ZnO Ceramics Research Articles

Ultrafast luminescence of Ga- and In-doped ZnO ceramics

In the presented work we compare luminescence characteristics of ZnO:Ga and ZnO:In ceramics prepared by hot uniaxial pressing method. Two types of initial powders were used. The first one was nanosized powder prepared by precipitation method. In the second case we used microsized powders and mechanical admixturing the oxides of intended dopant. In both cases doping by the trivalent ions lead to a significant quenching of ZnO visible emission and an increase in near-band-edge luminescence. The study has shown that the type of dopant greatly affects the transmittance of ceramics prepared from nano- and micro sized powders. Several reasons for the specific effect of powder preparation process and a type of introduced dopant, including changes in dopant solubility, their interaction with the grain boundaries, etc., were considered.

The mechanism of the effect of V doping on the thermoelectric properties of ZnO ceramics

In this paper, the mechanism of the effect of V doping on the thermoelectric properties of ZnO is studied by experimental test and first principles calculation. The results show that the carrier concentration and conductivity increase significantly after V doping, which is mainly due to the donor effect caused by V substituting for Zn site, and the band gap becomes narrower. Due to the influence of 3d orbit of V ion, the density of states near Fermi level increases. The effective mass decreases and the Seebeck coefficient decreases with V doping. The thermal conductivity decreases with the decrease of lattice thermal conductivity, which is mainly due to the decrease of Young's modulus due to the change of strain and stress field caused by V doping. While the power factor increases, the thermal conductivity decreases, and the ZT value increases significantly in the whole temperature range, with the highest value reaching 0.302 ​at 873 ​K for Zn0.985V0.015O, which is 3.02 times of undoped ZnO.

Simultaneously improving the electrical properties and long-term stability of ZnO varistor ceramics by reversely manipulating intrinsic point defects

Excellent electrical properties and the improved long-term stability of ZnO varistor ceramics were simultaneously achieved by doping NiO. The microstructural features were investigated using X-ray diffractometer, scanning electron microscopy, and energy dispersive spectroscopy, while the intrinsic point defects were characterized using frequency domain dielectric spectroscopy and verified by photoluminescence and Raman spectra. The results indicated that in the ZnO varistor ceramics, a reverse manipulation of donor point defects, i.e., suppressing mobile zinc interstitial but increasing stable oxygen vacancy, was achieved. The long-term stability of NiO-doped ZnO ceramics was improved via a decrease in zinc interstitial density, with a degradation rate of 0.064 μA cm−2 h−0.5. Meanwhile, due to an increase in oxygen vacancy density, the excellent nonlinear current–voltage performance, i.e., a high nonlinear coefficient (72.9), low leakage current density (0.08 μA cm−2), and low grain resistivity (13.43 × 10−3 Ω m), was maintained. The findings of this study provide a possible method for developing high-performance ZnO varistor ceramics by manipulating point defects.

Fabrication of ZnO Ceramics with Defects by Spark Plasma Sintering Method and Investigations of Their Photoelectrochemical Properties

ZnO, as an important semiconductor material, has attracted much attention due to its excellent physical properties, which can be widely used in many fields. Notably, the defects concentration and type greatly affect the intrinsic properties of ZnO. Thus, controllable adjustment of ZnO defects is particularly important for studying its photoelectric properties. In this work, we fabricated ZnO ceramics (ZnO(C)) with different defects through spark plasma sintering (SPS) process by varying sintering temperature and using reduction environment. The experimental results indicate that the changes of color and light absorption in as-prepared ZnO originate from the different kinds of defects, i.e., oxygen vacancies (VO), interstitial zinc (Zni), and Zinc vacancies (VZn). Moreover, with the increase in calcination temperature, the concentration of oxygen defects and interstitial zinc defects in the ceramics increases gradually, and the conductivity of the ceramics is also improved. However, too many defects are harmful to the photoelectrochemical properties of the ceramics, and the appropriate oxygen defects can improve the utilization of visible light.

