CA1200611A - Current restriction element responsive to applied voltage - Google Patents

Abstract

ABSTRACT
A current restriction device responsive to an applied electric field comprising a ferro-electric poly-crystalline ceramic with a main component of barium-titanate (BaTiO3). The device has an impedance ratio (Z2/z1) of less than 1/10, wherain Z2 is the impedance of the device measured with an alternating electric field lower than 500 volts/mm, and Z1 is the impedance of the device measured with an alternating electric field of 5 volts/mm. The device improves over prior art devices in its lower cost, smaller size and lower power loss during operation.

Description

6~

The present invention relates to a current ~estriction element responsive to applied voltage and, in particular, relates ~o polycrystalline ferro-electric ceramics havin~ rectan~ular hysteresis characteristics, in which the impedance of such cersmics depends upon the volta~e applied to same.
The current restriction el0ment of the invention has the main component BaTiO3, which may partially be replaced b~ Sr, Zr, Sn, and/or Pb. The present invention provides a low loss impedance element for alternati~g current, by usin~ the change of apparent dielectric constant of BaTiO3-type ceramics.
Generally, the power supplied by utilities for industry and domestic consumers is 50 or 60 Hz, 100 volts or 200 volts, of single phase or three phase alternating current. A passive element for varyin~ current through a low impedance load coupled with such a power source comprises, or instance, an inductance (L), a capacitance (C), or a resistance (R). of the fore~oin~, a resistance element is the cheapest, but it has the d;sadvanta~e that the heat generation is lar~e and, further, the generated heat represents not only undesired power loss but also decreases the reliability of the whole system by raisin~ the temperature of the same. Therefore, a resistance element is normally used only for restrictin~ very small currents. An inductance element (L) is the most commonly utilized at present - for instance, 8 choke coil (a ballast in a fluorescent lamp circuit is one example). An inductance element has the advanta~es that losses are small even with lar~e currents, heat generation is small and, therefore, the reliability of the system itself is not impaired by the use of such an element. Further, when the current is lar~e, only a small inductance element is required and, therefore, the number of turns of the inductance coil may be small. ~lowever, when the current is large, a thick conductor must be used for the coil and, therefore, the size and/or the weight of the element is large, which can make it difficult to install in small-scale apparatus.
When a capacitance element (C) is used, the capacity must be large for low frequencies like SO or 60 Hz. In particular, for lar~e current flow, the reactance must be small and, therefore, a lar~e capacitance must be utilized. Aluminum film electrolytic capacitors - which are typical large capacity capacitors - are, at present, not sufficiently reliable at high temperature. Accordingly, there is a need for an impedance element havin~
~5~3-1 small size, high capacity, hi~h breakdown voltage, hi~h thermal stability, and low cost.
A prior art approsch to satisfyin~ the above requirements comprises a cer~mic capacitor formed from the dielectric material BaTiO3.
The dielectric constant of BaTiO3 is higher than 20,000 while the dielectric constants (specific inductive capacity) of other materials like mica, paper or plastics film are in the range betw~en 3 and 15. Therefore, a - B~TiO3 capacitor is small in sl~e relative to its capacity. Further, ~ince ceramics are sintered at temperatures of 1300-1400C, the thermal stability is excellent, and the operational reliability is also excellent. However, a BaTiO3 capacitor has the disadvantage that the capacity per unit volume is limited, since the body is ceramic nnd, therefore, cannot be folded.
The capacity (C) of a capacitor with a pair of parallel electrodes is given by the equation ~0 x~s x s = t where C is capacity (Farads), sO is the dielectric ConStQnt in a ~acuu~, E iS the specific inductive cQpacity of the dielectric body (ratio of the dielectric constant to so)~ S is the area (m2) of the electrodes and t is the thickness (m) of the dielectric body. Accordingly, in order to obtain large capacity, the thickness (t) must be small, the specific inductive capacity (es) must be large, and the area (S) must be larse.
The present invention provides a current restriction element (impedance element) which is small in size, low in cost, and has low power loss. The basic concept of the present invention is to use the rectangular hysteresis characteristics of ferro-electric materials, and the main component is a ferro-electric polycrystalline BATio3 ceramic which has rectangular hysteresis characteristics such that the impedance measured by using an electric field of 500 volts/mm is les~ than one-tenth (l/lO) of the impedance measured by using an electrical field of 5 volts/mm.
