The densification,microstructure, and electrical properties of aluminum-doped zinc oxide sputtering target for transparent conductive oxide film

AZO films are regarded as a potential substitute for ITO due to their excellent performance. To optimize the performances of AZO films, the correlation between the target and film must be clearly clarified. Therefore, how the properties, particularly the electrical ones, of the sputtering targets evolve with the sintering parameters are rarely highlighted. To develop high-quality AZO and ZnO targets, the densification, microstructure, and electrical properties of the targets were investigated in this study.The results showed that after sintering at 1100 °C in air, the 2 wt% Al₂O₃ additive in ZnO results in retarded densification, the formation of ZnAl₂O₄ phase, and inferior electrical properties. However, after sintering at 1200 °C or higher temperatures, the Al₂O₃ additive leads to finer grain size, higher sintered density, and better electrical properties. In general, the AZO targets are also found to exhibit higher Hall mobility and lower carrier density than the AZO films do.     

The sintering behavior,microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets

The properties of sputtering targets have recently been found to affect the performances of sputtered films and the sputtering process. To develop high-quality GZO ceramic targets, the influences of Ga₂O₃ content and sintering temperature on the sintering behavior, microstructure, and electrical properties of GZO ceramic targets were studied.The results showed that the increase in Ga₂O₃ content from 3 wt% (GZO-3Ga) and 5 wt% (GZO-5Ga) not only inhibited the densification but retarded grain growth. During sintering, ZnGa₂O₄ phase formed before 800 °C, and Zn₉Ga₂O₁₂ phase was found after sintering at 1000 °C. Moreover, after sintering at 1200 °C, the number of Zn₉Ga₂O₁₂ precipitates increased at the expense of ZnGa₂O₄ and ZnGa₂O₄ disappearing completely. The relative density, grain size, and resistivity of GZO-3Ga sintered at 1400 °C in air were 99.3%, 3.3 μm, and 2.8 × 10⁻³ Ω cm, respectively. These properties of GZO ceramics are comparable to properties reported in the literature for AZO sintered in air.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Joining MgAl2O4 ceramics using ZnO–Al2O3–SiO2 glass ceramic with precipitated ZnAl2O4

In this work, crystallization, thermal expansion and wetting behavior of ZnO–Al₂O₃–SiO₂ (ZAS) glass were first investigated. The results showed that ZnAl₂O₄ was precipitated from ZAS glass after crystallization treatment. Crystallization increased the coefficient of thermal expansion (CTE) of ZAS glass ceramic due to the high CTE of ZnAl₂O₄. In addition, ZAS glass exhibited good wettability on the surface of MgAl₂O₄ substrate. On this basis, ZAS glass was used to join MgAl₂O₄ ceramic, and the microstructure and mechanical properties of joints obtained with different cooling methods were investigated. The flexural strength of joints was related to the content of ZnAl₂O₄ crystals in the brazing seams. Additional nucleation and crystallization treatment during cooling process improved the crystallinity of brazing seam, resulting in better matching of the CTE of brazing seam with that of MgAl₂O₄ ceramic. The maximum flexural strength of joints reached 201 MPa, which was equivalent to the strength of MgAl₂O₄ ceramic.     

Microstructure,Thermal Conductivity,and Electrical Properties of In Situ Pressureless Densified SiC–BN Composites

The microstructure, thermal conductivity, and electrical properties of pressureless densified SiC–BN composites prepared from in situ reaction of Si₃N₄, B₄C, and C were systematically investigated, to achieve outstanding performance as substrate materials in electronic devices. The increasing BN content (0.25–8 wt%) in the composites resulted in finer microstructure, higher electrical resistivity, and lower dielectric constant and loss, at the expense of only slight degradation of thermal conductivity. The subsequently annealed composites showed more homogeneous microstructures with less crystal defects, further enhanced thermal conductivities and electrical resistivities, and reduced dielectric constants and losses, compared with the unannealed ones. The enhanced insulating performance, the weakened interface polarization, and the reduced current conduction loss were explained by the gradual equalization of dissolved B and N contents in SiC crystals and the consequent impurity compensation effect. The schottky contact between graphite and p‐type SiC grains presumably played a critical role in the formation of grain‐boundary barriers. The annealed composites doped with 8 wt% BN exhibited considerably high electrical resistivity (4.11 × 10¹¹ Ω·cm) at 100 V/cm, low dielectric constant (16.50), and dielectric loss (0.127) at 1 MHz, good thermal conductivity [66.06 W·(m·K)^?1] and relatively high strength (343 MPa) at room temperature.     

