Relationships between microstructure,mechanical and dielectric properties of different alumina materials

Different alumina materials were elaborated in order to vary microstructural parameters (grain size, densification, porosity, inter-granular phase). These ceramic materials were then characterized from the mechanical point of view (hardness, toughness, friction and wear) and dielectric breakdown. The comparison of these various results shows that, for all these properties, the grain size and also, the nature of the secondary phases and the microstructural parameters were the most significant.Moreover, from the tribological point of view, the dielectric characteristic of materials (breakdown strength) has a fundamental role in the creation of agglomerated wear debris (“third body”) and its properties: a finely agglomerated third body will be obtained for high breakdown strength. Such third body will be able to protect the substrate and thus to reduce later wear. In the same logic a correspondence between breakdown strength and toughness was established, thus confirming the existence of mechanical–electrical correlation for non-conductive materials.     

Charge trapping characteristics of alumina based ceramics

The space charge dynamics is very important for electrical breakdown of alumina based ceramics. In this paper, the charge trapping/detrapping characteristics of alumina based ceramics were studied by means of isothermal surface potential decay (ISPD) method. For alumina and zirconia toughened alumina (ZTA) ceramic samples, the ISPD curves charged by corona discharge as well as microstructure characterization were carried out. For the first time, crossover phenomenon and hollow shaped potential profile were observed and reported in alumina based ceramics, indicating a surface potential decay process dominated by charge injection and volume conduction affected by the trap states in materials. In addition, the comparative trapping characteristics were evaluated based on a charge detrapping controlled decay model. The correlation between trap distribution and microstructure of alumina based ceramics was investigated. It was proposed that different charge trapping characteristics of alumina based ceramic samples was caused by varied shallow trap density of grain boundary.     

Integration and characterization of a ferroelectric polymer PVDF-TrFE into the grain boundary structure of ZnO via cold sintering

In this study, the Cold Sintering Process (CSP) is used to design ceramic-polymer composites with Polyvinylidene fluoride Trifluoroethylene (PVDF-TrFE), a ferroelectric co-polymer, as an active intergranular grain boundary phase in a semiconducting Zinc Oxide (ZnO) electroceramic matrix. The conductivity is modeled with Schottky thermionic emission and Fowler-Nordheim tunneling as a function of both temperature and voltage. In addition, through details of the dielectric characterization, the interfaces are also considered with the effective permittivity resulting with a space charge relaxation of the PVDF-TrFE. The Maxwell-Wagner-Sillars (MWS) model was used to predict ~ 3 nm as the thickness of the intergranular PVDF-TrFE phase controlling electrical properties of the composite. Transmission electron microscopy (TEM) investigation of the grain boundary phase confirms the polymer thicknesses to the dimensions predicted from the various electric measurements and subsequent modeling.     

Grain Size Effect on Creep Deformation of Alumina-Silicon Carbide Composites

A study of the flexural creep response of aluminas reinforced with 10 vol% SiC whiskers was conducted at 1200° and 1300°C at stresses from 50 to 230 MPa in air to evaluate the effect of matrix grain size. The average matrix grain size was varied from 1.2 to 8.0 μm by controlling the hot-pressing conditions. At 1200°C, the creep resistance of alumina composites increases with an increase in matrix grain size, and the creep rate (at constant applied stress) exhibits a grain size exponent of approximately 1. The stress exponent of the creep rate at 1200°C is approximately 2, consistent with a grain boundary sliding mechanism. On the other hand, the creep deformation rate of 1300°C was not sensitive to the alumina grain size. This was seen to be a result of enhanced nucleation and coalescence of creep cavities and the development of macroscopic cracks as the grain size increases. Observations also indicated that the prevalent site for nucleation and growth of creep cavities in coarsegrained materials is at two-grain junctions (grain faces), whereas in fine-grained materials cavities nucleate primarily at triple-grain junctions (grain edges). Electron microscopy studies revealed that the content of any amorphous phase present at whisker-alumina interfaces is independent of alumina grain size (and hot-pressing conditions). In addition, the alumina grain boundaries are quite devoid of amorphous phase(s). This variation in amorphous phase content does not appear to be a factor in the present creep results.     

High-Temperature Stress-Strain Behavior of MgO in Compression

Compressive stress-strain curves for several types of polycrystalline MgO specimens were correlated with those for single crystals and analyzed as a function of grain size and grain-boundary character at 1200° and 1400°C for several strain rates. The results for fully dense specimens were explained in terms of grain-boundary sliding and intergranular separation in addition to slip. The modification of grain-boundary nature concurrent with heat treatment for grain growth, caused by residual LUF, was associated with enhanced grain-boundary sliding and intergranular separation. For grain sizes <30 μm, it was concluded that the von Miss criteria for ductility could be relaxed by the Occurrence of dislocation climb and, to a limited extent, by intergranular separation. Yield drop corresponding to dislocation multiplication occurred when grain-boundary sliding was initially promoted. Specimens with a liquid phase of adequate viscosity also indicated plasticity accompanied by high strength. Specimens with clean grain boundaries exhibited ductility and normal strain hardening with no intergranular separation.     

