Enhanced piezoelectricity in 0.7BiFeO_(3)-0.3BaTiO_(3)lead-free ceramics:Distinct effect of poling engineering

BiFeO_(3)-BaTiO_(3) based ceramics are considered to be the most promising lead-free piezoelectric ceramics due to their large piezoelectric response and high Curie temperature.Since the piezoelectric response of piezoelectric ceramics just appears after poling engineering,in this work,the domain evolution and microscopic piezoresponse were observed in-situ using piezoresponse force microscopy(PFM)and switching spectroscopy piezoresponse force microscopy(SS-PFM),which can effectively study the local switching characteristics of ferroelectric materials especially at the nanoscale.The new domain nucleation preferentially forms at the boundary of the relative polarization region and expands laterally with the increase of bias voltage and temperature.The maximum piezoresponse(Rs),remnant piezoresponse(Rrem),maximum displacement(Dmax)and negative displacement(Dneg)at 45 V and 120C reach 122,69,127 pm and 75 pm,respectively.Due to the distinct effect of poling engineering in full domain switching,the corresponding d33 at 50 kV/cm and 120C reaches a maximum of 205 pC/N,which is nearly twice as high as that at room temperature.Studying the evolution of ferroelectric domains in the poling engineering of BiFeO_(3)-BaTiO_(3)ceramics provides an insight into the relationship between domain structure and piezoelectric response,which has implications for other piezoelectric ceramics as well.

Cu_(1.8)S基合金的热电性能和电稳定性增强:熵工程和Cu空位工程

无污染、低成本和高性能Cu_(1.8)S基类液态热电材料受到关注.但是,其过高的本征Cu空位和Cu离子迁移特性限制了其性能和电稳定性的进一步提升.本研究采用机械合金化结合放电等离子体烧结制备了一系列Cu_(1.8)S和Mn_(x)Cu_(1.8)S_(0.5)Se_(0.5)(0.01≤x≤0.06)块体热电材料.随着Se和Mn的引入,体系由低熵Cu_(1.8)S(0.4R^(*))转变为中熵MnxCu_(1.8)S_(0.5)Se_(0.5)(1.2R^(*)).构型熵的增加不仅提高了体系的结构对称性,MnxCu_(1.8)S_(0.5)Se_(0.5)室温下呈立方相结构,还增大了Mn的固溶度.高浓度Mn固溶有效填补了过高的本征Cu空位,降低了载流子浓度,优化了能带结构,提升了电输运性能.熵工程一方面增大了Cu离子迁移势垒,抑制Cu离子迁移.750 K下,即使电流密度达到24 A cm^(-2),Mn_(0.03)Cu_(1.8)S_(0.5)Se_(0.5)的电阻也几乎没有变化,显示出优异的电稳定性;同时可降低声速,软化晶格,降低晶格热导率.Mn_(0.06)Cu_(1.8)S_(0.5)Se_(0.5)的块体样品在773 K时获得最大ZT值0.79,相较于初始样品提高了两倍.结果表明熵工程结合Cu空位工程是提升Cu_(1.8)S基热电材料性能的有效策略.

Deciphering the leakage conduction mechanism of BiFeO_(3)–BaTiO_(3)lead-free piezoelectric ceramics

BiFeO_(3)–BaTiO_(3)(BF–BT)based piezoelectric ceramics are a kind of high-temperature lead-free piezoelectric ceramics with great development prospects due to their high Curie temperature(TC)and excellent electrical properties.However,large leakage current limits their performance improvement and practical applications.In this work,direct current(DC)test,alternating current(AC)impedance,and Hall tests were used to investigate conduction mechanisms of 0.75BiFeO_(3)–0.25BaTiO_(3)ceramics over a wide temperature range.In the range of room temperature(RT)−150℃,ohmic conduction plays a predominant effect,and the main carriers are p-type holes with the activation energy(Ea)of 0.51 eV.When T>200℃,the Ea value calculated from the AC impedance and Hall data is 1.03 eV with oxygen vacancies as a cause of high conductivity.The diffusion behavior of thermally activated oxygen vacancies is affected by crystal symmetry,oxygen vacancy concentration,and distribution,dominating internal conduction mechanism.Deciphering the conduction mechanisms over the three temperature ranges would pave the way for further improving the insulation and electrical properties of BiFeO_(3)–BaTiO_(3)ceramics.

