GB2242065A - Varistor composition - Google Patents

Abstract

The composition for manufacturing a varistor consists of the following constituents: (a) zinc oxide, (b) ceramic structure influencing additives selected from the group consisting of bismuth oxide, boric acid, chromium oxide, cobalt oxide, manganese oxide and tin oxide, (c) a grain growth influencing additive selected from the group consisting of antimony oxide, silicon dioxide and titanium dioxide, (d) an organic solvent carrier, (e) an organic viscosity influencing additive, and (f) an organic binder. The composition may serve as a ceramic ink for use in the manufacture of multilayer varistors, especially using a screen printing process. The organic materials are volatilised during the forming of the varistor. <IMAGE>

Description

(D C=, t5 Varistor ink formulations This invention relates to varistor

compositions.

Zinc oxide varistors are ceramic semiconductor devices based on zinc oxide. They have highly non-linear current/voltage characteristics, similar to back-to-back Zener diodes, but with much greater current and energy handling capabilities. Varistors are produced by a ceramic sintering process which gives rise to a structure consisting of conductive zinc oxide grains surrounded by electrically insulating barriers. These barriers are attributed to trap states at grain boundaries induced by additive elements such as bismuth, cobalt, praseodymium, manganese and so on.

The electrical characteristics of a metal oxide varistor, fabricated from zinc oxide, are related to the bulk of the device. Each zinc oxide grain of the ceramic acts as If it has a semiconductor junction at the grain boundary. The non-linear electrical behaviour occurs at the boundary of each semiconducting zinc oxide grain. Accordingly, the varistor can be considered as a multi-junction device, composed of many series and parallel connections of grain boundaries. The device behaviour may be analysed with respect to the details of the ceramic microstructure, and mean grain size and grain size distribution play a major role in electrical behaviour.

Fabrication of zinc oxide varistors has traditionally followed standard ceramic techniques. The zinc oxide and other constituents are mixed, for example by milling in a ball mill, and are then spray dried. The mixed powder is then pressed to the desired shape, typically tablets or pellets. The resulting tablets or pellets are sintered at high temperature, typically 1,000 to 1,400 0 C. The sintered devices are then provided with electrodes, typically using a fired silver contact. The behaviour of the device is not affected by the configuration of the electrodes or their basic composition. Leads are then attached by solder and the finished device may be encapsulated in a polymeric material to meet specified mounting and performance requirements.

In the device thus fabricated, the bulk of the varistor between its contact or electrode layers thus consists primarily of zinc oxide grains of a predetermined average grain size, yielding a specific resistivity per unit of thickness dimension. In designing a varistor for a given nominal varistor voltage, it is therefore basically a matter of selecting a device thickness such that the appropriate number of grains is in series between the electrodes. The voltage gradient of the varistor material, in terms of volts per unit of thickness dimension, can be controlled by varying the composition and manufacturing conditions of the varistor. Altering the composition of the metal oxide additives enables the grain size to be changed for this purpose. In practice, the voltage drop per grain boundary junction is approximately constant and does not vary greatly for grains of different sizes. Accordingly, varistor voltage is primarily determined by the thickness of the material and the size of the grains.

The construction and performance of varistors is discussed inter alia in "Zinc Oxide Varistors - A RevieW by L. M. Levinson and H. R. Philipp, Ceramic Bulletin, volume 65, No. 4 (1966), which article may be referred to for further detail.

A multiplicity of specific varistor compositions are known and described, inter alia, in the following patent specifications: US 3,598,763; US 3, 663,458; US 3,863,193; US 4,045,374 and GB 1,478,772. Methods of manufacturing varistors are described, inter alia, in US 3,863,193 and 4, 148,135.

It is an object of the present invention to provide improved compositions of material for use in producing varistors. It is a further object of the invention to provide suspensions of zinc oxide based materials in solvents for use in producing varistors using a wet screen printing method.

The present invention is especially directed to compositions facilitating the manufacture of multilayer varistors. While it has generally been accepted that a multilayer varistor would have a number of advantages as compared with the equivalent radial product, manufacturing problems have hitherto prevented any widespread move towards multilayer varistors.

The advantages of multilayer construction as applied to varistors include compact size for equivalent electrical characteristics, as compared with a conventional radial device. Multilayer varistors may also be completely symmetrical, fully passivated and have good IV characteristics. As against this, possible disadvantages include relatively high capacitance and potential reactions between the ceramic and the internal electrodes, especially the interaction of palladium and bismuth complexes.

