Cu-intercalated (Bi0.5Sb1.5Te3)0.96(Bi2Se3)0.04 single crystals

A procedure is described for converting the conductivity type (p → n) of single crystals of the Bi_0.5Sb_1.5Te₃-4 mol % Bi₂Se₃ solid solution via copper intercalation. Using this procedure, we have produced high-performance single-crystal n-legs with a room-temperature thermoelectric power α = ?200 μV/K. This procedure facilitates the fabrication of thermoelectric coolers because the same solid solution can be used to produce p-and n-legs, the only difference being the presence of copper in the n-legs. We have fabricated thermolements and determined their characteristics in the range 100–300 K.     

Composition and thermoelectric power factor variation of (Bi2Te3)0.96(Bi2Se3)0.04 crystal in growth direction

(Bi₂Te₃)_0.96(Bi₂Se₃)_0.04 crystal, which is an n-type thermoelectric semiconductor, has many applications in thermoelectric cooling systems. Single crystal of this composition was grown by Traveling Heater Method. A sensible gradient in thermoelectric power factor was observed in the first quarter length of the prepared crystalline ingot. Characterizing the crystallization procedure and ingot composition, the gradient was attributed to the variation of the Bi₂Se₃ concentration of Bi₂Te₃–Bi₂Se₃ quasi-binary solid solution system. The structural properties were characterized by means of XRD analyses. Results of composition variation (Bi₂Se₃ distribution function) were in good correlation with experimental thermoelectric power factor measured along the grown rod.     

Ba(Ti0.96Sn0.04)O3陶瓷的物性研究

采用传统固相反应法制备了Ba(Ti_0.96Sn_0.04)O₃无铅压电陶瓷, 对其压电性能、介电性能、铁电性能和微观结构等进行了研究。研究发现, 原料以及制备工艺对Ba(Ti_0.96Sn_0.04)O₃陶瓷的压电性质具有较大的影响。与BaTiO₃陶瓷相比, Ba(Ti_0.96Sn_0.04)O₃陶瓷的正交-四方相变温度T_O-T得到了一定的提高, 并且T_O-T附近的热滞只有1.8℃。陶瓷的微观形貌呈现出较为复杂的畴结构, 主要以90°平行带状畴为主, 偶尔有少量不同构型的180°畴。电滞回线呈现为理想的近似矩形饱和形状的曲线, 剩余极化强度P_r为18.9 μC/cm², 矫顽场E_c为 2.5 kV/cm。此外, 非180°畴的翻转是引起陶瓷逆压电常数d₃₃^*的主要因素, 其值可达550 pm/V。     

AlN-SiC固溶体的制备及介电性能

以碳化硅和铝粉为原料,均匀混合后在不同气氛中于2000℃下保温0.5h制得粉体.用X射线衍射分析(XRD)和扫描电子显微镜(SEM)对其进行表征.同时在8.2～12.4 GHz频率范围内测试其介电性能.结果表明:在N2气氛中,当铝的含量较少时,未出现AlN物相,当铝含量超过10at%时,AlN相开始出现,并且AlN的含量随着铝含量的增加而增加.掺杂铝样品与未掺杂铝样品相比,其介电常数实部ε'和介电损耗tanδ皆降低,且随着铝含量的增加而逐步降低,主要是因为AlN具有较低的介电常数实部和损耗.     

Cd0.96Zn0.04Te单晶生长及晶体成分偏析

采用常规Ｂｒｉｄｇｍａｎ法和ＡＣＲＴ－Ｂ法进行Ｃｄ０．９６Ｚｎ０．０４Ｔｅ晶体生长实验。结果表明：ＡＣＲＴ产生的强迫对流在很大程度上消除了侧壁形核，有利于获得大的晶体；ＡＣＲＴ的加入提高了有效分凝系数ｋｅｆｆ，使其向平衡分凝系数ｋｅｑ趋近，导致轴向的偏析增大。     

不同退火温度下Zn0.96Co0.04O薄膜的磁性研究

通过对磁控溅射方法制备的Zn_0.96Co_0.04O薄膜退火前后结构和磁性的研究发现,Co进入ZnO的晶格中并取代了Zn的位置,形成稀磁半导体结构,显示了室温铁磁性。后期真空退火可以产生更多的氧空位,提高BMP之间发生交叠的几率,从而使磁矩增大。因此可以认为氧空位是产生磁性的必要条件,其磁性来源机制符合J.M.D.Coey的BMP模型。     

