US3913057A - Oxygen ion transport type thermistors - Google Patents

Abstract

An oxygen ion transport type thermistor comprising an oxygen ion type solid solution consisting of 50-95 mol percent of an oxide of tetravalent metals and 5-50 mol percent of oxide(s) of divalent and/or trivalent metals and two metal lead wires embedded in said solid solution, in which said two metal wires are embedded and held oppositely in substantially the center line in the thickness direction of said solid solution, parallel and adequately space and that the porosity of said oxygen ion type solid solution is 7-55 percent.

Description

United States Patent Ushida et al.

OXYGEN ION TRANSPORT TYPE THERNHSTORS Inventors: Yoshiro Ushida, Toyoake; Shinji Nishio, Komaki, both of Japan Assignee: NGK Spark Plug Co., Ltd., Nagoya, Japan Filed: June 14, 1974 Appl. No.: 479,472

Foreign Application Priority Data June 21, I973 Japan 48-70493 US. Cl.... 338/22 R; 252/520; 252/521 Int. Cl. HOIC 7/04 Field of Search 338/22, 23, 25; 29/612;

References Cited UNITED STATES PATENTS 3/1961 lchikawa 338/22 R Oct. 14, 1975 2,985,700 5/1961 Johnston 338/22 R 3,044,968 7/1962 lchikawa 338/22 R 3,377,561 4/1968 Sauer 338/22 R 3,786,390 1/1974 Kristen 338/22 R Primary ExaminerC. L. Albritton Attorney, Agent, or FirmStevens, Davis, Miller & Mosher ABSI'RACT 2 Claims, 2 Drawing Figures U.S. Patent Oct. 14, 1975 F l6. l5

' F/GJA OXYGEN ION TRANSPORT TYPE THVERMISTORS The present invention relates to thermistors.

Oxygen ion transport type sintered oxide solid solutions having a stable crystal structure until a high temperature zone consisting of 50-95 mol percent of an oxide of tetravalent metals such as ZrO CeO l-ifO and ThO and 5-5O mol percent of oxide(s) of divalent alkaline earth metals and/or trivalent rare earth metals, such as CaO, MgO, SrO, La O Y O Yb O SC O Gd O and Nd O decrease rapidly the electrical resistance at a temperature'of 400-l ,200C and have excellent properties as a high temperature resistor of a thermistor. Such a solid solution is generally composed of polycrystalline fluorite structures, which have oxygen vacancies to preserve lattice neutrality. Accordingly, these oxide sintered bodies have been used for parts for measuring or controlling temperature of a high temperature furnace and an apparatus for purifying an exhaust gas of an internal combustion engine. Such ceramic resistors are referred to as oxygen transport type thermistors, because the electrical conductivity is shown to be due to the transport of oxygen ion of the oxide in the solid solution constituting the resistor. Heretofore, the electrodes of such oxygen ion transport type thermistors are provided by coating a platinum paste on both parallel main surfaces of the ceramic resistor fired in a disc form and arranging platinum wires to form lead wires thereon and bonding said wires to the resistors with the above described platinum paste and firing said paste at a temperature of l ,0OO1,500C, but the adhesion at the portions where the lead wires get out of the ceramic resistor, is weak and further the working steps are many and troublesome.

The adhesion at the bonded portions of the lead wires is degraded with the raising of the temperature owing to softening of vitreous components in the platinum paste at a high temperature. Accordingly, it is impossible at a high temperature, for example, higher than l,0()C to increase the bonding strength of the lead wires even by such oxidation resistant metal paste. Furthermore, the above described coated metal paste requires a moderate vapor permeability in order to make smooth the receiving and supply of oxygen ion in the atmosphere to be converted into electricity at the contact portion of the electrodes with the resistor, so that a thick coating exceeding a certain degree cannot be effected. Accordingly, the adhesion cannot be essentially improved.

While, the oxygen ion transport type thermistors according to the present invention which aim to obviate the above described drawbacks, are characterized in that as shown in FIG. 1, a molding is effected in such a manner that two fine metal wires 2, 2 composed of metals having a higher oxidation resistance, such as platinum or platinum-rhodium alloy are embedded in a resistor matrix leaving a space in parallel and the molded assembly is fired at an adequate temperature to form electrodes and lead wires which are embedded and held in a ceramic resistor 1 of the thermistor and that the porosity of the above described ceramic resistor 1 is made to be 755 percent, preferably l-35 percent, more particularly 25 percent.

The conventional oxygen ion transport type thermistor in which the electrodes are provided on the surfaces 2 of the ceramic resistor, are very tightly sintered and the porosity is usually less than 2 percent.

