ZnO Varistor Materials Research Articles

Segregation and properties at curved vs straight (000) inversion boundaries in piezotronic ZnO bicrystals

AbstractTEM and SEM investigations of ZnO bicrystal interfaces were undertaken with an aim to study the correlation of local grain‐boundary structure, segregation, and electrical transport perpendicular to the interface. To this end, varistor‐like ZnO bicrystals with piezotronic characteristics were chosen with (000)║(000) tail‐to‐tail orientation with respect to the c‐axis. In order to contrast different local grain‐boundary structures with different coherency and segregation of bismuth, but identical macroscopic polarization state, two complementary processing techniques were applied. A diffusion‐bonded bicrystal with an intermediate thin film containing Zn–Bi–Co–O provided a straight interface as reference. In contrast, a ZnO bicrystal prepared by epitaxial solid‐state transformation was manufactured by bonding two ZnO single crystals with a 100 µm thick polycrystalline ZnO varistor material with a typical dopant composition including bismuth and cobalt. This structure was annealed to the point that a bicrystal was formed with the varistor concentration at the boundary, which was strongly curved due to the polycrystalline microstructure still providing a shadow image at the interface. The results highlight a distinct correlation between local interfacial morphology, degree of segregation of bismuth, and degree of nonlinearity of the electrical transport across the interface.

A charge-based deep level transient spectroscopy measurement system and characterization of a ZnO-based varistor and a Fe-doped SrTiO3 dielectric

A charge-based deep level transient spectroscopy (Q-DLTS) method is applied to provide insights into the electronic behavior near grain boundaries and may provide new insights into mechanisms such as fatigue, degradation, dielectric aging, and dielectric breakdown. Here, we tested the Q-DLTS in both a ZnO varistor material and Fe-doped SrTiO3 materials. Comparisons are made to other data on ZnO varistors, and we obtain very good agreement for the energy levels. The status of deep traps in Fe-doped SrTiO3 dielectrics has been investigated where the relaxation was contrasted in a single crystal and polycrystalline ceramic materials. The relaxation is only observable in the polycrystalline materials, and was absent in single crystal Fe-doped crystals indicating that the deep traps originating from the Schottky barriers at the grain boundaries provide the DLTS signals. The energy associated with this grain boundary trap was found to be 1.26 eV.

Varistor piezotronics: Mechanically tuned conductivity in varistors

The piezoelectric effect of ZnO has been investigated recently with the goal to modify metal/semiconductor Schottky-barriers and p-n-junctions by application of mechanical stress. This research area called “piezotronics” is so far focused on nano structured ZnO wires. At the same time, ZnO varistor materials are already widely utilized and may benefit from a piezotronic approach. In this instance, the grain boundary potential barriers in the ceramic can be tuned by mechanical stress. Polycrystalline varistors exhibit huge changes of resistivity upon applied electrical and mechanical fields and therefore offer descriptive model systems to study the piezotronic effect. If the influence of temperature is contemplated, our current mechanistic understanding can be interrogated and corroborated. In this paper, we present a physical model based on parallel conducting pathways. This affords qualitative and semi-quantitative rationalization of temperature dependent electrical properties. The investigations demonstrate that narrow conductive pathways contribute to the overall current, which becomes increasingly conductive with application of mechanical stress due to lowering of the barrier height. Rising temperature increases the thermionic current through the rest of the material with higher average potential barriers, which are hardly affected by the piezoelectric effect. Hence, relative changes in resistance due to application of stress are higher at low temperature.

Effect of step sintering on breakdown voltage of varistors prepared from nanomaterials by sol gel route

ZnO varistor materials were prepared by a sol gel route with subsequent drying and calcination. Varistor discs fabricated from these materials were subjected to a two step sintering schedule. Therefore in a typical experiment, the samples were heated to 1000°C, then allowed to cool for over 30 min to 900°C and held there for 6 h. The results were compared with commercial varistor samples sintered in a similar way. Considerably higher breakdown voltages were obtained for the varistors made from nanosample (1192±30 V mm−1) compared with the commercial samples (723±30 V mm−1) sintered under the same experimental conditions. The sintered materials were characterised by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and density measurements.

