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Platinum in the Glass ZGS MATERIALS SUPPLEMENT CONVENTIONAL ‘ALLOYS By J. Stokes Johnson Matthey Materials Technology Division, Wembley

Platinum and its alloys have long been accepted as essential materials for the fabrication of equipment used in the glass industry. However the superior mechanical properties of the zirconia grain stabilised materials developed by Johnson Matthey have led to their successful utilisation for many of the most demanding applications; for others conventional platinum and alloys are still preferred and both types of material are considered here.

The metals are characterised high quality optical or crystal glasses are being by high melting points combined with excellent manufactured. Pure platinum is therefore chemical nobility. These properties are par- preferred in such high quality glass applica- ticularly useful in the glass industry which tions, even though the generally higher melting manufactures a multitude of end products. The points of these glasses limits the service life of range includes domestic glass as tableware or the platinum, due to grain growth and soften- cookware, flat glass for windows, crystal glass ing with time. for prestige domestic use, optical glass, radia- In some areas of the industry control of the tion shields, and fibres for reinforcement and glass flow rate is crucial to the success of a for- insulation, along with many others. Glasses in ming or processing operation. In such cases the their most basic form are combinations of soda, tendency for glass to wet platinum surfaces lime and silica together with other modifiers results in adhesion and poor flow and refiners, which melt at high temperatures characteristics. To overcome such problems 5 to form aggressive liquids. per cent -platinum can be used, an Platinum with a of 1769OC is which can be described as non-wetting to one of the most inert of the platinum group molten glass. Although limited by a lower max- metals, and is suitable for applications at imum operating temperature the use of this temperatures up to approximately q5o0C, alloy has enabled increased glass flow control although at these high temperatures softening and improved operating efficiencies. due to grain growth reduces the materials’ abili- All conventional platinum group metals and ty to withstand stresses and hence limits their alloys exhibit softening with time at service life. temperature, and consequent reduction in The addition of , typically 10 weight ultimate strength due to recrystallisation and per cent, to platinum produces a much stronger grain growth; in addition deformation occurs alloy which can be used under more highly by creep. These factors limit the life of many stressed conditions, and with some gain in max- fabrications so Johnson Matthey have imum operating temperature. However, the developed a range of Zirconia Grain Stabilised high initial cost of rhodium tends to limit its use (ZGS) platinum alloys which do not suffer from as a platinum strengthener to about 20 per cent. this undesirable loss of strength (1,2,3,4). One disadvantage in the use of rhodium is ZGS materials have a fine, evenly dispersed, that it tends to colour the glass yellow under discrete, second of zirconia, present certain conditions. This limits the applications throughout their matrix. These particles then of rhodium containing platinum alloys where slow down the processes of degradation

Platinum Metals Rev., 1987, 31, (2), 54-62 54 Fig. 1 During production of the discontinuous fibres used for heat insulation a steady stream of molten glass from an elec- trically heated feeder bushing is poured into a rapidly spinning basket. After hitting the of the basket the molten glass is thrown outwards by the cen- trifugal force, forming fine fibres as it passes through the orifices in the cylindrical wall of the basket. The stream of fine fibres leaving the basket and then falling downwards under the force of gravity can be seen on the left-hand side of the il- lustration Photograph by councry of Gvproc Insularion Limrtd

dramatically. They achieve this by pinning hence initial cost) for a given level of stress, dislocation networks formed during thermo- (iii) enhanced resistance to intergranular cor- mechanical processing, so inhibiting the move- rosion, ment of these dislocations to the grain boun- (iv) operation at higher service temperatures. daries. This process of dislocation movement is Zirconia grain stabilised versions of one of the steps that precedes grain boundary platinum, rhodium-platinum and gold- movement and competitive grain coarsening, platinum alloys are used extensively in many leading to the removal of the highly anisotropic areas of the glass industry. structure developed during the operations. As a consequence, softening occurs Glass Fibre Production and the ability of the materials to withstand The manufacture of continuous glass fibres stress with time at temperature is severely requires the use of a platinum containing com- diminished. With ZGS materials microstruc- ponent called a “bushing”. In essence, turd degradation is effectively restricted, giv- bushings are rectangular open topped boxes ing extended operating lives when compared to having bases with numerous precisely shaped conventional platinum group metals and alloys. orifices, called “jets”, through which the fibres Another benefit of ZGS materials is that of are drawn. In the process of glass fibre improved resistance to intergranular con- manufacture, molten glass or glass beads are tamination. This is achieved through retention fed from above into the bushing, heated to the of the high aspect-ratio grain structure, which operating temperature by direct electrical provides a long and tortuous route for in- resistance heating, and then drawn through the tergranular contaminants to travel. As a conse- jets in the baseplates. Individual glass fibres are quence, the critical intergranular crack length combined to form roving which in turn can be for failure is increased; this is shown in Figure woven into mat, and using a laying-up process 2. In summary, the use of ZGS alloys in the and a resin binder large high strength fabrica- glass industry gives the following advantages: tions can be made, for example car bodies. (i) greater strength with time at temperature, Chopped strands of roving can be added to a and hence longer service life, resin for use as a high strength moulding com- (ii) the ability to reduce cross-section (and pound or for local reinforcement of glass rein-