Dielectric anomalous peaks accenting ferroelectricity prospects of Li and Mg co-doped ZnO ceramics

This paper presents effects of lithium (Li) and magnesium (Mg) dopants on the dielectric properties of ZnO ceramics with emphasis on ferroelectricity prospects of co-doped samples with Zn0.7Li0.28Mg0.02O compositions. Ceramics with average relative density ∼97% were produced by solid-state-reaction of mechanically activated powder precursors using 1000 °C for 2 h sintering conditions. Room temperature A.C. conductivity σa.c(ω) and temperature-frequency (T-ω) dependence of dielectric permittivity (ε′) and loss (ε″) of the samples are measured within temperature range from −100 to 150 °C in air and in a vacuum cryostat with liquid nitrogen coolant. Undoped ZnO σa.c(ω) data exhibits a nearly insensitive dependence to the ω while that of doped samples show significant ω dependence and higher σa.c(ω) values than undoped ZnO ceramics. Anomalous ε′ and ε″ peaks were observed in Zn0.7Li0.28Mg0.02O specimens around TC of −25 °C during cooling and −10 °C during heating measurements in air. However, dielectric properties of the same Zn0.7Li0.28Mg0.02O ceramics obtained in vacuum cryostat and from undoped ZnO ceramics in air and in vacuum cryostat did not reveal any ε′ and ε″ transition peaks at low temperature.

Sb–doped ZnO ceramics: NTC thermistors with high temperature sensitivity and electrical stability

Sb–doped ZnO ceramics: NTC thermistors with high temperature sensitivity and electrical stability

Effect of Mo doping on the microstructures and mechanical properties of ZnO and AZO ceramics

ZnO ceramics should have good mechanical properties to meet their applications in semiconductor industry. Based on experiments and the first principles, the effect of Mo doping on the microstructure and the mechanical properties of ZnO and AZO ceramics were studied, and the model of the effect of Mo doping on ZnO crystal defects was established. The results of the structures and fracture-surface morphology of different systems show that doping Mo atoms is beneficial for ZnO and AZO ceramics to eliminate the lattice defects, promote the grain growth and reduce the porosity at low doping concentration. However, when the doping concentration is too high, the new phase of Zn3Mo2O9 is formed and precipitated in 4MZO crystal. Correspondingly, the lattice distorted of doping systems is so serious that hinders the growth of grains and further affects the mechanical properties. Then, the bilinear stress-strain (σ-ε) relationship was constructed by nanoindentation and finite element method. The results of mechanical properties by nanoindentation experiment and the first-principles simulation show that doping Mo can improve the hardness, elastic modulus, initial yield stress, plastic tangent modulus and creep resistance of ZnO and AZO ceramics. In particular, the mechanical properties of MZO ceramics are significantly improved because of the formation of Mo–O covalent bond between Mo6+ and O2−. In addition, it is worth noting that due to the synergistic and compensation effect of Al and Mo co-doped atoms, which is favor for reducing the lattice strain, improving the resistance to lattice deformation and forming the covalence of MAZO system, the mechanical properties of MAZO ceramics are increased remarkably.

Pressure‐assisted flash sintering of ZnO ceramics

AbstractImplementing pressure‐assisted flash sintering of ZnO powder without pretreatment by a new experimental configuration is presented. Rapid and energy‐concentrated heating of electrode‐sample‐electrode area by induction heating allows preheating and flash sintering of loose‐pack powder in the die with pressure assistance. Using an insulated die enables the current to flow through the sample during flash sintering. ZnO ceramics with a relative density of 95.1% can be achieved in less than 3 min. The whole process includes 104 s of preheating by a low‐power induction heating device and 30 s of flash sintering assisted by a pressure of 26 MPa using the pulsed direct current (DC). The process characteristics of pressure‐assisted flash sintering using the pulsed DC are discussed. The effect of pressure on densification and grain size is analyzed in detail, and some potential mechanisms are provided.

The Cold Sintering Process of ZnO and BaTiO3 ceramics under the electric current influence

The paper presents the preliminary study results of the influence of the electric current (direct or alternating), the values of voltage and current, the presence or absence of activating additives, and external heating of the mold on the process of cold sintering of ZnO and BaTiO3 powders. The microstructures of the obtained samples are analyzed. Approaches to further research are proposed. The article substantiates the prospects for using electric current in the process of cold sintering of ceramics.