Electrical properties of new tin dioxide varistor ceramics at high currents

New dense SnO₂-based varistor ceramics with high nonlinear current–voltage characteristics (nonlinearity coefficients are of approximately 50) in a system of SnO₂–CoO–Nb₂O₅–Cr₂O₃–Y₂O₃–SrO–MgO are reported. The current–voltage behaviour at high currents is studied by using exponential voltage pulses. The obtained SnO₂ varistor ceramics exhibit low grain resistivity values of 0.23–0.64 ohm cm. To date, such values are the lowest known for SnO₂ varistors, and are closely approaching the grain resistivity of the ZnO varistor. The current–voltage characteristics of the obtained SnO₂-based varistor materials are reproducible in a wide current range from 10^?11 to approximately 10⁴ A cm^?2. The minimum current density and the minimum electric field necessary to cause the irreversible electrical breakdown are measured. It is established that a decrease in the grain resistivity leads to an increase in the minimum current density necessary for irreversible electrical breakdown to occur.     

Room temperature preparation of high performance AZO films by MF sputtering

Aluminum-doped zinc oxide (AZO) thin films have been deposited by MF magnetron sputtering from a ceramic oxide target without heating the substrates. This study has investigated effects of sputtering power on the structural, electrical and optical properties of the AZO films. The films delivered a hexagonal wurtzite structure with (002) preferential orientation and uniform surface morphology with 27–33 nm grain size. The results indicate that residual stress and grain size of the AZO films are dependent on sputtering power. The minimum resistivity of 7.56×10^?4 Ω cm combined with high transmittance of 83% were obtained at deposited power of 1600 W. The films delivered the advantages of a high deposition rate at low substrate temperature and should be suitable for the fabrication of low-cost transparent conductive oxide layer.     

Alumina nanocrystalline ceramic by centrifugal casting

Centrifugal casting is an established molding method to prepare ceramics with high strength and high reliability and it has been well demonstrated in Al₂O₃. However, it has not yet been applied to Al₂O₃ nanocrystalline ceramic with < 100 nm grain size, primarily due to the unavailability of high-quality α-Al₂O₃ nanoparticles. In addition, ultrafine nanoparticles may be difficult to cast from the solution unless high-speed ultracentrifuge (e. g., >60,000 rpm) is used. Here we addressed these two challenges by home-made dispersed α-Al₂O₃ nanoparticles with 10 nm average particle size and HCl-assisted casting under a “normal” centrifuge condition and report the first attempt to produce Al₂O₃ nanocrystalline ceramic by centrifuge casting and pressureless sintering. The sintering kinetics and microstructure were analyzed, which assists the design of optimal two-step sintering schedule. We showed that dense Al₂O₃ nanocrystalline ceramic with 65 nm average grain size and ultra-uniform microstructure (the standard deviation of the grain size distribution to the average grain size is 0.358) can be obtained by two-step sintering at 1175 °C without holding followed by holding at 1025 °C for 20 h. The ultra-uniform microstructure may result from the denser and more uniform packing of particles in the green bodies produced by centrifugal casting. The two-step sintered Al₂O₃ nanocrystalline ceramic has a microhardness of 19.9 GPa. The microhardness indicates potential softening (inverse Hall-Petch relationship) of Al₂O₃ nanocrystalline ceramic at such a grain size.     

