US3430336A - Method of making a thermistor - Google Patents

Description

March 4, 1969 w. RIDDEL 3,430,336

METHOD OF MAKING A THERMISTOR Filed Sept. 15, 1965 Sheet ATTORNEY March. 4, v1969 ,J. w. RIDDEL 3,430,336

METHOD OF MAKING A THERMISTOR Filed Sept. 15, 1965 Sheet 2 of 2 ZnO C0 0 INVENTOR JohnWR/b'de/ ATTORNEY United States Patent 3,430,336 METHOD OF MAKING A THERMISTOR John W. Riddel, Fenton, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Sept. 15, 1965, Ser. No. 487,563 US. Cl. 29-612 Int. Cl. H01c 7/04 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates in general to thermistors and more specifically to an improved composition of the same.

A major problem in making thermistors for high tem perature use is lead attachment. In the past, platinum leads have been fired into the thermistor. This technique, however, is generally quite expensive. Usually, it is preferable to fire the thermistor as a disc, then fire on silver or other commercial coating for contacts. When the thermistor is intended for use below about 200 'C., the leads may be attached to the silver contacts with soft solder. For use above 200 C., the leads must be attached with the same material as used for the contact. This, however, is generally not very satisfactory because of poor lead adherence. A part of this invention is, therefore, concerned with improving the attachment of leads to a thermistor which is sufficiently bonded to enable the mounting to be used at high temperature.

One object of this invention is an improved method for mounting a thermistor to a thermally responsive body which provides a low thermal resistance between the thermistor and the metal housing.

Another object is an improved method for mounting a thermistor to a thermally responsive body wherein the thermal response is significantly increased.

A further object is to provide a thermistor mounting in which it is not necessary to ground either side of the thermistor.

Still a further object is to provide a thermistor mounting in which the thermistor is electrically insulated from the thermally responsive body but remains in thermal contact therewith.

Another object is to provide a mounting in which the thermistor contacts are not exposed to the atmosphere.

Still another object is to provide a thermistor mounting which has high thermal shock resistance.

A further object is to provide an improved method for attaching conductive leads to a thermistor in which the assembly can withstand temperature above 200 F.

Another object is to provide a method for attaching conductive leads to a thermistor which enables the use of silver paint for attaching the leads thus eliminating the use of special holding hardware.

Still another object is to provide an improved thermistor having a high temperature coefiicient.

A further object is to provide a thermistor composition that can be fired in air. v

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Another object is to provide a thermistor composition that has improved stability.

The accompanying drawing illustrates several preferred embodiments of the invention which will hereinafter be described, it being understood that the embodiments illustrated are susceptible of various modifications with respect to details thereof without departing from the scope of the appended claims.

In the drawings:

FIGURE 1 shows a cross-sectional view of an enlarged scale of one embodiment of the thermistor mounting according to the invention.

FIGURE 2 shows a cross-sectional view on an enlarged scale of another embodiment of the thermistor mounting according to the invention.

FIGURE 3 is a greatly enlarged cross-sectional view of a thermistor with leads mounted thereto according to the invention.

FIGURE 4 is a triangular diagram showing the preferred thermistor composition of cobalt, manganese .and zinc oxide according to the invention.

Referring to FIGURE 1, the numeral 10 designates one form of mounting a thermisor 12 to a cup-shaped metal housing 14. In this embodiment the thermistor 12 which preferably has the configuration of a circular water or disc is coated on the lower surface with a thin layer 16 of electrically conductive metal such as silver paint for contact with the inner surface of the metal housing 14. A portion of the upper surface of the thermistor 12 is coated with a thin layer 18 of silver paint for contact 'with a conductive lead 20. Another conductive lead 22 is spot welded to the inner surface of the metal cup 14. The thermistor is positioned in place and coated with a layer 24 of enamel. The enamel layer 24 is of sufficient thickness to cover the top and side surfaces of the thermistor as well as portions of the inner surface of the metal housing 14. When the enamel is initially applied to cover the thermistor it is in liquid form and uncured. After coating, the enamel is cured and caused to harden by firing the assembly in any suitable furnace heated to a sufii cient temperature. For example, the assembly may be fired five minutes at 15 50 F. in air.

If it is desired to mount the thermistor .12 so that it is not in electrical contact with the metal housing 14, a layer of insulating material 26 is disposed between the thermistor 12 and the metal housing 14 as shown in FIG- URE 2. In this embodiment, the conductive leads 28 and 30 are attached to electrically isolated areas on the top surface of the thermistor 12, each area being coated with a layer 32 and 34 of silver paint. The top surface is electrically isolated into two contact areas by cutting a groove 36 across the surface. The insulating material 26 must be a good electrical insulator as well as being characterized by a relatively high thermal conductivity. Examples of suitable materials that can be used include magnesium oxide (MgO) and aluminum oxide (A1 0 The insulating medium may be sprayed on the housing, inserted in the form of a thin wafer, or used in any other suitable manner. One important advantage of mounting these thermistors according to this method is that they have a higher temperature coefiicient of resistance than do conventionally glass bonded thermistors or other oxide thermistors of equal or lower resistivity. In addition, this type mounting is suitable for use at high temperatures since the enamel does not soften below about 1300 F. Normally, it is not feasible to coat thermistors with enamel because the characteristics of the thermistor change deleteriously. However, the particular composition of the thermistor according to this invention is not adversely affected by coating it with enamel.

