Preparation of Ni-doped ZnO ceramics for thermoelectric applications

Zn_1−xNi_xO dense ceramics were prepared from Zn_1−xNi_xO nanoparticles with x varying from 0 to 0.06. These nanoparticles were synthesized by liquid route. In the sintered samples, the solubility limit of Ni in the Zn_1−xNi_xO wurtzite structure was found to be 0.03. The increase of x until 0.03 led to a significant raise in both electrical conductivity (σ) and absolute value of Seebeck coefficient (|S|). Ni-richer samples (x > 0.03) contained in addition a small amount of Ni rich secondary phase (Zn_yNi_zO) with a cubic structure similar to NiO. The thermoelectric properties of all samples were investigated from room temperature to 1000 K. All doped samples showed a n-type semiconducting conductivity. For Ni contents higher than x = 0.03, the increase of the secondary phase content induced a decrease in σ and |S|. The highest power factor (0.6 mW m⁻¹ K⁻²) and ZT (0.09) were found for Zn_0.97Ni_0.03O at 1000 K.     

Tuning of defects in ZnO nanorod arrays used in bulk heterojunction solar cells

With particular focus on bulk heterojunction solar cells incorporating ZnO nanorods, we study how different annealing environments (air or Zn environment) and temperatures impact on the photoluminescence response. Our work gives new insight into the complex defect landscape in ZnO, and it also shows how the different defect types can be manipulated. We have determined the emission wavelengths for the two main defects which make up the visible band, the oxygen vacancy emission wavelength at approximately 530 nm and the zinc vacancy emission wavelength at approximately 630 nm. The precise nature of the defect landscape in the bulk of the nanorods is found to be unimportant to photovoltaic cell performance although the surface structure is more critical. Annealing of the nanorods is optimum at 300°C as this is a sufficiently high temperature to decompose Zn(OH)₂ formed at the surface of the nanorods during electrodeposition and sufficiently low to prevent ITO degradation.     

Bi2Te3-MWCNT nanocomposite: An efficient thermoelectric material

Bi₂Te₃–MWCNT nanocomposite has been synthesized by hydrothermal technique and demonstrate the role of MWCNT for thermoelectric properties. Herein, MWCNT has been used as conducting filler, which leads to the enhancement in the electrical conductivity in the case of nanocomposite. Bi₂Te₃–MWCNT nanocomposite shows ~22% decrease in the thermal conductivity as compared to Bi₂Te₃ nanostructures, which is attributed to the enhanced phonon scattering at the interfaces of Bi₂Te₃–MWCNT nanocomposite. Due to the increase in the electrical conductivity and decrease in the thermal conductivity, the overall enhancement in the figure of merit is ~45% in Bi₂Te₃–MWCNT nanocomposite as compared to Bi₂Te₃ nanostructures.     

Band structure and thermoelectric properties of inkjet printed ZnO and ZnFe2O4 thin films

The band structure and thermoelectric properties of inkjet printed ZnO and ZnFe₂O₄ thin films have been investigated. The bulk pellets were prepared by a solid-state method and thin films were deposited using an inkjet printing method. Multiple print cycles were required to fabricate homogeneous films and the composition of the thin films can be varied by varying the relative amounts of liquid deposited. It was possible to obtain high thermoelectric properties of ZnO by controlling the ratios of dopant added and the temperature of the heat treatments. XRD analysis showed that the fabricated samples have a wurtzite structure and an additional ZnAl₂O₄ phase was formed with increasing Al content and sintering temperature. It was found that the band gap of Al doped ZnO becomes smaller with increasing Al content and thus the electrical conductivity of Al_x doped ZnO (x=0.04) thin films showed the highest electrical conductivity (114.10 S/cm). The ZnFe₂O₄ samples were compared against the ZnO samples. The formation of single phase cubic spinel structure of the sintered ZnFe₂O₄ samples was found and confirmed by X-ray diffraction technique. Secondary phase Fe₂O₃ was also detected for compositions with Zn (x≤0.4). Finally, we want to report that the electrical conductivity of Zn_xFe_3−xO₄ was lower than the conductivity of the Al-doped ZnO.     

