ZnO Ceramics Research Articles

Cold Sintered ZnO-Polystyrene (PS) Composites with Modified Interfacial Structures and Properties.

Lightweight and integrated electronic devices with high performance have become an inevitable trend in the development of power electronic systems. However, it is challenging to obtain lightweight and miniaturized varistors due to the low threshold electric field. Herein, (1 - x)ZnO-xPS composites with a high threshold electric field are prepared through a cold sintering process at 220 °C for 50 min, utilizing lightweight PS to modulate the grain boundary structures of ZnO varistors. With PS content ≤1.5 wt %, the relative densities of cold sintered composites exceed 95%. PS thin layers with a thickness of 2-10 nm are distributed at the grain boundaries of ZnO, forming the Schottky barrier, which triggers nonlinear current-voltage responses. With the addition of PS, the threshold electric field is dramatically enhanced from 135 to 2703 V·mm-1, higher than that of commercial ZnO varistors, which is further demonstrated by the electric field simulation of the Finite Element Method. The interfacial resistance obtained from impedance analysis increases with the increase of the PS content, and the activation energy of (1 - x)ZnO-xPS composites is higher than that of pure ZnO ceramics. This study provides a promising approach for the development of lightweight and cost-effective varistor materials.

Large internal stress induced nonlinear current-voltage behavior in nanodiamond strengthened ZnO ceramics.

The modulation of the electrostatic potential barrier at grain boundaries determines the performance of many ceramic-based electronics such as varistors. However, conventional protocols relying on complex doping and annealing processes inevitably increase the inhomogeneity of microstructure, which may jeopardize the performance stability and mechanical reliability in service. Instead of doping, herein we demonstrate an effective strategy to modulate the potential barrier in ZnO-based low-voltage varistors by exploiting internal stress-induced piezoelectric polarization. The local residual stress as large as ~1 GPa can be created in the ZnO matrix by incorporating ultra-stiff nanodiamond particles using a cold sintering process. As a result, the composite with only 2 wt% of nanodiamond exhibits a prominent nonlinear current-voltage response at a low switch voltage of 15.7 V/mm, which is ascribed to the depressed barrier height induced by the distinct effects of positively and negatively charged polarization on grain boundaries. More strikingly, the large internal stress can significantly enhance the strength of the composite by more than 230% compared with the monolith, owing to the highly strengthened grain boundaries and crack-tip bridging from prestressed nanodiamonds. These findings add internal stress as a new dimension to design mechanically robust ceramic electronics with high performance.

Investigation of material ejection in laser decal transfer-based µ-3D printing of ZnO ceramics with microsecond pulsed CO2 laser

Investigation of material ejection in laser decal transfer-based µ-3D printing of ZnO ceramics with microsecond pulsed CO2 laser

Synthesis of ZnO and NiO nano ceramics composite high-performance supercapacitor and its catalytic capabilities

NiO and ZnO mixed nanocomposites were manufactured using the solution combustion process. As-prepared samples were analyzed using XRD. The XRD shows an average crystallite size of 35–40 nm. The elemental composition determined by EDS indicates a nearly equal proportion of Ni and Zn, with an atomic ratio of Ni/Zn = 0.96. The specific capacitances of NiO is 295.5 Fg-1, ZnO is 117.3 Fg-1 and ZnO/NiO nanocomposites is 561.75 Fg-1 which are more than NiO and ZnO alone. This study shows that constructing binary oxide nanocomposites is an approach for developing high-performance supercapacitor electrode materials. Experimental observations on catalytic activity revealed that NiO/ZnO increased catalytic activity. Furthermore, adding NiO to ZnO in the composite increased the overall amount of oxygen vacancies in the samples. Our research lays the door for a simple, inexpensive, nontoxic, and quick technique to synthesize binary transition metal oxide-based electrode materials for high-performance supercapacitors.

