Ceramic resistance elements

Abstract

 A resistance element based on barium titanate has an excessive donor dopant level such that the low temperature resistivity is greater than the high temperature resistivity maximum. The resultant NTC characteristic has an inflexion over which the resistivity is substantially constant. A similar effect is achieved with an excess of acceptor dopant. An advantage of the resistance element lies in automobile or other instrumentation or control devices where a resistance dwell is required over a specified temperature range.

Description

[0001] This invention relates to ceramic resistance elements.

[0002] It is a common practice to employ in electronic instrumentation a ceramic resistance element whose resistance varies linearly with temperature. In, for example, automobile instrumentation, an engine temperature sensor may employ an NTC element driving a dashboard temperature gauge. A typical resistance/temperature plot for an NTC device is shown in Figure 1. It will be appreciated that the temperature gauge driven by such a device will rise and fall with engine temperature and it is conventional to mark on the dashboard gauge upper and lower temperature limits. Whilst a rise in engine temperature to or beyond the high temperature limit will indicate a fault condition necessitating urgent attention, there are a variety of factors which influence the steady position of the gauge within the "safe" range and variations within this safe range are not usually of technical significance. Thus it is quite possible that an engine temperature display reaches a steady position which is close to the high temperature limit but where the engine is operating completely satisfactorily. In those circumstances, it has been experienced that drivers, alarmed by the seemingly high engine temperature, will waste time investigating a non-existent fault.

[0003] In an attempt to overcome this problem, a resistance element has previously been suggested which comprises a NTC element bonded to a PTC element, with the two elements being effectively connected in electrical series. It will be understood that a PTC resistance element has a relatively low resistance at low temperature with the resistance rising through several orders of magnitude at or around a transition temperature. A typical resistance/temperature plot of a PTC device is shown in Figure 2. It will be recognised that by the electrical series interconnection of NTC and PTC devices subject to the same temperature, a resistance temperature plot can be produced as shown in Figure 3. This, when used to drive a temperature display, will operate at high and low temperatures in the same manner as an NTC device but will maintain the temperature display constant over small variations in temperature in the safe working range defined by the plateau A. This deals with the problem as described above. However, the bonding together of two separate devices represents an additional manufacturing operation and there are obvious cost penalties as compared with a single NTC device.

[0004] It is an object of this invention to provide an improved resistance element which has an intrinsic temperature dependence of resistance similar to that of an NTC/PTC combination.

[0005] A typical PTC device is based on barium titanate which is rendered conducting by a suitable dopant such as holmium. It is well known that the characteristics of the PTC device can be controlled by the use of various additives and by variations in the conditions under which the ceramic is sintered. Thus, additives such as lead can be used to shift the transition or switching temperature to higher values whilst additives such as strontium can have the opposite effect. It will be understood that these particular additives serve to replace barium, forming lead and strontium titanate respectively. It is equally possible to use additives such as tin or zirconium effectively to replace the titanium. The proportions of such additives can vary widely and it is not essential for barium titanate to be the major constituent or, indeed, to be present at all.

[0006] A variety of donor dopants have been suggested, these including bismuth, niobium, tungsten, tantalum, antimony, yttrium and rare earth elements such as lanthanum and holmium. Acceptor dopants such as iron, copper, cobalt, manganese or nickel can also be used to control the PTC effect.

[0007] According to the present invention, there is provided a resistance element comprising a body of barium titanate or similar ferroelectric, provided with one or more dopant materials rendering the body conductive and electrode means provided on the body, wherein the nature and quantity of the or each dopant material is selected such that the variation of resistance with temperature of the element is inverse and exhibits an inflexion over a defined temperature range.

[0008] In one form of the invention, a donor dopant material selected from the group consisting of bismuth, niobium, tungsten, tantalum, antimony, yttrium and the rare earth elements is added in an amount substantially in excess of that required to provide maximum extrinsic conductivity.

[0009] In an alternative form of the invention, an acceptor dopant is selected from the group consisting of iron, aluminium, copper, cobalt, manganese and nickel. Suitably, the amount of acceptor dopant is at least about 15% measured in atomic % of the amount of donor dopant required to provide maximum extrinsic conductivity.

[0010] The invention will now be described by way of example with further reference to the accompanying drawings in which:

Figure 1 is a resistance/temperature plot for a known linear NTC device;

Figure 2 is a resistance/temperature plot for a known PTC device;

Figure 3 is a resistance/temperature plot for a known combination NTC/PTC device;

Figure 4 is a plot of room temperature resistance of a PTC device against donor dopant level; and

Figure 5 is a resistance/temperature plot of an element according to this invention.

[0011] It is found experimentally that the room temperature resistance of a PTC device is heavily dependent upon - among other factors - the level of donor dopant. Figure 4 is a plot of room temperature resistivity of samples of holmium doped barium titanate for varying levels of dopant quoted in atomic %. The samples in question were sintered at 1420°C. It will be seen that the optimum doping level, giving maximum conductivity, lies in the range 0.2 to 0.4 atomic % Ho. A significant increase in doping level would - on accepted teaching - reduce the PTC effect, as defined in terms of the high and low temperature resistivity differential, to a useless value.

[0012] It should be understood that the room temperature resistivity is influenced by a variety of other factors and by way of illustration, Figure 4 contains in broken lines a similar plot for samples sintered at 1460°C i.e. an increase in sintered temperature of 40°C. The level of acceptor dopant similarly has a marked effect.

[0013] The present applicants have recognised that by - for example - increasing the amount of donor dopant to levels that would previously be regarded as destroying any useful PTC effect, a device can be produced which has practical utility.

