本征导热聚合物研究:机理、结构与性能及应用

散热已成为制约超高频、大功率微电子器件和高电压电气绝缘设备日益微型化的技术瓶颈和发展面临的重要挑战,急需高性能的导热材料实现快速散热。相比导热高分子复合材料,本征结构的导热高分子材料因同步的高导热及高绝缘强度、优异柔韧性、轻质高强等优异的综合性能及优势受到了国内外学者的广泛研究和关注。本文首先讨论了聚合物的本征导热机理,系统深入地分析和评述了单体及分子链结构、结晶、取向、分子链间作用、交联、缺陷等结构因素,以及温度、压力、环境等因素对声子热传递及聚合物导热的影响机理,进一步归纳了本征导热聚合物的制备策略和途径。最后总结了当前本征导热聚合物研究面临的主要问题和挑战,展望了未来发展方向及其在众多领域的重要潜在应用。

基于枝条木材密度分级的鼎湖山南亚热带常绿阔叶林树高曲线模型

如何便捷准确地测量树高一直是林学及群落生态学所关心的问题。由于木材密度与树木生长密切相关,因此基于木材密度建立树高曲线模型能够为测量树高提供新的方法。本文以鼎湖山南亚热带常绿阔叶林1.44 ha塔吊样地内119个物种的4,032个个体为研究对象,利用树高、胸径和木材密度数据来探究基于枝条木材密度分级的树高曲线模型。首先,对个体进行随机抽样,将其划分为建模样本(占总样本量的70%)和检验样本(占总样本量的30%),并通过聚类分析将所有个体的木材密度划分为4级。其次,基于建模样本利用常见的5种理论生长方程(Richards、Korf、Logistic、Gompertz和Weibull方程)对不同分级建立树高–胸径模型;基于检验样本检验模型精度,并确定各分级的最适模型。最后,构建基于物种分类的树高曲线模型,并比较其与木材密度分级模型的差异。结果表明:基于木材密度分级的模型,各分级小组检验样本的平均绝对误差(MAE)和均方根误差(RMSE)最小值所对应的模型类型与建模样本结果一致,确定Gompertz模型和Weibull模型为鼎湖山南亚热带常绿阔叶林最适树高模型类型。比较基于木材密度分级的模型与基于物种分类的模型,发现二者的MAE、RMSE指数差异小。综上,基于木材密度分级的树高曲线模型对树高估测精度高,使用方便,为树高预测提供了新方法,可以更好服务森林调查等生产实践。

Towards suppressing dielectric loss of GO/PVDF nanocomposites with TA-Fe coordination complexes as an interface layer

In this work, graphene oxide(GO) nanosheets with surface modification by Tannic and Fe coordination complexes(TA-Fe) were incorporated into poly(vinylidene fluoride)(PVDF) to prepare high constant but low loss polymer nanocomposites, and the effect of TA-Fe interlayer on dielectric properties of the GO@TA-Fe/PVDF nanocomposites was investigated. The results indicate that the dosage, mixing ratio, and reaction time of TA-Fe complexes have obvious influences on the dielectric properties of the nanocomposites. Furthermore, the TA-Fe interlayer significantly influences the electrical properties of GO@TA-Fe nanoparticles and their PVDF composites, and the GO@TA-Fe/PVDF composites exhibit superior dielectric properties compared with raw GO/PVDF. Dielectric losses of the GO@TA-Fe/PVDF are significantly suppressed to a rather low level owing to the presence of TA-Fe layer, which serves as an interlayer between the GO sheets, thus preventing them from direct contacting with each other. Additionally, the dynamic dielectric relaxation of the GO/PVDF and GO@TA-Fe/PVDF nanocomposites was investigated in terms of temperature.

Mobile Assisted Chinese Learning as a Foreign Language:An Overview of Publications between 2007 and 2019

This paper presents a critical overview of studies on mobile assisted language learning(MALL)in teaching Chinese as a foreign language(CFL)during the period 2007−2019.In the review,keyword and reference searches were conducted to identify and select empirical studies during the review period.Thematic and frequency analyses were employed on the data.This identified methodological trends and research outcomes in the reviewed studies.As shown in the results,most of the reviewed studies used qualitative methods to examine the effect of mobile CFL learning on formal learning in higher education settings.These studies document the positive impact that mobile technology has on CFL learning.Their attention is primarily on the use of mobile learning in Chinese vocabulary acquisition,language skill development and mobile seamless learning.Suggestions are provided for further research to support continuous mobile assisted CFL teaching and learning.

Heat conductive h-BN/CTPB/epoxy with enhanced dielectric properties for potential high-voltage applications

To enhance the toughness and electrical insulation properties of cured epoxy,carboxyl-terminated polybutadiene(CTPB)liquid rubber was used to modify epoxy in this study,and the thermal conductivity,mechanical and dielectric properties of modified epoxy were investigated.The results indicate that the CTPB-epoxy exhibits higher impact strength and lower dielectric constant and loss compared with pure epoxy.Further,hexagonal boron nitride(hBN)was used to reinforce epoxy modified with 20 phr CTPB.It is found that compared with the hBN/epoxy under the same filler loading,the heat conductive hBN/CTPB/epoxy possesses a lower dielectric permittivity and dissipation factor in all frequency ranging from 10^(2)–10^(7)Hz,a higher electrical resistivity and dielectric breakdown strength,and improved mechanical toughness.Therefore,the prepared hBN/CTPB/epoxy composites are potentially useful in practical electrical insulation applications.

The Effects of Shared Singing Picture Book Instruction on Chinese Immersion Kindergarteners’Spoken Vocabulary Recall and Retention

The present study investigates the effects of shared singing of a picture book on the Chinese vocabulary recall and retention of young bilingual learners in Chinese immersion programs using an experimental study design.One hundred and six immersion kindergarteners from three different school districts in the U.S.Midwest were assigned to the experimental group(n=53)and the control group(n=53)at the class level.The experimental group listened to a song-based storybook(also known as a story song)and shared in singing it,while the control group heard the same story,read aloud and shared in reading it.The pre-test was administered to determine the participants’vocabulary level before the study.Immediate and delayed post-tests were used to assess their immediate recall and delayed retention of vocabulary targeted in the shared reading/singing.Independent t-test and one-way ANOVA analysis were employed to examine differences between the two groups in vocabulary recall and retention.The results showed that the experimental group achieved greater improvement in their vocabulary than the control group.This finding suggests that shared singing can be a useful pedagogical tool for spoken vocabulary acquisition and retention in picture books.