Synthesis and performance of TiB2–B4C composites by high pressure of TiC–B mixture

TiB₂–B₄C composites were in situ synthesized and consolidated by high pressure synthesis method from a mixture of TiC and B powders at the pressure and temperature of 5.0 GPa and 1500℃-1900℃. The phase composition, microstructure, density, hardness, thermal conductivity, and electrical resistivity of TiB₂–B₄C composites were analyzed. As the increase in the synthesis temperature, the products were TiB₂ and B₄C phases and that crystallinity improved. TiB₂–B₄C composites were dense without obvious pores. TiB₂–B₄C composites synthesized at 1800℃ obtained the optimized performance, including the relative density of 98.2%, the Vickers hardness of 31.7 ± 1.2 GPa with the load of 9.8 N, the thermal conductivity of 30.3 ± 0.7 W/(m K), and the electrical resistivity of 3.3 × 10⁻³ Ω cm, respectively. The grain size of the TiB₂–B₄C composites changed with the increase in synthesis temperature, leading to the changes in hardness, thermal conductivity, and electrical resistivity.     

Effect of Internal Current Flow During the Sintering of Zirconium Diboride by Field Assisted Sintering Technology

Effect of electric current on sintering behavior and microstructure evolution of zirconium diboride (ZrB₂) was investigated using three different configurations of Field Assisted Sintering Technology/Spark Plasma Sintering. The current flow through the ZrB₂ compact was controlled by modifying the interface between the graphite punches and the electrical conductive powder. Boron nitride discs, graphite foils or direct contact with the graphite punches were the three different interfaces used in order to deflect, conduct or promote, respectively, the current during the sintering process of the electrically conductive ZrB₂ ceramics. The current flow during the sintering process triggered the elimination/reduction in B₂O₃, leading to faster diffusion rates at high temperatures and limiting the formation of B₄C secondary phase. This allows to control the final density, grain size (from 19.6 to 43.2 μm) and secondary phase formation (from 5.95 to 11.61 vol%) as well as the electrical resistivity (from 7.7 to 9.4 μΩ·cm) of the specimens.     

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.     

Effect of applied pressure on microstructure and properties of hot-pressed Ca-doped LaCrO3 ceramics

High-density chromium deficient calcium-doped lanthanum chromite-based ceramics (La_0.8Ca_0.2Cr_0.98O₃) were prepared by hot pressing (HP) at different sintering pressures, and the highest density can reach 98.8%. The effects of sintering pressure on the microstructure, mechanical properties, and electrical conductivity of La_0.8Ca_0.2Cr_0.98O₃ materials were studied. The experimental results show that HP can increase the density of lanthanum chromite-based ceramic materials and significantly inhibit the growth of grain size. As the sintering pressure increases, the strength and hardness gradually increase, but the fracture toughness decreases. When the sintering pressure is greater than 58 MPa, the presence of the second phase CaCr₂O₄ can be detected in the XRD results of the sintered ceramics. The SEM results showed that CaCr₂O₄ had two completely different morphologies in the sintered ceramics, and it was initially speculated that the possible causes were due to two different generation pathways. The electrical conductivity decreases with increasing sintering pressure, whereas the maximum electrical conductivity obtained is 18.61 S/cm in vacuum at 800°C for pressureless sintering ceramic.     

Effect of mixing glass frits on electrical property and microstructure of sintered Cu conductive thick film