[0014] In one example, a ceramic resistance element was produced with the following composition:-

i)	100 mol%	BaTiO₃	(Commercial Grade)
ii)	0.5 mol%	Ho₂O₃
iii)	1 mol%	TiO₂

[0015] After milling and pressing, the sample was heated at 900°C per hour to a sintering temperature of 1420°C, this temperature being held for one hour. The sintered sample was then cooled to ambient at 300°C/hour.

[0016] It is clear from Figure 4 that the amount of Ho added as dopant (1.0 atomic %) is excessive by conventional standards and this has the effect of raising the low temperature resistance minimum of the conventional PTC plot above the high temperature resistance maximum. There is accordingly produced a resistance/temperature dependence which is generally inverse but which exhibits an inflexion. Reference is directed to Figure 5 which is an experimental resistance temperature plot for this ceramic composition. There is clearly shown an inflexion which approximates to a plateau of constant resistance. It will be recognised that this mimics the behaviour of the above described NTC/PTC composite device.

[0017] Titanium dioxide is added to the composition for stoichiometric adjustment. It may also be necessary to add further additives such as silica so as to provide for the sintering to take place in liquid phase. This helps to produce a uniform grain structure.

[0018] It has been found that a considerable degree of control can be exercised over the location of the inflexion or plateau by the introduction of a constant temperature annealing stage. In one experiement, samples were heated at 900°C/hour to a sintering temperature of 1320°C held for 0.5 hours. The overall cooling rate of 300°C/hour was interrupted at 1220°C for an annealing period of t hours. The dependence on t of the location of the inflexion in both resistance and temperature terms is set out below:-

Annealing Period t hours	Inflexion Resistance (ohms)	Inflexion Temperature Range
0	10³	200 - 300°
3	10⁴	180 - 230°
6	10⁵	170 - 190°
27	10⁶	140 - 150°

[0019] In a similar manner, control can be exercised over the slope of the linear NTC portions of the resistance plot. It will be understood that because of the cross coupling of the variables such as donor dopant level, acceptor dopant level, sintering conditions and other additives, the selection of one parameter cannot properly be made in isolation.

[0020] In an alternative embodiment of this invention, the ceramic composition includes a high concentration of acceptor dopant to produce the desired resistance behaviour. In one example, the composition comprises:-

i)	100 mol%	BaTiO₃	(Commercial Grade)
ii)	0.15 mol%	Ho₂O₃
iii)	0.025 mol%	Cr₂O₃
iv)	0.25 mol%	TiO₂

[0021] It is known that the use of an acceptor dopant such as chromium can be useful in stabilising the PTC effect. In a typical prior art composition, an acceptor dopant would be added at approximately 10% of the level of donor dopant. In a preferred form of this invention the acceptor dopant level is at least 15% of the donor level.

[0022] In the case where the desired inflexion is by adding an excessive amount of donor dopant, it is possible to use the level of acceptor dopant level to control the location of the inflexion. The following composition is again put forward as an example:-

i)	100 mol%	BaTiO₃	(Commercial Grade)
ii)	0.50 mol%	Ho₂O₃
iii)	0.015 mol%	Cr₂O₃
iv)	0.97 mol%	TiO₂

[0023] Increasing the amount of Cr₂O₃ would increase the resistance of the element at the inflexion temperature range.

[0024] It is expected that ceramic resistance elements in accordance with this invention will find application in a variety of instruments and control devices where a resistance "dwell" is required on changing temperature.

[0025] It should be understood that this invention has been described by way of examples only and a wide variety of further modifications can be made without departing from the scope of the invention.

Claims

1. A resistance element comprising a body of barium titanate or similar ferroelectric, provided with one or more dopant materials rendering the body conductive and electrode means provided on the body, wherein the nature and quantity of the or each dopant material is selected such that the variation of resistance with temperature of the element is inverse and exhibits an inflexion over a defined temperature range.

2. A resistance element according to Claim 1, provided with a donor dopant material in an amount substantially in excess of that required to provide maximum extrinsic conductivity.

3. A resistance element according to Claim 2, wherein said donor dopant material is provided in an amount which is at least double that required to provide maximum extrinsic conductivity.

4. A resistance element according to Claim 2 or Claim 3, wherein said donor dopant material is selected from the group consisting of bismuth, niobium, tungsten, tantalum, antimony, yttrium and the rare earth elements.

5. A resistance element according to Claim 4, wherein said donor dopant material is holmium added in an amount from 0.8 to 1.5 atomic %.

6. A resistance element according to Claim 1, provided with an amount of acceptor dopant material which is at least about 15% measured in atomic % of the amount of donor dopant required to provide maximum extrinsic conductivity.

7. A resistance element according to Claim 6, wherein said acceptor dopant material is selected from the group consisting of iron, aluminium, copper, cobalt, manganese, chromium and nickel.

8. A resistance element according to Claim 1, provided with a donor dopant material selected from the group consisting of bismuth, niobium, tungsten, tantalum, antimony, yttrium and the rare earth elements and an acceptor dopant material selected from the group consisting of iron, aluminium, copper, cobalt, manganese, chromium and nickel, wherein the acceptor dopant level measured in atomic % is at least 15% of the donor dopant level.

9. Electrical circuit means comprising a resistance element in accordance with any one of the preceding claims, means for applying a potential across the resistance element, and indicator means actuable in dependence upon the current flowing through the resistance element.

10. Electrical circuit means according to Claim 9, in the form of an engine temperature indicator.

Drawing