The preferred composition of the enamel used to mount the thermistor to the metal housing is described below.

The XG203, 2501, and 2502 flake frits listed above are described by the manufacturer as being alkali-boro-aluminum silicates.

FIGURE 3 illustrates a thermistor 38 with conductive leads 40 and 42 connected thereto according to another embodiment of this invention. A groove 44 is cut across the top surface of the thermistor so as to divide this surface into two electrically isolated areas coated with layers 46 and 48 of silver paint. The entire surface of the thermistor 38 is then coated with layer 50 of enamel of sufiicient thickness to satisfactorily adhere the leads 40 and 42 to the thermistor.

The conductive leads 40 and 42 are attached to the thermistor using the following procedure which is by way of example and without in any way limiting my invention thereto. After the thermistor is pressed into the desired shape and fired in a furnace, silver contact paint layers 46 and 48 are applied to the top surface and subsequently dried at a temperature of 400 F. for about minutes. A diametral groove 44 is then cut across the top surface of the thermistor to provide the surface with two electrically isolated contact areas. The conductive leads are shaped and a small amount of silver paint is applied to one end of each lead. The coated end of each lead is then mounted to the separate contact areas and the silver paint dried. The assembly is then placed in a furnace open to the atmosphere at a temperature of 1550 F. for 12 minutes. The thermistor is removed from the furnace and dipped in enamel having the composition described in Table I. The assembly is then placed in a suitable furnace at 400 F. for 10 minutes to dry the enamel. The assembly is then placed in a furnace open to the atmosphere at a temperature of 1550 F. for 20 to 30 minutes to cure the enamel. It has been found that in using silver paint a very thin glass film separates the lead from the contact silver thus causing resistance measurements to be erratic. A short voltage of 60 volts is applied for an instant to the thermistor to break down this film and the resistance reading is stabilized. This step may be eliminated by using a material different than silver paint. The thermistor is then permitted to cool and is checked for resistance. The enamel coating enables the thermistor to withstand temperatures up to about 1300 P. which, as previously mentioned, is the softening point of the enamel.

It is to be understood that the enamel coating can be used to secure conductive leads that are embedded in the thermistor. In the embodiment described above the use of silver paint in attaching the conductive leads is only made possible by the use of the enamel, thereby eliminating the use of special holding hardware used in connecting embedded leads.

For an understanding of the preferred thermistor composition, reference is made to FIGURE 4 in which a triangular-coordinate scale has been used to represent the relative proportion of the three oxides contained in the composition.

By reference to FIGURE 4, it is seen that the percentage of zinc oxide present may vary from to 37% by weight, the percentage of manganese oxide present may vary from 31 to 77% by weight and the balance to total 100% consists of cobalt oxide. A preferred composition is 23% zinc oxide, 46.1% manganese oxide and 30.8% cobalt oxide.

The manganese oxide may be manganese dioxide (MnO manganic oxide (Mn O manganese trioxide (MnO or manganese heptoxide (Mn O The cobalt oxide is in the form of cobaltous oxide (C00), cobaltousic oxide (C0 0 or cobaltic oxide (C0 0 When the various oxides are substituted one for another on the basis of the-metal in the oxide, the final product will be the same since its state of oxidation is determined by the atmosphere and temperature during firing.

The following example is given to illustrate the invention, it being understood that the invention is not to be limited to such specific details.

The following substances are crushed in distilled water for approximately 8 hours:

The product is preferably dried and fired at a temperature of about 1400 C. in a furnace open to the atmosphere for approximately 2 hours. The product is thereafter crushed and passed through a 320-mesh sieve and mixed with sufiicient Wax to form pellets. The pellets are then granulated to pass through a 28-Inesh sieve but be retained on an 80-mesh sieve using a wax binder. The granulated product is then pressed into a pellet shape at a pressure of approximately 10,000 p.s.i. The pellets are then dried and may be cut into pieces. These pellets are fired at a temperature of 2550 F, (1400 C.) for 1 hour. Silver contacts are then fired on the pellets at 1500 F. although platinum contacts may be embedded in the pellets if desired. In making the platinum type of contact, the pressed thermistor is bisque-fired to 1900 F., a hole drilled for a 0.020 platinum lead wire and fired with the lead in place. Either firing silver or glazing around the fired-in lead assures electrical contact.