Platinum Metals Rev., 1987, 31, (2) 55 Fig. 2 When both conventional and ZGS platinum and its alloys are subjected to high temperatures, of say 14OO0C, while under stress, there is an opportunity for any surface contaminants to move into the material along grain boundaries. In the case of conventional alloys (left) the worked structure is replaced by equiaxed grains between which the impurities (shown in blue) can move readily, especially normal to the direction of the stress, so weakening the section. However, with ZGS materials (right) there is only very limited grain growth and the grains retain their elongate shape. Although the impurities again move along the grain boundaries the direction of movement is mainly parallel to the direction of stress, so the section retains its mechanical in- tegrity for very much longer

forced plastics. Yarn which is produced from hundreds of filaments twisted together is usual- ly processed into various kinds of cloth, tapes, sleeves, etc., and used for electrical insulation and pipe mouldings. For the glass fibre producer the Metals Group at Johnson Matthey has developed a pro- prietary process for the manufacture of baseplates with high precision and excellent metallurgical integrity. Using this process, some 5,000 baseplates have been produced in the United Kingdom and over 30,000 worldwide by Johnson Matthey associate houses. The method of baseplate manufacture re- quires the use of special equipment and tools of the highest precision and quality. This is necessary to ensure a high degree of bore con- centricity. If concentricity is bad then thinning of the wall in part of the section will occur and in operation a hot spot will be formed. During fibre production the hot spot will lower the viscosity of the glass at that point and it may flow back up the outside of the jets. This to flooding of the baseplate, as glass flows among the jets, and prevents fibre production. Single baseplates with up to 2,000 jets and hav- ing widths up to I 10 mm have been produced; using a modular construction far greater numbers of jets are possible. The jets can be aligned in longitudinal or transverse rows, and within each row the jets can be arranged in a rectangular format or with 6oo offsets. Such

Platinum Metals Rev., 1987, 31, (2) 56 Fig. 3 Typical stress data for conventional rhodium-platinum alloys, and for the Johnson Mat- they range of ZGS platinum alloys at 14OO0C, showing the superiority of the zirconia grain stabilised materials

arrangements allow for logitudinal or transverse The latest development to improve bushing cooling fins, which may be , or economics is the use of ZGS 5 per cent . The jets themselves may have bore rhodium-platinum, which has superior diameters from 1.1 mm up to 2.5 mm and mechanical properties to conventional 10 per lengths 2.25 times their bore diameters. Wall cent rhodium-platinum. The reduced initial thicknesses may be as low as 0.25 mm while jets cost of the alloy, resulting from the lower with thicker walls can be counterbored. rhodium content, has proved most acceptable Baseplates manufactured from platinum and as it has not reduced the life of the bushing. rhodium-platinum have been used for many By establishing a close relationship with glass years for the production of glass fibres having fibre producers and working with them over diameters in the range 25 to 10pm. Below this many years, Johnson Matthey have gained con- the wetting of platinum by glass results in siderable experience in baseplate and bushing poor glass flow through the fine jets, and thus manufacture. Johnson Matthey have now continuity and diameter are severely affected. licensed their fabrication processes to glass pro- Use of the non-wetting gold-platinum alloys has ducers in a number of countries. enabled fibres with diameters as fine as 6 pm to be produced economically and with bushing ef- Discontinuous Fibre ficiencies in excess of 90 per cent. The use of High quality fibres have a high recovery fac- ZGS platinum, rhodium-platinum and gold- tor such that following compression during platinum alloys has overcome one of the major packing, delivery and long term storage, the operational limitations of a conventional mat will recover to its original thickness and bushing, namely baseplate distortion. give optimum heat insulation. This necessitates Since the development of ZGS materials the use of a platinum or rhodium-platinum bushing lives have increased by over IW per spinner basket, which is used in conjunction cent, significantly improving the economics of with a platinum group feeder bushing. fibre manufacture. Through the introduction of The feeder bushing can be heated by electrical ZGS baseplates the use of high rhodium con- resistance, either directly or indirectly through tent alloys has become less necessary. Addi- the use of a rhodium-platinum coil situated tionally, the higher strengths of the ZGS around the orifice . The purpose of the materials have enabled lighter bushing bodies, feeder bushing is to provide a steady stream of with thinner wall sections, to be used. glass of homogeneous composition and closely