Preferably, the barium component of the BaTiO3 is partially replaced by Sr or Pb, and the titanium component is partially replaced by Zr or Sn, in order to improve the impedance characteristics. Also, a mineralizer or additive agent composed of one or both of 0.005-0.3 weight percent chromium and 0.005-0.3 weight percent manganese is preferably added 6523-l i to improve the impedance characteristics. The impurities l~vel i5 preferably maintained at less than 0.5 weight percent.
The invention will now be described further by way of e~ample only nnd with reference to the accompanyin~ drawings 7 wherein:
Figs. lA and lB sho~ the structure of a ferro-electric cer~mics capacitor according to the prior art;
Figs. 2A and 2B show the structure of another ferro-electric - ceramics capacitor according to the prior art;
Fig. 3 shows the D-E curve ~Dielectric flux density - Electric field curve) of a conventional ferro-electric body;
Fig. 4 shows the D-E curve of a conventional ~aTiO3-type ceramic;
Fig. 5 shows the D-E curve of a current restriction element according to one embodiment of thls invention;
Fig. 6 is a circ~it dia~ram for measuring the characteristics of a current restriction element; and Figs. 7, 8 and 9 show the experiment~l results between the applied electric field and the measured impedance of a current restriction element according to the present invention.
Fi~s. lA and lB show a prior-art BaTiO3 capacitor. In these figures, the reference numeral l is a dielectric body, 2 and 3 are electrodes, 4 and 5 are lead wires, and & is an insulatin~ plastics material. The capacity with the structure of Figs. l~ and lB is limited to about 0.05 ~F due to restrictions imposed by the mechanical stren~th and the size of the structure.
When a laminated structure as shown in Figs 2A and 2B is used, the effective area of the electrodes becomes large depending upon the number of electrodes, and a lar~e capacity is obtained. In Figs. 2A and 2B, the reference numeral l is a dielectric body, 2 is an inner electrode, ~nd 2a is an externsl electrode. However, this laminated structure has the disadvantage that the voltags nandlin~ capability is low since the dielectric body is very thin, and the device is relatively expensive since rare metals such QS Pt, Au, Pd and Ag must be used for the electrodes.
When a current restriction element for alternating current is implemented by means of a ferro-electric body, the reactance component 6523-l i determined by its capacitance and the resistance component which is effectively coupled in series with the reactance component must be considered. The resistance component generates undesirable power loss and, therefore, the resistance component should be as s~sll as possible. However, the resistance component is rigidly coupled with the reactance component, and it is impossible to adjust the resistance and the reactance components separately. Accordingly, it i5 not enough to provide high capacity for ~ obtaining low reactance by using material with high specific inductive capacity. Therefore, the present invention uses the adaptive specific inductive capacity (e ~ which is dependent on the applied alternating voltage characteristic of 8aTiO3-type ceramic capacitors.
Japanese patent publication ~4440/77 teaches that the specific inductive capacity s of 8aTiO3-typ~ ceramics increases by 100-140 % when an alternating voltc~,e is applied to same. The fact thst the specific inductive capacity depends upon the applied voltaee suggests that the dielectric polarization which affects the specific inductive capacity depends upon the applied voltage (~ ). In a conven~ional dielectric body, the specific inductive capscity (s ) depends very little on the applied voltage.
It is well known that the dielectric flux density tD3 in a dielectric body is given by th following equation:
D = sE = EosE
where: D is dielectric flu~ density (C~m ), e is the dielectric constant, is the dielectric constant in vacuum, ES iS the specific inductive capacity of the dielectrlc body or the ratio of the dielectric constant of the dielectric body to the dielectric constant in vacuum, and E is the electric field (V/m).
The above relation is measured by measurin~, the D-E hysteresis curve by using a Sawyer-Tower circuit and Fig. 3 shows the D-E hysteresis curve of a conventional dielectric body. As shown in Fig. 3, the D-E curve of such a conventional dielectric body is almost linear.
Fig. 4 shows the D-E curve of a conventional BaTiO3-type ceramic with high specific inductive capacity, and it should be noted that the D-E curve is not linear, but the value (s) depends upon the applled ~L~f2~ 6~