Electron holography study of active interfaces in zinc oxide varistor materials

Electron holography measurements on individual interfaces in ZnO varistor material have been performed in the transmission electron microscope and correlated with the interfacial microstructure. The interfacial microstructure, including the local chemistry, has been determined using scanning transmission electron microscopy in combination with energy-dispersive x-ray spectrometry. The grain boundary charge was directly determined from electron holography to be negative, with a positively charged depletion region across it. The phase information has been analyzed to reveal several aspects of the double Schottky barrier, including the barrier height and the depletion region width. Variation in sample thickness was compensated for using a standard electron energy-loss log-ratio method combined with a total-inelastic mean free path calculated from scattering theory. From a potential line trace across the interface the barrier height was measured to be 0.71±0.20 V and the depletion region width estimated to approximately 150 nm. Evaluation of the mean inner potential of a ZnO grain from the phase map yielded a value of |V0|=21±4.2 V. Calculations of the mean inner potential using a nonbinding approximation and the Doyle–Turner scattering amplitudes, yielded, however, a lower value, |V0|=16.1 V. An asymmetric spatial extent of the depletion region was observed. These findings are compared to previously reported results from active interfaces in ZnO varistor materials.

Three-dimensional investigations of electrical barriers using electron beam induced current measurements

3D-investigations of electrically active interfaces in ZnO varistor materials have been performed using electron beam induced current (EBIC) contrast in the scanning electron microscope in combination with the focused ion beam workstation (FIB). The EBIC measurements were performed with spatial resolution better than 100 nm and the depth dependence of the signal was studied by variation of the acceleration voltage. The FIB was used for characterization of the subsurface geometry of electrically active grain boundaries. Experimental EBIC profiles, obtained by scanning the electron beam across grain boundaries, were compared with computer simulations using three different models for the electron–hole generation function. The EBIC contrast has been found to depend strongly on the geometric properties, i.e., the tilt of the grain boundary, and on asymmetries in the depletion region at the boundary. Calculations of the EBIC contrast taking these two effects into account show good agreement with experimental data. The “hot electron effect” close to the electrical breakdown voltage has been experimentally observed. The possibility to study asymmetry in the depletion region and the influence of the applied bias on the electrical properties of individual interfaces using the EBIC technique has been demonstrated.

On the microwave sintering technology for improving the properties of semiconducting electronic ceramics

Abstract Microwave (millimeter wave) sintering technology used for enhancing the densification behavior and electrical properties of ZnO electronics ceramic materials was described. Successfulness in application of such a novel technique to sinter ceramics relies heavily on the microwave (millimeter-wave) absorption efficiency of the materials. Semiconducting oxides such as Bi2O3-doped ZnO materials perform satisfactorily in microwave (millimeter-wave) sintering. Using susceptors to facilitate absorption efficiency of the samples greatly improves the sinterability of the materials. The advantage of the microwave (millimeter-wave) sintering process over the conventional sintering technique is best demonstrated by fast firing of Bi2O3-based ZnO varistor materials, in which, the whole sintering process required only 18 min. The varistor characteristics obtainable are: α=38, JL=55.5×10−6 A/cm2 and Vbk=600 V/mm.

Differences in Wetting Characteristics of Bi2O3 Polymorphs in ZnO Varistor Materials

Detailed analysis of the microstructure of grain boundaries, especially triple‐grain and multiple‐grain junctions, in ZnO varistor materials has been performed using transmission electron microscopy. Different polymorphs of Bi2O3 are shown to exhibit different wetting properties on ZnO interfaces. Recent investigations suggest that the equilibrium configuration consists of crystalline Bi2O3 in the triple‐grain and multiple‐grain junctions and an amorphous bismuth‐rich film in the ZnO/ZnO grain boundaries. The present investigation supports this suggestion for δ‐Bi2O3 and also adds to the microstructural image and wetting properties of α‐Bi2O3.