Plarinum Metals Rev., 1987, 31, (2) 57 baskets has been declining recently due to the use of modern high strength superalloys, which were introduced for economic reasons. However, the life of superalloy spinner baskets may be limited by erosion and distortion of the orifices, resulting in a reduction in the yield of dimensionally acceptable fibres. An improve- ment is achieved using a platinum-containing alloy, so most insulation fibre manufacturers do retain platinum group metal spinner baskets in their production lines. Through the introduction of ZGS rhodium- platinum alloys the operating lives of spinner baskets have been improved significantly. The added strength helps to reduce bowing of the basket walls during service. ZGS rhodium- platinum feeder bushings similarly have been introduced where the combination of high strength and resistance to contamination has in- creased operating lives. Optical Glass Production Glass for use in optical applications must be Fig. 4 By means of high speed photography it of extremely high quality. In this case the quality is possible to record the glass fibres leaving a determining factors are consistent homogenei- spinner basket and then falling downwards, as gravity overcomes any residual centrifugal force ty, a high degree of transparency and high Photography by counny of Cypm lnsulmian timired transmittance of light. Transparency is affected by coloration and controlled viscosity, from the forehearth to the bubbles, seed and blisters within the glass. spinner basket. Many of these problems were associated with Several different licensed methods of produc- the use of refractories within critical areas of the ing insulation fibre find applications glass manufacturing process. Through the throughout the world. The essentials of all selective introduction of platinum into the key these methods rely on a spinning basket bet- areas of melting, conditioning and forming, ween 150 and 600 mm in diameter whose high quality optical glass can be produced. walls contain accurately drilled orifices. Glass Pure platinum and ZGS platinum are the from the feeder bushing strikes the base of the preferred materials since, as stated earlier, the spinning basket and is then thrown outwards use of rhodium-containing alloys leads to col- and upwards whereupon it passes through the oration of the glass. orifices due to the centrifugal force. The In the melting furnace, platinum is used for streams of fibres produced are then broken up bubbler tubes, and, occasionally, as either by a second rotating spinner or by an at- a lining material in small furnaces. Skimmer tenuating flame, depending on the process blocks which remove surface debris from the employed. Fibres of glass, typically between 9 glass prior to conditioning are common, and and 25pm, then fall under gravity onto a con- consist of a refractory clad with platinum veyor belt where they are agglomerated through sheet, typically 0.5 mm thick. However with addition of a binder. ZGS platinum a typical cladding thickness The use of platinum group metals for spinner could be only 0.3 mm, giving a significant re-

Platinum Metals Rev., 1987, 31, (2) 58 duction in the weight of the platinum employed. high quality domestic ware. In order to achieve Optical glass is conditioned to remove bub- consistently high quality, platinum melting, bles, seed and blisters, to ensure homogeneity conditioning and processing equipment is now of composition and to give close control of used throughout the industry. In many cases temperature, all of which are crucial to the the low throughput of glass by crystal manufac- quality of the product. The most common turers dictates that batch melting techniques method of achieving these aims is through the are used. In these types of operation large use of a refining or fining system made from platinum , often up to 500 mm in platinum. Generally tubular in section, the diameter, are employed for the melting stage. design of these items is proprietary information The use of ZGS platinum has extended and varies from plant to plant. Following the operating lives, increased resistance to attack signing of non-disclosure agreements with glass from contaminants and in addition, in some producers, Johnson Matthey have manufac- cases, it has enabled the weight of the tured many such items. to be reduced. Larger crystal glass manufac- At the forming stage in the manufacturing turers use small continuous furnaces and in process secrecy again limits the amount of free- these cases platinum linings are often used. ly available information. However, platinum is Conditioning in platinum finers is carried out used for flow control devices, stirrers and gobb- in a similar manner to that for optical glass pro- ing units, most of which are made of clad duction. Again the objective is to produce glass refractory. for the forming operation that is homogeneous In summary, the high quality of modern op- with respect both to composition and tem- tical glasses results from significant use of perature, as well as being free from bubbles. platinum and ZGS platinum in key areas. At the forming end a plunger operating above an orifice ring controls the size of the glass gob, Crystal and Tableware Glass and rotation of the plunger ensures constant Production temperature throughout the gob. The orifice Crystal glass generally relies upon the addi- ring is made from platinum and the plunger is tion of to give it high transparency, generally a piece of refractory material clad colour purity, a high refractive index and to in- with platinum. crease its . It is a combination of these In the case of tableware the use of platinum factors which makes crystal glass such a is generally confined to the forming end of the desirable material for cutting and faceting in process where pressing or blowing takes place.