The present invention is further especially directed to compositions for use in the manufacture of multilayer varistors using printing techniques. A method of manufacturing a multilayer varistor by a sequence of printing operations forms a subject of a co-pending patent application by the present applicants. In a particular manufacturing method disclosed in said co-pending application, both the ceramic layers and the electrode patterns are successively screen AM SeApjpPP Bu[juanljul ainianals nyweaaa go Xlplpanld pleS -aapupq opue6ao up PUP MAPIpppU Oupjuanljup-AIPSOOSPA JUPTO Ue Capa= lUeAlOS OPUPS0 UP (P) lappxopp wnpuelpl pup apyopp uojplps 'appxo Xuowplue ispal JP go Oupispsuon dnoaB aqi Maj PalOalaS aA;Ipppe OUPOUOnljUP qlMOaB UPPA allO 1Seal JP (3) lappxo ull pup appxo asauubupw lappxo llpqoo lappxo wnywoaqn lp[ne n[aoq lappxo qlnwspq jsvaj JP go Bullsysuon dnoaB aql woaj palDalas SaAplyppP Bu;Duanljup anlinals Dpwuaaa go AlpleanLd P (q) lappxo nupz (R) Supspadwon a015paPA P Bupanl3pgnupw up asn ao, lplaalpw uoylpsodwon P POPPAoad sp aaaqi TOplUeAU; aql 01 6UPPIO3OV slueoplddp juasaid aq, go uolIP3plddp Ouppuad-oo P go loapqns aq, way spup qons up uopypaodaooup aoj alqPyns suopilsodwoa aaPM0d -510;SpaPA aaAplpllnw ao, salbolouqoa:

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In another aspect, the invention provides a composition material for use in manufacturing a varistor comprising (b) (d) (e) (f) 94 to 98 mole percent of zinc oxide, 1 to 4 mole percent of a plurality of ceramic structure influencing additives selected from the group consisting of at least bismuth oxide, boron oxide, chromium oxide, cobalt oxide, manganese oxide and tin oxide, 0.1 to 1.6 mole percent of at least one grain growth influencing additive selected from the group consisting of at least antimony oxide, silicon dioxide and titanium dioxide, an organic solvent carrier, an organic viscosity-influencing additive, and an organic binder.

Said composition material may comprise 0.002 to 0.01 mole percent of at least one electrical performance influencing additive selected from the group consisting of at least aluminium nitrate and silver oxide. The composition material may also comprise 0.6 to 1.1 mole percent of nickel oxide as a further additive, and at least 0.4 mole percent of magnesium oxide as an additional additive.

Preferably, the inorganic constituents of the composition material are in granular or powder form and the average grain size of the granular or powder form material is less than approximately 2 microns.

Suitably, the relative proportions of the organic constituents are selected so that the mixture of the organic and inorganic constituents of the composition material is in the form of a suspension.

The invention also provides a printing ink comprising a composition material as defined hereinabove, as well as a method of producing a composition material for use in manufacturing a varistor comprising the steps of:

(i) calcining a granular or powder form composition material comprising (a) (b) zinc oxide, a plurality of ceramic structure influencing additives selected from the group consisting of at least bismuth oxide, boron oxide, chromium oxide, cobalt oxide, manganese oxide and tin oxide, and at least one grain growth influencing additive selected from the group consisting of at least antimony oxide, silicon dioxide and titanium dioxide, and (ii) mixing the calcined composition material with an organic solvent carrier, an organic viscosity-influencing additive, and an organic binder.

Preferably said additive is added for a first stage of said mixing step, and said binder is added following said first stage of said mixing step, said binder being mixed with ot-her constituents of the material in a second stage of said mixing step. Said first stage of said mixing step suitably comprises a milling operation.

The invention will now be described having regard, inter alia, to the following drawings, in which:

Figure 1 is a block diagram showing a sequence of process steps for the manufacture of a multilayer varistor, and Figure 2 is a diagram showing a plot of shear stress against shear rate for a ceramic ink according to the invention.

As shown in Figure 1, zinc oxide powder and additives are weighed preparatory to mixing to provide a powder formulation suitable for manufacture of the particular varistor desired. The powder formulation Is then mixed with suitable organic constituents to provide the ceramic ink. A cycle of operations then takes place, In which, following the preparation of suitable screens, ceramic layers are deposited onto a substrate by screen printing and are interleaved with electrode layers similarly screen printed onto semi-dried layers of ceramic material.

