Humidity sensor using amorphous zirconium phosphate and its alkali salts

Abstract

It is well-known that inorganic electrolytes are formed by polybasic acids and certain hydrol3)sable polyvalent cations, and many of these salts are extremely insoluble in most reagents; a typical example is the phosphates of zirconium and titanium. Of these zirconium phosphate is known as an insoluble inorganic ion exchanger and proton conductor [1, 2]. Recently, it has been reported that the electrical conductivity of zirconium bis(monohydrogen orthophosphate)monohydrate and the sintered zircon with phosphoric acid and/or alkali phosphate is strongly affected by the moisture in the atmosphere [3-6]. Because these compounds are extremely insoluble in most reagents and the mechanically strengthened porous ceramics are easily prepared by burning at 1000°C or below, it would appear that zirconium phosphate is a suitable material for use in a humidity sensing device. In the present work the results obtained on the humidity sensitivity and its response to humidity changes of zirconium phosphate and its alkali salts are presented. Zirconylchloride was first dissolved in distilled and deionized water, then a large excess of phosphoric acid (phosphoric acid/zirconylchloride = 4 mole ratio) was added in the solution under agitation. After the white precipitate (zirconium phosphate) was separated from the solution by filtration, it was washed with distilled and deionized water and dried at 60 ° C. The sample thus obtained had no peaks in X-ray diffraction analysis. The alkali salt was obtained by titrating zirconium phosphate suspended in distilled and deionized water with the corresponding alkali hydroxide solution using the end-point titration method of up to pH --10.0. The products were washed with distilled and deionized water and dried at 60 ° C. The pulverized powder was pressed into a disc at 100 kg cm -2 and the disc was then shaped to 10ram x 10mm. The disc, ~0.5mm thick, was burned at each temperature in air. Next, gold electrodes, 4mm x 4mm, were applied to opposite faces of the disc by the vacuum evaporation method. The composition of amorphous zirconium phosphate may be expressed as ZrO(H2PO4)x(OH)2_xH20. By assuming that the dehydration goes essentially to completion by heating to ~800°C, the composition of the sample is predicted from the results of the pH titration measurements and the thermal gravimetrical analysis. These experiments suggest that the composition of starting zirconium phosphate may be expressed as ZrO(H2PO4)o3(OH)I7" H20, while no apparent plateaus are observed in the thermal TABLE I Surface area (m-'g i) of some samples