Fig. 5 With good insulation, direct electrically heated platinum forehearthe provide an eficient means of produc- ing consistently high quality optical glasses

Phoiqlrsph by COUK~~of EGLASSTREK GmbH

Platinum Metals Rev., 1987, 31, (2) 59 Reaction of Platinum with Miscellaneous Substances (5)

Substance Results of reaction, and/or comments

Sodium or potassium carbonate, fusion Only very small losses occur during mixture, alkali bifluorides, normal fusions Sodium nitrite or nitrate Up to 1-2 mg loss on fusion Alkali chlorides, alkaline earth chlorides Attack on metal above 1000°C, especially at air line Alkali bisulphates Attack occurs above 7OOOC Magnesium pyrophosphate Attack occurs above 900°C Alkali , peroxides, hydroxides, When fused will rapidly attack platinum sulphides Sulphuric acid, hydrofluoric acid, alkali Evaporations may be carried out with hydroxides, little loss No attack if oxidising agents absent Concentrated phosphoric acid or P,O, Noticeable reaction on prolonged heating Hydrochloric acid + oxidising agent (e.g. Rapid attack ) Fused Platinocyanides formed , , phosphorus, All attack platinum , antimony, lead, tin, , bismuth, etc. oxide, lead or bismuth oxides Reaction occurs above 1200-1 25OOC Air, , oxides of Negligible effect nitrogen, and vapour, dioxide , sulphur dioxide, , These atmospheres cause attack on volatile chlorides platinum Hydrogen, carburetted gases or Encourage reduction of a wide range of vapours, luminous gas flames, reducing substances at elevated temperatures. gases Must be avoided Organic ignitions, coal Burn off carbonaceous materials at as low a temperature as possible with access of air More stable oxides, AI,O,, MgO, TiO,. Ignitions possible without attack ZrO,, etc. ZnO, Co,O,, NiO Ignitions should be carried out in CdO and other reducible oxides oxidising conditions Silica, silicates Attack under reducing conditions above 1ooooc

The supply of glass to a pair of shear blades is glasses differ considerably with their composi- sometimes through a rhodium-platinum down- tion. In the case of (heat-resistant) glass draw tube. This conditions the glass to some manufacture, extensive platinum cladding is extent but also controls the flow to the forming necessary at the forming machine supply end of machine. the forehearth. With other glasses, platinum The corrosive and erosive properties of cladding of such as molyb-

Platinum Metals Rev., 1987, 31, (2) 60 denum is necessary only at or around the only an approximation of sub-surface tem- gladair interface. perature and viscosity can be made. A far better The use of a paddle plunger or a gobbing stir- temperature measurement and control arrange- rer clad in platinum or rhodium-platinum is ment is made possible by the use of immersion often used in high output operations. These in- in the furnace and forehearth. clude feeds to independent section presses or These assemblies contain multiple thermo- spinning machines used to produce tableware couples which enable temperature to be and stemware, such as wine glasses. measured at various depths within the glass. The temperature and aggressive chemical con- Temperature Measurement ditions require that these assemblies are In all sectors of the glass industry sheathed below the glass line with a rhodium- temperature measurement and control is of ex- platinum alloy. treme importance. With temperature change, ZGS 10per cent rhodium-platinum is prefer- glass viscosity and flow rate can vary red for this use because of its excellent con- significantly and the subsequent control of glass tamination resistance, and its ability to function forming and processing operations is impaired. at high operating temperatures. Additionally, Gob size and homogeneity are, for example, of the high strength of the alloy means that only prime importance in the production of crystal thin sheaths, typically 0.5 mm thick, are need- glass and precise temperature control is crucial ed to protect the alumina. in the control of these factors. The use of ZGS 10 per cent rhodium- The use of standard alumina sheathed platinum has proved extremely successful even platinum-rhodium/platinum thermocouples for in the most chemically aggressive glasses, for measuring furnace temperatures has example the glass which is used exten- been common for some time. However, the hot sively for container manufacture. atmosphere above the molten glass often con- tains exhaust gases from of the fuel Other Glass Industry Applications and vapour phases from the batch, either of Two-ply glass tubing is used for the manufac- which can attack the alumina sheathing and ture of low pressure sodium vapour lamps. affect accuracy. This problem is Here a 10 per cent rhodium-platinum mandrel compounded by the increasing number of is used to bring a controlled thickness of special refractory materials used in furnace lin- sodium-resistant glass into contact with the ings and by some of the more exotic glass addi- outer glass at a predetermined rate. tions which can be incompatible with alumina. In the aerospace industry limited use is made The use of 10 per cent rhodium-platinum as a of glass which is coated with thin films of sheathing material over the alumina had proved platinum and rhodium. to be successful in overcoming these problems. In the case of oil fired furnaces the presence of Conclusion traces of sulphur-containing gases can lead to The platinum group metals and alloys possess direct attack on platinum group metals, or to chemical and physical properties which occur the reduction of refractory oxides which can in unique combinations that make them par- then cause attack. The use of ZGS 10 per cent ticularly suitable for many applications in the rhodium-platinum as a sheathing material has glass industry. significantly reduced the number of failures As glass manufacturers have striven to im- due to contamination. prove the quality of their established products, Measurement of molten glass temperature is to achieve economies in production and to an area where radiation pyrometers are used ex- satisfy the demand for new materials, the tensively. However, such systems only measure platinum suppliers have contributed improved the temperature of the glass surface and as such fabrication techniques and alloys with superior