electric field E ~in the emhodiment shown, the value E iS 18,000). WQ
have made a capacitor with th~s~ ch~r~ct~ristic8 and have m~asured the relationship between the capacity of that capacitor and the applied field and confi7~ned that the c~pacity depends upon the applied volta~e, as shown in Fi~. 4. However, it has also been found tha~ the impedance of the capacitor itself depends little upon the applied voltQ~e.
It should be apprecia~ed that the fact that the D-~ curve is - Dot linear stems from the fact that the dielectric constant (~) depends upon the applied alternatin~7 voltsge, and the fact that the D-E curve increa5es rapidly stems from the fact ~hat ~he dielectric constant (e) increases rapidly ~ith increase of the applied voltage. Further, it should be noted that the area enclosed by the hysteresis curve relates to the equival~nt resistance or the power loss.
Conventionally, an AB03-type ferro-electric dielectric body, of which saTio3 is a typical example, is used for its high dielectric constant, and piezo-electric properties. However, the D-E hysteresis characteristic has not been enough because of the lack of a satisfactory material which provides a rectangular hysteresis cur~e.
~ e have developed dielectric ceramics which provide e~cellent rectangular hysteresis and steep rise characteristics~ and have also developed a current restriction element or a variable impedance element for an alternating current circuit by using the fact that the capacitance and the impedance of such ceramics depend to a large e~tent upon the applied alternatin~ field.
We have found that an excellent current restriction element is obtained by usin~ 8aTiO3 itself, and~or an element wherein the Ti component of BaTiO3 is partially substituted by Sn or Zr, and/or the Ba component is partially substituted by Sr or Pb. Ceramics other than the BaTiO3 series are not found to be practical since their capacity increase coefficient is small and/or their impedance does not decrease until a very high voltage7 which is higher than the breakdown voltage, is applied. ~urther, we have found that addition of some additives including oxides of Mn and/or Cr to BaTiO3 improves the polycrystallinity by preventin~ re-oxidation in the sintering process. The characteristics depend upon the additives and their amount, and we have found experimentally that Mn and Cr are the best, the ~z~

desired amount of those additives being 0.005-0.3 weight percent in the case of Mn, and 0.005-0.3 weight percent in the case of Cr. It is possible to ~dd a mixture of the two (Mn and Cr) additiv~s. The Sr, Sn, Zr, or Pb enters the BaTiO3 crystal, and forms a solid solution. Thus, the characteristiCs of the crystal are improved without deteriorating the ferro-electricity of the crystal. An additive of Mn or Cr functions as a mineralizer which improves the sintering process and prevents the re-oxidation.
~- Each of these additives is a para-electric dielectric body itsel~, and has differen~ electrical properties from those of a cryst~l particle. Since the purpose o~ the novel element is to make the dielectric constant dependent on the volta~e, the amount of the para-electric additive should preferably be as small as possible, as has been confirmed by experiment with ~arious amount of minerali~ers. Some impurities like A1203 or SiO2 which are included in raw materials from which the present element is found, or added in the manufacturing process, produce a para-electrical dielectric body by being deposited from the crystal part;cles or form a solid solution with BaTiO3 and, therefore, the ferro-electricity itself of the element is reduced. Therefore, the allowable amount of those impurities is very small. Accordingly, the amounts and types of mineralizers or impurities present in the novel element are very restricted compared with other ferro-electric dielectric bodies. Therefore, in the production process, the raw material and the process conditions must be monitored very carefully compared with the production of prior art dielectric bodies.
As a result,the novel current restriction element has excellent voltage characteristics, whereby the impedance at 500 volts is less than one hundredth of the impedance at 5 volts. The novel current restriction element is small, lightweight, and has high operational reliability with low production cost. Therefore, it has a wide field of application ;n the electrical industry.

