@article{chen_qin_liu_lin_zhang_lu_kim_bernholc_wang_zhang_2023, title={Interfacial origin of dielectric constant enhancement in high-temperature polymer dilute nanocomposites}, volume={122}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0143938}, DOI={10.1063/5.0143938}, abstractNote={The origin of dielectric constant enhancement in high-temperature (high glass transition temperature Tg) polymer dilute nanocomposites is investigated via Infrared (IR) Spectroscopy applied through Atomic Force Microscope (AFM) and density functional theory (DFT) calculations. The dielectric constant can be greatly enhanced by trace nanofiller loadings (<0.5 vol. %) in a broad class of high-temperature polymers without affecting or even with a positive influence on breakdown strength and dielectric loss. This avenue provides attractive polymer systems for high-performance polymer-based capacitive energy storage in a wide temperature range. In the dilute nanocomposites, the interface regions between the polymers and trace nanofillers are the key to the observed dielectric constant enhancement. This Letter employs AFM-IR to study chain packing in the interface regions of polyetherimide (PEI) dilute nanocomposites. The experimental results and DFT calculations indicate that flexible linkages, i.e., ether groups in PEI, play a crucial role in inducing heterogeneous morphologies in the interface regions. These results are confirmed by studies of PI(PDMA/ODA) and other dilute polymer nanocomposites in the literature as well as by lack of dielectric constant enhancement in PI(Matrimid® 5218) that does not contain flexible linkages.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Chen, Xin and Qin, Hancheng and Liu, Yang and Lin, Yen-Ting and Zhang, Bing and Lu, Wenchang and Kim, Seong H. H. and Bernholc, J. and Wang, Qing and Zhang, Q. M.}, year={2023}, month={May} }
 @article{han_qin_chen_xu_liu_bernholc_wang_2023, title={Microstructures and Ferroelectric Properties of Poly(vinylidene fluoride-ter-trifluoroethylene-ter-vinyl fluoride) Terpolymers}, volume={6}, ISSN={["1520-5835"]}, url={https://doi.org/10.1021/acs.macromol.3c00207}, DOI={10.1021/acs.macromol.3c00207}, abstractNote={The incorporation of comonomers as structural defects has been extensively exploited to modulate the structures and physical properties of poly(vinylidene fluoride) (PVDF)-based ferroelectric polymers. Previous studies have shown that the large-sized comonomers can induce relaxor ferroelectricity in the PVDF-based terpolymers as a result of the stabilization of the helical conformation. Here, we show that, with incorporating a small-sized defect, vinyl fluoride (VF), into the copolymer, the resulting terpolymers preferentially adopt the polar all-trans conformation, as revealed by the structural characterizations and confirmed by density functional theory simulations. Consequently, the terpolymers exhibit robust ferroelectricity with an increased ferroelectric-to-paraelectric transition temperature and a higher coercive field as the VF content increases. This work systematically investigates the compositional dependence of the microstructures, crystalline structures, and ferroelectric and electromechanical properties of the terpolymers and stresses the vital role of comonomer size in tailoring the structures and properties of ferroelectric polymers.}, journal={MACROMOLECULES}, author={Han, Zhubing and Qin, Hancheng and Chen, Xin and Xu, Wenhan and Liu, Yang and Bernholc, J. and Wang, Qing}, year={2023}, month={Jun} }
 @article{qin_lu_bernholc_2022, title={Ab initio simulations of metal contacts for graphene-based devices}, volume={131}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0091028}, DOI={10.1063/5.0091028}, abstractNote={The precise atomic structure of a metal contact significantly affects the performance of nanoscale electronic devices. We use an accurate, DFT-based non-equilibrium Green’s function method to evaluate various metal contacts with graphene or graphene nanoribbons. For surface metal contacts not chemically bound to graphene, Ti contacts have lower resistance than those of Au, Ca, Ir, Pt, and Sr. However, as an edge contact, Ti has larger resistance than Au. Bridging O atoms at Ti and Au edge contacts lowers the transmission by over 30%.}, number={21}, journal={JOURNAL OF APPLIED PHYSICS}, author={Qin, Hancheng and Lu, Wenchang and Bernholc, J.}, year={2022}, month={Jun} }