Understanding the thermal evolution of defects in carbon-implanted ZnO single crystal

Defects and impurities play a major role in controlling the electrical and optical properties of semiconductor materials. Herein, hydrothermally grown ZnO single crystals have been implanted with carbon (C) dopants at room temperature and then annealed in argon atmosphere at various temperatures between 400 and 800 °C. The thermal evolution of C-related defects and their effects on the structural, optical and electrical properties of ZnO single crystals were systematically characterized and discussed. The results show ion implantation induces serious lattice disorder, and post-implantation annealing could promote the lattice renormalization, accompanied by an increase in crystal quality and average visible transmittance. Furthermore, it is found that the diffusion of octahedral carbon interstitial (C_i) along parallel to c-axis facilitates the growth of carbon sp² clusters due to its low migration barrier during annealing, which energetically contribute to the decrease of the resistivity. Meanwhile, abundant C_i will be able to enter into V_Zn to form C_Zn or combine with lattice O to form (C_O)_O donor defects upon annealing, dominating the increase of electron carrier concentration and enhancing the anomalous Raman mode at 510-525 cm⁻¹. These findings strengthen the fundamental understanding of the donor behavior of C impurities in ZnO.     

Characterization of Al-doped ZnO thermoelectric materials prepared by RF plasma powder processing and hot press sintering

RF plasma processing technique was used to prepare Al-doped (up to 4 at%) ZnO nanopowders. The as-prepared powders were subsequently hot pressed to form sintered pellets. The temperature-dependent electrical and thermal properties of the sintered materials were measured from room temperature to 800 °C. All the doped samples showed metallic electrical conductivity and the overall thermoelectric performances were comparable to doped ZnO thermoelectric materials obtained by other nanoprocessing techniques. The results showed that RF plasma processing technique can be effectively used to produce doped nanopowders. The thermoelectric performance of the doped samples was discussed and related to the microstructure of the materials.     

Interior-architectured ZnO nanostructure for enhanced electrical conductivity via stepwise fabrication process

Fabrication of ZnO nanostructure via direct patterning based on sol-gel process has advantages of low-cost, vacuum-free, and rapid process and producibility on flexible or non-uniform substrates. Recently, it has been applied in light-emitting devices and advanced nanopatterning. However, application as an electrically conducting layer processed at low temperature has been limited by its high resistivity due to interior structure. In this paper, we report interior-architecturing of sol-gel-based ZnO nanostructure for the enhanced electrical conductivity. Stepwise fabrication process combining the nanoimprint lithography (NIL) process with an additional growth process was newly applied. Changes in morphology, interior structure, and electrical characteristics of the fabricated ZnO nanolines were analyzed. It was shown that filling structural voids in ZnO nanolines with nanocrystalline ZnO contributed to reducing electrical resistivity. Both rigid and flexible substrates were adopted for the device implementation, and the robustness of ZnO nanostructure on flexible substrate was verified. Interior-architecturing of ZnO nanostructure lends itself well to the tunability of morphological, electrical, and optical characteristics of nanopatterned inorganic materials with the large-area, low-cost, and low-temperature producibility.     

Influence of annealing environments on the conduction behaviour of KNN-based ceramics

The electrical conduction behaviour of the lead-free (1-x)K_0·5Na_0·5Nb_0·95Sb_0·05O₃-xBi_0.5Na_0·41K_0·09ZrO₃ (x = 0.05) ceramics annealed in air, oxygen, and argon environments at various temperatures was investigated. Post-sintering annealing in different atmospheres affected the temperature of dielectric phase transition and electrical conductivity values significantly. The ceramic sample annealed in an oxygen environment showed the lowest conductivity for grain (σ_g ～ 1.22 × 10^?5 S/m) and grain boundary (σ_gb ～ 1.56 × 10^?6 S/m) regions at 250 °C which is due to the reduction in oxygen vacancy defects. The activation energy E_a for DC conduction in this sample obeyed the Arrhenius relation and was found to be ～ 0.99 ± 0.03 eV. An additional anomaly observed at T ～ 250 °C dielectrics vs. temperature plots for the as-sintered and argon-annealed samples is ascribed to defect relaxation, which was inconspicuous in the samples annealed in air and oxygen. In addition, the in-situ impedance measurement was performed to analyze the impact of argon atmosphere on the electrical conductivity of the O₂-annealed samples with high-temperature electrode curing.     