Application of ZnO Nanoparticles in Sn99Ag0.3Cu0.7-Based Composite Solder Alloys

The properties of Sn99Ag0.3Cu0.7 (SACX0307) solder alloy reinforced with ZnO nanoparticles were investigated. The primary ZnO particle sizes were 50, 100, and 200 nm. They were added to a solder paste at a ratio of 1.0 wt %. The wettability, the void formation, the mechanical strength, and the thermoelectric parameters of the composite solder alloys/joints were investigated. Furthermore, microstructural evaluations were performed using scanning electron and ion microscopy. ZnO nanoparticles decreased the composite solder alloys’ wettability, which yielded increased void formation. Nonetheless, the shear strength and the thermoelectric parameters of the composite solder alloy were the same as those of the SACX0307 reference. This could be explained by the refinement effects of ZnO ceramics both on the Sn grains and on the Ag3Sn and Cu6Sn5 intermetallic grains. This could compensate for the adverse impact of lower wettability. After improving the wettability, using more active fluxes, ZnO composite solder alloys are promising for high-power applications.

Impact of mechanical activation of reactant powders on the solid-state-densification of Zn1-xLixO and Zn0.7Li0.28Mg0.02O ceramics

This paper presents effects of powder mechanical activation and controlled sintering on the densification properties of ZnO ceramics with Lithium (Li) and Magnesium (Mg) dopants. Dense bodies of Zn1-xLixO and Zn0.7Li0.28Mg0.02O ceramics were prepared via solid-state-reaction of mechanically activated mixtures of ZnO, Li2CO3 and MgO reactant raw powders. The density, surface microstructure, porosity and phase purities of the ceramics were investigated with a Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD) techniques. The study highlights mechanical activation processes such as milling, calcination and compaction steps that are needed to overcome complicated interactive reactions during conventional sintering of Zn1-xLixO and Zn0.7Li0.28Mg0.02O ceramics that have been reported to show ferroelectric behavior. The results show that, densely packed single phase Zn1-xLixO and Zn0.7Li0.28Mg0.02O ceramics with average relative density values above 96% and single phase hexagonal wurtzite crystalline structure were achieved at a sintering temperature of 1000 °C for 2 h in air.

Ultraviolet Luminescence of ZnO Whiskers, Nanowalls, Multipods, and Ceramics as Potential Materials for Fast Scintillators

The presented work is dedicated to the study and comparison of scintillating properties of zinc oxide samples prepared in different morphologies: whiskers, nanowalls, multipods, and ceramics. It was shown that total transmittance, photo- and radioluminescence spectra, and radioluminescence kinetics can vary significantly depending on sample structure and preparation conditions. The highest total transmittance was registered for ZnO ceramics (>50% at 0.5 mm thickness). Differences in the transmittance of whiskers, nanowalls, and multipods can be attributed to their shape and thickness which affects the amount of light refraction and scattering. The study of radioluminescence demonstrated that all samples, except undoped ceramics and air annealed whiskers, have predominantly fast luminescence with a decay time <1 ns. High transmittance of ceramics opens the way for their use in the registration of high energy X-ray and gamma radiation, where a large volume of scintillators is required. In cases, where large scintillator thickness is not a necessity, one may prefer to use other ZnO structures, such as ensembles of whiskers and nanowalls. Studies of near-band-edge luminescence components at low temperatures showed that the structure is quite similar in all samples except Ga doped ceramics.

Effect of Impurities on Varistor Properties of High-Voltage ZnO Ceramics

Effect of Impurities on Varistor Properties of High-Voltage ZnO Ceramics

Comparative Study on Micromechanical Properties of ZnO:Ga and ZnO:In Luminiscent Ceramics

Abstract Indium (0.038 at.%) and gallium (0.042 at.%) doped ZnO ceramics were prepared by hot pressing. Ceramics were investigated to determine their structural and mechanical characteristics for the prospective use in scintillators. Based on results of nanoindentation, atom force and scanning electron microscopy as well as energy dispersive X-ray spectra measurements, locations of gallium within grain, indium at grain boundaries (GBs) and their different effect on the mechanical properties of ZnO ceramics were detected. Doping of gallium led to the increased modulus of elasticity in grain, decreased hardness near GBs, stabilization of micropores and brittle intercrystalline fracture mode. ZnO:In ceramic has modulus of elasticity and hardness values close to ZnO characteristics, the increased fracture toughness and some plasticity near GBs. Differences in the micromechanical properties of the ceramics correlate with the location of dopants. Results demonstrate that the ZnO:In ceramic has a greater stress relaxation potential than the ZnO:Ga.