Preparation and thermoelectric properties of Cu1.8S/CuSbS2 composites

Chalcostibite(CuSbS2)is composed of earth-abundant elements and has a proper band gap(Eg=1.05 eV)as a thermoelectric(TE)material.Herein,we report the TE properties in the CuSbS2 based composites with a mole ratio of(1–x)CuSbS2–x Cu1.8S(x=0,0.1,0.2,0.3),which were prepared by mechanical alloying(MA)combined with spark plasma sintering(SPS).X-ray diffraction(XRD)and back-scattered electron image(BSE)results indicate that a single phase of CuSbS2 is synthesized at x=0 and the samples consist of CuSbS2,Cu3SbS4,and Cu12Sb4S13 at 0.1≤x≤0.3.The correlation between the phase structure,microstructure,and TE transport properties of the bulk samples is established.The electrical conductivity increases from 0.14 to 50.66 S·cm–1 at 723 K and at0≤x≤0.03,while the Seebeck coefficient holds an appropriate value of 190.51μV·K–1.The highest ZT value of 0.17 is obtained at 723 K and at x=0.3 owing to the combination of a high PF183μW·m–1·K–2 and a lowκ0.8 W·m–1·K–1.

Effect of Ni content on high power laser ablation behavior of coatings sprayed by Ni covering graphite/SiO2 powders

Faced with the challenge of high energy ablation problems, especially for laser ablation, effective energy dissipation protective materials fabricate by efficient preparation method is a feasible solution. The Ni-graphite/Si O2 coatings with different Ni content were prepared by plasma spraying method with optimized plasma spraying parameters. All coatings are pure without oxidation and dense. Their ablation behaviors were investigated by high power continuous wave laser. The results indicate that the Ni-graphite/Si O2 coating with appropriate Ni content could realize the purpose of energy consumption by endothermal reaction of graphite/Si O2 and reflection improvement. High Ni content will block the occurrence of endothermal reaction of graphite/Si O2 and increase the heat diffusion to interior part of coating, which can make the ablation situation of coating more serious.

A sound velocity method for determining isobaric specific heat capacity

Isobaric specific heat capacity(Cp)is an important parameter not only in physics but also for most materials.Its accurate measurement is particularly critical for performance evaluation of thermoelectric materials,but the experiments by differential scanning calorimetry(DSC)often lead to large uncertainties in the measurements,especially at elevated temperatures.In this study,we propose a simple method to determine Cp by measuring the sound velocity(υ)based on lattice vibration and expansion theory.The relative standard error of theυis smaller than 1%,showing good accuracy and repeatability.The calculated Cp at elevated temperature(>300 K)increases slightly with increasing temperature due to the lattice expansion,which is more reasonable than the Dulong–Petit value.

Enhanced thermoelectric performance in MnTe due to doping and insitu nanocompositing effects by Ag2S addition

Extremely low lattice thermal conductivity is always the pursuit of thermoelectric materials research.In this work,we reported an exceptional effect of Ag2S addition in MnTe,an emerging promising midtemperature thermoelectric material,to enable the realization of minimum lattice thermal conductivity,namely-0.4 Wm^(-1) K^(-1).Such a low lattice thermal conductivity is guaranteed by the incorporation of in-situ formed Ag rich phase(Ag2Te)with ultralow lattice thermal conductivity and further scattering of phonons from the partial doping effects induced point defects and boundaries between various phases.Apart from the dramatically decreased lattice thermal conductivity,the partial doping of Ag and S simultaneously enhance the electrical conductivity,further contributing to enhanced thermoelectric performance.Meanwhile,an inverse sign of Seebeck and Hall coefficient was observed and rationalized by the influence of highly electron-conductive Ag_(2)Te phase.Thanks to the synergetic modulation of electrical and thermal transport properties by in-situ formed composite,a high ZT value of 1.1 was achieved in MnTe based thermoelectric materials,which also demonstrates the importance of compositing approaches to design state-of-the-art thermoelectric materials.