Barium Titanate Ceramic

1. Field of the Invention

This invention relates to a barium titanate ceramic having a positive temperature coefficient of electric resistance (to be referred to as "PTC ceramic"), and more particularly to a ceramic which maintains a high electric resistance over a wide temperature range and a high temperature coefficient of electric resistance at temperatures just below the aforesaid range.

2. Description of the Prior Art

When a PTC ceramic is used for temperature sensing and electric current limiting, inclusive of current interruption, it is required to simultaneously meet the following two conditions; namely, to maintain a high electric resistance over a wide temperature range and to have a large temperature coefficient of electric resistance at temperatures just below the aforesaid range. For instance, when a ceramic is used to sense the presence of a flame, it is required to keep a high electric resistance as long as flame exists, large or small, and to rapidly decrease its electric resistance upon loss of the flame. However, there has not been any PTC ceramic, heretofore, which satisfactorily meets both of the two conditions.

Therefore, an object of the present invention is to provide an improved PTC ceramic which simultaneously meets the aforesaid two conditions.

The inventors have found out that high electric resistance over a wide temperature range and a large temperature coefficient of resistance at temperatures just below the aforesaid range can be achieved by using a PTC ceramic which consists essentially of

100 PARTS BY WEIGHT OF FOUR INGREDIENTS INCLUDING

A. 1 mol of one barium titanate composition selected from the group consisting of barium titanate, barium titanate plus lead titanate, barium titanate plus strontium titanate, and barium titanate plus barium stannate,

B. 0.003 to 0.05 mol of titania,

C. 0.003 to 0.05 mol of alumina, and

D. 0.005 to 0.06 mol of silica;

0.05 TO 0.3 PART BY WEIGHT OF

E. at least one oxide selected from the group consisting of tantalum pentoxide and antimony pentoxide; and

0.005 to 0.03 part by weight of

f. at least one metal selected from the group consisting of manganese and copper.

In the above composition, titania constituting the aforesaid ingredient (b) is added to reduce the firing temperature of the PTC ceramic and to improve the stability thereof. If the amount of titania in the PTC ceramic is in short of 0.003 mol for 1 mol of the barium titanate composition, the desired improvement of the firing temperature and the stability cannot be achieved. The addition of titania in excess of 0.05 mol for 1 mol of the barium titanate composition tends to result in a small temperature coefficient of electric resistance of the PTC ceramic.

Alumina, which is the aforesaid ingredient (c), is added to reduce the firing temperature of the PTC ceramic and to provide a wide temperature range in which the electric resistance of the PTC ceramic is kept high. If the amount of alumina in the PTC ceramic is less than 0.003 mol for 1 mol of the barium titanate composition, the desired effect of providing a wide temperature range for high electric resistance cannot be achieved. The addition of alumina in excess of 0.05 mol for 1 mol of the barium titanate composition tends to result in a small temperature coefficient of resistance of the PTC ceramic.

The aforesaid ingredient (d), i.e., silica, is added to produce a large temperature coefficient of the resistance of the PTC ceramic and to provide a PTC ceramic which can withstand a high electric voltage. The desired effects cannot be achieved unless silica is added in excess of 0.005 mol for 1 mol of the barium titanate composition. On the other hand, the addition of more than 0.06 mol of silica for 1 mol of barium titanate composition tends to result in a small temperature coefficient of resistance and a low melting point of the PTC ceramic which may lead to ceramic fusion in the firing process.

The aforesaid ingredient (e), i.e., tantalum pentoxide and/or antimony pentoxide, is effective in improving the stability of the PTC ceramic, as long as its content in the ceramic is more than 0.05 part by weight. The addition of the ingredient (e) in excess of 0.3 part by weight tends to result in an excessively high resistance and to deteriorate the stability of the PTC ceramic.