Following completion of the printing operations, the printed layered product supported on the substrate is separated to provide a multiplicity of multilayer varistor units. Subsequent treatment steps are broadly conventional, In that the binder and other organics are burnt out, the product is then fired and rumbled, terminations are applied for electrical communication between the varistor and other circuit elements, and the finished varistor is then tested.' Optional final steps include the attachment of leads and overall encapsulation.

Co-pending patent applications by the present applicants entitled 'Waristor structures", 'Waristor configurations" and 'Waristor manufacturing method and apparatus" disclose details of multilayer varistor constructions derived from a screen printing manufacturing process together with a novel manufacturing method for multilayer varistors. The present invention provides novel compositions of inks suitable for use in this method and in the production of multilayer varistors.

Table 1, following, lists materials present in powder formulations for varistors, and in particular in ceramic powder compositions for use in the manufacture of multilayer varistors:

i TABLE 1

Material Zinc Oxide Bismuth Oxide Cobalt Oxide Manganese Oxide Nickel Oxide Antimony Oxide Silicon Dioxide Magnesium Hydroxide Aluminium Nitrate Chromium Oxide Barium Carbonate Boric Acid Titanium Dioxide Tin Oxide Silver Oxide fo I I ows, Formu 1 a ZnO Bi203 C0203 Mn02 Nio Sb 203 Si02 Mg (ON) 2 MgO A120 3 added as A12(NO3)3' 9H 20 Cr203 BaCO 3 BaO HBO 3 Ti02 SnO 2 A920 These various materials as listed in Table 1 may be grouped as 1. Zinc oxide Zinc oxide makes up typically in the range 85 to 95 mole-percent of the bulk of a varistor formulation, for example 92 mole-percent of the bulk. For low voltage varistors, barium grains may be added, in the form of barium carbonate, transformed to barium oxide during the manufacturing process. The function of the barium is to promote the growth of zinc oxide grains and this additive disappears after sintering of the varistor.

2. Glass-related materials (ceramic structure influencing additives) These additives serve to enhance the development of the ceramic structure. They include the following materials listed in Table 1:

Bismuth oxide, added in the trioxide form. This is a glass-forming additive. Cobalt oxide is another glass additive agent, assisting the glass frit and serving to maintain phase stability in the ceramic. Manganese oxide has an augmenting effect similar to that of bismuth oxide. Chromium oxide is a further glass additive functioning to stabilise the ceramic product. Boric acid is another glass former. Tin oxide is yet another stabiliser for the glass structure, albeit less commonly used than those already cited.

3.

^L- 1 Grain growth modifiers (grain growth influencing additivesi Antimony oxide is an additive controlling grain growth. It acts as an inhibitor to keep grain size small. This is particularly important in high voltage devices. Silicon dioxide is a strong grain growth inhibitor and is added to compositions or powder formulations to get high values of voltage per millimetre thickness. Silicon dioxide per se is however highly conducting and absorbs energy when the junctions at the depletion layer break down and conduct. In a varistor, the grain structure is almost entirely zinc oxide and the additives go into the glass matrix which surrounds the grains. It is this aspect of the construction of varistors that results in the highly significant impact of small quantities of additives on the performance of the device, as exemplified by the action of the conductive silicon dioxide at the grain boundaries. The remaining grain growth enhancing additive of those listed in Table 1 is titanium dioxide, also usable in multilayer varistors.

4. Nickel oxide Nickel oxide is a unique additive, having properties not achieved by any of the other additive materials and directed to stabilisation of the microstructure. The nickel oxide assists the formation of a microstructure in the ceramic material suitable for handling both DC and AC stress.

5. Junction-related additives (electrical performance influencing additives) Aluminium nitrate is a crucial additive under this category. The nitrate is transformed into an oxide in the course of the manufacturing method. As in the case of the majority of other additives, it goes into the glass matrix surrounding the grains. Aluminium oxide in very small parts per million enhances the conductivity of the zinc oxide. For higher additive levels however, the aluminium oxide diffuses into the grain boundaries and can create a leaky device by reducing intergranular activity. Conversion of the nitrate takes place during sintering. Silver oxide is used in combination with the aluminium additive in various formulations, to achieve certain desirable results in the varistor.