The reason why the lower limit of the porosity of the ceramic resistor of the thermistor according to the present invention is defined to be 7 percent is as follows. When the oxygen ion transport type thermistor is supplied with a given direct current voltage of about 12V usually used between the electrodes, the oxygen molecule is absorbed from the atmosphere near the cathode in order that the oxygen ion 0 which transports in the ceramic resistor and is concerned in the electric transmission mechanism, is supplied at the cathode side as /2O +2e 0 and said oxygen molecule is discharged into the atmosphere near the anode in order that the above described oxygen ion is taken out at the anode side as 0 /2O +2e. But in the thermistor of the present invention, both the electrodes are provided by embedding the fine metal wires in the ceramic resistor, so that the contact area of the electrodes to the resistor is small and consequently if the porosity in the resistor is less than 7 percent, the diffusion movement of the oxygen molecule which is effected through pores in the resistor between air in the pores near the electrodes and the ambient atmosphere, becomes not free and particularly when the oxygen to be supplied at the cathode side is deficient, the oxygen ion in the crystal structure of the sintered body of oxides is used for the electric transmission after the electric current flows for a given time under a high temperature condition and the oxides are reduced and blackened and the resistor becomes a semiconductor having a very small resistance and loses the normal function. Accordingly, in order to prevent such a fact and to ensure the stable circulation mechanism of oxygen during use for a long time, the porosity must be not less than 7 percent. On the other hand, the above described upper limit of the porosity is defined for on the following reason. When the porosity is more than 55 percent, the variation of the electric resistance with the lapse of time in the ceramic resistor not only becomes larger but also the mechanical strength lowers and the electrodes are readily removed and cracks and breaks are apt to be caused. When the porosity is too large, the contact resistance between the electrodes and the resistor is large and further the adhesion is gradually lowered owing to the ion conversion at the electrode portions.

The present invention will be explained in more detail.

For a better understanding of the invention, reference is taken to the accompanying drawings, wherein:

FIG. 1A is a plan view of the oxygen ion transport type thermistor according to the present invention; and

FIG. 1B is a side view of said thermistor.

The following examples are given for the purpose of illustration of this invention and are not intended as limitations thereof.

EXAMPLE 1 Each of mixtures of powdery ZrO and CaCO having compositions as shown in the following Tables 1-3 (83 mol percent of ZrO +l7 mol percent of CaO(CaCO 87 mol percent of ZrO +l3 mol percent of CaO(- CaCO mol percent of ZrO +25 mol percent of CaO(CaCO was added with 4 percent by weight of an emulsion consisting of equal weight amounts of stearic acid and water as a binder and pulverized in a wet process in a trommel for 20 hours. Each of the resulting 3 powdery mixtures was filled in a mold and two 0.4 mm platinum wires were held in the center portion and the filled mixture in the mold was pressed at a pressure of 2,000 Kg/cm and the molded mixture was fired at vari- 4 cording to the present invention did not cause such a variation and further even after the test was continued for l0,000 hours, the resistance increases only slightly and satisfactory durability was attained. However,

ous temperatures of about 1,400l ,800C in air to 5 when the porosity exceeds 55 percent, the adhesion produce oxygen ion transport type disc-shaped therm- (pulling strength) of the platinum electrodes is insuffiistors having different porosities as shown in the atcient and there is problem in practical use. The test retached drawing, which have an outer diameter of about sults are shown in the following Tables 1-3 showing 3.8 mm and a thickness of about 1.4 mm and in which data from the. Process for testing the tension strength a pair of platinum wires 2, 2 are embedded and fixed in 10 of the electrodes. parallel at the center line of the thickness direction at a The two electrode wires are separately secured with spacing of about 2 mm. The resulting thermistor was clips at a distance of 3 mm'from the ends of the ceramic applied with DC12V at 800C between the two platiresistor and pulled in such a direction that the two elecnum electrodes for 1,000 hours. This test showed that trode wires are diverged, through a spring system tenin the thermistors having the porosity of less than 7 per- 15 sion gauge fixed between the two clips and the strength cent, the vicinity of the electrodes became grey or when the electrode wires are removed from the above black and the run away phenomenon occurred, while described resistor, is determined and is shown by an avthe thermistors having the porosity of 7-55 percent acerage value of 10 samples.