The aforesaid ingredient (f), i.e., manganese and/or copper, is effective in increasing the magnitude of the temperature coefficient of the resistance of the ceramic, provided that it is added in excess of 0.005 part by weight. The ingredient (f) in excess of 0.03 part by weight tends to deteriorate the stability of the ceramic and to make the ceramic into an electrically insulating substance. Preferably, to make the ceramic, the ingredient (f) is added in the form of an aqueous solution of its sulphate or nitrate.

FIG. 1 is a graph, illustrating the temperature-resistance characteristics of PTC ceramics, each lacking one of essential ingredients of the present invention;

FIGS. 2A to 2C are graphs, showing the temperature-resistance characteristics of PTC ceramics according to the present invention;

FIG. 3 is a graph, showing the temperature-resistance characteristics of PTC ceramics according to the present invention, which contain various compositions for the ingredient (a), respectively; and

FIG. 4 is a graph, showing the temperature-resistance characteristics of conventional PTC ceramics.

Reference specimens R-1 to R-5 were prepared, each of which contained barium titanate (BaIiO.sub.3) and four of five other indispensable ingredients of the ceramic of the present invention, and their resistances at different temperatures were measured. The results are shown in Table 1 and FIG. 1. The measurement of the resistances was carried out by securing ohmic electrodes on opposing surfaces of each PTC ceramic specimen and applying D.C. 1 volt across the electrodes.

The size of sample was 16 mm of diameter and 2.5 mm of thickness (thereafter measurement was carried out as the same manner).

Specimens No. 1 to No. 35 of the PTC ceramic according to the present invention were prepared, and their electric resistances were similarly measured. The results are shown in Table 2 and FIGS. 2A to 2C.

The method of producing the PTC ceramic according to the present invention will be explained with respect to Sample No. 2 in Table 2. BaCO.sub.3, TiO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2, Sb.sub.2 O.sub.5 and an aqueous solution of manganese sulphate were mixed so that the fired barium titanate ceramic would be composed of BaTiO.sub.3, TiO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2, Sb.sub.2 O.sub.5 and Mn in amounts as shown in Sample No. 2 in Table 2, and the resulting mixture was pulverized and mixed in a wet state by means of agate pebbles in a pot having a polyethylene lining, the resulting slurry was dried by means of a spray drier, granulated and press molded in a conventional manner, and the molded article was fired at 1,325.degree. C for 2 hours to obtain the barium titanate ceramic.

As apparent from the comparison of Table 1 with Table 2 and the comparison of FIG. 1 with FIGS. 2A to 2C, in order to fulfill the object of the present invention, all of the aforesaid six ingredients (a) to (f) are essential. More particularly, with the ceramic of the present invention, the width of the temperature range for the maximum resistance R.sub.max to 0.1R.sub.max is 150.degree. to 208.degree. Celsius while ensuring a temperature coefficient of electric resistance in terms of (resistance at 170.degree. C)/(resistance at 120.degree. C) above 10,600. With reference Specimens R-1 to R-5, the aforesaid temperature range width is generally less than 140.degree. Celsius, and when that temperature range width is 240.degree. Celsius, the aforesaid temperature coefficient is reduced to 700. In any case, none of the reference specimens R-1 to R-5 has both of the aforesaid wide temperature range for high electric resistance and the aforesaid high temperature coefficient of electric resistance.

As far as the embodiments of Table 2 are concerned, the present invention provides barium titanate ceramic which consists essentially of

100 parts by weight of four ingredients including

a. 1 mol of barium titanate,

b. 0.003 to 0.05 mol, preferably 0.01 to 0.03 mol, of titania,

c. 0.003 to 0.05 mol, preferably 0.005 to 0.025 mol, of alumina, and

d. 0.005 to 0.06 mol, preferably 0.01 to 0.04 mol, of silica;

0.05 to 0.3 part, preferably 0.1 to 0.2 part, by weight of

e. at least one oxide selected from the group consisting of tantalum pentoxide and antimony pentoxide; and

0.005 to 0.03 part, preferably 0.01 to 0.02 part by weight of

f. at least one metal selected from the group consisting of manganese and copper.