Influence of carbon nanotubes on thermal and electrical conductivity of zirconia-based composite

Carbon nanotubes (CNTs) are widely used in ceramic-matrix composites (CMC) as a filler. An individual carbon nanotube exhibits extremely high thermal conductivity, however, the influence of CNTs on the thermal conductivity of CMCs is moderate. In contrast, even a small quantity of CNTs significantly increases the electrical conductivity of CMCs. The present paper studies this contradictory influence for ZrO₂-CNTs composites with 3, 5, 10 and 20 vol% multi-wall carbon nanotubes (MWCNTs). Their thermal and electrical conductivity was studied by the laser flash method and electrochemical impedance spectroscopy. The analysis reveals that the moderate influence of MWCNTs on the thermal conductivity of composites originates from the similar thermal conductivity of MWCNTs in a bundle and zirconia. On the other hand, the substantial difference in the electrical conductivity of MWCNTs and zirconia leads to an exponential increase in the electrical conductivity of the ZrO₂-CNTs composite even with small additions of nanotubes.     

Al doping influences on fabricating ZnO nanowire arrays: Enhanced field emission property

Increase of free electrons concentration and decrease of defects density are the most desirable solution for stimulating the field emission property in semiconductor nanostructures. To implement this, herein we study the field emission efficiency of Al-doped ZnO nanowire arrays with different Al doping concentration, which were prepared by a simple facile hydrothermal method. The Al doping concentration plays a very important role on the structure, morphology, photoluminescence and field emission properties of the as-assembled samples. The results indicate that Al-doped ZnO nanowire arrays with Al doping concentration of 7 at% exhibit the highest quality crystalline structure and lowest defect density relative to others samples thanks to the suitable modification of Al doping, and this led to the increase of free electrons concentration and decrease of defects density, which have an excellent field emission performance with the lower turn on field of 1.03 V/μm and higher field enhancement factor of 20658. This remarkable field emission performances of the Al-doped ZnO nanowire arrays may provide promising applications for different field emission devices.     

Improved electrical and thermal conductivities of polysiloxane-derived silicon oxycarbide ceramics by barium addition

The electrical and thermal conductivities of bulk barium-added silicon oxycarbide (SiOC-Ba) ceramics are investigated. The SiOC-Ba ceramics exhibited improved electrical and thermal conductivities upon increasing the sintering temperature from 1450 °C to 1650 °C. Precipitation of graphitic carbon clusters observed by Raman spectroscopy and high-resolution transmission electron microscopy is attributed to the phase separation during the fabrication process. The increase in the electrical conductivity can be rationalized in terms of an increase in the density of the sp² CC bonds within the carbon clusters. The increase in the thermal conductivity is mainly attributed to the formation of interconnected graphitic clusters in the SiOC matrix and SiC embedded in the clusters. The electrical and thermal conductivities of the SiOC-Ba ceramics sintered at 1650 °C are 14.0 Ω^?1 cm^?1 and 5.6 W/m K, respectively, at room temperature. The electrical conductivity of SiOC-Ba sintered at 1550 °C is 5.3 Ω^?1 cm^?1 and 7.0 Ω^?1 cm^?1 at 2 and 300 K, respectively.     

Synergistically optimizing thermoelectric performance of ZnO ceramics by interfacial band alignment and self-doping defects

ZnO is a promising thermoelectric ceramic material due to non-toxicity and abundance in resources. However, its thermoelectric performance is limited by the intrinsic low carrier concentration and high thermal conductivity. In this work, we synthesized the (1 ? x)ZnO/xZnS (x = 0–0.05) powders by a two-step solution method followed by microwave sintering in an oxygen-deficient environment at 1000 ℃, and then produced the self-doped ZnO ceramics with ZnO/ZnS interfaces. The electrical and thermal properties was investigated from room temperature to 900 K. The ZnO/ZnS interface and self-doping significantly increased the electrical properties of ZnO ceramics, the electrical conductivity (σ) and Seebeck coefficient (α) increased simultaneously with temperature for (1 ? x)ZnO/xZnS (x > 0), and the highest power factor (PF, 3675 µW·m^?1·K^?2) was obtained from 0.98ZnO/0.02ZnS at 900 K. At the same time, the ZnO/ZnS interfaces and self-doped defects greatly reduced the lattice thermal conductivity. Finally, the highest ZT value of 0.94 has been reached in 0.95ZnO/0.05ZnS at 900 K.     