The response of nano-ceramic doped fluids in heat convection models: A characteristics-based numerical approach

In this paper, forced, free, and mixed convections in incompressible flow were studied numerically. Nano-sized Al2O3, TiO2, MgO, and ZnO ceramics with water were considered as nano-fluids. Simulations were carried out for cavity flow with different boundary conditions and aspect ratios, as well as flow over stationary and rotating cylinders. The mean Nusselt number ((Nu) ̅) and friction factor for cavity flow and (Nu) ̅ for flow over a cylinder were compared for different nano-fluids. A new code was developed in FORTRAN 95 for numerical simulations. A fifth-order Runge-Kutta method for time discretization and a characteristic-based scheme for convective terms were used in this code. The averaging scheme on the secondary cells is used to obtain viscous fluxes. Primary results are validated with other researcher's outputs. Results showed that MgO-water and ZnO-water had maximum and minimum heat transfer rates, respectively. Moreover, maximum and minimum shear stresses were recorded for the Al2O3-water and TiO2-water, respectively. Using nanofluid increases the heat transfer rate between 15 and 37 percent depending on the Richardson number and selected nano-particles.

3D printing of TPMS structural ZnO ceramics with good mechanical properties

In this study, ZnO ceramics with two types of TPMS structure (Gyroid structure and Schwartz P structure) were fabricated by 3D printing. The solid content of ZnO ceramics powder was 38 wt%. The solid content of SR454NS Acrylic was 57 wt%. The other components of the slurry were photoinitiator, dispersant and sintering additive. The ZnO ceramics with high densification could be achieved after sintering at 1250 °C for 2 h. The average grain size was 37 μm and the true density of the printed ZnO ceramics was 4.95 g/cm3. The shape of printed ZnO ceramics can be controlled and the minimum size of thin rod is 200 μm. The compressive strength for Gyroid structural ZnO ceramic with 55% porosity and the Schwartz P structural ZnO ceramic was approximately 6.87 MPa and 4.02 MPa, respectively. The result shows that the Gyroid structural ZnO ceramics can bear much more deformation than the Schwartz P structure during the compression tests. The TPMS structural ZnO ceramics can be used in the field of sensing and detection in the future.

Influence of reducing heat treatment on the structural and magnetic properties of MnO:ZnO ceramics

Polycrystalline MnO:ZnO bulk ceramics with a Mn proportion of 6, 11, 17 and 22 at% are prepared through a solid-state reaction and subjected to a heat treatment in a reducing atmosphere (Ar (95%) and H2 (5%)). The samples are studied with particular emphasis on their composition and structural and magnetic properties. A detailed microstructural and chemical analysis confirms the Mn doping of the wurtzite ZnO structure mainly at the surface of the ZnO grains. For the samples with higher Mn proportions, the secondary phases ZnMn2O4 and Mn1−xZnxO (Zn-doped MnO) are detected for the as-prepared and heat-treated samples, respectively. The structural change of the secondary phases under heat treatment, from ZnMn2O4 to Mn1−xZnxO, confirms the effectiveness of the heat treatment in reducing the valence of the Mn ions and in the formation of oxygen vacancies into the system. In spite of the induced defects, the magnetic analysis presents only a paramagnetic behavior with an antiferromagnetic coupling between the Mn ions. In the context of the bound magnetic polaron theory, it is concluded that oxygen vacancies are not the necessary defect to promote the desired ferromagnetic order at room temperature.