The barium titanate ceramic of the present invention is not restricted to the aforesaid composition. More particularly, the ingredient (a) of the PTC ceramic according to the present invention need not be pure barium titanate, but it can be one of the mixtures containing barium titanate, as will be described hereinafter. The inventors prepared Specimens No. 36 to No. 41, which are based on ingredients (a) made of barium titanate compositions, or mixtures containing barium titanate. The temperature-resistance characteristics of such Specimens were similarly determined. The results are shown in Table 3 and FIG. 3, together with the chemical compositions thereof.

For comparison, Specimens R-6 and R-7 were prepared, which contained conventional barium titanate compositions. Properties of such Specimens R-6 and R-7 were determined, and the results are shown in Table 4 and FIG. 4, together with the chemical compositions of the Specimens. (e) Antimony (f) Ingredients pentoxide Manganese (a) (Sb.sub.2 O.sub.5) (Mn) Temperature Barium (b) (e) (d) Weight parts Rate of range for Curve Sam- titanate Titania Alumina Silica (100 parts repre- Resistances (Ohms) at different temperatures (.degree. C) resistance top 10% No. ple (BaTiO).sub.3 (TiO.sub.2) (Al.sub.2 O.sub.3) (SiO.sub.2) sent total weight (A) (B) variation resistance* in No. Mol of (a) to (d) 70 120 170 220 270 320 370 (B/A) (.degree. C) FIG. 1 R-1 -- 0.01 0.02 0.2 0.01 2.0.times.10.sup. 2 5.1.times.10.sup.2 1.2.times.10.sup.6 1.0.times.10.sup.6 -- -- -- 2,400 80 1 R-2 0.02 -- 0.02 0.2 0.01 8.5.times.10 1.4.times.10.sup.2 1.22.times.10.sup.6 3.0.times.10.sup.6 2.3.times.10.sup.5 -- -- 8,700 100 2 R-3 1 0.02 0.01 -- 0.2 0.01 1.4.times.10.sup.2 3.1.times.10.sup.2 2.3.times.10.sup.6 4.2.times.10.sup.6 5.1.times.10.sup.5 -- -- 7,500 110 3 R-4 0.02 0.01 0.02 -- 0.01 4.0.times.10 9.5.times.10 8.9.times.10.sup.5 8.3.times.10.sup.6 4.1.times.10.sup.6 -- -- 9,400 140 4 R-5 0.02 0.01 0.02 0.2 -- 1.3.times.10 2.6.times.10 1.8.times. 10.sup.4 1.1.times.10.sup.5 1.6.times.10.sup.5 9.4.times.10.sup.4 2.6.times.10.sup.4 700 >240 *In this temperature range, the resistance is between its maximum R.sub.max and 0.1R.sub.max. Temper- Ingredients ature (e) (f) Rate of range Curve (a) (b) (c) (d) Antimony Tantalum Mang- Cop- resist- for top No. Barium Ti- Alum- Sil- pentoxide pentoxide anese per ance 10% in Sam- titanate tania ina ica (Sb.sub.2 O.sub.3) (Ta.sub.2 O.sub.3) (Mn) (Cu) Resistances (Ohms) at different temperatures (.degree. C) varia- resist- FIGS. ple (BaTiO.sub.3) (TiO.sub.2) (Al.sub.2 O.sub.3) (SiO.sub.2) Weight parts (100 parts represent (A) (B) tionance* 2A to No. Mol total weight of (a) to (d)) 70 120 170 220 270 320 370 (B/A) (.degree. C) 2C 1 0.005 1.0.times.10.sup.22.3.times.10.sup.22.6.times.10.sup.61.4.times.10.sup.71 .1.times.10.sup.77.8.times.10.sup.5--11,400 150 6 2 0.01 9.0 2.3.times.10 1.2.times.10.sup.4 5.7.times.10.sup.6 4.5.times.10.sup.4 1.1.times.10.sup.4 -- 60,000 170 7 3 1 0.02 0.025 0.02 0.2 -- 0.61 -- 3.8 9.0 7.3.times.10.sup.5 5.6.times.10.sup.4 1.4.times.10.sup.7 6.3.times.10.sup.4 -- 81,000 165 5 4 0.03 5.4 1.3.times.10 5.2.times.10.sup.3 7.0.times.10.sup.6 1.0.times.10.sup.7 4.6.times.10.sup .6 -- 40,000 170 9 5 0.05 2.9.times.10 1.1.times.10.sup.2 1.2.times.10.sup.2 4.5.times.10.sup.6 4.6.times.10.sup.4 2.6.times.10.sup .6 -- 10,900 190 10 6 0.003 7.1.times.10 2.0.times.10.sup.2 2.6.times.10.sup.4 1.4.times.10.sup.5 1.0.times.10.sup.3 1.7.times.10.sup .6 -- 13,000 150 -- 7 0.005 3.4.times.10 1.2.times.10.sup.2 3.1.times.10.sup.4 1.8.times.10.sup.7 1.0.times.10.sup.7 1.1.times.10.sup .4 -- 25,800 150 -- 1 0.02 0.02 0.2 -- 0.01 --10.02 0.020.2 -- 0.01 -- 80.025 6.2 1.6.times.10 8.3.times.10.sup.7 9.0.times.10.sup.4 1.2.times.10.sup.7 2.8.times.10.sup.4 -- 52,000 163 -- 9 0.05 9.3 3.3.times.10 3.5.times.10.sup.5 1.7.times.10.sup.4 1.9.times.10.sup.4 9.5.times.10.sup.5 1.9.times.10.sup.5 10,600 208 -- 10 0.005 2.9.times.10 1.2.times.10.sup.2 3.8.times.10.sup.4 1.3.times.10.sup.7 9.6.times.10.sup.4 1.0.times.10.sup.6 -- 31,700 153 -- 11 0.01 6.1 1.4.times.10 8.4.times.10.sup.5 7.1.times.10.sup.4 5.9.times.10.sup.4 1.8.times.10.sup.6 -- 60,000 167 -- 1 0.02 0.015 0.2 -- 0.01 -- 12 0.04 9.2 2.6.times.10 3.9.times.10.sup.5 1.9.times.10.sup.6 1.3.times.10.sup.6 6.3.times.10.sup.5 1.1.times.10.sup.5 15,000 195 -- 13 0.06 1.2.times.10 4.4.times.10 4.9.times.10.sup.4 1.9.times.10. sup.4 1.1.times.10.sup.4 5.0.times.10.sup.5 2.2.times.10.sup.5 11,300 202 -- 14 0.05 8.6 2.2.times.10 2.6.times.10.sup.3 1.9.times.10.su p.4 4.0.times.10.sup.6 2.1.times.10.sup.6 2.6.times.10.sup.5 11,800 192 11 15 1 0.02 0.015 0.02 0.1 -- 0.01 -- 5.1 1.1.times.10 7.6.times.10.sup.6 7.4.times.10.sup.6 1.0.times.10.s up.7 4.6.times.10.sup.2 -- 69,000 177 12 16 0.3 1.8.times.10 7.2.times.10 1.7.times.10.sup.4 1.9.times.10.sup.7 1.6.times.10.sup.7 2.8.times.10.sup.6 -- 23,600 153 13 17 0.05 1.1.times.10 3.2.times.10 3.5.times.10.sup.5 2.4.times.10.sup.4 3.5.times.10.sup.4 2.5.times.10.sup.4 3.4.times.10.sup.5 10,900 185 -- 18 0.1 7.3 2.8.times.10 6.6.times.10.sup.4 3.9.times.10.sup.4 6.5.times.10.sup.4 3.8.times.10.sup.4 -- 23,600 180 -- 1 0.62 0.015 0.02 -- 0.01 -- 19 0.2 5.6 