Thermistor with more stable beta

#This invention relates to thermistors. It more particularly relates to a thermistor composition that provides a more stable temperature coefficient over a wide temperature range.

#A thermistor composition consisting essentially of cobalt oxide, manganese oxide, and zinc oxide is disclosed in U.S. Pat. No. 3,652,463 Riddel, assigned to the assignee of the present invention. The Riddel patent discloses particular ternary ratios of such oxides that provide a high temperature coefficient (i.e. beta) for discrete thermistors. These particular compositions can be fired in air and have moderate electrical resistivity, high stability, and good mechanical properties. On the other hand, these compositions generally exhibit a temperature coefficient which changes about 5%-20% over a wide temperature range such as about 50.degree.-350.degree. F. In other words beta is not stable. This produces a corresponding error in temperature measurement based on thermistor resistance change, unless this deviation in beta is compensated. Such compensation would have to be made for each significant span in the temperature range measured. If a computer is used for temperature measurement, considerable memory and extensive computer time is required to perform this compensation. If beta is constant over the temperature range measured, temperature measurement can be done by computer much more simply and quickly.

#I have found how to make the temperature coefficient of certain high beta compositions more constant over a wide temperature range without detrimental effects on resistivity, stability or mechanical properties. Temperature measurement with such compositions can now be more simply and quickly done.

#Other objects, features and advantages of the invention will become more apparent from the following description of the preferred examples thereof and from the drawing, in which:

#FIG. 1 is a ternary compositional diagram showing selected ratios of the three predominant oxides in my thermistor composition;

#FIG. 2 is a block diagram illustrating the titanium oxide addition of this invention; and

#As mentioned, this invention is an improvement on the prior ternary cobalt oxide-manganese oxide-zinc oxide thermistor compositions. However, to realize the improvement, only a selected few ternary oxide ratios are useful. Most of my ratios are different from those disclosed in the aforementioned U.S. Pat. No. 3,652,463 Riddel. The selected ternary ratios useful in my invention are shown in FIG. 1.

#The lines ABCD of FIG. 1 in the drawing of this patent application enclose an area representing selected ternary oxide ratios that can be used in this invention. Such oxide mixtures, when combined with titanium oxide, provide thermistors with temperature coefficients (beta) more constant over 77.degree.-330.degree. F. It is believed that beta will be constant over an even wider range, including 50.degree.-350.degree. F. In the area ABCD, cobalt oxide varies from 27% to 65%. Manganese varies from 15% to 46%. Zinc oxide varies from 4% to 34%. Best results were obtained with more than about 30%, by weight, manganese oxide.

#The cobalt oxide is in the form of cobaltic oxide (Co.sub.2 O.sub.3). However, cobaltous oxide (CoO), or mixtures of the two are expected to be useful. The manganese oxide is manganese sesquioxide (Mn.sub.2 O.sub.3). However, manganese dioxide (MnO.sub.2), manganese trioxide (MnO.sub.3), or manganese heptoxide (Mn.sub.2 O.sub.7) should be substitutable. Titanium oxide means titanium sesquioxide (Ti.sub.2 O.sub.3). However, other oxides of titanium are expected to be useful too. Since the thermistor composition of this invention is fired in air, the final product should be the same regardless as to the particular cobalt, manganese, zinc or titanium oxide used as a starting ingredient. Analogously, precursors of such oxides may also prove to be useful, particularly if one elects to calcine the mixture prior to pressing and sintering. For example, materials which decompose during calcination into their related oxides, such as carbonates, may prove to be useful in forming the thermistor composition of this invention.