Influence of Thermal Treatment on Electrical and Physical Properties of Coated Ceramics

Technical dielectric materials and ceramics are used in many different high technology industrial areas and especially for spacecraft applications. On satellites, these materials are subjected to extreme conditions due to the space plasma environment. To survive, these ceramic insulators must have exceptional electrical and thermal properties. Boron Nitride (BN) and Aluminum Oxide (Al 2O3) are used in particular because they combine good electrical insulation and high thermal conductivity. However, BN and Al2O3 used in spacecraft interiors are exposed to critical radiation demands, where these insulators are irradiated by electrons with high energies and flux. Charged particles are trapped in the ceramics, producing high electric fields. Subsequently, internal disturbances and electrical breakdowns can occur. Over time, these phenomena may cause degradation or failure of various components and embedded systems. Consequently, this study endeavors to understand the physical mechanisms which occur in these ceramics materials under electron irradiation. These dielectrics materials have been characterized at ONERA Toulouse (DESP) in the CEDRE (Chambre d’Etude De Revêtement Electrisés) irradiation chamber. A parametric study was performed to assess the influence of incident energy and flux, temperature, coating s, annealing, and ionizing dose on the charging and relaxation kinetics of BN and Al2O3. Surface and thermal treatments were found to limit BN’s charging. Dedicated treatments enhanced charge transport. To identify the effect of thermal annealing on electrical behavior in these materials, a thorough study of electron trapping processes was performed using cathodoluminescence in the Electron Emission Test facility at Utah State University. These tests explored differences in the nature and density of defect states. Together, these investigations determined correlations between chemical, structural and physical properties for each insulator’s configurations. Further, we observed degradation of coatings and an evolution of the concentration of their chemical defects. Contamination and ageing effects were identified on the rough material surfaces of ceramics exposed under a critical electron flux. Therefore, treatments applied to optimize electrical properties were found to be ineffective, especially for long-term charging mitigation. We will discuss these results and compare them for each ceramic configuration. The goal of this investigation is to understand the predominant physical mechanisms and main structural and chemical differences between these ceramic configurations in order to perform an exhaustive correlation between the properties. In future studies, we propose to define a defect-based model which can be used to optimize a material to limit both its charging and degradation over the time

Ceramic Substrates for High-temperature Electronic Integration

One of the most attractive ways to increase power handling capacity in power modules is to increase the operating temperature using wide-band-gap semiconductors. Ceramics are ideal candidates for use as substrates in high-power high-temperature electronic devices. The present article aims to determine the most suitable ceramic material for this application

Investigation of the thermal expansion and heat capacity of the CaCu3Ti4O12 ceramics

The thermal expansion of the CaCu3Ti4O12 ceramics has been measured over a wide temperature range 120–1200 K. The high quality of the samples under study has been confirmed by good agreement of the results of measurements of the heat capacity in the range 2–300 K and in the vicinity of the phase transition of magnetic nature at 25 K with the data for the single crystal. No anomalies in the thermal expansion that can be associated with the phase transition at 726–732 K assumed by other investigators have been found. The influence exerted on the thermal expansion by the heat treatment of the sample in a helium atmosphere and in air has been investigated

Colossal dielectric permittivity of BaTiO3-based nanocrystalline ceramics sintered by spark plasma sintering

In pursuit of high permittivity materials for electronic application, there has been a considerable interest recently in the dielectric properties of various perovskite oxides like calcium copper titanate or lanthanum doped barium titanate. When processed in a particular way, this later material present at ambient temperature and at f=1 kHz unusual interesting dielectric properties, a so called “colossal” permittivity value up to several 106 with relatively low dielectric losses. Moreover and contrary to what is classically expected and evidenced for this type of materials, no temperature dependence is observed. This behavior is observed in nanopowders based ceramics. An assumption to explain the observed properties is proposed. These results have important technological applications, since these nanoceramics open a new route to the fabrication of very thin dielectric films

Cation distribution in manganese cobaltite spinels Co3−xMnxO4 (0 ≤ x ≤ 1) determined by thermal analysis

Thermogravimetric analysis was used in order to study the reduction in air of submicronic powders of Co3−x Mn x O4 spinels, with 0 ≤ x ≤ 1. For x = 0 (i.e. Co3O4), cation reduction occurred in a single step. It involved the CoIII ions at the octahedral sites, which were reduced to Co2+ on producing CoO. For 0 < x ≤ 1, the reduction occurred in two stages at increasing temperature with increasing amounts of manganese. The first step corresponded to the reduction of octahedral CoIII ions and the second was attributed to the reduction of octahedral Mn4+ ions to Mn3+. From the individual weight losses and the electrical neutrality of the lattice, the CoIII and Mn4+ ion concentrations were calculated. The distribution of cobalt and manganese ions present on each crystallographic site of the spinel was determined. In contrast to most previous studies that took into account either CoIII and Mn3+ or Co2+, CoIII and Mn4+ only, our thermal analysis study showed that Co2+/CoIII and Mn3+/Mn4+ pairs occupy the octahedral sites. These results were used to explain the resistivity measurements carried out on dense ceramics prepared from our powders sintered at low temperature (700–750 °C) in a Spark Plasma Sintering apparatus

Development of nano oxide αCoMoO

Molybdates oxides as AMoO4 (A = Cu, Co), have remarkable properties. These properties depend strongly on the crystallite size. Nanostructured Powders of these molybdates make their applications more usable. The aim of our work is to synthesize oxide CoMoO4 by soft chemistry method, which has the advantage of producing very fine and homogeneous powders, which increases their reactivity. The products obtained were characterized by XRD, SEM and TEM