Fabrication and high-temperature thermoelectric properties of Ti-doped Sr0.9La0.1TiO3 ceramics

Ti-doped Sr_0.9La_0.1TiO₃ ceramics with high density were successfully prepared in argon atmosphere by conventional solid state reaction. The influences of titanium doping content on the microstructure and thermoelectric properties were investigated. The results showed that titanium was oxidized during the calcination procedure. TiO₂ phase survived and coexisted with Sr_0.9La_0.1TiO₃ phase in the sintered ceramics. The Seebeck coefficients were increased from −163 to −259 μV/K as the temperature increased from 350 K to 1073 K. The thermal conductivity can be significantly reduced by doping Ti. Thermoelectric figure of merit (ZT) first decreased and then increased with increasing Ti doping content. Ceramics showed the best thermoelectric properties when Ti doping amount was 5 wt%, the maximum PF was 7.13 μW/K²/cm, and ZT value was 0.144 at 1073 K.     

Fabrication of highly textured Ca3Co4O9 ceramics with controlled density and high thermoelectric power factors

Ca₃Co₄O₉ ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca₃Co₄O₉ ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)₂ as a template and CaCO₃ as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m⁻¹ K⁻².     

Microstructure and thermoelectric properties of La0.1Dy0.1SrxTiO3 ceramics

La_0.1Dy_0.1Sr_xTiO₃ (x=0.80, 0.78, 0.75, 0.70) powders were synthesized via a sol-gel method, followed by sintering at 1550 °C in a reducing atmosphere of 5 vol% hydrogen in nitrogen. The microstructure and thermoelectric properties of the Sr-deficient La and Dy co-doped SrTiO₃ were investigated. The result of XRD revealed that La_0.1Dy_0.1Sr_xTiO₃ consisted of SrTiO₃ with a cubic crystal structure as the main phase and of a small amount of Dy₂Ti₂O₇ as the second phase. All the Sr-deficient samples exhibited a step-like microstructure. As the nominal Sr deficient content increased, the electrical conductivity of the Sr-deficient La_0.1Dy_0.1Sr_xTiO₃ ceramics enhanced due to the increasing Sr and oxygen vacancies, the absolute value of the Seebeck coefficient increased a little, and the thermal conductivity decreased to ~3.0 W m⁻¹ K⁻¹, leading to a high ZT value of 0.19 for La_0.1Dy_0.1Sr_0.75TiO₃ at 500 °C.     

Thermoelectric properties of the textured Bi1.9Gd0.1Te3 compounds spark-plasma-sintered at various temperatures

Elemental composition, crystal and grain structures, specific electrical resistivity, Seebeck coefficient, thermal conductivity, and thermoelectric figure-of-merit of n-type grained Bi_1.9Gd_0.1Te₃ compounds, spark-plasma-sintered at T_S = 690, 720, 750, 780 and 810 K, have been studied. All the samples are highly textured along the 001 direction parallel to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction. A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction was found within the 290 ÷ 630 K interval. Electrical resistivity decreases and thermal conductivity increases for parallel orientation as compared to these properties for perpendicular orientation. The T_S - effect on thermoelectric figure-of-merit of textured Bi_1.9Gd_0.1Te₃ compounds has been found and analyzed. Highest thermoelectric figure-of-merit (∼0.75) was observed for sample with T_S = 750 K at perpendicular orientation.     

Microstructural analyses and improved ionic conductivity of La0.62Li0.16TiO3 ceramics prepared by a reactive-templated grain growth (RTGG) process

Impedance measurements were conducted for textured lithium lanthanum titanate (LLTO) ceramics prepared by the reactive-templated grain growth (RTGG) process. Four La_0.62Li_0.16TiO₃ cuboid ceramics with different degree of <110> crystal orientation were used for the measurement at temperatures from 298 to 423 K. The ionic conductivity of the specimen processed with 20% of the reactive template was one order of magnitude higher than that without a template. The high conductivity was attributed to the decrease of grain boundaries caused by the templated grain growth. No obvious anisotropic conductivity was detected for the specimens by the influence of grain and domain boundaries. Domain structures in the LLTO grains were observed by scanning electron microscopy (SEM) for all specimens.     