Raman spectroscopy study of Ga-doped ZnO ceramics: An estimative of the structural disorder degree

The structural disorder degree of Ga-doped ZnO (Zn1−xGaxO, with x=0.005,0.01,0.02,and 0.03) ceramics was investigated by means of Raman spectroscopy analyzes at room temperature. The Raman spectra for Ga-doped ZnO samples exhibits both activated Raman modes for wurtzite and spinel structures. The asymmetry parameter, line-width, and Raman shift could be fitted to the Breit-Wagner-Fano (BWF) function and the structural disorder degree in the wurtzite structure were analyzed by spatial phonon confinement model (PCM). For Raman E2high mode, the correlation length, L and defect concentration, N were found to be 2.72, 2.62, 2.56, and 2.41 nm and 1.19×1019, 1.33×1019, 1.21×1019, and 1.70×1019cm−3 for x=0.005,0.01,0.02,and0.03 samples, respectively. Additional Raman modes were detected in Ga-doped ZnO (x≥0.01) spectra that could be attributed to ZnGa2O4 spinel structure. This is an indication that the solubility limit of Ga in ZnO ceramics is certainly less than 1 mol at% which was not possible to be detected by means of x ray diffraction (XRD) experiments.

Critical positive temperature coefficient of resistivity of Li/Y co-doped ZnO ceramics modified by Cr-ions

ZnO-based ceramics doped with Li/Y and modified by various contents of Cr-ions, (Zn0.9475Y0.0025Li0.05)1-xCrxO (0 ≤ x ≤ 0.025), were prepared by a wet chemical route followed by a traditional ceramic sintering technology. The effect of Cr-ion content on the electronic conductivity and resistance–temperature characteristics of the prepared ceramics was investigated. All the ceramics have hexagonal wurtzite ZnO structure, and exhibit critical positive temperature coefficient of resistivity (C-PTCR) effect. With various contents of Cr-ions, the maximum temperature coefficient of resistivity (TCR) reaches 61.1%·°C−1 or the resistivity jump log(ρmax/ρmin) is up to 4.74. The critical transition temperature of resistivity can be adjusted in the temperature range of 76–147 °C for different various contents of Cr-ions. The complex impedance analysis shows that the C-PTCR effect of the ZnO ceramics resulted from both grain effect and grain boundary effect. According to the analysis of the dielectric temperature spectra, the ZnO-based ceramics have ferroelectricity at room temperature, and the ferroelectric–paraelectric transition around the critical transition temperature of resistivity during the temperature increases.

Механічна поведінка та електрична провідність оксидно-цинкової кераміки

Ceramics sintered from zinc oxide powders, which differ in crystal structure, particle size and amount and type of impurities, have been studied for their mechanical behavior (strength and micromechanisms of biaxial bending at room temperature) and electrical conductivity depending on the purity of ZnO powder (99,9% byweight — type I and 99,5% byweight — type II) and its sintering temperature in the interval from 800 to 1250 ºC for 2 hours. It is found that the maximum values of strength and electrical conductivity are achieved in ZnO-ceramics sintered at temperatures of 1100—1200 and 1000—1150 ºC, respectively, and their micromechanism of fracture is the cleavage only. ZnO-powder developed (type II), being twice as large as the purchased (type I), 300—350 nm instead of 150—200 nm, provides close to 100% density at 1100 °C, the type II powder is sintering at almost 100 °C lower temperature than the purchased one. Type I ceramics provide biaxial strength at room temperature of 150—170 MPa; type II — 120—160 MPa. ZnO-ceramics from powders of both types provide maximum electrical conductivities of 8,54 10-3S/ cm and 1,6·10-3 S / cm at temperatures of 265 and 600 ºC, respectively. The activation energy of the electrical conductivity of ZnO-ceramics is dependent significantly on the properties of the powder and, accordingly, the structure of the ceramics and the test temperature. Type I ZnO ceramics have a lower conductivity activation energy than type II, 0,2—0,3 eVand 0,3—0,5 eV, respectively. The mechanism of electrical conductivity of ZnO-ceramics type I is practically unchanged in all the interval of testing temperatures, from the room one to 600 °C. In ZnO-ceramics of the type II, it changes at least twice. Keywords: zinc oxide, ZnO ceramics, sintering temperature, porosity, grain size, micromechanism of fracture, bending strength, electrical conductivity, activation energy.