Table l 83 mol7r of ZrO +l7 mol7 of CaO(CaCO Resistance 800C, 12V

Tension Firing After After After strength tempcri 100 1,000 between Test ature Porosity Initial hours hours hours electrodes N (K (K g) 1 1,450 65 26 17 16 120 0.2 2 1,480 58 13 9.3 11 21 0.5 3 1,485 55 8.7 7.6 7.0 13 0.6 4 1,500 52 7.2 7.0 6.0 10 0.7 5 1,520 45 5.1 4.6 4.2 5.7 1.0 6 1,535 40 4.1 3.8 4.4 1.2 7 1,550 35 3.5 3.5 3.5 3.8 1.4 8 1,570 30 3.0 3.3 3.3 3.7 1.6 9 1,600 25 2.8 3.1 3.5 3.7 1.8 10 1.650 20 2.6 3.0 3.2 3.7 2.1 11 1,665 2.5 3.0 3.2 3.6 2.1 12 1,680 10 2.5 2.9 3.2 3.3 2.2 13 1,700 6.3 2.5 1.4 0.2 run 2.2

away 14 1,730 4.3 1.4 run 2.2

away

Table 2 87 mol% of ZrO +l3 mol% of CaO(CaCO Resistance 800C. 12V

Tension Firing After After After strength temperl0 100 1,000 between Tes't uture Porosity Initial hours hours hours electrodes No. ("C) (Zr) 1 K0 (K0) (K0) (K9 (Kg) 27 1,780 6 1.5 0.8 run 2.2

away 28 1,800 4 1.0 run 2.2

Table 3 75 mol% of ZrO +25 mol% of CaO(CaCO Resistance 800C, 12V

Tension Firing After After After strength temper- 100 1 .000 between Test ature Porosity lnitial hours hours hours electrodes N0. (C) 7:) (KO) (KQ) (K0) (K9) (Kg) 29 1,470 55 44 37 38 88 0.4 30 1,485 50 28 27 28 34 0.6 31 1,500 45 22 21 22 26 0.8 32 1,515 40 19 18 18 21 1.0 33 1,535 35 17 17 17 18 1.2 34 1.550 30 15 16 16 1.4 35 1,570 25 14 14 15 16 1.6 36 1,600 13 14 15 16 1.8 37 1,620 15 13 13 14 16 1.9 38 1,630 12 14 14 15 17 2.0 39 1,645 10 14 15 15 17 2.1 40 1,665 8 14 15 15 17 2.1 41 1,690 5 12 8 5 1 2.2 42 1,710 3 10 0.1 run 2.2

away

EXAMPLE 2 EXAMPLES 3 AND 4 70 mol percent of a mixture of 12 mol percent of CaO and 88 mol percent of ZrO was added with mol percent of spinel composed of MgO and A1 0,; as a resistance controlling agent. The resulting mixture was calcined at 1,350C for 2 hours and then added with 4 percent by weight of an emulsion consisting of equal weight amounts of stearic acid and water and pulverized in a wet process in a trommel for 20 hours. The pulverized mixture was treated in the same manner as described in Example 1 except that the heating temperature lower than 1,700C to produce the oxygen ion transport type thermistors. DC 12V was applied to each of the thermistors at 800C. The obtained results are shown in the following Table 4. As seen from this table, the thermistor having a porosity of 5 percent obtained by sintering at a temperature of 1,670C showed run away after 100 hours and was not able to be practically used.

Table 4 A mixture of 90 mol percent of ZrO and 10 mol percent of Y O and a mixture of 90 mol percent of ThO and 10 mol percent of Y O were calcined at 1,400C for 2 hours respectively. Then each of the mixtures was added with 3.2 percent by weight of an emulsion consisting of equal weight amounts of stearic acid and water. The resulting mixture was pulverized in a wet process in a trommel for 8 hours. The pulverized mixture was treated in the same manner as described in Examples 1 and 2 at various temperatures as shown in the following Tables 5 and 6 by using 0.4 mm alloy wires having and consisting of 70 percent of platinum and 30 percent of rhodium as a pair of electrodes to produce oxygen ion transport type thermistors.

DC 12V was applied to the resulting thermistors and the results as shown in the following Tables 5 and 6 were obtained.

70 moF/z (12 mol7l Ca0+88 mol?! ZrO )+30 mow (MgAl O Resistance 800C, 12V

Tension Firing After After After strength tern per- 10 100 1,000 between Test ature Porosity Initial hours hours hours electrodes g) 43 1,420 55 16 13 14 24 0.5 44 1,465 50 11 9.8 10 14 0.7 45 1,475 45 8.2 7.6 7.8 9.6 0.9 46 1,505 40 6.4 6.0 6.2 7.3 1.2 47 1,520 35 5.2 5.0 5.1 5.8 1.4 48 1,545 30 4.4 4.3 4.4 5.2 1.6 49 1,560 25 4.2 4.3 4.4 4.8 1.7 50 1,575 20 4.2 4.3 4.4 5.0 2.0 51 1,610 15 4.2 4.3 4.5 5.0 2.1 52 1,620 13 4.3 4.3 4.6 5.0 2.1 53 1,635 11 4.4 4.4 4.6 5.0 2.1 54 1,660 7 4.4 4.4 4.6 4.9 2.2 55 1,670 5 4.4 1.6 run 2.2

away 56 1,695 3 2.7 run 2.2

Resistance 800C, 12V

Tension Firing After After After strength temper- 10 100 1,000 between Test ature Porosity lnitial hours hours hours electrodes No. (C) (/c) (K) (K0) (KS1) (KS2) (Kg).