Temperature-induced modification on the structural, optical and morphological properties of Zn0.96Cu0.04O nanoparticles

Zn_0.96Cu_0.04O nanoparticles synthesized by co-precipitation method were annealed at different temperatures, 400, 500, 600 and 700 °C for 2 h in air atmosphere. Crystalline phases and optical studies of the nanoparticles were studied by X-ray diffraction (XRD) and ultraviolet (UV)–visible photo-spectrometer. Elemental composition was studied by the energy dispersive X-ray (EDX) analysis and the microstructure was examined by scanning electron microscope. The XRD showed that the prepared nanoparticles had different microstructure without changing a hexagonal wurtzite structure. The average crystallite size increased from 28 to 60 nm when the annealing temperatures increased from 400 to 700 °C. The EDX analyses confirmed the presence of Cu in ZnO system and the weight percentage was nearly equal to their nominal stoichiometry within the experimental error. The optical band gap was varied between 3.75 and 3.86 eV and found maximum, 3.86 eV at 500 °C. Existence of functional groups and bonding were analyzed by fourier transform infrared spectra. The observed blue shift in UV emission from 400 to 500 °C in photoluminescence spectra was due to the intrinsic and extrinsic impurities whereas the red shift after 500 °C was due to the increase of crystalline size and relaxation of tensile strain. The reduction in intensity of green band emission with temperature was due to the reduction of intrinsic and extrinsic defects in Zn–O–Cu lattice.     

Two-step growth of Cd0.96Zn0.04Te/Si(111) epilayers by HWE

High quality Cd_1−xZn_xTe, x = 0.04 epilayers are successfully grown directly on hydrogen-terminated Si(111) substrates by hot wall epitaxy method. Growth conditions are optimized in order to grow single crystal films with desired composition. It is found that surface morphology of the epilayers is dramatically affected by the growth temperature and the growth rate at the early stage of the crystal growth. Applying limited high substrate temperature of T_sub = 440 °C and low growth rate of 0.04 μm/h, the crystallinity is significantly improved and for the first time a pseudomorphic 2D growth is observed notwithstanding of the large lattice mismatch. Designing a suitable two-step growth process makes it possible that Cd_1−xZn_xTe/Si(111), x = 0.04 epilayers are fabricated with good surface morphology, which could be used as lattice matched substrates for MCT and MCZT epitaxy.     

(0.96NaNbO3-0.04CaZrO3)-xFe2O3反铁电陶瓷的介电及储能性能研究

0.96NaNbO₃-0.04CaZrO₃(简称NNCZ)陶瓷在室温下展现出稳定的双电滞回线, 但是其储能密度、储能效率和击穿强度都比较低, 限制其成为储能材料。本工作通过掺杂Fe₂O₃, 利用Fe ³⁺离子变价的特点, 实现NNCZ储能性能的优化。采用传统固相法制备了(0.96NaNbO₃-0.04CaZrO₃)-xFe₂O₃(简称NNCZ-xFe)反铁电储能陶瓷, 并对样品的相结构、微观形貌、电学性能和储能性能进行了表征, 重点研究了Fe₂O₃掺杂量对NNCZ陶瓷介电和储能性能的影响规律。结果表明, 样品均具有单一的钙钛矿结构, 掺杂Fe₂O₃能明显降低NNCZ陶瓷的烧结温度, 晶粒平均尺寸随着掺杂量增大先减小后增大, 掺杂量x=0.02时, 晶粒平均尺寸最小(5.04 mm), 且具有较好的储能性能。室温下, NNCZ-0.02Fe击穿强度为230 kV/cm, 击穿前的有效储能密度和储能效率分别为1.57 J/cm ³和55.74%。在125 ℃和外加电场为180 kV/cm下, NNCZ-0.02Fe的储能密度为4.53 J/cm ³。掺杂Fe₂O₃使NNCZ陶瓷的烧成温度降低, 氧空位的迁移速率下降, 抑制晶粒的长大, 同时降低了介电损耗, 使得击穿强度增加; 适量氧空位钉扎使得反铁电相向铁电相相翻转变得困难, 避免出现哑铃状双电滞回线, 从而提高储能效率。本研究结果表明NNCZ-xFe在电介质储能领域具有潜在应用价值。     

Effect of solvents on the structural, optical and morphological properties of Zn0.96Cu0.04O nanoparticles

