由于一级相变磁制冷材料发生磁相变时有晶胞体积的突变,相变过程中有相变潜热存在,其磁化过程中有许多磁学问题有待于进一步探究.本文以LaFe13-xSix合金为研究对象,在现有对磁一级相变基础问题的分析基础上,对一级相变材料中系统熵变、等温熵变、绝热温变、热滞、磁滞、铁磁与顺磁态两相共存的温度区间和磁场区间、制冷能力的计算等磁学基础问题进行了较为细致的探究.分析表明,在忽略完全铁磁态和顺磁态对磁热效应的贡献时,Maxwell方程和Clausius-Clapeyron方程计算熵变的值具有等效性.等温磁化过程中升温和降温曲线包围的面积SABCE(磁滞的大小),实际上是升温过程和降温过程中磁场做的净功,等于相变潜热之差.磁滞和热滞的大小与磁化过程数据测量的时间有关,测量时间越长则滞后越小,当相变是平衡相变则滞后为零.另外,对温度和磁场诱导磁相变过程进行了分析,提出了一级相变磁制冷材料制冷能力的不同计算模型.本文对一级相变磁制冷材料的磁学基础问题研究有一定的参考价值.

Due to the cell volume mutations and the phase transition latent heat existing during phase transition of the first-order phase transition magnetic refrigeration material, many basic problems need to further explore in the magneti-zation process. In this paper, taking LaFe13-xSixalloys as the research object, we discuss in detail some problems, such as a phase-change, entropy change, isothermal entropy change, adiabatic temperature change, thermal and magnetic hysteresis, the temperature range and magnetic field range in which the ferromagnetic and paramagnetic state coexist, and magnetic refrigeration capacity calculation, The analysis shows that the magnetic entropies calculated by Maxwell equation and Clausius-Clapeyron equation are equivalent when neglecting the contributions of ferromagnetic and param-agnetic state to magnetocaloric effect. The area surrounded by the curve in heating of isothermal magnetization process and curve in cooling of isothermal magnetization process (hyst

根据Nd2Fe14B的冲击加载实验,计算了3.3—7.2 GPa压力范围内冲击波阵面上压力与温度的关系.基于分子场理论,引入压力等效场,改进了双亚点阵理论模型,并分析了在不同温度和压力下Nd2Fe14B的磁性转变机理.计算了压力对Nd2Fe14B磁致伸缩系数、磁化率、磁化强度以及居里温度的影响,给出了Nd2Fe14B发生铁磁–顺磁相变的压力和温度判据.计算结果表明:压力使Nd2Fe14B的居里温度逐渐向低温区转移,当压力从0 GPa增加到1.15 GPa时,居里温度从584 K降至292 K;随着压力的增加,Nd2Fe14B的磁化强度不断下降,且临界去磁压力随温度的升高呈下降趋势;在3.3—7.2 GPa压力范围内,Nd2Fe14B发生了铁磁-顺磁相变.

According to the shock wave experiment on the Nd2Fe14B ferromagnet, the relationship between pressure and temperature on the shock front is calculated in a pressure range from 3.3 GPa to 7.2 GPa. In order to analyze the magnetic transition mechanism of Nd2Fe14B under different temperatures and applied pressures, the equivalent pressure field is introduced to improve the two-sublattice model based on the molecular field theory. The pressure dependence of magnetostriction coefficient, susceptibility, magnetization, and Curie temperature of Nd2Fe14B are calculated. The criteria of the ferromagnetic-paramagnetic phase transition occurring in Nd2Fe14B at different temperatures and pressures are obtained. The results indicate that the Curie temperature of Nd2Fe14B decreases as pressure increases. The Curie temperature reduces from 584 K at 0 GPa to 298 K at 1.142 GPa. With the increasing of pressure, the magnetization of Nd2Fe14B declines. The critical demagnetization pressure of Nd2Fe14B al