The thermistor may then optionally be coated with a glaze to improve its stability. The glaze can be applied by dipping, spraying or brushing. The preferred glaze composition is given below:

TABLE 11 Material: Parts by weight Ferro Corporation 2501 Flake Frit 500 Ferro Corporation 2502 Flake Frit 500 Clay 50 Borax 2.5 Sodium nitrite 1.0

TABLE III Temperature, F. Resistance Percent chaugel I Three thermistors having the above composition increased in resistance an average of 2.3% at 140 F. and 4.2% at 200 F. during a 500 hour stability test. This test consisted of cycling the thermistor mounted in a probe from cold tap water to hot water (200 F.) at 24 cycles per hour with 14.5 volts applied to the thermistor in series with a 50 ohm resistor. The thermistor probe assembly contained a dielectric fluid. No significant resistance change occurs when the thermistor is operated for the same time in an air atmosphere.

An additional feature of the thermistor composition according to this invention is that the resistivity can be increased by a factor of up to a thousand or more by adding alumina. The resistivity increase depends on the method of mixing the alumina into the material and on the percentage of alumina. For a particular method of adding alumina the following linear dependence of log of room temperature resistance versus percent alumina was obtained (milled for one hour in a steel mill followed by an acid leach):

Percent Alumina R/25 C. (K ohm) Log Rl25 C.

As noted above the temperature coefiicient is only slightly increased by the addition of alumina. A batch of material containing approximately 20% alumina milled for 8 hours had a resistivity of 640 times that of the above material with no alumina. Although the above composition contains up to 9% alumina, these compositions can contain up to 50% alumina. The advantage of adding alumina to increase resistance is that it gives an easy method for controlling resistivity over wide ranges. In addition, it can be used to shift the useable resistance range to higher temperatures.

The long term stability at 1400 to 1600 F. of a group of thermistors manufactured according to the above was investigated as follows:

Chromel wires were welded to the embedded platinum leads of six rod type thermistors approximately /2" long by .090 square.

The thermistors were mounted in a furnace. Resetability of the furnace temperature controller was /2" F. A 12- volt battery was connected to each thermistor in series with a 2000 ohm resistor. Provisions were made for measuring thermistor voltage and current to /2% accuracy.

The furnace temperature was set and held at 1400 F. for 2 months, increased to 1600 F. for A month, returned to 1400 F. for 2% months, reset to 1600 F. for 1 month, and then alternated weekly between 1400 and 1600 F.

The following table gives the average resistance of the thermistors at the times indicated.

TABLE IV Thermodynamic considerations lead to the conclusion that the stable state during the last /2 months of the test can be achieved in a short time at a higher temperature. After this aging process the thermistor changes less than 1% during 5 /2 months alternated weekly between 1400 and 1600 F. Since the thermistors have a temperature coefficient of resistance of .353%/ F. at 1400 F., the 1% resistance change corresponds to a temperature diiference of less than 3 F.

This thermistor is suitable for many engine exhaust sensing and control applications since it has long term stability comparable to a thermocouple and operates at much higher current levels. The higher currents permit the use of less sensitive and therefore less expensive control or sensing instruments.

The most preferred composition is 33 /3 mole percent zinc oxide, 33 /3 mole percent cobaltic oxide, and 33 /3 mole percent manganic oxide arrived at by recent investigation.

An X-ray diffraction analysis of thermistors of this composition prepared essentially as in the previous example revealed that the final product had a slightly distorted spinel structure.

This composition was found to have the same temperature coefficient as the previous example but a resistivity of only 4500 ohm-cm, at room temperature.

As may be noted, this composition is designed to yield a single phase spinel structure according to any of the following reactions:

The lower resistivity obtained from this material is assumed to result from the formation of a single phase according to one of these reactions. Consideration of the previous composition shows that it cannot form a single phase except as a solid solution of a manganese cobalt zinc spinel and a manganous manganic spinel.

What is claimed is:

1. The method of attaching conductive lead means to a thermistor body comprising the steps of coating the thermistor body with a conductive material, drying the conductive material, dividing the coated surface into a plurality of electrically isolated areas, temporarily securing a conductive lead member to each of said electrically isolated areas, immersing said thermistor body and said temporarily attached lead members into a bath of enamel to coat said thermistor body and the areas of contact between said conductive lead members and said thermistor body.

2. The method according to claim 1 wherein said enamel comprises 800 parts by weight alkali-boro-aluminurn silicates, 200 parts by weight chromium oxide, 70 parts by weight clay, 5 parts by weight bentonite, 50 parts by weight feldspar, 1 part by weight sodium nitrite, and 2.5 parts by weight borax.

3. The method according to claim 1 wherein the enamel coating is cured in a furnace open to the atmosphere for about 5 minutes at 1550 F.

4. The method according to claim 1 wherein the conductive material comprises silver paint.

References Cited UNITED STATES PATENTS 2,481,371 9/1949 Van Dyke 29--619 X 2,491,965 12/1949 Ganci 106-48 X 2,686,244 8/1954 Dahm et a1 338-23 X 2,720,573 1 0/ 1955 Lundqvist 33822 3,044,968 7/1962 Ichikawa 338-22 X 3,184,320 5/1965 Michael 10648 JOHN F. CAMPBELL, Primary Examiner.

JOHN L. CLINE, Assistant Examiner.

US. Cl. X.R. 33822, 23

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