Thermoelectric efficiency has three Degrees of Freedom

Thermal energy can be directly converted to electrical energy as a result of thermoelectric effects. Because this conversion realises clean energy technology, such as waste heat recovery and energy harvesting, substantial efforts have been made to search for thermoelectric materials. Under the belief that the material figure of merit $zT$ represents the energy conversion efficiencies of thermoelectric devices, various high peak-$zT$ materials have been explored for half a century. However, thermoelectric properties vary greatly with temperature $T$, so the single value $zT$ does not represent device efficiency accurately. Here we show that the efficiency of thermoelectric conversion is completely determined by \emph{three} parameters $Z_{\mathrm{gen}}$, $\tau$, and $\beta$, which we call the \emph{thermoelectric degrees of freedom}. The $Z_{\mathrm{gen}}$, which is an average of material properties, is a generalisation of the traditional figure of merit. The $\tau$ and $\beta$, which reflect the gradients of the material properties, are proportional to escaped heat caused by the Thomson effect and asymmetric Joule heat, respectively. Our finding proposes new directions for achieving high thermoelectric efficiency; increasing one of the thermoelectric degrees of freedom results in higher efficiency. For example, thermoelectric efficiency can be enhanced up to 176\% by tuning the thermoelectric degrees of freedom in segmented legs, compared to the best efficiency of single-material legs.Comment: main articles with 9 pages, 4 figures, supplementary information with 35 pages, 9 figures, 6 table

A NOVEL SPLIT SELECTION OF A LOGISTIC REGRESSION TREE FOR THE CLASSIFICATION OF DATA WITH HETEROGENEOUS SUBGROUPS

A logistic regression tree (LRT) is a hybrid machine learning method that combines a decision tree model and logistic regression models. An LRT recursively partitions the input data space through splitting and learns multiple logistic regression models optimized for each subpopulation. The split selection is a critical procedure for improving the predictive performance of the LRT. In this paper, we present a novel separability-based split selection method for the construction of an LRT. The separability measure, defined on the feature space of logistic regression models, evaluates the performance of potential child models without fitting, and the optimal split is selected based on the results. Heterogeneous subgroups that have different class-separating patterns can be identified in the split process when they exist in the data. In addition, we compare the performance of our proposed method with the benchmark algorithms through experiments on both synthetic and real-world datasets. The experimental results indicate the effectiveness and generality of our proposed method

Counterintuitive example on relation between ZT and thermoelectric efficiency

The thermoelectric figure of merit ZT, which is defined using electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature T, has been widely used as a simple estimator of the conversion efficiency of a thermoelectric heat engine. When material properties are constant or slowly varying with T, a higher ZT ensures a higher maximum conversion efficiency of thermoelectric materials. However, as material properties can vary strongly with T, efficiency predictions based on ZT can be inaccurate, especially for wide-temperature applications. Moreover, although ZT values continue to increase, there has been no investigation of the relationship between ZT and the efficiency in the higher ZT regime. In this paper, we report a counterintuitive situation by comparing two materials: although one material has a higher ZT value over the whole operational temperature range, its maximum conversion efficiency is smaller than that of the other. This indicates that, for material comparisons, the evaluation of exact efficiencies as opposed to a simple comparison of the ZTs is necessary in certain cases.Comment: 12 pages, 2 tables, 2 figure

“詭辭”以見義 : 論《太史公自序》的書寫策略

司馬遷通過一系列特别的書寫策略，將恢國、致孝、繼聖與發憤這四種表達意圖同時注入《太史公自序》之中，這當中既包括對《國語·楚語》、《孝經》等援據文獻的微妙改寫，也包括對“五百年”這一神秘數字的刻意渲染，在與壺遂的對話中呈現出的前後矛盾，以及對《春秋》、《吕氏春秋》、《孤憤》等文本生成背景的時空重塑。這一書寫方式源於《春秋》公羊學所強調的“詭辭”以見義的表現方式，對於這一書寫策略的梳理，有利於我們進一步理解《太史公自序》，乃至《史記》全書的立場與語言風格。 This paper argues that Sima Qian 司馬遷, author of Shiji 史記 (Records of the Grand Historian) implanted various expressive intentions into his autobiographic writing “Taishigong zixu” 太史公自序 (The Autobiography of the Senior Archivist) by applying a series of special writing techniques, including the delicate rewriting of Guoyu 國語 (Discourses of the States) and 沿孝經 (Book of Filial Piety), deliberate rendering of the mysterious number “five hundred years”, contradictions within his dialogue with Hu Sui, and reconstructions of the temporal and spatial backgrounds of Chunqiu 春秋 (Spring and Autumn Annals), Lushi chunqiu 呂氏春秋 (Master Lü’s Spring and Autumn Annals), Gufen 孤憤 (Solitary Indignation) and other historical texts. This writing strategy originated from the Gongyang School, a commentary of Spring and Autumn Annals emphasizing guibian yi jianyi 詭辯以見義 “sophism for righteousness,” a way of manifestation allows writers to violate the facts for a better presentation of certain righteousness of the text generated from individualized writing. To examine this writing strategy will shed light on the authorial stance and language style of The Autobiography of the Senior Archivist and even the entire Records of the Grand Historian

Hybrid-functional and quasi-particle calculations of band structures of Mg2Si, Mg2Ge, and Mg2Sn

We perform hybrid functional and quasi-particle band structure calculations with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge, and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the characteristics of band edge states, i.e., band and valley degeneracies, and orbital characters, are found to be conserved, independent of the computational schemes such as density functional generalized gradient approximation, hybrid functionals, or quasi-particle calculations. However, the magnitude of the calculated band gap varies significantly with the computational schemes. Within density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge, and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely underestimated compared to the experimental gaps, due to the band gap error in the density functional theory and the significant relativistic effect on the low-energy band structures. By employing hybrid-functional calculations with a 35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and 0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively.Comment: 23 pages, including 84 references, 5 tables, 3 figure

Study on the activation of calcined kaolin

Calcined temperature is a key factor to the activity of metakaolin. Structure characteristics and alkali activation of kaolin and its calcined products at different temperatures were analyzed by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), infrared spectrometry (IR) and isothermal calorimetry. The results show that the activity of kaolin calcined at 900°C is best. The characteristic absorption peak of kaolin disappears, a large amount of Al atoms convert from 6-coordination to 5-coordination; some characteristic vibration peaks of kaolin disappear while characteristic absorption peaks of metakaolin appear; There is much heat evolution after mixing it with alkali and the compressive strength is the highest. The strength of samples cured at 80°C for 3 days and 7 days reaches 33.8 and 35.3 MPa respectively