6. Other additives Magnesium hydroxide, which transforms to magnesium oxide in the finished product, comes under this category. The function and action of this additive is obscure, and the nature of its contribution to the performance of the varistor device is not completely understood. It is however a traditional additive material in radial-type varistors.

The foregoing categorisation of the materials comprised in a 1 powder formulation for a varistor product represents only one manner of viewing the purpose and function of each of the various materials and additives comprised in the product. The particular analysis set forth is however effective in explaining certain of the performance characteristics of varistor devices, and in particular multilayer constructions of such devices, and has been found beneficial In the development and preparation of useful novel powder formulations, especially formulations particularly suited to use in ceramic inks for application in screen printing varistor production methods. It is not however claimed to be a definitive categorisation or subdivision of these additives and materials, given that many aspects of varistor operation and performance remain obscure or not fully understood. value of the present classification resides in its ability to facilitate understanding of certain aspects of varistor performance and in assisting in the development of suitable formulations for, in particular, varistor products manufactured by screen printing methods.

Table 2 (following) lists a number of powder formulations found especially suited to the preparation of ceramic inks for use in the manufacture of multilayer varistors by screen printing techniques. For each of the formulations listed, the quantity of both the basic zinc oxide and of each category of additive, as identified above, is quoted in mole-percent. Desirable physical features of the powder formulations listed will be subsequently identified, in discussing the preparation of ceramic inks. It will however be noted from Table 2 that the additives are present in different quantities from formulation to formulation. The precise quantities of additives selected in each category depend on the purpose and performance desired of the varistor. For example, silicon dioxide is a stronger inhibitor of grain growth than antimony oxide. However, the use of silicon in larger quantities may tend to cause a reduction in the resistance of the grain boundaries of the zinc oxide structure. In order to avoid possible problems, for example in regard to product life, which the use of silicon in large quantities might entail, alternative compositions involving a different balance of additives may therefore be substituted or favoured depending on the characteristics and/or performance 1 i required of the finished device. The balance of functions and the interrelationship between the various additives is also complex and not fully understood. In order to achieve a desired performance from a finished varistor product, reformulation of, for example, the glass aspects of the composition may be required and not necessarily variation of, for example, only a grain growth modifier, such as the silicon dioxide. The various materials and additives react and co-operate together in such a complex way that the adverse consequences of an increased level of silicon may, for example, be offset by modification of the ceramic glass structure. To an extent, therefore, the development of effective and advantageous formulations is an empirical art, guided however by theoretical considerations derived from the known characteristics of each material and additive of the composition.

j j.

i 1 1 i i TABLE 2

Material BASIC CONSTITUENT Zinc oxide Formulation ID 1 2 3 4 5 mole mole mole mole mole percent percent percent percent percent 96.9 94.9 96.3 97.3 97.2 GLASS-RELATED ADDITIVE 2.1 2.5 3.2 1.7 1.6 - selected from Bismuth oxide Boric acid Chromium oxide Cobalt oxide Manganese oxide Tin oxide GRAIN-GROWTH MODIFIERS - selected from Antimony oxide Silicon oxide Titanium dioxide JUNCTION-RELATED ADDITIVES - selected from Aluminium nitrate Silver oxide Nickel oxide Magnesium hydroxide 1.0 1.5 0.5 0.5 0.2 0.005 0.005 0.003 0.004 0.009 0.7 - - 0.5 - 0.5 1.0 In all of the formulations shown in Table 2, the mole-percentages relate to dry product.

For use as an ink, it is required that the ceramic powder formulation be maintained in suspension in suitable solvents and that the ink product thus formed be thixotropic. i.e. exhibit a variable viscosity depending on shear rate. A thixotropic product typically behaves like a very thick or sticky medium when the rate of application of shear force is low, but is capable of flow in the manner of a low-viscosity liquid under high rates of shear. A combination of organic materials as solvents and carriers is used to achieve this, in combination with a preferred range of particle size for the dry powder product.

Typically, the preferred grain size is approximately 1.5 microns. Varistor powder as received from the preparatory powder manufacturing stage has usually grains of considerably smaller size, for example from 0. 1 to 0.2 microns. The range of grain sizes is also generally relatively wide, and the powder is not fully homogeneous. In order to render this dry powder as received suitable for incorporation in a varistor ceramic ink, the particle size must be increased and the powder homogenised. This is achieved by calcining, a step which is not normally required for most conventional powders as used for radial varistors, but is however nonetheless occasionally used. The calcining step consists of firing the powder as received at between 800 0 C and 9200C and then reducing the fired powder in a milling operation.