Zn_0.96Cu_0.04O nanoparticles were synthesized by co-precipitation method using different solvents like ethanol, water and mixer of ethanol and water in 50:50 ratios. Crystalline phases and optical studies of the nanoparticles were studied by X-ray diffraction (XRD) and UV–visible photo-spectrometer. The XRD showed that the prepared nanoparticles have different microstructure without changing a hexagonal wurtzite structure. The calculated average crystalline size was high for ethanol (27.3 nm) due to the presence of more defects and low for water (26 nm) due to the reduction of defects and vacancies. The energy dispersive X-ray analyses confirmed the presence of Cu in ZnO system and the weight percentage is nearly equal to their nominal stoichiometry within the experimental error. The presence of lower Zn and Cu percentage in the sample prepared using ethanol than other solvents was due low reaction rate which was confirmed by XRD spectra. Water solvent has relatively stronger transmittance in the visible region which leads to the industrial applications especially in opto-electronic devices. The average crystalline size is slowly decreased from 27.3 nm (ethanol) to 26 nm (water) whereas energy gap is steadily increased from 3.56 eV (ethanol) to 3.655 eV (water) when water concentration is increased from 0 to 100 % in ethanol. Existence of functional groups and bonding were analyzed by FTIR spectra. The observed blue shift of UV emission from ethanol (349 nm) to water (340 nm) solution and the high I_UV/I_G ratio in water solution in photoluminescence spectra was due to the decrease of crystalline size and defects/secondary phases. The intensity of blue–green band emission was gradually decreased due to the reduction of defects and vacancies when water concentration is increased from 0 to 100 % in ethanol solution, which was consistent with the XRD observation.     

Dielectric and piezoelectric properties of CeO2-added nonstoichiometric (Na0.5K0.5)0.97(Nb0.96Sb0.04)O3 ceramics for piezoelectric energy harvesting device applications

In this study, nonstoichiometric (Na(0.5)K(0.5))(0.97)(Nb(0.96)Sb(0.04))O(3) ceramics were fabricated and their dielectric and piezoelectric properties were investigated according to the CeO(2) addition. In this ceramic composition, CeO(2) addition improved sinterability, electromechanical coupling factor k(p), mechanical quality factor Q(m), piezoelectric constant d(33), and g(33). At the sintering temperature of 1100°C, for the 0.2wt% CeO(2) added specimen, the optimum values of density = 4.359 g/cm(3), k(p) = 0.443, Q(m) = 588, ε(r) = 444, d(33) = 159 pC/N, and g(33) = 35 × 10(-3) V·m/N, were obtained. A piezoelectric energy harvesting device using 0.2 wt% CeO(2)- added lead-free (K(0.5)Na(0.5))(0.97)(Nb(0.96)Sb(0.04))O(3) ceramics and a rectifying circuit for energy harvesting were fabricated and their electrical characteristics were investigated. Under an external vibration acceleration of 0.7 g, when the mass, the frequency of vibration generator, and matching load resistance were 2.4 g, 70 Hz, and 721 Ω, respectively, output voltage and power of piezoelectric harvesting device indicated the optimum values of 24.6 mV(rms) and 0.839 μW, respectively-suitable for application as the electric power source of a ubiquitous sensor network (USN) sensor node.     

Enhanced thermoelectric properties of solution grown Bi2Te(3-x)Se(x) nanoplatelet composites

We report on the enhanced thermoelectric properties of selenium (Se) doped bismuth telluride (Bi(2)Te(3-x)Se(x)) nanoplatelet (NP) composites synthesized by the polyol method. Variation of the Se composition within NPs is demonstrated by X-ray diffraction and Raman spectroscopy. While the calculated lattice parameters closely follow the Vegard's law, a discontinuity in the shifting of the high frequency (E(g)(2) and A(1g)(2)) phonon modes illustrates a two mode behavior for Bi(2)Te(3-x)Se(x) NPs. The electrical resistivity (ρ) of spark plasma sintered pellet composites shows metallic conduction for pure Bi(2)Te(3) NP composites and semiconducting behavior for intermediate Se compositions. The thermal conductivity (κ) for all NP composites is much smaller than the bulk values and is dominated by microstructural grain boundary scattering. With temperature dependent electrical and thermal transport measurements, we show that both the thermoelectric power S (-259 μV/K) and the figure of merit ZT (0.54) are enhanced by nearly a factor of 4 for SPS pellets of Bi(2)Te(2.7)Se(0.3) in comparison to Bi(2)Te(3) NP composites. Tentatively, such an enhancement of the thermoelectric performance in nanoplatelet composites is attributed to the energy filtering of low energy electrons by abundant grain boundaries in aligned nanocomposites.     