采用溶胶-凝胶法制备不同粒径的La缺位La0.9MnO3钙钛矿氧化物.用X射线衍射、扫描电子显微镜、振动样品磁强计对材料的晶体结构、微结构、磁性和磁热效应进行了研究.结果表明,不同温度下退火(700oC~1100oC)的样品均具有菱形钙钛矿结构.退火温度从700oC增加到1100oC,样品的平均晶粒大小从25 nm增加到1000 nm.随着晶粒大小的增加,材料在低温的饱和磁化强度逐渐增强,铁磁到顺磁的相变变得陡峭,从而使材料的磁熵变逐渐增加.当材料的晶粒尺寸大于500 nm时,材料的磁熵变趋于稳定(-2.7 J/kg·K).

La-deficient La0.9MnO3 perovskite nanoparticles with different grain size were prepared by sol-gel method. The crystal structure, microstructure, magnetic properties, and magnetocaloric effect in La0.9MnO3 nanoparticles were studied using x-ray diffraction, scanning electron microscopy and vibrating sample magnetom-eter. All samples that annealed at different temperatures (700 ℃~1100 ℃) have rhombohedral structure. With the increase of annealing temperatures from 700 ℃ to 1100 ℃, the average crystal size increases gradually from 25 nm to 1000 nm. The magnetization and magnetic entropy change (ΔSM) increase with the increase of crystal size. The value of ΔSM becomes stable when the crystal size is larger than 500 nm.

大多数相变材料由固相转变为液相时,吸收相变潜热,相变材料体积膨胀;从液相转变为固相时放出相变潜热,体积缩小。利用相变材料的这种体积随相变而变化的特性,将一定量的相变材料封装在密闭容器中,通过环境温度引起相变材料相变,相变过程中因体积变化产生的力,为某些需要微小位移量的运动机构提供驱动动力。最后,实例说明相变材料这种特性在航天器微驱动上的应用。

Most of the phase change material when they convert from the solid phase to the liquid phase,absorb the la-tent heat of the phase change material,and the volume of the phase change material expands;when they convert from the liquid phase to the solid phase,release latent heat of the phase change material,and the volume of the phase change materi-al reduce. By use of the characteristic of the volume of the phase change material variation with the phase transition,a cer-tain amount of the phase change material closed in a sealed container,the phase transition of the phase change materials with the variation of the ambient temperature,the stress which is produced due to the volume changes during the phase transformation,provides drive force for moving mechanism requires some tiny displacement. Finally,the property of the phase change materials is applied for micro drive in spacecraft with example.

为了探讨相变回填材料固液相变对地埋管换热器蓄能传热性能的影响,建立了带有相变的垂直U型埋管换热器传热数学模型,并利用显热容法对相变材料的相变问题进行了处理。基于模型的数值求解,分析了夏冬季运行工况下相变材料固液相变对U型埋管换热器蓄能性能及其周围土壤温度热响应特性的影响规律,结果表明:同样条件下,相变材料固液相变会减缓埋管周围土壤温度变化趋势,缩小埋管热影响区域;夏季工况采用较低相变温度、冬季采用较高相变温度的相变材料均可以明显改善其换热效果,同时相变潜热大的相变材料可以明显增加地埋管的蓄能效果。研究结论对于缓解土壤热影响区域、改善地埋管换热器的蓄能传热性能具有重要意义。

Ground source heat pump (GSHP) has been recognized as being among the cleanest, most energy efficient and cost effective systems for residential and commercial space’s heating and cooling applications. The main advantage of using the ground as the heat source or sink of the system is that the soil temperature at tens to hundreds of meters in depth is relatively constant and is generally lower in summer and higher in winter than that of ambient air temperature. This results in an overall improvement of the system performance and thus reduces operation costs. Therefore, GSHP systems have become increasingly popular in commercial and institutional buildings. Heat transfer around vertical ground heat exchanger (GHE) is a common problem for the design and operation of GSHP. The energy storage performance of GHE and its influences on the temperature thermal response characteristics of soil around it are important for a long-term high-efficient and steady operation of GSHP systems.