Shoichi Kimura

Electrical Characteristics of TiO2 and TiO2-BaTiO3 Coatings Produced by Flame Spraying as Thermistor Elements

This paper describes the semi-conductive properties of flame-sprayed TiO2 and TiO2-BaTiO3 coatings and discusses about the applicability of these coatings to thermistor elements.The following conclusions were drawn from the results of various tests.1) Specific resistance of the coatings increased with the increase in the addition amount of BaTiO3. Then, the change of resistance with temperature was also greater; that is, the value of B (thermistor constant) increased.2) The heat dissipation constant (k) of single TiO2 was 0.5-1; but the value increased with the increasing addition of BaTiO3 to TiO2. It reached the maximum at a certain amount of BaTiO3, and then, decreased with the increase of BaTiO3. The maximum value of k would be at 50-60 wt% of BaTiO3.3) Coatings of low resistance with large values of B and k could be obtained by proper selection of blending ratio, temperature of heat treatment, and electrode material.4) In almost all the cases, TiO3-based coatings exhibited negative resistance.5) Hall coefficient of flame-sprayed TiO2 was extremely small so that it could not be measured by the ordinary direct-current method.

Analysis of Coating Structure by Means of Microanalyzer and Thermal Analyzer

Analysis of the barium titanate coating produced by flame spraying was made by means of microanalyzer and thermal analyzer.The oxygen-vacant lattices were present in the coating; but most of them disappeared when the coating was treated at high temperatures. Although it was treated at the temperature as high as 1, 400°C, some of them still remained on the interfacial layer between the coating and Ni substrate. The ratio of Ba: Ti in the coating was approximately equal to that in ceramic barium titanate.Ceramic barium titanate showed a sharp endothermie peak at 110-120°C by differential thermal analysis; while, it was not observed in flame-sprayed product. However, when the coatings was heattreated at the temperatures of 1, 200 and 1, 400°C respectively broad and high peaks at 120°C were observed. The figures were evidently different from that of ceramic product.

Studies of Heat Treatment on Properties of Flame-Sprayed Barium Titanate Coatings

Physical, mechanical, and electrical properties of flame-sprayed barium titanate coatings were considerably affected by heat treatment.Micro-crystallites were developed with the elevation of annealing temperature. The structure of crystallites changed to tetragonal system at 1, 400°C. The effects of the substrate on the crystallites structure was remarkable in rod sprayed coatings. The structure of the coatings with substrate annealed at 1, 400°C turned into tetragonal system, whereas in case of the structure without the substrate annealed at that temperature still remained in cubic system.The bond strength increased with the elevation of annealing temperature up to 1, 200°C.The surface roughness after heat treatment reached its maximum at 1, 400°C. The dielectric constant and dissipation factor proportionally increased with the elevation of annealing temperature. Furthermore, the dependence of properties on the temperature more increased at higher annealing temperature.The breadth of the peak of Curie point increased as compared with that of ceramic barium titanate when heat treated at above 1, 200°C (in rod-spraying) or above 1, 000°C (in powder-spraying).When the annealing temperature was higher, the insulation resistance was lower; however, the dependence of the resistance on the temperature was less.

Physical, Mechanical and Electrical Properties of Flame-sprayed BaTiO3 (Part 2)

BaTiO3 rods were sprayed with a commercial rod spray gun and the physical, mechanical and electrical properties of the deposited film were investigated under various spraying conditions.As the results of X-ray analysis of the film, the following 3 sorts of phases were determined according to the relation between the film structure and the distance from nozzle to substrate.When the distance was up to 8cm, the degree of amorphous was lower with the decrease of the distance between the gun nozzle and substrate. When the distance was 8-10cm, a structural change rapidly occurred, whereby the structure of film was converted into tetragonal system at 10cm. When the distance was beyond 10cm, the degree of amorphous was gradually higher with the increase of the distance.The tensile strength of the film was much greater than that of flame sprayed alumina.The maximum dielectric constant attainable was 350, but higher values of dielectric constant will be obtained under some other spraying conditions.The flame sprayed BaTiO3 did not represent a transition point such as Curie point of 120°C as seen in the conventional BaTiO3. The temperature coefficients of capacitance at the temperatures of -55-160°C ranged from 100 to 200ppm/°C, depending on the spraying conditions.Peculiar characteristics of the film was observed when the film was subjected to aging tests at various temperatures. The rate of decrease of dielectric constant was lower when the aging temperature was higher.