The use of MgO–ZnO ceramics to record pressure and temperature conditions in the piston–cylinder apparatus

Abstract. The factors affecting the calibration of pressure in the piston–cylinder and other solid-media apparatus are so multifaceted and complex as to challenge existing approaches. Here we demonstrate how MgO–ZnO ceramics may be used in piston–cylinder assemblies to routinely record the pressure–temperature conditions experienced by samples in each run. The miscibility gap between rock-salt- and wurtzite-structured phases in the binary system MgO–ZnO is well suited for this purpose as it is capable of recording pressure and/or temperature in situ with a typical sensitivity to pressure of ± 0.02 GPa (1 standard deviation) if temperature is known, or variations in temperature around a sample of ∼ 10 °C assuming pressure is constant. MgO–ZnO ceramics can simply replace the widely used MgO surrounding samples under most conditions, since they are almost as inert chemically as MgO and have similar mechanical properties. As a demonstration, we apply the method to a redetermination of the quartz–coesite univariant phase transition in the piston–cylinder, using different assembly materials, sizes, and pressure–temperature path protocols. Continuous monitoring of piston travel during the entirety of each run helps reveal the differences in behaviour of the apparatus under these variables. We show that several assumptions about the behaviour of the piston–cylinder apparatus are ill-founded, that there may be a discrepancy of ∼ 10 % in pressure between otherwise identical experiments conducted using slightly different experimental protocols, and that the effects of the various options for assembly materials are complex, depending on the pressure–temperature path of the experiment throughout its duration. We have also used the sensitivity of the miscibility gap to temperature to map the temperature distribution in two dimensions surrounding a platinum capsule in a piston–cylinder experiment. The routine inclusion of the ceramic in piston–cylinder assemblies would provide an archive of actual experimental P–T conditions experienced by samples.

Record-High Thermoelectric Performance in Al-Doped ZnO via Anderson Localization of Band Edge States.

Oxides are of interest for thermoelectrics due to their high thermal stability, chemical inertness, low cost, and eco-friendly constituting elements. Here, adopting a unique synthesis route via chemical co-precipitation at strongly alkaline conditions, one of the highest thermoelectric performances for ZnO ceramics ( 21.5µW cm-1 K-2 and 0.5 at 1100K in ) is achieved. These results are linked to a distinct modification of the electronic structure: charge carriers become trapped at the edge of the conduction band due to Anderson localization, evidenced by an anomalously low carrier mobility, and characteristic temperature and doping dependencies of charge transport. The bi-dimensional optimization of doping and carrier localization enable a simultaneous improvement of the Seebeck coefficient and electrical conductivity, opening a novel pathway to advance ZnOthermoelectrics.

In-situ rapid nondestructive characterization of multiple irregular three-dimensional grain boundary structures and electrical properties by nanorobot with SEM in ZnO

To address the problem of spatial quantification of irregular grain boundaries, a unique approach for the nondestructive characterization of multiple irregular three-dimensional grain boundaries is proposed using nanorobots with scanning electron microscopy (SEM) to accomplish in-situ rapid synchronized quantification of structures and electrical properties. By constructing a three-dimensional trigonometric geometric mapping relationship, multiple irregular three-dimensional grain boundary structures and electrical properties can be rapidly quantified. Experimental results demonstrate that it is feasible to accomplish in-situ rapid nondestructive characterizations. The two-dimensional profile shapes of the targeted grain boundaries at each layer exhibit irregular and non-uniform features in the Y-Z plane. The hypotenuse length between the virtual space tilting lines presents a three-dimensional characteristic in the X-Z plane. The spatial tilt angle of the virtual space tilting line can be calculated using the right-triangle relation. The unequal numbers of grains along the same tilted hypotenuse direction with equal lengths directly illustrate the existence of irregular and nonlinear grain boundaries in bulk ZnO ceramics. The relatively uniform spatial sizes of the grain boundary illustrate the coupled spatial characteristics existing in the irregular micro-structures. The relatively uniform equivalent circuit fitting parameters confirm the existence of relatively uniform doping components in bulk ZnO ceramics. Furthermore, this method provides a unique approach for the rapid nondestructive quantitative analysis of multiple three-dimensional irregular grain boundaries in bulk polycrystalline materials.

Structural and electromagnetic shielding of ZnO ceramics in X-band

Structural and electromagnetic shielding of ZnO ceramics in X-band

An Ultralow Concentration of Cr2O3 Dopants‐Driven Lower Temperature Sintering ZnO‐Based Varistor Ceramics

Low working voltage‐driven ZnO‐based varistor ceramics play an important role in the multilayer chip varistors, which require a low sintering temperature of ZnO varistor for its low energy consumption. Herein, a remarkable reduction of the sintering temperature from the usual 1100–1300 °C to 950 °C is successfully achieves in the ZnO ceramics via a certain 0.05 mol% of Cr2O3 dopants. The underlying mechanism is found to be involved with the formation of basal‐plane inversion boundaries between the ZnO grains, which can promote the rapid grain growth within the ceramics. Furthermore, the ZnO varistors with 0.05 mol% Cr2O3 dopant exhibit excellent performance. A low breakdown voltage of 416 V mm−1, a high nonlinear coefficient of 39, and a low leakage current of 3.4 μA are obtained simultaneously. This work presents an effective and promising approach for the cost‐efficient preparation of high‐performance ZnO‐based varistors, which have particular significance for the application of multilayer chip varistors with low working voltage.