Control of grain boundary in alumina doped CCTO showing colossal permittivity by core-shell approach

Grain boundaries of CaCu₃Ti₄O₁₂ (CCTO) materials have been shown to play leading role in colossal permittivity. Core-shell design is an attractive approach to make colossal dielectric capacitors by controlling the grain boundaries. Core-shell grains of CCTO surrounded by Al₂O₃ shell were synthesized by ultrasonic sol-gel reaction from alumina alkoxide precursor. The influence of alumina shell by comparison with bare CCTO grains was studied. Particularly, microstructure, dielectric and electric effects on sintered ceramics are reported. The average grain size and the density are increased compared to undoped CCTO leading to an improvement of permittivity from 58,000 to 81,000 at 1?kHz. Furthermore a decrease of dielectric loss is found in a frequency range of 10²–10³?Hz. Moreover, the activation energy of grain boundaries is increased from 0.55 to 0.73?eV and the electrical properties such as breakdown voltage, non-linear coefficient and resistivity are improved with the aim of making industrial capacitors.     

Co-substitution tailored dielectric relaxation and electrical conduction in lanthanum orthoferrite

Effect of co-substitution on structural, dielectric and electrical conduction property of LaFeO₃ is reported. Partial co-substitution of La by Na and Fe by Mn has been observed to cause substantial modification in structural property including lattice distortion in LaFeO_3. Evidence of Jahn teller distortion has been noticed in Raman analysis due to presence of Mn³⁺ Jahn teller active ion at Fe lattice site. Raman analysis also indicated Fe–O–Fe bond weakening in LaFeO₃ on co-substitution. Dielectric response provides evidence of temperature dependent polydispersive relaxations contributed by combined effect of dipolar and space charge relaxations. Temperature response of dielectric parameter (ε_r and tan δ) provided indication of phase transition that is similar to found in ferroelectric materials. The dielectric relaxations are studied in the framework of complex impedance and electrical modulus spectrum. The electrical response comprises both grain and grain boundary contribution in the reported polycrystalline sample and it seems consistent with brick layer model of electrical equivalent circuit. It is evident from the activation energy estimation from modulus and conductivity plot that the small polarons hoping are the key factor in both dielectric ordering as well as electrical conduction mechanism in co-substituted LaFeO₃.     

Particle/Matrix Interface and Its Role in Creep Inhibition in Alumina/Silicon Carbide Nanocomposites

This study focuses on interfacial bonding between intergranular silicon carbide particles and an alumina matrix, to determine the creep inhibition mechanism of alumina/ silicon carbide nanocomposites. It is revealed that the silicon carbide/alumina interface possesses much stronger bonding than the alumina/alumina interface through three approaches: investigation of fracture toughness and fracture mode and consideration of internal thermal stresses acting at grain boundaries, estimation of equilibrium thickness of intergranular glassy films by force balance, and direct observation of grain boundaries by TEM. The rigid bonding of alumina/silicon carbide interfaces causes inhibition of vacancy nucleation and annihilation at the interfaces, causing remarkably improved creep resistance of the nanocomposite.     

Microcrystalline BaTiO3 by Crystallization from Glass

The properties and composition of glasses suitable for crystallization of BaTiO₃ are described. The crystallization of certain glasses results in a nearly complete recovery of BaTiO₃, besides the feldspar BaAl₂SiO₃ as a minor phase. The mechanism of crystallization was investigated by thermal analysis, viscosity, and grainsize measurements as a function of the temperature whereas density data were used for evaluation of the BaTiO₃ content. Within the range 30 to 60% by volume of BaTiO₃ at about 1μ grain size, the measured dielectric constant increased from 100 to 1200. The calculated partial dielectric constant of the Titanate phase at this grain size was about 3500. As the grain size approached 0.1μ, the dielectric constant decreased and became nearly independent of the temperature because of the predominance of surface states. Other effects were attributed to special structural characteristics, such as absence of porosity and clamping of the titanate particles within the microcrystalline matrix. Data are also presented on dielectric constant and loss tangent at different frequencies, dc breakdown strength, dc resistivity, and ferroelectric properties as a function of the grain size of the crystallized material.     