Nonlinear electrical properties of ZnO varistors fast-fired by using millimeter-wave sintering process

Fast firing of Bi2O3-based ZnO varistor materials, which includes zero minutes soaking at 1100°C with 120°C/min heating and 145°C/min cooling rate, was made possible using millimeter-wave sintering (mS) technique. The overall sintering time of the process is less than 18 minutes, and the varistor characteristics obtained are α = 38, JL = 5.55 × 10−6 A/cm2 and Vbk = 600 V/mm, whereas the intrinsic parameters of the materials are φb = 2.84 eV, Nd = 1.85 × 1024 m−3 and Ns = 7.02 × 1011 cm−2. By contrast, conventional sintering (cS) process needs higher sintering temperature (1200°C), longer soaking time (60 min) and slower ramping rate (30°C/min) to obtain ZnO materials with the same marvelous nonlinear properties as those prepared by mS-process. Moreover, millimeter-wave sintering (24 GHz, mS) process enhances the densification kinetics and grain growth behavior more efficiently than the microwave sintering (2.45 GHz, μS) process, resulting in better varistor characteristics for ZnO materials. However, sintering by millimeter-wave for too long period induces overfiring of the samples, which results in a density reversion phenomenon. Such a phenomenon leads to the decrease in surface state (Ns) and the potential barrier height (φb), which are presumed to be the mechanism leading to the degradation of ZnO materials' nonlinear properties.

Electrical Properties of Microwave-Sintered ZnO Varistors

Bi2O3-based ZnO varistor materials were prepared by 2.45 GHz microwave, 24 GHz millimeter-wave or conventional sintering processes. Both millimeter-wave and microwave sintering processes are superior to the conventional sintering process for densifying Bi2O3-based ZnO materials. Moreover, the millimeter-wave sintering process not only densified ZnO materials at a higher rate, but also resulted in significantly better varistor characteristics, as compared with the microwave sintering process. A large nonlinear coefficient and low leakage current density were attained by millimeter-wave sintering the samples under 1100°C for 5 min. A density as high as 95% theoretical density (T.D.) has been achieved, accompanied by a nonlinear coefficient (α) larger than 36, and a leakage current density (JL) smaller than 4.1×10-6 A/cm2. The samples had a donor density (Nd) around 2.5× 1024 m-3, a surface density (Ns) around 7.23×1011 m-2 and a potential barrier height (φb) around 2.22 eV. However, the varistor characteristics, including the nonlinear coefficient (α) and leakage current density (JL), degraded markedly for samples sintered at too high a temperature or soaked for too long. This was ascribed to the occurrence of abnormal grain growth, accompanied by the loss of Bi2O3 and ZnO species.

Electron Holography of Potential Barriers in Zno Varistors

ZnO varistor materials are polycrystalline materials that exhibit strong non-linear current-voltage characteristics. Their non-ohmic behaviour makes them suitable for overvoltage protection devices. It is the interfaces between ZnO grains that are the key to the non-linear behaviour of the varistor materials. Previous work has shown that the local microstructure determines the breakdown voltage of the individual ZnO/ZnO interface [1]. The pre-breakdown conductivity and the non-linearity in the breakdown region, which are both crucial to the performance of the varistor, are also affected by the height and the width of the potential barriers. In the present work, electron holography has been used to investigate the potential barriers in ZnO varistor materials.Electron holograms were recorded at 200kV using a Hitachi HF-2000 field emission gun transmission electron microscope [2]. The electrostatic bi-prism was located near the second image plane. The bi-prism voltage was about 9 V and the fringe spacing in the holograms was about 1 nm.

