David Houivet

High temperature NTC ceramic resistors (ambient-1000 °C)

New temperature sensors operating from ambient to 1000 C, stable in diesel exhaust gas, and cheap, are needed. Such components may be ceramics. The value of their electrical resistance versus temperature is classically expressed as R=a.exp(B/T). Compounds from the ternary diagram Y 2 O 3 -Mn 2 O 3 Cr 2 O 3 may permit the obtaining of such performances. We have synthesized some compositions by solid-state reaction. Dense ceramics sintered at 1600 C are two-phased, compound of a Y 2 O 3 phase and an orthorhombic perovskite YCr 0.5 Mn 0.5 O 3 phase. Their resistivity at room temperature can be in the range 4 kΩ cm to 10 6 MΩ cm together with a B coefficient value of about 3600 K. Such ceramics can be used to make NTC resistors with a resistance value ranging from some hundreds kΩ at room temperature to some Ω at 1000 °C.

Effect of annealing on the microwave properties of (Zr,Sn)TiO4 ceramics

Abstract (Zr,Sn)TiO 4 dielectric ceramics containing La 2 O 3 and NiO as sintering aids were prepared by the conventional solid-state reaction. Ceramics sintered at 1370°C for 20 h exhibit dielectric constants k around 37.1 and a QF value of 41,500 GHz, measured at 4 GHz. After annealing, k remains constant. However, the QF value is strongly affected by the annealing process. At 1300°C, annealing has no effect on the quality factor Q . Annealing at temperatures 1225 and 1200°C induce a ≈50% decrease of QF values. Intermediate annealing temperatures of 1275 and 1250°C induce a ≈25% improvement of QF values. These ceramics were investigated by XRD, SEM and EDS. The matrix phase ZST was observed together with three different secondary phases: TiO 2ss , La 2/3 TiO 3 and TiNiO 3 . Appearance and following diffusion of these secondary phases and other impurities to the ceramic surfaces during annealing can explain the QF behaviour.

Microwave properties and microstructures of LaTiO3 stabilized with NiO

Abstract (1– x )La 2/3 TiO 3 – x TiNiO 3 ceramics with x ranging from 0.01 to 0.2, were elaborated by conventional solid-state reaction starting from TiO 2 , La 2 O 3 , and NiO powders. These compositions have been identified as secondary phases detected in (Zr,Sn)TiO 4 ceramics sintered with La 2 O 3 and NiO. Microwave resonators were sintered at temperatures ranging from 1340 to 1380°C. These sintering temperatures have been determined taking into account thermodilatometric data. Microwave dielectric properties and microstructures were investigated. Perovskite phase La 2 3 TiO 3 was identified by XRD, SEM and EDS together with minor phases. Dielectric constants are in the 50–70 range and QF values, measured at 3 GHz, are close to 20 000 GHz. One significant composition is characterized by k =69 and QF =17,000 GHz at 3 GHz.

Effect of hygrometry on dielectric materials

Abstract We have investigated the electrical properties of sintered disks elaborated from different type I and type II dielectric compositions by measurement of their insulation resistance versus the moisture content in air. The insulation resistance of the disks can reach values such as 10 13 ohm when measured in dry air and 10 8 ohm when measured in a wet atmosphere. This behaviour may be due to reversible surface reactions occurring at the surface of the dense ceramics.

Image analysis of multiphased ceramics

Abstract In this study, we focus on the recognition and characterization of the different phases present in a (Zr,Sn)TiO4 ceramic (with some amounts of La2O3 and NiO as sintering aids). Our work deals with the study of segmentation techniques that automatically extract each of the phases (ZST, TiO2, La2/3TiO3,…) from a scanning electronic microscope image of a polished section of the ceramic. The main difficulty encountered at this stage of the work is that their properties in terms of gray levels are very close together, thus, imposing the combination of simple image processing operators (such as thresholding) with advanced methods based on region growing processes.

Ultrafine Grinding of MgTiO3 Based Ceramic Influencing the Material Properties

The effect of grinding MgTiO3 on the particle size and electrical properties was studied. Initially, good dispersion conditions were optimized via rheological characterizations of the aqueous slurry in variant dispersant amounts. Then, 50% MgTiO3 slurry was grinded using balls of different diameter sizes (0.8 and 0.4 mm) for two consecutive cycles. The milled powder was characterized using laser granulometry, BET method, X-ray diffraction and scanning electron microscopy techniques. The crystallite size was calculated using Halder-Wagner methods. The electrical and dielectrical properties were also monitored for different particle size. These results revealed impurities from the zirconium balls, however, without affecting the particle size even if prepared with different grinding operations.