The raw materials 8aC03, TiO2, SnO2, ZrO2, SrO2, PbO, and/or MnC03 in various combinations were wet mixed usin~ a polyethylene pot and an agate ball so that the desired mixtures of the samples 1 throu~h lS of the Table 1 were obtained. After dehydrating and drying, each mixture was held at 1150 C for 2 hours for pre-sintering, and then crushed again 6523--l into powder with Q polyethylene pot and an ~7ate ball. The moisture WQS then evsporated off t R proper quantity of binder was ad~ed, and the powders were then fabricRted into discs with diameter of 16.5 n~l and thickness of 0.50 7nm using a 10-ton press. The discs were then sintered at 1300-1400 C for 2 hours.
A silver electrode was baked onto each ceramic device thus obtained, and lead wires were soldered to the electrode. After cleanin~7 and - paintin~7 with insulatin~ paint, efFective current flow in the circuit ofFi~7ure 6 was measured by applying7 50 Hz alternatin~7 current volta~7e. The impedance was thus calculated. In Fig. 6, the reference numeral 10 is a sample, 12 is an ammeter, 14 is a voltmeter, and 16 is a S0 1l~ power supply.
The experimental results are shown in Table 1 and are also illustrated by Fir7s. 5 and 7. In Table 1 and Fig. 7, samples No. 14 and No.
15 are not within the scope of the present invention, but are sho~n for the purpose of comparison.
5amples Nos. 1 through 13 in Table 1 and Fig. 7 show that the ratio (~2/~1~ of the impedance Z2 for the field of 500 volts/mm to the impedance æl for the field of S volts/mm (rms) is in the ran~7e from 1/20 to 1/108 and, therefore, each of those samples satisfies the requirement that a practical current restriction element have a ratio less than 1/10.
Fig. 7 shows the relation between the impedance and the applied field in volts/n~m (rms), and it should be noted that the samples belon~7in~7 tothe group P, which are within the scope of the present invention, have the characteristic that the impedance initially decreases with increase of the field but increases a~7ain when the field increases above 50-100 volts/m~n.
The theoretical reason why the impedance increases when the field is higher than 50-100 volts/~m is not clear at present, but it should be appreciated that sufficiently low impedance is obtained when the field is less than S00 volts/mm. The impedances illustrated in Fi~. 7 were measured by using dlscs havin~7 a diameter of 16.5 mrn and thickness of 0.50 n~n, with electrodes.
Fig. 5 shows the experimental D-E hysteresis curve of sample No. 3 of Table 1, and it should be noted that it has excellent hysteresis characteristics. In this experiment, the thickness of the sample is O.S0 n~
and the applied alternatin~ field is 280 volts (rrns) - therefore7 a field of 560 volts/rnrn is applied to the sample.

As will be apparent from the above description, a resistance element having as its mcin component BaTiO3 decreases in impedance considerably under high electrical field conditions, and it has been found th~t B~TiO3 is unique in this regard compare~ with other ferro-electric dielectric bodies.