Fabrication of highly conducting undoped ZnO ceramics by flash sintering

Highly conducting undoped ZnO ceramics were fabricated by flash sintering using commercial pure ZnO powder as a model system. The flash-sintered samples with diameters of 15 mm and heights of 2 mm exhibited relative densities ranging from 93.6% to 97.1% and room-temperature electrical conductivities between 205 and 590 S/cm. X-ray diffraction and Raman spectra indicated a high concentration of oxygen vacancies and zinc interstitials in the flash-sintered samples, increasing the carrier concentration. Detailed microstructural analyses show that clean grain boundaries and the morphology and distribution of impurities in flash-sintered samples are favorable for improving carrier mobility. Additionally, flash sintering has the advantage of reducing conditions, which reduces acceptor defects at ZnO grain boundaries, lowers Schottky barriers, and increases carrier mobility. The results show that flash sintering can synergistically enhance the carrier concentration and mobility in ZnO ceramics, making it a promising technique for fabricating highly conducting ZnO ceramics.

Influence of compacting pressure on the electrical properties of ZnO and ZnO:Mn ceramics

Undoped and Mn-doped ZnO ceramics were prepared from the powders compacted at different pressures and sintered in air at high temperature. Their structural, optical, light emitting and electrical characteristics as well as the distribution of chemical elements were studied. It was found that an increase in compacting pressure stimulates an increase in direct current conductivity in both undoped and doped samples. In the case of doped samples, this effect was accompanied by a decrease in the height of potential barriers at the grain boundaries. It is found that electron concentration in ceramic grains, estimated from the modelling of infrared reflection spectra, remained relatively constant. The analysis of luminescence spectra and spatial zinc distribution revealed that the increase in compacting pressure results in the accumulation of interstitial zinc at the grain boundaries forming channels with enhanced conductivity. These findings provide an explanation for the evolution of electrical properties of ceramic samples with compacting pressure.

Flash sintering of ZnO ceramics with combined power supplies at room temperature

A flash sintering method at room temperature based on combined power supplies is proposed in this paper. Flash sintering is performed in two steps. First, a high voltage is used to break down the sample and then disconnect the power supply. Second, a low-voltage power supply is used to provide a large current to densify the sample. The method does not require changing the properties of the material or the sintering atmosphere. In addition to reducing the capacity of the required power supply, there is no need for external auxiliary equipment. Different types of high-voltage breakdown are also studied. The temperature and grain size corresponding to different currents of low-voltage flash sintering are studied. The results show that the sintered ZnO ceramics reach 98.5% relative density. A possible mechanism of flash sintering using combined power supplies is analysed.

Investigation of Frequency-Stable Colossal Permittivity in ZnO Ceramics using Impedance Spectroscopy

Investigation of Frequency-Stable Colossal Permittivity in ZnO Ceramics using Impedance Spectroscopy

Deposition and photoluminescence of zinc gallium oxide thin films with varied stoichiometry made by reactive magnetron co-sputtering

This paper reports on the deposition and photoluminescence of amorphous and crystalline thin films of zinc gallium oxide with Ga:Zn atomic ratio varied between 0.3 and 5.7. The films are prepared by reactive direct current magnetron co-sputtering from liquid/solid gallium/zinc targets onto fused quartz substrates; the temperature of the substrate is varied from room temperature (RT) to 800 °C. The sputtering process is effectively controlled by fixing the sputtering power of one of the targets and controlling the power of the other target by plasma optical emission spectroscopy. The method, in conjunction with oxygen flow adjustment, enables the production of near-stoichiometric films at any temperature used. The composition analysis suggests a few at% oxygen deficiency in the films. The resulting deposition rate is at least an order of magnitude higher compared to the commonly used radio-frequency sputtering from a ceramic ZnO:Ga2O3 target. Deposited onto unheated substrates, the films with Ga:Zn ≈ 2 are X-ray amorphous. Well-defined X-ray diffraction peaks of spinel ZnGa2O4 start to appear at a substrate temperature of 300 °C. The surface of the as-deposited films is dense and exhibits a fine-featured structure observed in electron microscopy images. Increasing the deposition temperature from RT to 800 °C eliminates defects and improves crystallinity, which for the films with Ga:Zn ratio close to 2 results in an increase in the optical band gap from 4.6 eV to 5.1 eV. Room temperature photoluminescence established the main peak at 3.1 eV (400 nm); a similar peak in Ga2O3 is ascribed to oxygen-vacancy related transitions. A prominent feature around 2.9 eV (428 nm) is attributed to self-activation center of the octahedral Ga-O groups in the spinel lattice of ZnGa2O4. It was found that photoluminescence from ZnGa2O4 depends significantly on the stoichiometric ratio between Ga and Zn and the deposition/annealing temperature.