Voronoi cell finite element modelling of the intergranular fracture mechanism in polycrystalline alumina

The mechanisms of fracture in polycrystalline alumina were investigated at the grain level using both the micromechanical tests and finite element (FE) model. First, the bending experiments were performed on the alumina microcantilever beams with a controlled displacement rate of 10 nm s^–1 at the free end; it was observed that the intergranular fracture dominates the failure process. The full scale 3D Voronoi cell FE model of the microcantilever bending tests was then developed and experimentally validated to provide the insight into the cracking mechanisms in the intergranular fracture. It was found that the crystalline morphology and orientation of grains have a significant impact on the localised stress in polycrystalline alumina. The interaction of adjacent grains as well as their different orientations determines the localised tensile and shear stress state in grain boundaries. In the intergranular fracture process, the crack formation and propagation are predominantly governed by tensile opening (mode I) and shear sliding (mode II) along grain boundaries. Additionally, the parametric FE predictions reveal that the bulk failure load of the alumina microcantilever increases with the cohesive strength and total fracture energy of grain boundaries.     

Possible Electrical Double-Layer Contribution to the Equilibrium Thickness of Intergranular Glass Films in Polycrystalline Ceramics

The plausibility of the entropic repulsion of electrical double layers acting to stabilize an equilibrium thickness of intergranular glass films in polycrystalline ceramics is explored. Estimates of the screening length, surface potential, and surface charge required to provide a repulsive force sufficiently large to balance the attractive van der Waals and capillary forces for observable thicknesses of intergranular film are calculated and do not appear to be beyond possibility. However, it has yet to be established whether crystalline particles in a liquid-phase sintering medium possess an electrical double layer at high temperatures. If they do, such a surface charge layer may well have important consequences not only for liquid-phase sintering but also for high-frequency electrical properties and microwave sintering of ceramics containing a liquid phase.     

Dielectric investigations on ‘MgAlON’ compounds: role of nitrogen content

The dielectric properties of dense polycrystalline magnesium aluminium oxynitride have been investigated up to 90 °C. The oxygen/nitrogen substitution on the anionic lattice of this solid solution enhances the charge trapping ability and the dielectric breakdown of ‘MgAlON’ compounds. Oxygen vacancies, which play the role of electron traps, can partially explain the good dielectric properties of this solid solution at low temperature; but they are ineffective for higher temperature where the charge trapping phenomenon remains active. This behaviour, specific to ‘MgAlON’ solid solution, is explained on the basis of a crystallographic model of atom repartition: this spinel structure offers rich-nitrogen zones randomly located as AlN₄ clusters which generate locally the heterogeneous zones of electric charge supposed to be responsible for the good dielectric performance of ‘MgAlON’ compounds. The local polarizability is modified which enhances the charge-trapping phenomenon.     

Microstructural study of dielectric breakdown in glass-ceramics insulators

Novel glass-ceramic materials based on Na and Ca-rich feldspar crystallizations with a hierarchical micro-nanostructure shown the largest dielectric strength, >57 kV/mm, reported at room temperature in ceramic insulators, due to a large amount of interfaces that favor scattering processes of charge carriers. Dielectric breakdown tests with temperature indicated they withstood up to 200 °C, with dielectric strengths of 30 kV/mm and 44 kV/mm for anorthite and albite-based glass-ceramics, respectively. These values are even larger than the ones obtained at room temperature for most of the current ceramic insulators. Microstructural characterization and micro-Raman spectroscopy carried out after breakdown allow determining the dielectric breakdown mechanisms. Glass phases in the surroundings of the crater because of local melting and fast cooling are identified. These results make feldspar based glass-ceramics suitable for electrical insulator applications at room and high temperature. Moreover, dielectric breakdown mechanism may allow tailoring new high insulating application in the future.     

Wet Erosive Wear of Alumina Densified with Magnesium Silicate Additions

A study was made of the wet erosive wear of polycrystalline alumina of mean grain size >1 μm, containing up to 10 wt% of magnesium silicate sintering aid. For pure polycrystalline alumina, the dominant wear mechanism was grain-boundary microfracture, leading to partial or complete grain removal. In the case of the liquid-phase-sintered materials, wear rates could be as low as 25% of those of pure alumina of the same mean grain size, and the main material removal mechanism was transgranular fracture combined with tribochemical wear. The use of Cr³⁺ photoluminescence line broadening showed much higher levels of local stress in the magnesium silicate-sintered materials (∼450 MPa) than in the pure-alumina materials (∼200 MPa). Grain-boundary compressive hoop stresses, caused by the thermal expansion mismatch between a continuous magnesium silicate film and the alumina grains, provided an explanation for the improved wear resistance of the alumina sintered with magnesium silicate.     

Effect of Porosity and Grain Size on the Microwave Dielectric Properties of Sintered Alumina

The real part of the permittivity (epsilon') and the tan of sintered alumina (Al₂O₃) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, and sintered grain size. The tan is found to depend very strongly on the pore volume, purity, and grain size. ɛ' is far less sensitive to impurities and grain size. The dependence of ɛ' on porosity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these materials where extrinsic factors such as porosity, random crystal orientation, grain boundaries, microcracks, and impurities dominate. These factors have been studied in an attempt to describe the tan δ and ɛ' of sintered polycrystalline alumina. In this work, the tan δ for alumina has been studied in near-theoretical density ranges between 9.1 × 10₋₅ and 2.4 × 10₋₅ depending on grain size.     