 @article{chen_qin_qian_zhu_li_zhang_lu_li_zhang_zhu_et al._2022, title={Relaxor ferroelectric polymer exhibits ultrahigh electromechanical coupling at low electric field}, volume={375}, ISSN={["1095-9203"]}, DOI={10.1126/science.abn0936}, abstractNote={Electromechanical (EM) coupling—the conversion of energy between electric and mechanical forms—in ferroelectrics has been used for a broad range of applications. Ferroelectric polymers have weak EM coupling that severely limits their usefulness for applications. We introduced a small amount of fluorinated alkyne (FA) monomers (<2 mol %) in relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) terpolymer that markedly enhances the polarization change with strong EM coupling while suppressing other polarization changes that do not contribute to it. Under a low–dc bias field of 40 megavolts per meter, the relaxor tetrapolymer has an EM coupling factor (k33) of 88% and a piezoelectric coefficient (d33) >1000 picometers per volt. These values make this solution-processed polymer competitive with ceramic oxide piezoelectrics, with the potential for use in distinct applications. Description Polymer piezo The best-performing piezoelectric materials are oxide ceramics, which are widely used for sensors and actuators. X. Chen et al. added two additional components to poly(vinylidene difluoride) trifluoroethylene to improve the electromechanical coupling (see the Perspective by Wang and Liao). The resulting tetrapolymer has piezoelectric properties that are dramatically improved and it appears to be competitive with traditional oxides. The pliability and relative ease of fabrication of this tetrapolymer piezoelectric makes it attractive for a range of interesting applications. —BG An improved polymer has properties that make it competitive with commercially available ceramic piezoelectrics.}, number={6587}, journal={SCIENCE}, author={Chen, Xin and Qin, Hancheng and Qian, Xiaoshi and Zhu, Wenyi and Li, Bo and Zhang, Bing and Lu, Wenchang and Li, Ruipeng and Zhang, Shihai and Zhu, Lei and et al.}, year={2022}, month={Mar}, pages={1418-+} }
 @article{qian_han_zheng_chen_tyagi_li_du_zheng_huang_zhang_et al._2021, title={High-entropy polymer produces a giant electrocaloric effect at low fields}, volume={600}, ISSN={["1476-4687"]}, DOI={10.1038/s41586-021-04189-5}, abstractNote={More than a decade of research on the electrocaloric (EC) effect has resulted in EC materials and EC multilayer chips that satisfy a minimum EC temperature change of 5 K required for caloric heat pumps1-3. However, these EC temperature changes are generated through the application of high electric fields4-8 (close to their dielectric breakdown strengths), which result in rapid degradation and fatigue of EC performance. Here we report a class of EC polymer that exhibits an EC entropy change of 37.5 J kg-1 K-1 and a temperature change of 7.5 K under 50 MV m-1, a 275% enhancement over the state-of-the-art EC polymers under the same field strength. We show that converting a small number of the chlorofluoroethylene groups in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer into covalent double bonds markedly increases the number of the polar entities and enhances the polar-nonpolar interfacial areas of the polymer. The polar phases in the polymer adopt a loosely correlated, high-entropy state with a low energy barrier for electric-field-induced switching. The polymer maintains performance for more than one million cycles at the low fields necessary for practical EC cooling applications, suggesting that this strategy may yield materials suitable for use in caloric heat pumps.}, number={7890}, journal={NATURE}, author={Qian, Xiaoshi and Han, Donglin and Zheng, Lirong and Chen, Jie and Tyagi, Madhusudan and Li, Qiang and Du, Feihong and Zheng, Shanyu and Huang, Xingyi and Zhang, Shihai and et al.}, year={2021}, month={Dec}, pages={664-+} }