率,添加0.2%α-Fe₂O₃使热导率提高了90.8%;循环150次后,复合相变储能材料的相变潜热值为252J/g,相比于循环前衰减了7.4%,相变温度为35.4℃,过冷度为1.3℃,热导率为2.054W/(m·K),相比纯材料提高了100.2%。改性后的复合相变储能材料相变温度适宜,潜热值大,热导率高,热性能稳定,可推广应用到热泵蓄热、温室生产和电子器件散热等领域。     

Stability of a crystal surface in fractional crystallization on a band

The problem on the morphological and thermal stability of a crystal surface in fractional crystallization on a band has been investigated theoretically. A model to describe the dynamics of the development of a striped perturbation of the crystal surface is suggested. It is used as a basis for analyzing the influence of the main factors on the stability, and a stability criterion has been derived. It has been established theoretically that the thermal instability must appear at small Reynolds numbers (Re < 10). Numerical experiments have shown that an increase in the length of the band, its inclination angle, as well as a decrease in the rate of melt feed and in the band motion velocity increase the thermal instability. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 1, pp. 106–113, January–February, 2009.     

高碳当量灰铸铁的变质研究

采用微分热分析法、液淬法及电镜能谱分析法观察研究了阻碍石墨化变质剂对高碳当量灰铸铁一次结晶过程及凝固组织的影响机理，并结合生产实际研究了它对机械性能的影响规律．试验结果分析表明：阻碍石墨化变质剂对高碳当量（ＣＥ＝３．９％～４．２％）灰铸铁具有强烈地增大共晶过冷的作用，初生奥氏体枝晶数量及细小石墨含量随变质剂加入量的增加而增加，高碳当量灰铸铁通过阻碍石墨化变质其机械性能显著提高．     

精氩塔工况波动的原因分析与调整

介绍了粗氩塔和精氩塔的工况调整，分析了精氩塔内波动的原因，最终确认引起精氩塔波动的主要原因是液氮过冷度的影响，进行调整后，达到了稳定的工况。