Thermoelectric properties of Yb-doped La0.1Sr0.9TiO3 ceramics at high temperature

The effect of ytterbium doping on the thermoelectric properties of La_0.1Sr_0.9TiO₃ ceramics has been investigated at the temperature range between 300 K and 1000 K. Samples with different ytterbium concentrations have been synthesized by the conventional solid state reaction technique. An X-ray diffraction pattern suggests that the dominant crystal structure is of perovskite, with a small amount of pyrochlore phase of Yb₂Ti₂O₇. The electrical resistivities of all samples exhibit a minimum in the temperature range between 300 K and 1000 K. The minimum values of electrical resistivity are 1.5 mΩ cm, 2.0 mΩ cm, 3.9 mΩ cm, 7.1 mΩ cm, and 9.0 mΩ cm for x=0.01, 0.03, 0.05, 0.07, and 0.10, respectively. With the increasing ytterbium content, the electrical resistivity enhanced dramatically, the Seebeck coefficients are increased marginally, and the thermal conductivities are reduced moderately. The lowest thermal conductivity of 3.9 W/mK is obtained in sample of La_0.1Sr_0.89Yb_0.01TiO₃, which exhibits maximum figure of merit 0.20 at 963 K.     

Densification,microstructure, and thermoelectric properties of AZO/SrTiO3 composites

Al-doped ZnO (AZO) has emerged as a potential high-temperature thermoelectric material with an appropriate Seebeck coefficient and high thermal stability, and hence is considered as a promising material for power generation applications. Herein, we report the fabrication of AZO/SrTiO₃ composites with improved thermoelectric performance. The densification, microstructure, and thermoelectric properties of the AZO/SrTiO₃ composites were investigated. The significant increase in the relative density of AZO from 89.1 to 98.0% after the addition of SrTiO₃ indicates that SrTiO₃ promoted the densification of the composites. Furthermore, the electrical conductivity of AZO increased after the addition of SrTiO₃, which can mainly be attributed to its enhanced relative density. The AZO/SrTiO₃ composite with 2.0 wt% SrTiO₃ showed the highest power factor at 1000 K because of its highest electrical conductivity. In addition, the composite showed the highest ZT value, which was 1.8 times higher than that of pure AZO.     

过量TiO2助烧剂对Ba0.998La0.002TiO3陶瓷晶粒生长的影响

采用电性能测定和微观结构分析等方法研究了过量TiO2助烧剂对Ba0.998La0.002TiO3陶瓷晶粒生长的影响.结果表明:对于Ba0.998La0.002TiO3陶瓷在有液相参与烧结的晶粒溶解-淀析过程中,晶粒生长的必要条件是不仅要有液相的形成,而且BaTiO3晶粒必须在液相中有一定的溶解度.由于BaTiO3晶粒在Ba6Ti17O40和Ba2TiSi2O8液相中有足够的溶解度,促进了晶粒的生长,同时旌主掺杂离子进入晶格而释放氧,形成半导性的陶瓷,而BaTiO3晶粒不能溶解于Ba2Ti2SiP2O13液相中,抑制了晶粒的生长.     

Cu-modified Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 textured ceramics with enhanced electromechanical properties and improved thermal stability

Grain-oriented Pb(Mg_1/3Nb_2/3)O₃-PbZrO₃-PbTiO₃ (PMN-PZ-PT) based ceramics were synthesized through templated grain growth via using BaTiO₃ (BT) templates. Although BT templates are partially destroyed by PMN-PZ-PT matrix, CuO addition remarkably promotes [001]_c-oriented grain growth behavior of the ceramics, resulting in an improvement of Lotgering factor F₀₀₁ from ~86% to 98%. Both crystallographic texture and CuO doping increase tetragonality and reduce average domain size of the ceramics dominated by rhombohedral phase. Consequently, 0.50 wt% CuO-doped ceramics (F₀₀₁~98%) exhibit optimum electromechanical properties: d₃₃~860 pC/N, d₃₃×g₃₃~48.6 × 10^?12 m²/N, k_p~0.80, E_c~7.2 kV/cm, tan δ~0.8% and T_c~222 °C. In addition to ~3.7 times and 6.6 times higher d₃₃ and d₃₃×g₃₃, those ceramics possess about 240% enhanced piezoelectric strain and much better thermal stability (S_max/E_max variation ≤2% between RT and 150 °C) relative to non-textured counterpart. This work offers a good paradigm for simultaneously exploring high piezoelectric response and good temperature stability in piezoceramics, benefiting the development of next-generation advanced electromechanical devices.     

