High‐Performance Triboelectric Nanogenerators Based on Electrospun Polyvinylidene Fluoride–Silver Nanowire Composite Nanofibers

The preparation of ferroelectric polymer–metallic nanowire composite nanofiber triboelectric layers is described for use in high‐performance triboelectric nanogenerators (TENGs). The electrospun polyvinylidene fluoride (PVDF)–silver nanowire (AgNW) composite and nylon nanofibers are utilized in the TENGs as the top and bottom triboelectric layers, respectively. The electrospinning process facilitates uniaxial stretching of the polymer chains, which enhances the formation of the highly oriented crystalline β‐phase that forms the most polar crystalline phase of PVDF. The addition of AgNWs further promotes the β‐phase crystal formation by introducing electrostatic interactions between the surface charges of the nanowires and the dipoles of the PVDF chains. The extent of β‐phase formation and the resulting variations in the surface charge potential upon the addition of nanowires are systematically analyzed using X‐ray diffraction (XRD) and Kelvin probe force microscopy techniques. The ability of trapping the induced tribocharges increases upon the addition of nanowires to the PVDF matrix. The enhanced surface charge potential and the charge trapping capabilities of the PVDF–AgNW composite nanofibers significantly enhance the TENG output performances. Finally, the mechanical stability of the electrospun nanofibers is dramatically enhanced while maintaining the TENG performances by applying thermal welding near the melting temperature of PVDF.     

聚合物前驱体法制备CTNA陶瓷及其性能

采用DSC-TGA、XRD和SEM对聚合物前驱体法制备的0.7CaTiO_3-0.3NdAlO_3(CTNA)陶瓷粉末进行了分析。结果表明:经550℃预烧后的粉末为无定形态;但是当预烧温度提高到600℃时,形成了钙钛矿结构的CTNA单相。这表明CTNA晶相是未经中间相而直接从无定形态的前驱体中结晶形成。与传统固相反应法相比,合成温度从1300℃大幅下降到600℃。经900℃预烧,1375℃烧结的样品,其εr为43.3,Q·f为34862GHz,τf为1.4×10–6℃~(–1)。     

Bi3.25La0.75Ti3O12薄膜的化学溶液淀积方法制备及表征

采用新颖的化学溶液淀积方法,配制出可长时间稳定的先体溶液,采用红外分析和热分析等手段,对成膜过程中发生的反应变化进行了分析,结果表明采用新的方法可以增加先体的稳定性,并且确定了薄膜的析晶温度.XRD测试表明薄膜在不到500℃即开始析晶,并且析晶情况与Bi含量有关.对薄膜的电学性能研究表明随着Bi含量的增加,薄膜的漏电性能和抗疲劳特性变差,矫顽场变大,而2Pr在Bi过量10%的情况下达到最大31.6 μ C/cm2.     

Effect of mechanical force on domain switching in BiFeO_3 ultrathin films

Ferroelectric polarization can be switched by an external applied electric field and may also be reversed by a mechanical force via flexoelectricity from the strain gradient.In this study,we report the mechanical writing of an epitaxial BiFeO3(BFO)thin film and the combined action of an applied mechanical force and electric field on domain switching,where the mechanical force and electric field are applied using the tip of atomic force microscopy.When the applied force exceeds the threshold value,the upward polarization of the BFO thin film can be reversed by pure mechanical force via flexoelectricity;when an electric field is simultaneously applied,the mechanical force can reduce the coercive electric field because both the piezoelectricity from the homogeneous strain and the flexoelectricity from strain gradient contribute to the internal electric field in the film.The mechanically switched domains exhibit a slightly lower surface potential when compared with that exhibited by the electrically switched domains due to no charge injection in the mechanical method.Furthermore,both the mechanically and electrically switched domains exhibit a tunneling electroresistance in the BFO ferroelectric tunnel junction.     

(Ba1-xSrx)(Zn1/3Nb2/3)O3固溶体系统第二相的研究

研究了(Ba1-xSrx)(Zn1/3Nb2/3)O3(BSZN)固溶体系统在烧结中生成的第二相,分析了第二相的分布、形貌、形成规律、机理以及对介电性能的影响。研究表明,系统的主晶相为BSZN,同时生成了大量的第二相——Ba5Nb4O15(、Ba5-xSrx)Nb4O15(x=0~5)、Ba2Nb2O7、BaNb2O6等。第二相的生成是由于ZnO的挥发造成的;烧结温度越高、烧结时间越长,ZnO的挥发越多,第二相的含量也越多。第二相的形貌大部分为长方形的板状,样品的表面部分或完全被第二相所覆盖,且有少量呈粒状散布于晶粒之间。第二相的生成会对系统介质损耗产生不利的影响。     