57 1,420 60 2.6 2.3 2.5 5.4 0.3 58 1,480 55 2.2 1.9 2.1 3.7 0.5 59 1,520 50 1.8 1.7 1.8 2.8 0.7 60 1,540 45 1.6 1.4 1.5 2.2 0.9 61 1,570 40 1.4 1.2 1.3 1.8 1.1 62 1,590 35 1.2 1.1 1.1 1.5 1.3 63 1,620 30 1.0 1.0 1.0 1.2 1.5 64 1,640 25 0.90 0.88 0.9 1.0 1.6 I 65 1,675 20 0.86 0.85 0.88 0.98 1.7 66 1,710 15 0.85 0.86 0.89 0.98 1.8 67 1,750 10 0.86 0.86 0.92 0.98 1.9 68 1,790 7 0.87 0.87 0.92 0.40 2.0 69 1,805 0.87 0.80 0.20 run 2.0

away 70 1,820 3 0.70 run 2.1

away

Table 6 90 mol'71 of ThO +l0 mol7r of Y O Resistance 800C, 12V

. v 1 Tension Firing After After After strength temper- 100 1.000 between I Test ature Porosity lnitial hours hours hours electrodes No. (C) "/11 K0 (K0) (K0). (K0 (Kg) i 71 1,590 60 72 64 '75 156 -0.4 I 1 72 1,615 55 65 61 .69 I 112 0.5 I 1 73 1,630 50 58 55 60 71 0.7 i 74 1,655 45 44 41 45 56 0.9 1 75 1,670 40 36 34 1.1 76 1,695 35 i 27 4 26 27 30 1.3 77 1,720 '30 22 22 23 26 1.5 78 1.730 25 20 20 21 24 1.8 79 1,745 20 l9 19 20 22 2.0 80 1,760 15 17 18 18 20 2.0 '8! 1,780 11 16 17 17 19 2.1 82 1,805 8 15 17 17' 17. 2.1 83 1,840 5 14 16 13 run 2.2

away 84 1,870 2 14 7 run 2.2 laway present invention as mentioned above, there is no fear A that the electrodes will fall from the ceramic resistors,

oxygen can be transported freely between both the electrodes, the variation of the resistance due to the reduction of the ceramic resistor can be prevented, the' 1. An oxygen ion transport type thermistor comprising an oxygen ion 'type solid solution consisting of 5 O9 5 mol percent of at least one metal oxide selected from the group consisting of ZrO CeO HfO and ThO and 5-50 mol'percent of at least one metal oxide selected from the group consisting of CaO, MgO, SrO, La O 1 0 Yb O S0 0 Gd O ancl Nd O and two metal lead wires are embedded in said solution and held in an adequately spaced parallel relationship to each other and lying substantially in the center line in the thickness direction of said solution and that the porosity of said oxygen ion transport type solid solution is 7 percent. v

2 The thermistor as claimed in claim 1, wherein said porosity is 5-35 percent.

Claims (2)

1. AN OXYGEN ION TRANSPORT TYPE THERMISTOR COMPRISING AN OXYGEN ION SOLID SOLUTION CONSISTING OF 50-95 MOL PERCENT OF AT LEAST ONE METAL OXIDE SELECTED FROM THE GROUP CONSISTING OF ZRO2, CEO2, HFO2 AND THO2, AND 5-50 MOL PERCENT OF AT LEAST ONE METAL OXIDE SELECTED FROM THE GROUP CONSISTING OF CAO, MGO, SRO, LA2O3, Y2O3, YB2O3, SC2O3, GD2O3 AND ND2O3, AND TWO METAL LEAD WIRES ARE EMBEDDED IN SAID SOLUTION AND HELD IN AN ADEQUATELY SPACED PARALLEL RELATIONSHIP TO EACH OTHER AND LYING SUBSTANTIALLY IN THE CENTER LINE IN THE THICKNESS DIRECTION OF SAID SOLUTION AND THAT THE POROSITY OF SAID OXYGEN ION TRANSPORT TYPE SOLID SOLUTION IS 7-55 PERCENT.

2. The thermistor as claimed in claim 1, wherein said porosity is 15-35 percent.

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