A novel high-entropy perovskite ceramics Sr0.9La0.1(Zr0.25Sn0.25Ti0.25Hf0.25)O3 with low thermal conductivity and high Seebeck coefficient

In this work, a novel high-entropy n-type thermoelectric material Sr_0.9La_0.1(Zr_0.25Sn_0.25Ti_0.25Hf_0.25)O₃ with pure perovskite phase was prepared using a conventional solid state processing route. The results of TEM and XPS show that various types of crystal defects and lattice distortions, such as oxygen vacancies, edge dislocations, in-phase rotations of octahedron and antiparallel cation displacements coexist in this high-entropy ceramic. At 873 K, the high-entropy ceramics showed both a low thermal conductivity (1.89 W/m/K) and a high Seebeck coefficient (393 μV/K). This work highlights a way to obtain high-performance perovskite-type oxide thermoelectric materials through high-entropy composition design.     

Synthesis of platelike CaTiO3 particles by a topochemical microcrystal conversion method and fabrication of textured microwave dielectric ceramics

Platelike CaTiO₃ particles with an orthorhombic perovskite structure have been synthesized by topochemical microcrystal conversion (TMC) from platelike precursor particles of the layer-structured CaBi₄Ti₄O₁₅ at 950 °C. The CaTiO₃ particles inherited and retained the shape of the precursor particles with a thickness of approximately 0.3 μm, and a width of 2–6 μm. XRD analysis showed that in the TMC reaction, the crystallographic {0 0 1} plane of CaBi₄Ti₄O₁₅ is converted into the {1 0 0} plane of CaTiO₃. Using the platelike CaTiO₃ particles as templates in the templated grain growth method, dense {1 0 0} grain-oriented CaTiO₃ ceramics having a {1 0 0} orientation could be fabricated at sintering temperatures between 1350 and 1500 °C. The maximum orientation factor reached 99.7% at 10% of template. It was found that texturing improves microwave dielectric low-loss properties, providing a 1.55 times higher Q_f value of 9310 GHz in textured ceramics compared to that of 6005 GHz in non-textured ceramics.     

织构化（Na_（1/2）Bi_（1/2））TiO_3-BaTiO_3无铅压电陶瓷晶粒定向生长机制

以熔盐法合成的片状SrTiO3晶粒为模板,利用模板晶粒生长（TGG）技术制备晶粒沿[001]方向为取向的0.94（Na1/2Bi1/2）TiO3-0.06BaTiO3（简写为BNBT6）无铅压电陶瓷,采用X线衍射仪（XRD）、扫描电子显微镜（SEM）对陶瓷试样进行表征,采用透射电子显微镜（TEM）观察SrTiO3与BNBT6基体界面的微观结构.结果表明,BNBT6陶瓷晶粒定向生长过程分为2个阶段：首先是异质外延生长阶段,即在片状模板晶粒的诱导下,BNBT6基体粉体在SrTiO3模板晶粒表面外延生长,形成与模板取向完全一致的单晶生长层的过程;其次是同质外延生长阶段,即单晶生长层生成后吞噬BNBT6基体粉体逐步生长得到各向异性的高取向BNBT6陶瓷的过程.     

Structure and enhanced thermoelectric properties of InGaO3(ZnO)m (m=1, 2, 3, 4, and 5) ceramics

InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics are a series of n-type oxide thermoelectric materials with layered structures and low thermal conductivities. Herein, InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics were fabricated by spark plasma sintering (SPS). Two different trends in the thermoelectric properties of the InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics were observed depending on the odevity of the m value. The InGaO₃(ZnO) sample exhibited a relatively high electrical conductivity and was therefore selected for vacuum annealing to further improve the electrical transport performance. Oxygen vacancy defects were introduced to the matrix during the annealing procedure, which improved the thermoelectric performance. A maximum ZT of 0.45 was obtained at 973 K for the InGaO₃(ZnO) sample with a 96 h vacuum annealing treatment, which is 30 times higher than that of the pristine sample.     