Influence of electric current on microstructure and electrical property of Al-doped ZnO ceramic consolidated by spark plasma sintering

Electrical current passing through SPS-ed Al-doped ZnO (AZO) ceramic in four conditions with same heating rate and dwell time has been conducted to investigate the influence of current on microstructure and electrical property. The applied current and thermal distribution of AZO ceramic are analyzed by finite element modeling. The local microstructure and electrical property of SPS-ed AZO in center and edge has been discussed in comparison. It is indicated that current density determines the thermal distribution and the coupling effects of current/thermal significantly influence the grain size, impurity phase ZnAl₂O₄ and dislocation defects. The increased current density contributes to the refinement of grain size and enhancement of impurity ZnAl₂O₄ which dramatically affects the resistivity. The elevated Hall mobility of SPS-ed AZO ceramic by the reduction of current density is mainly governed by weakening grain boundary scattering due to the enlarged grain size, which is confirmed to be the crucial factor on the variation of resistivity. The lowest resistivity of 6.1 × 10⁻⁴ Ω cm for SPS-ed AZO ceramic is achieved with extremely low current density passing through specimens.     

超声辅助均匀沉淀法由前躯体ZnS制备ZnO纳米颗粒及其表征

前躯体ZnS在超声辅助60℃的低温条件下,采用醋酸锌为锌源、硫代乙酰胺为硫源来制备,然后采用在空气中热处理前躯体ZnS的方法制备了直径约为20~40 nm的ZnO纳米颗粒。所得产物分别采用红外光谱(FTIR)、热重-差热分析(TGA-DTA)、X射线衍射(XRD)、场发射扫描电镜(FE-SEM)、透射电镜(TEM)、电子能谱(EDS)和荧光光谱(PL)进行表征。实验结果表明,所得产物ZnO为六方纤锌矿结构,且结晶性很好,并且随着超声时间的延长其粒径有所降低。室温PL光谱表明,样品在400~550 nm内有3个较强的荧光发射峰。     

Influence of Ba2+ doping on the thermoelectric properties of BiCuSeO fabricated by spark plasma sintering

We studied the influence of Ba²⁺ doping on the thermoelectric properties of the p-type Bi_1–xBa_xCuSeO (0?≤?x?≤?0.21) fabricated by spark plasma sintering. The substitution of Ba²⁺ for Bi³⁺ gradually increased the electrical and thermal conductivities and decreased the Seebeck coefficient, which were due to the increased hole concentration. The largest value of dimensionless figure-of-merit (0.57) was obtained for the Bi_0.86Ba_0.14CuSeO at 500?°C, which was over three times greater than that of pristine BiCuSeO (0.18) at 500?°C. We believe that the thermoelectric properties of BiCuSeO were substantially enhanced through the partial substitution of Ba²⁺ for Bi³⁺.     

Shift of tellurium solid-solubility limit and enhanced thermoelectric performance of bismuth antimony telluride milled with yttria-stabilized zirconia balls and vessels

As a thermoelectric material, Bi_0.3Sb_1.7Te_3.0+x (x = 0‒0.05) was fabricated by mechanical alloying using yttria-stabilized zirconia (YSZ) ceramic balls and vessels, followed by hot pressing. The effects of the added tellurium on the thermoelectric properties of Bi_0.3Sb_1.7Te_3.0 fabricated with YSZ milling media were investigated. All sintered samples were isotropic and showed p-type conduction. The tellurium solid-solubility limit for Bi_0.3Sb_1.7Te_3.0 was determined to be x = 0.01 by differential thermal analysis (DTA). The solid-solubility limit of the sample fabricated using YSZ was narrower than that of the congener prepared with Si₃N₄ balls and stainless-steel metal vessels. Among the evaluated compositions, the Bi_0.3Sb_1.7Te_3.01 sintered disk had the highest dimensionless figure of merit, ZT = 1.30, at room temperature. This value was superior to that of Bi_0.3Sb_1.7Te_3.0+x fabricated using metal vessels. Thus, selection of the milling media affected the optimum doping amount and maximum ZT.     

Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering

Al-doped ZnO ceramics were sintered by conventional sintering method and spark plasma sintering (SPS) respectively. Electrical properties and microstructure have been investigated by various measurements. The samples sintered via SPS exhibit a huge electrical conductivity, up to 3.0 × 10⁵ S/m at room temperature, which was much higher than that of the sample sintered via the conventional sintering. Structural and morphorlogical characterizations pointed out that the further incorporation of Al ions and the absence of a secondary phase, contribute to the increase of the carrier concentration. Raman spectroscopy revealed the occurrence of structural distortions and a disorder induced by Al doping. Photoluminescence spectra were interpreted by different electronic active defects such as the defect complexes (Al_Zn-Zn_i) which play a key for the high electrical conductivity. Thus, SPS and Al doping modified the microstructure and the concentration of the electronic active defects to ensure high electrical conductivities in doped ZnO-based ceramics.     

High improvement of degradation behavior of ZnO varistors under high current surges by appropriate Sb2O3 doping

The effects of Sb₂O₃ content on the microstructures and electrical properties of the ZnO varistors, especially on the degradation behavior under pulse current stress were studied. The results showed that the degradation behavior was effectively improved by doping appropriate amount of Sb₂O₃, which attributed to the homogenized microstructure and the compression of the interstitial void. The change rates of positive and negative breakdown voltage gradients of samples doped with 0.92 mol% Sb₂O₃ under 20 * 20 kA + 2 * 30 kA surges were -1.76 % and 1.56 % respectively, which was one of the best levels reported in the previous literatures. In addition, the sample exhibited excellent comprehensive electrical properties, with breakdown voltage gradient of 207.74 V mm^?1, nonlinear coefficient of 119.1, leakage current density of 1.03 × 10^-2 μA cm^-2, and clamping voltage ratio of 2.17 under 20 kA surge, making it a promising candidate for surge protector devices.     

Effects of SmF3 addition on aluminum nitride ceramics via pressureless sintering

Aluminum nitride (AlN) ceramics with dense structure, high thermal conductivity, and exceptional mechanical properties were fabricated by pressureless sintering with a novel non-oxide sintering additive, samarium fluoride (SmF₃). The results showed that the use of a moderate amount of SmF₃ promoted significant densification of AlN and removed the oxygen impurity. This led to the formation of fine and isolated secondary phase that cleaned the grain boundaries and increased the contact between AlN grains, remarkably enhancing thermal conductivity. Furthermore, SmF₃ also exhibited grain refinement and grain boundary strengthening effects similar to traditional sintering additive, samarium oxide (Sm₂O₃), leading to high mechanical properties in SmF₃-doped AlN samples. The most optimal characteristics (thermal conductivity of 190.67 W·m⁻¹·K⁻¹, flexural strength of 403.86 ± 18.27 MPa, and fracture toughness of 3.71 ± 0.19 MPa·m^1/2) were achieved in the AlN ceramic with 5 wt% SmF₃.     

Facile synthesis of ZnO nanostructures and enhancement of their sinterability via short-time cryo-milling

Zinc oxide (ZnO), a wide band-gap semiconductor, has received a great interest due to its potential applications in various fields both as nanostructures and as sintered compacts. In this study, we report on the synthesis of the ZnO nanostructures and facilitation of their sintering for the production of fine-grained dense compacts. The facile synthesis of gram scale ZnO nanostructures was achieved by thermal decomposition of zinc acetate dihydrate (Zn(Ac)₂·2H₂O) or Zn(Ac)₂·2H₂O/graphite mixtures at 300 °C for 12 h. Thermal decomposition of Zn(Ac)₂ resulted in the formation of mostly ZnO nanoparticles with wurtzite structure along with ZnO nanorods, while the addition of graphite significantly promoted the growth of ZnO nanowires. Microstructural and phase properties of the obtained ZnO nanostructures were determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) techniques, all of which revealed the successful synthesis of high quality ZnO nanostructures. In addition to synthesis and characterization of the ZnO nanostructures, we report on the enhancement of their sinterability by a subsequent cryogenic milling for a short duration of 5 min. As a result of the applied cryo-milling, fabrication of highly dense (96.2%) sintered compacts with fine grain sizes (572 nm) could be achieved after pressureless sintering at 1000 °C for 2 h.