The samples Nos. 1 ~hrough 13, which have as their main ~ component BaTiO3 and showed the excellent results in Example 1, are examined in more detail by addition of certain additives. Typical results are listed in the Table 2 and illustrated in Fig. 8. In Table 2, the main component is a mixture of 94 mol-70 BaTiO3 and 6 mol-% BaSnO3 and the additives are MnC03 or Cr203 or mixtures thereof in the amounts shown in Table 2. The experiment is performed in similar fashion to that o~
Example 1. As may be seen from Table 2, a finer, more homogeneous and enhanced polycrystalline substance which prevents re-oxidation and has improved sintering property without deterioration of the electrical properties of the finished has been obtained. In this experiment, the additives Mn or Cr were added in the form of MnC03 or Cr203, but the present invention is not restricted to them, since other compounds containing ~n or Cr may also be effective. The amount of the additive depends upon the Mn or Cr compound, and in the form of Cr203, a wei~ht percent in the range 0.005 percent to 0.5 percent is preferable. When that weight percent is higher than O.S percent, ths decrease in the impedance becomes small, and is not preferred.
Fig. 8 shows the curves of field versus impedance for the various amount of additives, and it should be noted in Fig. 3, that the ran~e between 0.05 percent and 0.3 percent which belon~s to the group P is preferable for the best electrical properties - in particular, for decrease in the impedance. When the amount is less than 0.005 weight percent, no measurable effect is achieved by using an additive. In the case of MnC03, the practical range is between 0.005 weight percent and 0.3 weight percent, and when it is less than 0.005 weight percent, no measurable effect is achieved.
Further, the addition of MnCOI or Cr203 prevents reduction or re-oxidation in the sintering process, and improves the fineness of particles in the sinterin~ process.
In the Table 2 and FiK. 8, samples 6 and 12 ar~ not within the scope of the present invention.

The samples which show e~cellent characteristics in the Examples 1 and 2 are tested a~ain by adding some of the impurities which can be present in the raw materials or added during production. Our experimental -- results are shown in Table 3 and Fi~. 9, in which it is found that such impurities are desirably less than 0.5 weight percent (group P), otherwise the impedancc is too high. It should be appreciated that the 0.5 weight percent i5 quite small compared with the allowable impurity level in prior ceramics, or the samples listed, except sample No. 1 and sample No. 2 which include impurities of 0.6-1.0 weight percent. The reason that only small amounts of impurities are allowed to be present is that additives and/or impurities which do not form a solid solution with BaTiO3 are deposited from the crystal, and form a para-electric dielectric layer, which degrades the ferro--electric properties of the ceramic. Therefore, the amounts of impurities allowable in the present invention are severely restricted.
In Table 3, sample No. 4 and the sample No. 8 are not included in the scope of the present invention, since the ratio (Z2/zl) in those samples is not sufficient. The curve (a) in Fig. 9 shows the case where no impurity A1203 is included (wei~ht percent of A1203 is zero).

6523~1 Table 1 Sample Composition Electric Characteristics (50 Hz) ~o (Mineralizer MnCO = 0.01 wt%) z 3 Impedance Zl Minimum Impedance Z2 Ratio (z with 5 volts/mm with 500 volts/mm 1BaTiO3 + BaSnO3 2 mol% 1300 12 1/108