Significantly enhanced breakdown field with high grain boundary resistance and dielectric response in 0.1Na0.5Bi0.5TiO3-0.9BaTiO3 doped CaCu3Ti4O12 ceramics

Electrical performances are strongly associated with the electrical heterogeneity of grains and grain boundaries for CaCu₃Ti₄O₁₂ (CCTO) ceramics. In this work, the dielectric ceramics of 0.1Na_0.5Bi_0.5TiO₃-0.9BaTiO₃ (NBT-BT) doped CCTO were fabricated by a conventional solid-state reaction method, and the ceramics were sintered at 1100 °C for 6 h. Relatively homogeneous microstructures are obtained, and the average grain sizes are characterized about 0.9∼1.5 μm. Impressively, a significantly enhanced breakdown field of 13.7 kV/cm and a noteworthy nonlinear coefficient of 19.4 as well as a lower dielectric loss of 0.04 at 1 kHz are achieved in the 0.94CCTO-0.06(NBT-BT) ceramics. It is found that the improved electrical properties are attributed to the increased grain boundary resistance of 3.7 × 10⁹ Ω and the Schottky barrier height of 0.7 eV. This is originated from the NBT-BT compound doping effect. This work demonstrates an effective approach to improve electrical properties of CCTO ceramics by NBT-BT doping.     

Enhanced high temperature microwave absorption of La0.9Sr0.1MnO3/MgAl2O4 composite ceramics based on controllable electrical conductivity

The high temperature microwave absorbing efficiency (HTMAE) of xLa_0.9Sr_0.1MnO₃/(1 − x)MgAl₂O₄ composite ceramics was investigated by studying the crystal structure, electrical conductivity, and permittivity. The crystal structure of La_0.9Sr_0.1MnO₃ and MgAl₂O₄ were maintained, but the Mn³⁺ and Al³⁺ ions were exchanged with each other through doping. The conductivity and permittivity of the composite ceramics increased with the increase of La_0.9Sr_0.1MnO₃ content and test temperature. When x = 0.36, the electrical conductivity in La_0.9Sr_0.1MnO₃ significantly enhanced the microwave polarization of the composite ceramics at high temperature. According to transmission/reflection modelling, the composite ceramics with x = 0.24 showed excellent HTMAE near the optimal thickness of 1.8 mm. Although the optimal thickness of the composite with x = 0.36 was reduced to 1.1 mm, the HTMAE was seriously lessened due to an impedance mismatch. xLa_0.9Sr_0.1MnO₃/(1 − x)MgAl₂O₄ are promising as thin and efficient microwave absorbing materials at high temperatures and the microwave permittivity can be further enhanced by adjusting the conductivity of La_0.9Sr_0.1MnO₃.     

Grain size-driven effect on the functional properties in Ba0.6Sr0.4TiO3 ceramics consolidated by spark plasma sintering

Dense fine-grained Ba_0.6Sr_0.4TiO₃ ceramics with submicronic grains sizes (GS) have been prepared using nanopowders synthesized via sol-gel route and consolidated by Spark Plasma Sintering (SPS). By changing SPS parameters, the GS was reduced from 214 nm to 74 nm. Diffuse ferroelectric-paraelectric phase transitions and low values of dielectric permittivity (<1000) at the Curie temperature (T_C ∼280 K) were revealed by Impedance Spectroscopy in all sintered ceramics. The GS reduction from submicron to nanoscale range reflects in a gradually diminishment of dielectric constant, tunability, polarisation and storage energy properties. Raman spectroscopy investigations pointed out the presence of polar nanoclusters above the T_C. The short-range polar order is affected by the GS decrease, but becomes more thermally stable. The observed properties of Ba_0.6Sr_0.4TiO₃ nanostructured ceramics are interpreted by considering the interplay between the GS reduction, the role of low-permittivity grain boundaries and the diffuse character of the ferroelectric-to-paraelectric transformation.     