1.5.times.10 7.5.times.10.sup.5 6.4.times.10.sup.4 9.9.times.10.sup.4 5.0.times.10.sup.4 -- 50,000 170 -- 20 0.3 6.5.times.10 1.2.times.10.sup.2 3.0.times.10.sup.4 2.5.times.10.sup.7 1.7.times.10.sup.7 1.9.times.10.sup.4 -- 25,000 150 -- 21 0.025 0.025 1.0.times.10 3.1.times.10 3.7.times.10.sup.5 2.5.times.10.sup.4 3.4.times.10.sup.4 2.5.times.10.sup.4 3.6.times.10.sup.5 11,900 190 -- 22 0.05 0.05 7.9 3.1.times.10 9.2.times.10.sup.3 4.5.times.10.sup.6 7.1.times.10.s up.4 3.6.times.10.sup.4 -- 29,700 170 -- 1 0.02 0.015 0.02 0.01 -- 23 0.1 0.1 4.2 9.5 5.1.times.10.sup.4 6.3.times.10.sup.4 8.8.times.10.sup.4 3.7.times.10.sup.4 -- 53,700 170 -- 24 0.15 0.15 2.7.times.10 4.2.times.10 8.5.times.10.sup.5 1.4.times.10.sup.7 1.1.times.10.sup.7 2.3.times.10.sup.6-- 20,200 155 -- 25 0.005 4.0 1.1.times.10 1.1.times.10.sup.4 4.8.times.10.sup.7 5.5.times.10.sup.4 3.0.times.10.sup .5 -- 10,000 200 14 26 1 0.02 0.015 0.02 0.2 -- 0.02 -- 2.1.times.10 9.0.times.10 9.7.times.10.sup.4 7.0.times.10.sup.7 7.8.times.10.sup.7 1.5.times.10.sup.7-- 108,000 167 15 27 0.03 6.0.times.10.sup.2 1.2.times.10.sup.3 3.7.times.10.sup.4 3.7.times.10.sup.4 2.8.times.10.sup .8 2.9.times.10.sup.7 -- 30,700 150 16 28 0.005 3.7 1.0.times.10 1.2.times.10.sup.4 4.7.times.10.sup.4 5.4.times.10.sup.5 1.9.times.10.sup .5 -- 12,000 185 -- 29 0.01 8.4 2.0.times.10 1.1.times.10.sup.4 5.4.times.10.sup.6 4.2.times.10.sup.6 9.6.times.10.sup.5-- 55,000 170 -- 1 0.02 0.015 0.02 0.2 -- -- 30 0.02 2.5.times.10 9.4.times.10 5.4.times.10.sup.7 3.5.times.10.sup.7 2.5.times.10.sup.7 5.1.times.10.sup .4 -- 59,700 163 -- 31 0.03 7.1.times.10.sup.2 1.4.times.10.sup.3 2.2.times.10.sup.7 2.5.times.10.sup.4 1.4.times.10.sup.6 2.4.times.10.sup .7 -- 15,700 150 -- 32 0.0025 0.0025 6.0 1.6.times.10 2.2.times.10. sup.4 1.0.times.10.sup.4 8.5.times.10.sup.6 2.7.times.10.sup.5 -- 13,800 152 -- 33 0.005 0.005 1.4.times.10 6.3.times.10 1.2.times.10.sup.4 5.1.times.10.sup.4 5.9.times.10.sup.4 2.4.times.10.sup.6 -- 17,000 170 -- 1 0.02 0.015 0.02 0.2 -- 34 0.01 0.01 1.0.times.10 1.7.times.10 .sup.7 4.5.times.10.sup.4 6.2.times.10.sup.7 6.3.times.10.sup.7 1.3.times .10.sup.7 -- 38,2000 160 -- 35 0.015 0.015 1.1.times.10.sup.1 1.9.times.10.sup.1 3.2.times.10.sup.7 2.0.times.10.sup.4 1.9.times.10.sup .6 2.4.times.10.sup.7 -- 16,800 167 -- *In this temperature range, the resistance is between its maximum D.sub.min and 0.1R.sub.max. Ingredients (a) (b) (c) (d) (e) (f) Barium Lead Strontium Barium Antimony titanate titanate titanate stannate Titania Alumina Silica pentoxide Manganese (BrHO.sub.1) (SbTiO.sub.3) (SrTiO.sub.3) (BaSnO.sub.3 ) (TiO.sub.2) (Al.sub.2 O.sub.3) (SiO.sub.2) (Sb.sub.2 O.sub.3) (Mn) Weight parts (100 parts repre- Sample sent total weight No. Mol of (a) to (d)) 36 0.9999 0.0001 -- -- 37 0.65 0.35 34 0.9999 0.0001 -- -- 0.02 0.015 0.02 0.2 0.01 35 0.6 0.4 40 0.9999 0.0001 -- -- 41 0.65 0.15 Temperature Rate of range for Curve resistance top 10% No. Resistances (Ohms) at different temperatures (.degree. C) variation resistance* in -10 0 50 100 120 170 220 270 320 370 420 470 520 (B/A) (.degree. C) FIG. 5 (A) (B) 36 -- 7.1 4.5 5.6 9.1 7.2.times.10.sup.5 1.0.times.10.sup.7 1.4.times.10.sup.7 7.9.times.10.sup.6 -- -- -- -- 79,000 170 17 (A) (B) 37 -- -- 6.0.times.10.sup.2 3.5.times.10.sup.2 2.8.times.10.sup.2 1.0.times.10.sup.2 1.1.times.10.sup.2 1.3.times.10.sup.7 9.0.times.10.su p.5 1.2.times.10.sup.7 1.8.times.10.sup.7 7.5.times.10.sup.61.3.times.10. sup.6 6,9000 190 14 (A) (B) 38 -- 7.0 4.5 5.6 8.9 7.1.times.10.sup.5 1.0.times.10.sup.7 1.4.times.10.sup.7 7.8.times.10.sup.6-- -- -- -- 80,000 170 19 (A) (B) 39 2.1.times.10.sup.2 4.0.times.10.sup.2 2.2.times.10.sup.6 1.2.times.10.sup.7 1.6.times. 10.sup.7 1.9.times.10.su p.7 5.7.times.10.sup.4 -- -- -- -- -- -- 5,500 193 20 (A) (B) 40 -- 7.1 4.6 5.7 9.0 7.2.times.10.sup.6 1.1.times.10.sup.7 1.4.times.10.sup .7 7.8.times.10.sup.6-- -- -- -- 80,000 170 21 (A) (B) 41 3.0.times.10 1.1.times.10.sup.2 6.0.times.10.sup.5 7.4.times.10.sup.6 1.1.times.10.sup .7 1.3.times.10.sup.7 4.4.times.10.sup.4 -- -- -- -- -- -- 5,500 186 *In this temperature range, the resistance is between its maximum R.sub.max and 0.1R.sub.max. Temperature Resistances (Ohms) at different Rate of range for temperatures (.degree. C) resistance top 10% Sample (A) (B) variation resistance* Curve No. No. Ingredients 70 120 170 220 270 320 (B/A) (.degree. C) in FIG. 4 Barium titanate (BaTiO.sub.3) : 1 mol R-6 Manganese oxide (MnO.sub.2) : 1.36 .times. 10.sup.-3 mol 2.3 .times. 10.sup.2 1.0 .times. 10.sup.4 1.5 .times. 10.sup.10 1.0 .times. 10.sup.8 -- -- 1,500,000 60 23 Niobium pentoxide (Nb.sub.2 O.sub.5) : 2.2 .times. 10.sup.-3 mol Barium lanthanum titanate R-7 (Ba.sub.0.997 La.sub.0.003 TiO.sub.3) 4.0 .times. 10 3.6 .times. 10.sup.3 2.0 .times. 10.sup.5 3.4 .times. 10.sup.5 1.4 .times. 10.sup.5 4.0 .times. *In this temperature range, the resistance is between its maximum R.sub.max and 0.1R.sub.max.

It is apparent from the foregoing that the PTC ceramic according to the present invention has not only a large electric resistance over a wide temperature range, but also a large temperature coefficient of electric resistance at temperatures just below the aforesaid range.