Microstructure and Electrical Characteristics of Plasma Sprayed Thick Film Mn-Co-Ni Oxide Thermistor
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Microstructure and Electrical Characteristics of Plasma Sprayed Thick Film Mn-Co-Ni Oxide Thermistor S. Liang, B.G.Ravi, S.Sampath, R. Gambino Center for Thermal Spray Research, Department of Materials Science and Engineering, State University of New York, Stony Brook, NY 11794-2275. ABSTRACT Mn-Co-Ni-O spinel, owing to its large temperature dependant resistivity, is an attractive ternary system for negative temperature coefficient (NTC) thermistor applications, particularly in oxygen rich environments. This research explores the potential of plasma spray as a method for the fabrication of thick film Mn-Co-Ni-O thermistors. Nanostructured Mn-Co-Ni-O spinel powders were synthesized by gel combustion method and were air-plasma-sprayed on to alumina substrates. The as-sprayed deposit formed a rocksalt structure rather than a cubic spinel structure. Annealing in air at 600oC for 3 hours resulted in transformation to the spinel structure while higher temperature annealing resulted in phase separation of rocksalt phase at 1250oC and recombination into spinel monophase at 1350oC. Temperature dependent measurement of resistivity showed -4 to -1% temperature coefficient of resistivity (TCR) from room temperature to 250oC with high reproducibility, making the low temperature annealed coating suitable in thermistor applications. Keywords: Plasma spray, NTC thermistor, combustion synthesis, electrical resistivity INTRODUCTION Mn-Co-Ni-O spinel, due to its large temperature dependent resistivity, is an attractive candidate for negative temperature coefficient (NTC) thermistor applications [1], and infrared detecting bolometers [2]. The material resistivity and temperature coefficient of resistivity (TCR) depend on its relative content of metallic species. Mn1.56Co0.96Ni0.48O4 is a composition of specific interest since it is very near the resistivity minimum for the ternary oxide, and its coefficient of thermal expansion (CTE=9.3 ppm/K) is close enough to that of alumina (7~8ppm/K) making it suitable to make film sensors on alumina substrate. The site distribution for this composition has been determined by other researchers to be [Co 02.+557 Mn02.+443 ]T [ Ni 02.+48 Co 03.+403 Mn04.+48 Mn 03.+637 ] o O4 [3]. Electrical conduction is believed to be due to electron hopping between Mn3+ and Mn4+ [4] in the octahedral sites. Thick film spinel-based NTC thermistor sensors are traditionally fabricated using ceramic paste printing and subsequent sintering [1]. This requires lengthy high temperature sintering cycles making it cumbersome from the manufacturing point of view. Other alternate routes have been used, such as rf magneton sputtering [2,3], electron beam evaporation [5], and metal organic decomposition [6]. In recent years, the potential for plasma spray process as a possible
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technique in making thick film sensors have been explored, for example, magnetoresistive sensors [7] and strain gauges. It has several advantages: (1) simple process (2) suitable for making complex geometry (3) cost efficie
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