共查询到17条相似文献,搜索用时 78 毫秒
1.
2.
层间稀薄气体传热对多层绝热材料性能的影响分析 总被引:1,自引:2,他引:1
通过建立的热量传递模型,分析了不同的气体稀薄程度(Knudsen数)时,气体传热对多层绝热材料有效热导率和各层温度分布的影响。分析表明:由多层绝热材料真空度变化引起的稀薄气体传热量波动较大,在10—60层/cm层密度范围,真空度低于100Pa时,Kn数属于自由分子状态区域和中间压强区域,此时材料的有效热导率随残留气体热适应系数的增大而减小,并随着真空度的降低而增大;当残留气体为空气时,为保证多层材料的绝热性能,尽量维持真空度不低于10-2Pa。同时分析表明,为有效降低低真空下稀薄气体传热对多层绝热性能的影响,可以采用综合热适应系数较低的气体置换夹层中的空气,以减少低真空多层绝热材料的有效热导率,改善绝热性能。 相似文献
3.
4.
5.
6.
7.
8.
9.
文中通过建立的能进行夹层气体置换的稳态量热器试验系统,试验分析了夹层气体传热对多层绝热材料有效热导率的影响,重点对置换气体种类、气体压强、材料层数及冷热边界温度对多层材料的影响进行试验研究。试验表明在10—60层/cm层密度范围,真空度低于100Pa时,Kn数属于自由分子状态区域和中间压强区域,此时材料的有效热导率随残留气体热适应系数的增大而减小,并随着真空度的降低而增大,当残留气体为空气时,为保证多层材料的绝热性能,应尽量维持真空度不低于10-2Pa。同时,分析表明为有效降低低真空下稀薄气体传热对多层绝热性能的影响,可以采用综合热适应系数较低的气体置换夹层中的空气,以减少低真空多层绝热材料的有效热导率,改善绝热性能。 相似文献
10.
11.
为探究临床常用的7 MHz高频聚焦超声在多层生物组织中的声传播以及毫秒级时间内的生物传热规律问题,基于Westervelt方程和Pennes传热方程,使用有限元方法建立高频聚焦超声辐照多层组织的非线性热黏性声传播及传热模型。首先分析了线性模型和非线性模型之间的差异,然后在非线性模型下探究换能器的参数对声场和温度场的影响。仿真结果显示:在7 MHz频率下,当换能器输出声功率超过5 W时,声波传播的非线性效应不可忽视(p <0.05);当声功率从5 W增大到15 W时,非线性模型与线性模型预测的温度偏差从20%增加到34.703%;高频聚焦超声波的非线性行为比低频更加显著,基频能量向高次谐波转移的程度增大,声功率为10 W和15 W时4次谐波与基波之比分别达到7.33%和12.12%;高频换能器参数的改变对组织中声场和温度场分布的影响较大,换能器焦距从12 mm减小到11.2 mm,焦点处最高温度增加了77%。结果表明,7 MHz聚焦超声的非线性声传播需要考虑到4次谐波的影响。该文提出的多层组织非线性仿真模型可为高频聚焦超声换能器参数优化及制定安全、有效的术前治疗方案提供理论参考。 相似文献
12.
13.
14.
Preparation and study on performance of submicron nickel powder for multilayer chip positive temperature coefficient resistance 下载免费PDF全文
Base metal nickel is often used as the inner electrode in multilayer chip positive temperature coefficient resistance (PTCR).The fine grain of ceramic powders and base metal nickel are necessary.This paper uses reducing hydrazine to gain submicron nickel powder whose diameter was 200-300 nm through adjusting the consumption of nucleating agent PVP properly.The submicron nickel powder could disperse well and was fit for co-fired of multilayer chip PTCR.It analyes the submicron nickel powder through x-ray Diffraction (XRD) and calculates the diameter of nickel by PDF cards.Using XRD analyses it obtains several conclusions:If the molar ratio of hydrazine hydrate and nickel sulfate is kept to be a constant,when enlarging the molar ratio of NaOH/Ni 2+,the diameter of nickel powder would become smaller.When the temperature in the experiment raises to 70-80 C,nickel powder becomes smaller too.And if the molar ratio of NaOH/Ni 2+ is 4,when molar ratio of (C 2 H 5 O) 2 /Ni 2+ increases,the diameter of nickel would reduce.Results from viewing the powders by optical microscope should be the fact that the electrode made by submicron nickel powder has a better formation and compactness.Furthermore,the sheet resistance testing shows that the electrode made by submicron nickel is smaller than that made by micron nickel. 相似文献
15.
16.
栅极组件热变形是影响离子推力器工作性能及工作寿命的主要因素,为研究栅极组件升温过程中温度场分布及变化规律,探索能较准确模拟栅极温度场的方法,建立了栅极组件1/12全尺寸有限元模型进行温度场仿真计算。同时,基于实验室搭建的温度测量平台,测量了大气环境下加热时栅极组件的瞬态温度变化。对比有限元分析求解与试验过程中的温度场,加速栅平均误差为14.4%,屏栅平均误差为9.7%,双栅最大误差不超过18.4%,验证了有限元模型及方法的可信度和合理性。 相似文献
17.
Evaluation of a gate-first process for AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with low ohmic annealing temperature 下载免费PDF全文
In this paper, TiN/AlO_x gated Al Ga N/Ga N metal–oxide–semiconductor heterostructure field-effect transistors(MOSHFETs) were fabricated for gate-first process evaluation. By employing a low temperature ohmic process, ohmic contact can be obtained by annealing at 600℃ with the contact resistance approximately 1.6 ?·mm. The ohmic annealing process also acts as a post-deposition annealing on the oxide film, resulting in good device performance. Those results demonstrated that the TiN/AlO_x gated MOS-HFETs with low temperature ohmic process can be applied for self-aligned gate Al Ga N/Ga N MOS-HFETs. 相似文献