2 " 6 mol% 970 9.2 1/105

3 " 10 mol% 960 7.2 1/106

4 " 12 mol% 680 7.0 1/97 " 16 mol% 1050 19 1/55 6BaTiO3 + SrTiO3 4 mol% 900 26 1/35 7 " 6 mol% 800 18 1/44 8 " 8 mol% 1100 21 1/52 9BaTiO3 + BaZrO3 4 mol% 700 22 1/32 " 8 mol% 500 21 1/24 11 " 12 mol% 700 30 1/23 12BaTiQ3 + PbTiO3 1 mol% 1400 28 1/50 13 " 3 mol% 1400 70 1/20 X14PbTiO3-Pb(M~1/3 Nb2/3)03-Pb (Mnl/3 Nb2/3)03 5mol%+95mol%~0.5wt% 310 220 1/1.4 X15 (PbO.962-SrO.04)[(Nb2/3-Col/3)0.01-Tio.458-zro.532]o3+wo3o-6 230 78 1/2.9 Table 2 Electric Characteristics Impedance Zl Minimum Im~edance Z2 Z2 No. CompositionAdditive Ratio (z ) with 5 volts/mm with 500 volts/mm 1 BaTiO3+BaSnO36mol%MnC03 0.005wt% 800 8.6 1/93 2 " "0.01 wt% llQO 9.2 1/119 3 " "0.1 wt% 900 lZ~0 1/75 4 " "0.2 wt% 850 23.5 1/36 " "0.3 wt% 1100 62.0 l/18 X6 " "0.5 wt% 1000 230.0 1/4.3 7 "Cr203 0.005wt% 700 8.0 1/88 8 " 0.01 wt% 720 9.0 1/80 9 " 0.1 wt% 800 10.5 1176 " 0.2 wt% 900 20.0 1/~5 ll " 0.3 wt% 980 58.0 1/17 ~12 " 0.5 wt% 1050 108.0 1/9.7 13 "MnCO 0 05 wt~
Cr2O3 0 05 wt% 1200 13.0 1/92 14 Cr23 0 l wt% 1000 32.5 1/31 Table 3 Electric Characteristics Sample Impedance Zl Minimum Impedance Z2 Z
No. Composition Impurity with 5 volts/mm with 500 volts/mm Ratio ( 2) BaTiO +BaSnO3+MnC03 3 Al O O.lwt~ 1600 13.5 1/119 94mol%+6mol%+0.01wt% 2 3 2 " " 0.3wt% 2250 21.5 1/105 3 " " 0.5wt% 1600 85 1ll9 X4 " " 0.7wt% 1600 340 1/4.7 " SiO2 O.lwt% 1500 15 l/lOC
6 " " 0.3wt% 2350 3~ 1/60 7 " " 0.5wt% 1800 120 1/15 X8 " " 0.7wt% 1500 5GO 1/3

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A current restriction device responsive to an applied electric field comprising a ferro-electric polycrystalline ceramic with a main component of barium titanate (BaTiO3), said device having an impedance ratio (Z2/z1) of less than 1/10 wherein Z2 is the impedance of said device measured with an alternating electric field lower than 500 volts/mm, and Z1 is the impedance of said device measured with an alternating electric field of 5 volts/mm.

2. A current restriction device responsive to an applied electric field comprising a ferro-electric polycrystalline ceramic with a main component of barium titanate (BariO3), a first additive selected from Sr and Pb substituted for part of the barium of said barium titanate, a second additive selected from Zr and Sn substituted for part of the titanium of said barium titanate, said device having an impedance ratio (Z2/zl) of less than 1/10, wherein Z2 is the impedance of said device measured with an alternating electric field lower than 500 volts/mm, and Z1 is the impedance of said device measured with an alternating electric field of 5 volts/mm.

3. A current restriction device responsive to an applied electric field comprising a ferro-electric polycrystalline ceramic with a main component of barium titanate (3aTiO3), a first additive selected from Sr and Pb substituted for part of the barium of said barium titanate, a second additive selected from Zr and Sn substituted for part of the titanium of said barium titanate, a mineralizer comprising at least one of 0.005-0.3 weight percent Cr and 0.005-0.3 weight percent of Mn, said device having an impedance ratio (Z2/z1) of less than 1/10, wherein Z2 is the impedance of said device measured with an alternating electric field lower than 500 volts/mm, and Z1 is the impedance of said device measured with an alternating, electric field of 5 volts/mm.

4. A current restriction device according to claim 1 or claim 2 wherein the ratio of impurities except of said main component and said additives is less than 0.5 weight percent.

5. A current restriction device according to claim 3 wherein the ratio of impurities except of said main component, said additives, and said mineralizer is less than 0.5 weight percent.