Electrical properties of ZnO varistors prepared by microwave and conventional sintering process

The Bi2O3-based ZnO varistor materials were prepared by 2.45 GHz microwave, 24 GHz millimeter-wave or conventional-sintering processes. Both millimeter-wave and microwave sintering processes are superior to the conventional sintering process in densifying the Bi2O3-based ZnO materials. Large nonlinear coefficient and low leakage current density were attained by sintering the samples under the 1200°C sintering temperature and 5 min soaking time, A high density as 96.7% T.D. (theoretical density) has been achieved. However, the varistor characteristics, including breakdown voltage (Vbk), nonlinear coefficient (α) and leakage current density (JL), degraded markedly for the samples sintered at too long soaking time (10 min), which was ascribed to the occurrence of abnormal grain growth, in accompany with the loss of Bi2O3 and ZnO species.

Microstructures and Electrical Properties of V2O5-based Multicomponent ZnO Varistors Prepared by Microwave Sintering Process

The V2O5-based ZnO varistor materials were successfully densified using microwave sintering process. The materials can reach a high density as 94.6% T.D. (theoretical density) when sintered at 800 °C (10 min) but the grain growth was initiated only when sintered at a higher temperature than 1000 °C (10 min). The varistor characteristics, including breakdown voltage (V bk), nonlinear coefficient (α) and leakage current density (J L), degraded markedly for the samples sintered at too high temperature (i.e., T≥1000 °C) and for too long period (i.e., t≥10 min) that was ascribed to the occurrence of abnormal grain growth. Contrarily, the intrinsic characteristics, including potential barrier height (φ b) and donor density (N d), varied only moderately with these sintering conditions. The V2O5-based ZnO materials sintered at 1000 °C (5 min) possessed good varistor characteristics as V bk=248 V/mm, α=31 and J L=4.6×10-5 A/cm2. The corresponding intrinsic parameters are φ b=0.63 eV and N d=2.37×1024 m-3.

Electrical Properties of ZnO Varistors Prepared by Microwave Sintering Process

Microwave sintering processes were observed to substantially enhance the densification rate for ZnO varistor materials. Samples microwave sintered at 900–1200° C for 10 min reached the same high density as those conventionally sintered at the same temperature for 60 min. A similar tendency was observed for the grain size. Prolonged microwave sintering, however, resulted in fast grain growth and degraded electric field–current density ( E–J ) properties. Capacitance–voltage ( C–V ) analysis indicated that the degradation in E–J behavior primarily resulted from the decrease in grain boundary barrier height. The modification of the grain boundary electrical properties, in turn, is ascribed to the loss of volatile Bi species along the grain boundaries.

Processing-induced resistive barriers in ZnO varistor material

Evidence is presented for the origin of high-resistivity barriers at certain interfaces in high-field varistor materials. The barriers give rise to terrace contrast which can be observed using remote electron beam induced current imaging in the SEM. It is proposed that these interfaces, which are often associated with a thick intergranular layer of a compositionally distinct bismuth rich oxide, are relicts of the powder structure and correspond to agglomerate surfaces. It is suggested that the presence of these resistive surfaces ultimately limits the current that can flow at breakdown.

Development of Functional Microstructure during Sintering of a ZnO Varistor Material

The effects of sintering time and rate of cooling on the microstructure and current/voltage characteristics of a ZnO varistor material have been investigated. The results provide information about the development of microstructure during sintering and have been interpreted in terms of the role which microstructure plays in the determination of electrical properties. A continuous network of intergranular Bi2O3, which lies mainly along the triple junctions between ZnO grains, can give a substantial contribution to the leakage current. The conductivity of this skeleton depends upon its internal microstructure.

Development of interfacial microstructure during cooling of a ZnO varistor material

The development of interfacial microstructure during cooling of a ZnO varistor material from the sintering temperature has been studied by a combination of analytical electron microscopy and x-ray diffractometry. It was found that pyrochlore is the first Bi-rich phase to form from the liquid-phase sintering medium during cooling from the sintering temperature. When cooling at normal rates, pyrochlore forms between 940 and 740 °C, and γ-Bi2O3 forms at about 740 °C. Quenching from higher temperatures results in the formation of δ-Bi2O3 from the Bi-rich intergranular liquid. The formation of barriers to electrical conduction at individual ZnO junctions was studied by use of microelectrodes. The barriers started to form while cooling between the temperatures of 1000 and 800 °C. Quenching from higher temperatures did not result in varistor behavior, but barriers to electrical conduction could be created in such specimens by post-sintering heat treatments.