Microstructure and thermoelectric properties of pristine and Al-doped ZnO ceramics fabricated by cost-effective and eco-friendly wet chemistry methods

ZnO has potential as a thermoelectric material for high-temperature applications, but its performance is limited by the strong interconnection between electrical and thermal transportation. In this study we proved that ZnO bulk samples produced from cost-effective and environmentally friendly nitrates via ultrasonic spray pyrolysis and modified chemical precipitation methods followed by spark plasma sintering, demonstrate significantly higher thermoelectric performance than those fabricated from commercially available ZnO powder. To further explore the potential of the developed chemical precipitation technique, we synthesized a series of Al-doped samples with a nominal composition of Zn1–xAlxO (x = 0.02, 0.04, 0.06). The substitutuion of Al into the ZnO structure led to two significant effects: (i) a partial substitution of Zn with Al boosts the power factor; (ii) the formation of the ZnAl2O4 spinel phase acting as scattering center for phonons in addition to the mass and strain fluctuations induced by the partial Zn-Al substitution, both leading to the thermal conductivity reduction. As a result, Al doping led to a threefold enhancement of the thermoelectric performance with zT value of ∼0.12 achieved at 1100 K for the Zn0.94Al0.06O sample.

Processing and enhancement of thermoelectric performance of Nb doped ZnO (NZO) ceramics

In this investigation, we extensively studied the effect of various parameters on the processing, microstructure, sintering and thermoelectric characteristics of ZnO ceramics. Different Nb-doped ZnO-based ceramics, ranging from 0 to 0.25 wt %, were fabricated and sintered at the temperature of 1450 °C for 2 h in an inert atmosphere of argon (Ar). Various characterizations were employed to discern the performance attributes of these ceramics, encompassing phase analysis, densification, and microstructure evaluations. Thermal conductivity properties were studied from room temperature up to the temperature of 800 °C. Thermoelectric properties and efficiency of the sintered ceramics were evaluated across the temperature range of 100–800 °C. Results indicated that the incorporation of Nb into ZnO ceramics resulted in a harmonious structure due to the attained homogeneous and complete solid solution diffusion reaction. It was found that the optimal values for thermal conductivity, electrical resistivity, and thermoelectric properties were achieved at 0.15 wt % Nb doping. ZnO doped ceramics exhibited typical behavior of n-type semiconductors, showcasing exceptional thermoelectric properties at elevated temperatures. Our findings indicate that native defects, such as oxygen vacancies (OV), contribute to decreased band gap energy values and increased carrier concentration, thereby enhancing electrical conductivity and power factor. Hence, this study underscores the viability and practicality of Nb-doping ZnO sintered in an argon atmosphere as an effective approach to enhance its thermoelectric performance.

Tweaks to intrinsic point defects in ZnO ceramics via MnCO3 and their effect on electrical properties

Transition metal elements are necessary dopants in ZnO ceramics to improve electrical properties by defects engineering. In the present study, the tweaking effect of MnCO3 on the intrinsic point defects, i.e., the zinc interstitial and the oxygen vacancy, are investigated to optimize electrical properties of ZnO ceramics. After analyzing microstructure and dielectric response, it was found that Mn2+ was further oxidized to Mn4+ during sintering and Mn4+ solved into ZnO grains and inhibited the formation of the intrinsic point defects. On the other hand, due to the increase of the interface states contributed by Mn separation at grain boundaries, the barrier width broadened to keep charge neutrality. As a result, although the barrier height decreased from 0.68 eV to 0.54 eV as MnCO3 content increased from 0 mol% to 0.63 mol%, the nonlinear electrical properties were enhanced, that is, the breakdown field increased from 188.47 V/mm to 234.82 V/mm, the nonlinear coefficient increased from 12.87 to 74.76, and the leakage current density decreased from 22.08 μA/cm2 to 0.04 μA/cm2. Combined with the stability during DC aging tests, it is suggested that the optimal content of MnCO3 is 0.38 mol%. The study also indicates that the barrier width is a sensitive parameter in optimizing electrical properties of ZnO ceramics, instead of the sole focus on higher barrier height.