In order to then form the thixotropic ceramic ink, organic solvents are then added to the calcined powder. These may include butyl dioxitol acetate or a terpene alcohol. The organic material acts as a carrier for the particles in suspension. Viscosity influencing materials may be added to control the rheology of the ceramic ink, in conjunction with the main solvent additive(s).

The ceramic ink, which is typically green in colour, is prepared from the foregoing ingredients in the following manner: The - i 1 calcined powder is mixed together with the solvent and the viscosity modifier by ball milling or other mill methods. Suitable proportions or quantities of these constituents are quoted in the following Table 3. A further organic product is added after milling to achieve the desired ink properties and fulfil] a binder function. The binder may be ethyl cellulose, ethyl hydroxy cellulose or a rosin derivative. The binder has a significant impact on the viscosity of the organic and ceramic powder mixture. For this reason, it is added to the mixture following the milling step. If the full quantity of the binder required to achieve the thixotropic qualities of the finished ink was added before willing, the viscosity of the mixture would be increased to an excessive degree and this would impair milling performance.

In summary therefore, considering the powder and ink preparation process as a whole, a typically 0.25 or less micron grain size powder represents the starting point. This is calcined and then ground to provide particles of 2 microns average size. Solvents in the proportions to be specified are then added to this larger particle size product and ball milling of the organic material and ceramic powder product takes place during which the grain size is once again somewhat reduced to approximately an average figure of 1.6 microns, typically + or - 10%. This level of particle size allows the powder to remain in suspension in the ink product over a relatively long period. The particle size is of considerable importance in providing a successful ceramic ink. If the particle size is too small, an undue quantity of solvent may be required and it may also be difficult to maintain a suspension of a homogeneous nature. By contrast, if the particle size is too large, the particles will settle out under gravity, so that there is separation of the powder grains from the organic solvent materials and the binder.

Table 3 following gives the weight percentages of the powder and the organic ingredients, along with that of the zirconia cylinders, required for ball milling to give certain specified quantities of ceramic ink, also identified in the Table. The limits on the organic quantities as set forth in this Table are typically + and - 1%.

TABLE 3

Solvent (gm) Ink Volume (gal) 3.0 2.5 2.0 1.5 1.0 0.5 Calcined Powder (gm) 5635 4695.8 3756.7 2817.5 1878.3 939 1980 1650 1320 990 660 330 Viscosity Modifier (gm) 38 31.7 25.3 19 12.7 6.4 Zirconia Cylinders (gm) 16000 13300 10600 8000 5300 2700 The solvents, viscosity modifiers and binders used in the ceramic ink of the present invention are natural materials and offer advantages in terms of safety, having both low toxicity and high flash points. Alternative materials meeting the same criteria are not readily available in substitution for these preferred solvents, although alternative materials are nonetheless not excluded from the scope of the invention.

In the detailed preparation of ceramic ink in accordance with the foregoing specifications, as described and set forth in Table 3, the required quantities of the various ingredients are carefully weighed out and placed in the ball mill. The ball mill is preferably rotated at a speed of between 36 and 42 r.p.m. for a period of approximately 24 hours. The binder is added to the ceramic ink as constituted following the ball milling step during a further mixing step. The mixed product is then stored in a sealed container so that.

none of the volatile organic materials are lost. Following this shear mixing, adverted to above, the ink must be left in sealed storage for at least 24 hours, after which its viscosity is measured to establish its quality and suitability for printing.

Viscosity is measured on any suitable viscometer such as for example a Naake viscometer. This enables a plot of shear stress against shear rate to be provided for any particular ceramic ink sample, a typical such plot is shown in Figure 2. The plot is preferably provided with a standard "curve" or desired relationship between shear stress and shear rate, with which the figures for the sample should comply within specified predetermined limits. In the event of the shear stress performance of the sample being different from that of the standard curve, the ink may be treated to adjust its viscosity. The standard curve may also allow for changes in viscosity during shelf storage of the ceramic ink prior to its use for printing purposes.