Biomorphous SiC ceramics prepared from cork oak as precursor

Porous ceramic materials of SiC were synthesized from carbon matrices obtained via pyrolysis of natural cork as precursor. We propose a method for the fabrication of complex-shaped porous ceramic hardware consisting of separate parts prepared from natural cork. It is demonstrated that the thickness of the carbon-matrix walls can be increased through their impregnation with Bakelite phenolic glue solution followed by pyrolysis. This decreases the material’s porosity and can be used as a way to modify its mechanical and thermal characteristics. Both the carbon matrices (resulted from the pyrolysis step) and the resultant SiC ceramics are shown to be pseudomorphous to the structure of initial cork. Depending on the synthesis temperature, 3C-SiC, 6H-SiC, or a mixture of these polytypes, could be obtained. By varying the mass ratio of initial carbon and silicon components, stoichiometric SiC or SiC:С:Si, SiC:С, and SiC:Si ceramics could be produced. The structure, as well as chemical and phase composition of the prepared materials were studied by means of Raman spectroscopy and scanning electron microscopy.     

钛酸锶钡材料应用于超高密度动态随机存储器的研究

铁电钛酸锶钡材料具有十分优越的介电性能 :高的介电常数 ,较低的介电损耗 ,好的绝缘漏电性能 ;而且 ,通过调节材料中的Ba/Sr成分比 ,可改变材料的居里相变温度TC,以满足特定应用环境的温度需要 ,在超高密度集成的动态随机存储器 (DRAM)方面表现出广阔的应用前景 .文章概括介绍了BaxSr1-xTiO3 薄膜材料在DRAM应用中已取得的最新研究进展 ,并对这一应用所面临的问题也进行了详细讨论     

Crystal structure and ferroelectric properties of Mn-doped ((Ka0.5Na0.5)0.935Li0.065)NbO3 lead-free ceramics

In this study, ceramics in Mn-doped ((Ka_0.5Na_0.5)_0.935Li_0.065)NbO₃ ceramics (Mn content = 0.25, 0.50, 1.00 and 1.50 mol%) were successfully prepared by the conventional mixed-oxide technique. The crystal structure was identified by XRD as a single-phase perovskite structure with tetragonal symmetry. The valence of Mn ions, characterized by the synchrotron XAS technique, was seen to change from Mn⁴⁺ to Mn³⁺ during the formation of the crystal. The valence state of Mn strongly affected the crystal structure and ferroelectric properties of the ceramics. The ferroelectric parameters show the decrease of remnant polarization and the increase of the coercive field with increasing MnO₂ content. The results confirm the Mn³⁺ in KNNL perovskite lattices, leading to the formation of oxygen vacancies and hardening effects.     

Effect of temperature-dependent surface heat transfer coefficient on the maximum surface stress in ceramics during quenching

We study the difference in the maximum stress on a cylinder surface σ_max using the measured surface heat transfer coefficient h_m instead of its average value h_a during quenching. In the quenching temperatures of 200, 300, 400, 500, 600 and 800°C, the maximum surface stress σ_mmax calculated by h_m is always smaller than σ_amax calculated by h_a, except in the case of 800°C; while the time to reach σ_max calculated by h_m (f_mmax) is always earlier than that by h_a (f_amax). It is inconsistent with the traditional view that σ_max increases with increasing Biot number and the time to reach σ_max decreases with increasing Biot number. Other temperature-dependent properties also have a small effect on the trend of their mutual ratios with quenching temperatures. Such a difference between the two maximum surface stresses is caused by the dramatic variation of h_m with temperature, which needs to be considered in engineering analysis.     

Critical behaviour of the smectic A*–C* phase transition and electroclinic effect of ferroelectric liquid crystals with terphenyl rigid core

The SmA*–SmC* phase transition was studied by measuring the temperature and electric field dependences of the optical tilt angle, the electric polarisation and the dielectric spectra collected in a wide frequency range. Critical behaviour of the phase transition was analysed by varying the length of the fluorinated part of the alkyl terminal chain and by differing fluorine substitution in the terphenyl core. Both tilt and polarisation show tricritical mean-field behaviour for all homologues with n?>?2. Almost all coefficients that describe the SmA*–SmC* transition in the frame of the Landau theory were derived for homologue series. Double fluorine substitution in the central ring of the core seems to promote the ‘de Vries'-type smectic A*–C* phase transition with a little layer shrinkage. These well correspond with the lower tilt angle and smaller changes of the birefringence at the phase transition compared to the other homologues.