Influence of Eu2O3 on microstructure and electrical properties of ZnO-Pr6O11 based varistors

ZnO-Pr6O11-based ZnO nonlinear ceramics are well studied for their stability. Improving the electrical properties and stability of varistors has been an important part of the research and development of ZnO ceramics. The doping of rare-earth oxides in previous studies did not result in better performance of ceramics. In this study, the doping of rare earth element europium oxide is carried out by the traditional solid phase sintering method using metal oxides ZnO-Pr6O11–CoO–La2O3 (ZPCL) as the substrate, and the microstructure and electrical properties of the samples are tested. It is found that the doping of Eu2O3 helps to increase the grain boundary barrier, effectively improves the nonlinear coefficient of varistors, which is a very important electrical parameter, and reduces the leakage current. The best performance of the varistors is obtained at 1.5 mol% Eu2O3 doping, with the nonlinear coefficient of 18.7, the leakage current of 0.04 mA/cm2, the highest barrier height φb of 0.45 eV and the breakdown field of 1283 V/mm. This study provides a reasonable theoretical basis and experimental support for the improvement of the electrical properties of ZnO-Pr6O11-based ZnO ceramics, and lays a certain foundation for the doping of rare earth oxides in ceramics in the future.

Oxygen vacancy-activated thermoelectric properties of ZnO ceramics

Understanding the structural and thermoelectric (TE) properties of a pure material is essential for developing a practical approach to improve its TE performance. This study focuses on the high-temperature TE properties of ZnO ceramics synthesized by the spark plasma sintering (SPS) technique. The analyses of crystallinity and microstructure along with photoluminescence and Raman spectroscopy indicated an increase in oxygen vacancy in the ZnO ceramics with SPS temperature, which resulted in unit-cell shrinkage, lattice modification, grain densification, and TE modification. Specifically, oxygen vacancies were found to be a crucial factor affecting the TE performance of the spark-plasma-sintered ZnO ceramics at temperatures exceeding 773 K. Oxygen vacancies can act as carrier donors when they are ionized, contributing to an increase in electrical conductivity and a decrease in thermal conductivity. Additionally, this study proposes a potential way to engineer native defects in ZnO ceramics by controlling the SPS process.

Hybrid low‐temperature sintering processes of electro‐ceramics

AbstractCold sintering process (CSP) developed recently has attracted much attention due to the ultralow sintering temperatures and high efficiencies with the assist of transient liquid phase (TLP). Based on CSP, we have further established a protocol for low‐temperature sintering processes by combining TLP with other sintering technologies, including hybrid sintering process with microwave and TLP (MW‐TLP), and hybrid sintering process with spark plasma sintering and TLP (SPS‐TLP). Three typical electro‐ceramics (TiO2, CaWO4, and ZnO) are selected, which are highly densified (>97%) with excellent electrical properties at reduced sintering temperatures, applied pressures or holding times, demonstrating the feasibility of MW‐TLP and SPS‐TLP in fabricating electro‐ceramics. Especially, the Q × f value of TiO2 ceramics (38,020 GHz) prepared by MW‐TLP at 1000°C is 46.2% and 23.4% higher than that of microwave and spark plasma sintered samples, respectively, and comparable to traditional thermal sintered (TTS) samples at 1300–1400°C. The dielectric properties of MW‐TLP CaWO4 ceramics sintered at 900°C for 2 h are comparable to TTS samples sintered at 1000–1300°C for 2–5 h. ZnO ceramics can be highly densified (∼98%) by SPS‐TLP with mild sintering conditions (200–300°C and 3.8–50 MPa) compared to SPS (>500°C) and CSP (>100 MPa). The frameworks of fundamental mechanisms are outlined together with the experimental data. It is expected that this work will provide promising sintering methods to fabricate electro‐ceramics and offer inspirations on sintering combinations to develop low‐temperature sintering processes with high efficiencies.