US 2664486 A
Description (OCR text may contain errors)
H. C. COLPITTS THERMISTOR AND METHOD OF HEAT TREIIINGy IT Dec. 29, 1953 Filed June 15, 1951 Patented Dec. 29, 1953 V"IHERMISTGR. .AND METHOD rOF "HEAT-TREATI'NG IT 'HarryC'..Colpitts, Montreal, Quebec, Canada, as-
signor to Northern Electric Company, Limited, -MontreaL Quebec, Canada, a corporation of Canada Application June 15, 1951,"Seri'al No. 231,814
- 5' claims.
This invention relates 6to -thermistors and more particularly toy a method` of heat treating the thermistor element.
Thermistors, or semi-conductors having a negative resistance-.temperature coeiilcient, have been known for several years. Their value as possible circuitl elements has long been recognized. I-Iowever, one of the reasons for the slow adoption'fof thermistors into general use has been a diiiculty experienced With all semi-conductor' elements, that is, the diiiiculty of obtaining uniform characteristics.
It is the object of this invention to provide a simplified method for the manufacture of thermistors. v
It is the further object of this invention to provide a method for producing thermistors of uniform resistance.
It is the further object of this invention to provide means for making an adjustment of the cold resistance after the thermistor element has "been assembled in a protective envelope.v
It is the further object of this invention to provide a method of reclaiming thermistors whose resistance characteristics have `changed due to aging or abnormal use.
An understanding of the invention may be had by reference to the drawings in which:
Fig. l illustrates a string of thermistor beads.
Fig. 2 illustrates an individual bead cut from the string.
Fig. 3 shows the thermstor element mounted in a protective envelope.
Fig., 4 shows the resistance distribution of a group of thermistor beads.
Fig. 5 is a charge showing the relation between heating current and bead temperature.
The present art used in the manufacture of bead type thermistors is illustrated in Fig. 1 in which a string of small spheroidal beads l2 of semi-conductor material is formed on a pair of lead wires ll which support the bead and make electrical contact with it. To do this the pair of lead wires I I, which may be of platinum alloy, are strung on a suitable support. The lead wires used to form the string of Fig. V1 may be about twelve inches in length, four-thousandths of. an inch in diameter, and spaced one thirty-second of an inch apart. The semi-conductor material is ground into a paste with avolatile vehiclesuch as distilled water. Several kinds oi"V semi-.conductor materials are known to the art, of which the oxides of cobalt, iron, nickel, zinc, manganese, and uranium have been widely used. A ceramic binder. 91 ller may A be included in.theepaste` 2 e Smalldaubs ofthe paste are placed on theparallel wiresat quarter inch intervals. Surface tensionpulls. the paste into a spheroidal shape.
v*The rbeads are dried by evaporating the vehicle,. hardened by passing the string of beads shown in Fig. 1 through a small flame, and then sintered by passing through furnace. Individual beads illustrated in Fig. 2 are then cut from the string and then tested. Beads having the desired resistance characteristic are selected and mounted in an evacuated glass envelope 1.3,
vgiving following'information on the changing of resistance of thermistors no mention will bev made of delay-time although it must be kept'in mind that the delay-time is a function of the re,- sistance.
In the ring process described above, it isy necessary to control the furnace temperature. the speed with which the string of beads is pulled through the furnace, the rate of cooling, and the amount of air passing through the furnace. n
spite of these controls it is diflicult `to obtain a' uniform resistance characterisie of the bead.
, The bead and wire assembly is fragile, and is easily subject to mechanical change. Handling of the assembly tends to increase the resistancev of the bead. Since it is necessary to handle the bead several times before assembly in the glass` envelope is complete, beadsv which have a desired characteristic after firing may be spoiled or changed during assembly. ln addition, differ--l ences in individual beads cannot be corrected..V
since ring a string does not permit of individual adjustment.
Fig. 4 shows a representative distribution of cold resistance values for -a group of thermistor beads made according to the ypresent art and tested` while in the stage illustrate-d in Fig. 2. The horizontal axis of Fig. 4 is in thousands ofohms, while the vertical axis is in per cent' of the total group for a class intervalfof 5,000 ohms.Y
Thusthe point P indicates that 10 per cent of-a representative group of thermistor beads would have a cold resistance in the range 45,000 ohms to 3 50,000 ohms. From Fig. 4, it is apparent that the yield of thermistors lying within practical resistance limits of 125 per cent is very small. As mentioned above, this is aggravated by further handling of the bead after selection at this stage.