Effects of SrTiO3 on dielectric and piezoelectric properties of K0.48Na0.48Li0.04Nb0.96Ta0.04O3-based piezoceramics

In this study, (100 ? x) K_0.48Na_0.48Li_0.04Nb_0.96Ta_0.04O₃ ? xSrTiO₃ (0 ≤ x ≤ 10) ceramics were fabricated via normal sintering of synthesized powder by using solid state reaction. All ceramics revealed pure perovskite structure, indicating formation of solid solution between KNNLT and ST up to 10%. With increasing x, the crystal structure of ceramics changed from orthorhombic to tetragonal and finally pseudocubic symmetry when x = 10. Ceramic containing 1% ST had orthorhombic and tetragonal symmetries, simultaneously. Investigation of the variation of dielectric constant of ceramics versus temperature revealed that for ceramic with x = 1, polymorphic phase transition (PPT) temperature between orthorhombic and tetragonal is less than room temperature. Thus coexistence of two different structures in this ceramic is due to vicinity of its composition to morphotropic phase boundary (MPB). As a result, the maximum piezoelectric constant was measured for this ceramic. Ceramics containing 5 and 7.5% ST tend to appear relaxor ferroelectric behavior which is because of chemical inhomogeneities in both A- and B-sites of the ABO₃ perovskite structure.     

Thermodynamic properties of Tl5Se2Cl-based solid solutions

The equilibrium subsolidus phase diagram of the TlCl-Tl₂Se-TlSe system has been mapped out using X-ray diffraction analysis and emf measurements on thallium concentration cells. The Tl₅Se₂Cl compound has been shown to be a nonstoichiometric phase with a homogeneity region extending over a considerable part of the Tl₂Se-Tl₅Se₂Cl-Tl₅Se₃ composition triangle. The emf results have been used to evaluate the relative partial thermodynamic functions of the thallium in the alloys studied and the standard integral thermodynamic functions (ΔG ⁰(298 K), ΔH ⁰(298 K), and S ⁰(298 K)) of Tl₅Se₂Cl and Tl₅Se₂Cl-based solid solutions.     

The solid solution and magnetic properties of (Mg,Cu)3 (PO4)2 phosphates

The powder synthesis of Mg_3−xCu_x(PO₄)₂ (0 ⩽ x ⩽ 3) between 1173 and 1273 K, shows a discontinuous solid solution. The X-ray diagrams of compositions 0 ⩽ x ⩽ 1.70 are similar to that of Mg₃(PO₄)₂. The compounds with 2.30 ⩽ x ⩽ 3 are isotypic with the stanskiite-related structure of Cu₃(PO₄)₂. Magnetic measurements (4.2–300K) achieved on powder samples with x = 0.50, 1.00, 1.50, 1.70 and 3 are reported. An antiferrodistorsive ordering induced by the cooperative Jahn — Teller effect of Cu²⁺ ions results in a one dimensional magnetic behaviour with the composition x = 3. The Bonner and Fisher's model for Heisenberg chains was used to determine the average value of the antiferromagnetic exchange.     

Phase composition and microwave dielectric properties of (Zn,Ni)TiNb2O8 solid solution

The Zn_1?xNi_xTiNb₂O₈ (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were synthesized by the conventional solid-state reaction method. The crystal structure and microwave dielectric properties were investigated by XRD, SEM and dielectric measurements. The XRD patterns of Zn_1?xNi_xTiNb₂O₈ ceramics showed that (Zn, Ni)TiNb₂O₈ phase and (Ni, Zn)_0.5Ti_0.5NbO₄ phase were obtained. With the increase of sintering temperature and Ni content, the amount of the second phase [(Ni, Zn)_0.5Ti_0.5NbO₄] enhanced, resulting in the increase of dielectric constant and the decrease of Q × f value. Moreover, the temperature coefficient of resonant frequency (τ_f) shifted to a positive value with increasing Ni content. In conclusion, the excellent microwave dielectric properties were obtained for Zn_0.7Ni_0.3TiNb₂O₈ ceramics sintered at 1,125 °C for 4 h: ε_r = 41.36, Q × f = 31,760 GHz, τ_f = ?9.2 ppm/°C. Furthermore, to realize the zero value for τ_f, a proper adjustment of Ni doping and sintering temperature can be feasible in this system.