The organic materials included in the ceramic ink serve only to enable flow and laying down of the ink in the production of multilayer ceramic varistor products. During the subsequent firing of the formed products, all of the organic materials are volatilised, leaving only the ceramic powder in a sintered structure, together with the interleaved layers of electrode material.

Claims (19)

1. A composition material for use in manufacturing a varistor comprising: (a)- zinc oxide, (b) a plurality of ceramic structure influencing additives selected from the group consisting of at least bismuth oxide, boric acid, chromium oxide, cobalt oxide, manganese oxide and tin oxide, (c) at least one grain growth influencing additive selected from the group consisting of at least antimony oxide, silicon dioxide and titanium dioxide, (d) an organic solvent carrier, (e) an organic viscosityinfluencing additive, and (f) an organic binder.
2. 2. A-composition material according to Claim 1, wherein said plurality of ceramic structure influencing additives includes at least bismuth oxide, cobalt oxide and manganese oxide.
3. 3. A composition material according to Claim 1 or Claim 2, comprising at least one electrical performance influencing additive selected from the group consisting of at least aluminium nitrate and silver oxide.
4. 4. A composition material according to any of Claims 1 to comprising nickel oxide as a further additive.
5. 5. A composition material according to any of Claims 1 to 4, comprising magnesium hydroxide as an additional additive.
6. 6.

   3# A composition material for use in manufacturing a varistor comprising (a) 94 to 98 mole percent of zinc oxide, (b) 1 to 4 mole percent of a plurality of ceramic structure influencing additives selected from the group consisting of at least bismuth oxide, boron oxide, chromium oxide, t j i.

   cobalt oxide, manganese oxide and tin oxide, (c) 0.1 to 1.6 mole percent of at least one grain growth influencing additive selected from the group consisting of at least antimony oxide, silicon dioxide and titanium dioxide, an organic solvent carrier, an organic viscosity influencing additive, and an organic binder.

   (d) (e) (f)
7. 7. A composition material according to Claim 6, comprising 0.002 to 0.01 mole percent of at least one electrical performance influencing additive selected from the group consisting of at least aluminium nitrate and silver oxide.
8. 8. A composition material according to Claim 6 or 7, comprising 0.6 to 1. 1 mole percent of nickel oxide as a further additive.
9. 9. A composition material according to any of Claims 6 to 8, comprising at least 0.4 mole percent of magnesium oxide as an additional additive.
10. 10. A composition material according to any of Claims 1 to 9 wherein the inorganic constituent materials are in granular or powder form and the average grain size of the granular or powder form material is less than approximately 2 microns.
11. 11. A composition material according to any of Claims 1 to 10, wherein the relative proportions of the organic constituents are selected so that the mixture of the organic and inorganic constituents of the composition material is in the form of a suspension.
12. 12. A printing ink comprising a composition material according to any of Claims 1 to 11.
13. 13. A method of producing a composition material for use in manufacturing a varistor comprising the steps of:

    \1 (i) calcining a granular or powder form composition material comprising (a) zinc oxide, (b) a plurality of ceramic structure influencing additives selected from the group consisting of at least bismuth oxide, boron oxide, chromium oxide, cobalt oxide, manganese oxide and tin oxide, and (c) at least one grain growth influencing additive selected from the group consisting of at least antimony oxide, silicon dioxide and titanium dioxide, and (ii) mixing the calcined composition material with an organic solvent carrier, an organic viscosity-influencing additive, and an organic binder.
14. 14. A method according to Claim 13, wherein said additive is added for a first stage of said mixing step, and said binder is added following said first stage of said mixing step, said binder being mixed with the other constituents of the material in a second stage of said mixing step.
15. 15. A method according.-o Claim 13 or Claim 14, wherein said first stage of said mixing step comprises a milling operation.
16. 16. A method of producing a composition material for use in manufacturing a varistor substantially as described herein by way of Example and with reference to the accompanying drawings.
17. 17. A varistor composition formulation prepared by the method of any of Claims 13 to 16.
18. 18. A varistor composition formulation according to any of Claims 1 to 12.
19. 19. A varistor composition formulation substantially as described herein by way of Example.

    0298S Published 1991 at The Patent Office. Concept House. Cardiff Road. Newport. Gwent NP9 I RH. Further copies maybe obtained from Sales Branch. Unit 6. Nine Mile Point. Cwmfelinfach. Cross Keys. Newport. NP1 7HZ. Printed by Multiplex techniques ltd. St Mary Cray. Kent.

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