In the method according to this invention, the beads are made in strings as shown in Fig. 1 and passed through the sintering furnace. The individual beads of Fig. 2 are then cut from the string and completely asse-mbled. After assembly, the resistance of the thermistor beads are individually adjusted to the required value by heat treatment. Since the bead is protected by envelope i3, further handling does not alter the characteristics.
According to this invention, the resistance of uranium oxide beads may be adjusted by heat treatment at temperatures from 600 degrees centigrade to 1100 degrees centigrade. This range is well below the initial sintering temperature which may be 1300 degrees centigrade. It has been found that beads which have a resistance below the average of a group may be brought nearer the average by heat treatment at the lower end of the temperature range 600-1100 degrees centigrade. Furthermore, beads which have a resistance above the average of a group mty be brought nearerthe average by heat treatment at the upper end of the temperature range 600-1100 degrees centigrade.
According to this invention, heating may be accomplished by the passage of a current many times the working current through the bead. Fig. 5 shows a representative relation between surface temperature and heating current for uranium oxide-pyrex beads il@ inch in diameter. The normal working current for such beads may be a few milliamperes. Quantities expressed in the discussion below rre for thermistor beads oi this type.
If the bead resistance is above the average of the group, it may be lowered by the momentary application of a heating current of approximately 0.7 ampere. Generally speaking, the larger the heating current (or the higher the temperature), the gre: ter the drop in resistance. However, currents above one ampere yield inconsistent results and may damage the bead permanently. The proper value of heating current may be determined for a group by trial on a few beads. The resistance may not be lowered indefinitely, but behaves as though it were rpproaching a stable range of values. The beads, or completed thermistors are allowed to cool in air.
If the bead resistance is below the average of the group, it may be increased by successive momentary applications ci a heating current of 2pproxirnateli7 0.3 ampere. Generally speaking, increasing the number of hot and cold cycles by repeated applications of heating current followed by a cooling period increases the resistance of the bead. The resistrnce may not be increased indeinitely, but behaves as though it were approaching a stable range of values.
In general, beads which are above the group average for resistance are not affected by heat trerting at the lower end of the range 600-1100 degrees centigrade, and beads which are below the group average are not aiiected by heat treatment at the upper end of the range 60G-1100 degrees centigrade.
The heat treating temperatures mentioned above are surface temperstures estimated by means of an optical pyrometer. It will be appreciated that the interior temperature of the bead may be considerably above these values.
Thermistor beads which have changed values due to aging or overloading may be readjusted by the method outlined above.
It is convenient to use a relatively high voltage source of poor regulation to supply the heating current. Thus a 70 volt supply working through a 3500 ohm resistor which may be variable has been found suitable for the heat treatment outlined above.
The behavior of thermistors of initially high resistance is illustrated in the Table I below in which the resistance is expressed in thousands of ohms, and the heating current in amperes:
TABLE I Initial Eat rea lllgW-l a Thermistor sample cold re current 0f sistance The behavior of thermistors of initially low resistance is illustrated in Table II below in which the resistance is expressed in thousands of ohms:
Cold resistance after hefaieng with a cureit Initial o ampere an a Thermistor sample cold relowmg to e001 sistance Three Once Twice times What is claimed is:
l. A method of making uranium oxide thermistors with a temperature-resistance characteristic of high accuracy which comprises a preforming of the semiconductor element and thereafter heat treating by the momentary dissipation of electric energy in the said element, said energy being very large compared to the normal working capacity of the thermistor.
2. A method of making uranium oxide thermistors with a temperature-resistance characteristic of high accuracy which comprises a preforming of the semiconductor element and assembling said element in its protective envelope or holder, and thereafter heat treating the said element by the momentary dissipation of electric energy in it, said energy being very large compared to the normal working capacity of the thermistor.
3. A method of adjusting the resistance of uranium oxide thermistors to obtain a temperature-resistance characteristic of high accuracy whereby the semiconductor element is heat treated by the momentary dissipation in it of electric energy, said energy being very large compared to the normal working capacity of the thermistor. K
4. A method of making uranium oxide thermistors with a temperature-resistance characteristic of high accuracy whereby the resistance is adjusted according to the method of claim 3 after the semiconductor element has been assembled into its protective envelope or. holder. References Cited in the le of this patent 5. A method of making thermistors with a UNITED STATES PATENTS temperature-res1stance characteristic of high accuracy having a semiconductor element con- Number Name Date taining uranium oxide in which said element is 5 119061853 Hedlger May 2, 1933 heat treated in the range 600 to 1100 degrees 19u15 Randolph Apr- 16, 1940 centigrade by the momentary dissipation in the 2,271'975 Y Hau Feb- 31 1942 said element of electric energy.
HARRY C. COLPITTS.