The resistivity of the sintered Cu thick film decreases with the weight percentage of the SiO₂–ZnO–B₂O₃ additive in the mixing glass frits up to 50 wt%. As the weight percentage of the SiO₂–ZnO–B₂O₃ additive in the mixing glass frits is over 50 wt%, the resistivity of the sintered Cu thick films is quite similar. The lowest resistivity (6.62 × 10⁻⁶ Ω-cm) of the sintered Cu thick films occurs at 75 wt% of the SiO₂–ZnO–B₂O₃ additive. Also, we observe the extensive glass phase framing around the large Cu grains in the Cu thick films sintered with low SiO₂–ZnO–B₂O₃ additives (less than 50 wt%) narrows the cross-section area of the electrical path. On the contrary, the round-shaped glass phase solidified among the small Cu grains allows a larger cross-section of the electrical path (a possible lower resistivity) for the Cu thick films sintered with higher SiO₂–ZnO–B₂O₃ additives (larger than 50 wt%). The above results imply that the resistivity of the sintered Cu thick film correlates well with the microstructure (Cu grain size and the glass/Cu composite structure) of the sintered Cu thick films. Twin grain boundaries can clearly be observed in the sintered Cu thick films, especially for the Cu thick film sintered with the higher SiO₂–ZnO–B₂O₃ additives. Owing to small Cu grains size and high density of Cu grain boundary, the probability of the grain boundaries with a high grain-boundary energy in the Cu thick film sintered with high SiO₂–ZnO–B₂O₃ additive would be much larger, comparing to that in the Cu thick film sintered with low SiO₂–ZnO–B₂O₃ additive. Thus, more annealing twin boundaries formed in the Cu thick film sintered with high SiO₂–ZnO–B₂O₃ additive. Hence, the formation of the twin boundary in the sintered Cu thick film helps reducing the resistivity of the sintered Cu films.     

Spark Plasma Sintering and Densification Mechanisms of Conductive Ceramics under Coupled Thermal/Electric Fields

In spark plasma sintering (SPS), thermal and electric fields are applied simultaneously as a material is densified under pressure. The interactions between these two types of physical fields influence the densification behavior during SPS. Moreover, the uniformity and spatial distribution of these fields are also influenced by sample size. In the current investigation, the densification behavior of electrically conductive aluminum‐doped zinc oxide (AZO) ceramics is studied to provide insight into the role played by the thermal and electric fields on densification mechanisms, as a function of sample size. Our results demonstrate that field uniformity and densification behavior depend on sample size, and that ultimately, this behavior can be rationalized in terms of the electrical conductivity characteristics. Our results show that in small samples with a diameter of 20 mm, both thermal and electric fields are spatially uniform, which result in homogeneous microstructure. In large samples with a diameter of 80 mm, however, spatial variations in both thermal and electric fields lead to microstructural inhomogeneities, such as incomplete particle–particle bonding. Furthermore, as the density of the AZO sample increases, the effective electrical conductivity increases due to a decrease in void/pore volume, which changes the densification mechanisms, especially for the larger sample. Thus, for effective sintering of larger samples, a two‐stage sintering sequence is proposed, which relies on the thermal field that evolves once the effective electrical conductivity increases in the sample. We provide experimental confirmation to this suggestion on the basis of results which demonstrate that by extending the hold time from 3 to 30 min, high‐density (99.4%), homogeneous AZO ceramic samples with a diameter of 80 mm can be achieved after sintering at 1200°C.     

Thermal ageing of nanostructured tetragonal zirconia ceramics: Characterization of interfaces

This paper reports the preparation of nanometric powders of 3.5 mol% Y₂O₃-doped ZrO₂, with controlled microstructure, by the spray pyrolysis process, assisted by ultrasonic atomizer, at relatively low temperature. As-prepared powders were found crystalline and consisted of dense and chemically homogeneous spherical particles. Conventional sintering at 1500 °C for 2 h in air yields dense ceramics of 83 nm of average grain size. The electrical properties of electrode/electrolyte interface were determined by impedance spectroscopy measurements before, during and after thermal ageing for 2000 h at 700 °C in dry air. The effect of thermal ageing on the electrical responses of the ceramic and interfaces with platinum electrodes was investigated.     

Flash sintering of Al2O3–ZrO2 ceramics under alternating current electric field

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al₂O₃–ZrO₂) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al₂O₃–ZrO₂ composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al₂O₃–ZrO₂ composite ceramic was fabricated via AC flash sintering under 60 mA mm^?2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering.     