Modeling of Grain-Size-Dependent Microfracture-Controlled Sliding Wear in Polycrystalline Alumina

To quantify grain-size-dependent sliding wear of polycrystalline alumina induced by grain-boundary microfracture, an attempt is made to extend and combine Cho et al. 's fracture mechanics analysis and Fu and Evans' simple micro-cracking theory. An analytical equation is derived to relate wear with microstructural parameters. Wear by intergranular microfracture occurs, provided that the combined stresses are greater than a threshold value. The critical sliding time to the wear transition decreases with increasing grain size, and the wear rate after the threshold is proportional to the grain size. The theoretical predictions are correlated with the lubricated sliding wear data of aluminas with different grain sizes reported by Cho et al.     

Electrically conductive alumina–carbon nanocomposites prepared by Spark Plasma Sintering

Carbon nanotubes (CNTs) and carbon black were added to alumina to convert it into a good electrical conductor. Alumina–CNT and alumina–carbon black nanocomposites were fabricated by Spark Plasma Sintering (SPS). The electrical conductivity of alumina–CNT nanocomposites was found to be four times higher as compared to alumina–carbon black nanocomposites due to the fibrous nature and high aspect ratio of CNTs. The electrical conductivity of alumina–CNT nanocomposite increased with increasing grain size due to increasing density of CNTs at the grain boundaries. This effect was not observed for alumina–carbon black nanocomposite due to the particulate geometry of the carbon black.     

Computational modeling of grain boundary electrostatic effect in polycrystalline SrTiO3 thin film

Understanding the leakage current caused by charge transport and local accumulation in dielectric oxides is critical for predicting and extending the lifetime of dielectric-based electronic devices. The internal interfaces such as grain boundaries (GBs) inside a dielectric induce local strain and charge segregation and thus further influence the charge transport behavior. In this work, we employ computational modeling based on the Schottky barrier model and nonlinear Nernst-Planck transport equation is used to study the oxygen vacancy transport and leakage current evolution in a SrTiO₃ thin film under a DC bias with planar electrodes. It is found that in polycrystalline SrTiO₃, the GB-bounded donors create an electric potential barrier and a local depletion region near the GBs, impeding the oxygen vacancy transport and suppressing the leakage current increase compared to a single crystal SrTiO₃ thin film. The effects of temperature, the magnitude of an applied field, the number density of GBs, the GB-bounded donor concentration, and the depletion layer width on the leakage current evolution are systematically investigated. The simulation results are compared with the analytical solutions, as well as with existing theoretical and experimental reports. This work thus helps shed light to the grain-structure dependent electrostatic behaviors in dielectric thin films under different intrinsic and extrinsic conditions.     

The search for enhanced dielectric strength of polymer-based dielectrics: A focused review on polymer nanocomposites

This report traces the leading scientific endeavors to enhance the dielectric strength of polymer dielectrics for energy storage and electrical insulation applications. Remarkable progress has occurred over the past 15 years through nanodielectric engineering involving inorganic nanofillers, coatings, and polymer matrices. This article highlights the challenges of dielectric polymers primarily toward capacitors and cable/wire insulation. It also summarizes several major technical approaches to enhance the dielectric strength of polymers and nanocomposites, including nanoparticle incorporation in polymers, filler-polymer interface engineering, and film surface coating. More attention is directed to interface contributions, including rational design of core-shell structures, use of low-dimensional fillers and thermally conducting fillers, and inorganic surface coating of polymer films. These efforts demonstrated the enhancement in dielectric strength by 40–160% when controlling the fillers below 5 wt% in polyvinylidenedifluoride (PVDF) composites. This article also discussed the possible dielectric mechanisms and the positive role of interfaces against charge transport traps for attaining higher breakdown strength. The investigation of low-dimensional filler/coating materials of high thermal conductivity can be key scientific subjects for future research.     

Improved Flaw Tolerance in Alumina Containing 1 vol% Anorthite via Crystallization of the Intergranular Glass

The intergranular phase in an alumina containing 1 vol% anorthite glass was crystallized in order to enhance internal residual stresses within the microstructure. The influence of crystallization on the mechanical behavior was investigated by the indentation–strength method. Such crystallization was found to result in a marked improvement in the flaw tolerance of this alumina, indicative of strong R -Or T -curve behavior. These results are discussed in the light of a theoretical model which assumes grain-localized crack bridging to be the predominant toughening mechanism. Particular reference is made to the influence of residual stresses and interfacial properties on grain pullout across the crack walls in the wake of the crack tip.