二氧化硅掺杂钛酸锶钡陶瓷的介电性能与电卡效应

采用传统的固相反应法制备二氧化硅（SiO₂）掺杂钛酸锶钡（Ba_0.65Sr_0.35TiO₃,BST）陶瓷（BST+x%SiO₂）,研究了掺杂二氧化硅对BST陶瓷的物相、微观形貌、介电性能及电卡效应的影响。结果表明：掺杂二氧化硅并未改变BST陶瓷的晶型结构,但有助于提升晶粒的均匀性和材料介电性能频率的稳定性。随着二氧化硅掺杂量增加,BST陶瓷的介电常数呈现单调递减趋势,介电弥散特性逐渐增强。二氧化硅的掺杂有利于提升BST陶瓷的电卡性能,其中BST+3%SiO₂陶瓷具有最优的电卡效应,在30 ℃下可获得最大电卡绝热温变（ΔT_max）,ΔT_max和ΔT_max/ΔE分别为1.6 ℃、8.00×10 ^-7 ℃·m/V,电卡效应半峰宽（T_span）为10 ℃左右。     

Core-shell structure strategy and oxygen vacancy engineering of 0.1LaMnO3@0.9(Ba0.5Sr0.5)TiO3 ceramics towards tuning electrical properties

In recent years, core-shell structures have attracted much attention for their potential in tuning the functional properties of materials and have become an effective method for preparing high-performance materials. In this work, core-shell structured 0.1LaMnO₃@0.9(Ba_0.5Sr_0.5)TiO₃ nanoparticles were fabricated by sol-gel method and dense ceramics were prepared at different sintering temperatures. The phase composition, morphology, chemical state and electrical properties were systematically investigated. Structural analysis confirmed the formation of 0.1LaMnO₃@0.9(Ba_0.5Sr_0.5)TiO₃ core-shell nanostructures. The results show that the prepared samples exhibit typical negative temperature coefficient characteristics in the range of 50–1000 °C, which is derived from the effect of carrier pairs and oxygen vacancies on the electrical properties. This study provides new insights from structure to material into guiding the development and design of advanced thermistor materials for temperature sensing applications.     

Luminescence and electrical properties of Eu-modified Bi0.5Na0.5TiO3 multifunctional ceramics

The Eu³⁺-modified Bi_0.5Na_0.5TiO₃ (BNT) ceramics have been fabricated by the solid-state reaction method. The impact of Eu³⁺ doping on the structure, photoluminescence, and electrical properties has been studied by XRD, SEM, PL spectra, and LCR meter. X-ray diffraction analysis reveals that the crystal structure of the samples is well matched with the trigonal perovskite, and the optimal temperature of presintering is 880°C. The Eu³⁺-doped BNT ceramics show excellent red fluorescence at 614 nm corresponding to the ⁵D₀→⁷F₂ transition of Eu³⁺ under 466 nm excitation and relatively long fluorescence lifetime. The BNT-0.02Eu ceramic density is up to 5.68 g/cm³ and the relative density is up to 94.6% with sintering temperature 1075°C. The piezoelectric constant (d₃₃) of samples has been significantly improved up to 110 pC/N by Eu³⁺ doping. The BNT-0.03Eu ceramic presintered at 880°C and sintered at 1050°C has good dielectric properties and excellent luminescence properties. Eu³⁺-doped BNT ceramics make it potential applications for novel integrated electro-optical and multifunctional devices.     

Band gap narrowing and magnetic properties of transition-metal-doped Ba0.85Ca0.15Ti0.9Zr0.1O3 lead-free ceramics

Transition metal (TM = Mn, Fe, Co, and Ni)-doped Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ (BCZT) lead-free ceramics with excellent optical and magnetic properties are synthesized via a solid-state reaction method. The effects of TM elements on the sintering, structure, optical, and magnetic properties of BCZT ceramics are investigated in detail. Structural phase transition from the coexistence of rhombohedral and tetragonal phases to a single rhombohedral phase is observed owing to grain refinement. A narrow band gap of 1.68 eV is achieved in the Co-doped BCZT. The optical absorption of TM-doped BCZT is enhanced, which is ascribed to the molecular orbital rearrangement caused by lattice distortion. Moreover the magnetic behaviors of TM-doped BCZT are observed. The Fe-doped BCZT presents the most evident ferromagnetism, resulting from the exchange coupling interaction between the Fe³⁺ ions and oxygen vacancies. These results provide additional insight into the use of TM-doped BCZT lead-free ferroelectric ceramics for various applications.