Sintering kinetics and properties of NiFe2O4-based ceramics inert anodes doped with TiN nanoparticles

Sintering kinetics of NiFe₂O₄-based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe₂O₄-based ceramics possessed high-temperature semiconductor essential characteristics. The electrical conductivity of NiFe₂O₄-based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe₂O₄-based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.     

Microstructures and mechanical properties of brazed Al2O3/Cu joints with bismuth glass

Alumina ceramics and copper were vacuum brazed using a bismuth-borate-zinc glass at temperatures between 660 and 720 °C for 20 min. The interfacial phases were characterized and the influence of brazing temperatures on microstructure of the joints were investigated. Shear tests of brazed joints under different conditions were also performed, and the joint fracture was observed and analyzed to determine the influence of brazing temperatures on the joint mechanical properties. Al₂O₃ ceramic/Al₂O₃ + glass phase/ZnAl₂O₄ + glass phase/(Ni, Cu)O/Ni(s.s) + BiNi/copper was identified as the main structure of the Al₂O₃ joints brazed with the glass. As the brazing temperature increased, the (Ni, Cu)O oxide layer in the joint was observed to thicken and extend to both sides gradually. The amount of BiNi formed in the Ni coating layer increased, and the scattered ZnAl₂O₄ particles gradually grew. When the brazing temperature reached 700 °C, ZnAl₂O₄ particles agglomerated on the alumina ceramic side, and glass permeated into the alumina base material. The shear strength of the joint first increased and then decreased with the increase of brazing temperature. The shear strength reached the maximum value of 21.1 MPa when brazed at 680 °C.     

Electrical properties of Y/Mg modified NiO simple oxides for negative temperature coefficient thermistors

The semiconductors based on simple oxide have unique features with controllable electrical property by element doping. Y³⁺ doped NiO (Ni_1−xY_xO, x ≤ 0.01) and Mg²⁺ substituted Ni_0.995Y_0.005O (Ni_0.995−yY_0.005Mg_yO, y ≤ 0.5) powders were synthesized by a wet chemical method. The related ceramics were obtained by conventional ceramic processing. Phase component, microstructure, electrical property and temperature sensitivity of the prepared ceramics were investigated. All ceramics have a rock-salt type crystalline structure. The room-temperature resistivity of the ceramics can be widely adjusted from 254 to 12 322 Ω·cm by changing the concentrations of Y³⁺ and Mg²⁺ ions. The samples show typical characteristics of negative temperature coefficient of resistivity and have high temperature sensitivity with material constants higher than 4745 K. The analysis of impedance spectra indicates that the electrical properties resulted from both grain effect and grain boundary effect. Both band conduction and small polaron hopping were proposed as possible conduction mechanisms in the studied ceramics.     

Enhanced sintering of Ce0.8Nd0.2O2−δ-La0.8Sr0.2Ga0.8Mg0.2O3−δ using CoO as a sintering aid

Ce_0.8Nd_0.2O_1.9 (NDC) and La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) electrolytes were prepared using a sol-gel method. NDC-LSGM composite electrolytes were subsequently prepared by adding 5% (w, mass fraction) precalcined LSGM powders to NDC sols. The electrolyte materials of NDC-Co and NDC-LSGM-Co were obtained by adding 1 mol% CoO to NDC sols and NDC-LSGM composite electrolytes, respectively. The microstructure and phase composition of the pellets were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray spectroscopy (EDS). The electrical conductivities of the pellets were measured using alternative current (AC) impedance spectroscopy. The results indicate that a single perovskite phase is observed for the LSGM ceramic, while NDC-Co, NDC-LSGM and NDC-LSGM-Co have a cubic fluorite structure similar to that of NDC. As a sintering aid, CoO can further promote grain growth and increase relative density (＞95%) of the NDC-LSGM composite electrolyte. The enhancement of the total conductivity is primarily attributed to the large increase in the conductivity of the grain boundary. However, the slight decrease of the grain boundary conductivity of the NDC-LSGM-Co electrolyte is caused by the presence of trace amounts of impurity phases in the grain boundaries.