The effect of Bi2O3 content on the microstructure and electrical properties of ZnO varistor materials

The microstructures of three ZnO varistor materials with different Bi2O3 contents have been evaluated by analytical electron microscopy in combination with x-ray diffractometry. The results have been correlated to microelectrode measurements, where breakdown voltages of individual ZnO junctions were measured, and also to current/voltage characteristics of bulk specimens. The volume fraction of the continuous intergranular network of Bi-rich phases, which lies along the triple junctions of the ZnO grains, increases with increasing Bi2O3 content, The conductivity of this network is strongly influenced by its internal microstructure. It was found that increased volume fractions of δ-Bi2O3 and less interpenetration between α-Bi2O and δ-Bi2O3 increases the conductivity of the network. Individual ZnO/ZnO grain boundaries exhibited breakdowns at 3.2 and 3.6 V, depending upon whether they contained segregated Bi atoms or thin Bi-rich amorphous films. The current/voltage characteristics of heterojunctions between ZnO and intergranular Bi2O3 were asymmetrical with respect to the polarity of the applied voltage. It was found that α-Bi2O3 and δ-Bi2O3 give rise to different breakdowns for electrons traveling from the Bi2O3 into an adjacent ZnO grain.

Characterization of individual interfacial barriers in a ZnO varistor material

The role of different types of interfaces in a ZnO varistor material has been investigated by directly measuring the current/voltage characteristics of individual interfacial junctions. The electrically characterized interfaces were subsequently studied by analytical transmission electron microscopy. It was shown that barriers to electrical conduction form during cooling from the sintering temperature. The ZnO/ZnO junctions exhibited breakdowns at either 3.2 or 3.6 V, where the higher breakdown was observed at boundaries which contained a thin (∼2 nm) intergranular amorphous Bi-rich film, while the breakdown at 3.2 V was found at boundaries without secondary phases but which contained intergranularly segregated Bi atoms. Junctions between ZnO and intergranular Bi2O3 exhibited asymmetric current/voltage characteristics with breakdowns at 3.2 V for one polarity of the applied voltage and either 0.4 or 0.9 V for the other polarity. No varistor behavior was observed at interfaces between ZnO and pyrochlore, Zn2Sb3Bi3O14. This gives rise to a significant number of ‘‘inactive’’ grain boundaries in the material.

Grain boundaries in zinc oxide-based varistor ceramics

Since the development of the zinc oxide-based varistor with highly nonlinear current-voltage characteristics and high energy absorption capabilities [1], the electro-physical behavior of non-ohmic ZnO varistors has been studied and related to the microstructure of the material, the conduction and degradation mechanisms, the dielectric properties, and the high pressure memory. An extensive research effort has been aimed at characterizing ZnO/ZnO grain boundary regions in order to explain the origin of the nonlinear current/voltage characteristics of these materials [2-4]. A typical ZnO-varistor material contains small concentrations of several metal oxides (e.g., Bi2O3, CoO, MnO, Sb2O3, and Cr2O3). Co and Mn are contained within the ZnO grains, while the other “impurities are present as several polymorphic forms of Bi2O3, the spinel, Zn7Sb2O12, and the pyrochlore Zn2Bi3Sb3O14, are present as intergranular phases [1,5-7]. The breakdown voltage depends on the number of grain boundaries between the electrodes of the ZnO varistor device [8]. Therefore, the breakdown voltage is influenced by the presence and form of these intergranular phases, and the size, shape and distribution of the ZnO grains. In ZnO-Bi2O3-MnO-TiO2-based varistor materials, the morphology of the ZnO grains is strongly influenced by their tendency to grow preferentially along the directions perpendicular to the prism planes [9] (i.e., the basal plane becomes a common grain boundary facet plane). The aim of the present study is to advance the understanding of the role of the special grain boundaries which are found in air-quenched Zn0-Bi2O3-MnO-TiO2-based varistor materials.