Platinum Metals Rev., 1987, 31, (2) 61 properties. In particular the use of ZGS References platinum and alloys has given improved service I G. L. Selman, J. G. Day and A. A. Bourne, life, reduced metal inventories, and improved Platinum Metals Rev., 1974, 18, (z), 46 z G. L. Selman and A. A. Bourne, Platinum Metals resistance to contamination, thus extending the Rev.,1976, 20, (31, 86 applications for platinum in the glass industry. 3 R. B. McGrath and G. C. Badcock, Platinum The continuing close relationship between sup- Metals Rev., 1987, 31, (I), 8 and users will present further oppor- 4 A. E. Heywood and R. A. Benedek, Platinum Metals Rev., 1982, 26, (3), 98 tunities for progress to be made in glass 5 A. G. Knapton, Platinum Metals Rev., 1979, 23, production technology. (I), 2 A Study of Platinum Alloy Welds THE EFFECT OF HYDROGEN CONCENTRATION Considerable experience of the use of Russian journal Avtomaticheskayu Svarka being platinum alloys for glass-melting apparatus has sponsored by the British Library Lending Divi- now accumulated, and this shows that a key sion, and published by The Welding Institute, factor determining service life can be the in- Abington, Cambridge, England. tegrity of the alloy in the proximity of welded joints. Brittle fracture of platinum alloys can Solidification in Microgravity result from contaminants entering the weld during fabrication or use, and a recently Gravity induces significant disturbances in reported investigation by 0. D. Smiyan, B. I. gaseous and liquid systems. These disturbances Shnaider, D. M. Pogrebiskii, L. A. Potapenko are manifested in the form of buoyancy, and E. I. Butkova of the E. 0. Paton Welding sedimentation, thermal convection and Institute of the Ukraine SSR Academy of hydrostatic pressure. All these phenomena af- Sciences has considered the effect of welding fect the solidification characteristics of liquid technique on crack initiation and development metals and alloys. By carrying out a solidifica- (Avt. Svarku, 1986,395, (2), 10-12, 29). tion process in a microgravity environment, The alloy used was platinum-20 rhodium-Io gravity-induced disturbances are virtually palladium-o. I -o. I gold, in the form of eliminated and the structures produced a disc of 0.5 mm thick sheet which had been an- are unique in uniformity of composition and nealed for 30 minutes at 10ooOC.Concentric properties. welds were made by plasma micro-welding, Undercooling experiments have been carried either under an argon +hydrogen atmosphere out on a number of refractory elements, in- or without a protective shielding, while the cluding platinum, rhodium, iridium and perimeter of the disc was held rigid. , in the free-fall microgravity Microstructure, hardness and composition (Io-6g,,; IXIO-~Torr) environment of the were examined and it was concluded that this drop-tube facility of the Marshall Space Flight alloy is highly resistant to crack formation dur- Center by W. H. Hofmeister, H. B. Robinson ing welding, that the use of the protective at- and R. J. Bayuzick (Appl. Phys. Lett., 1986~49, mosphere does not affect the results, and that (201, I342-!344). there is a relationship between welding se- Both platinum and rhodium achieved an ex- quence and the hydrogen distribution. Under pected cooling rate of between 17 and 20 per unfavourable conditions the local hydrogen cent of their melting temperatures whereas concentration can vary by more than 2: I, and iridium and ruthenium undercooled between may account for the reduced strength of some 10 and 13 per cent. The latter effect was welded joints. associated with heterogeneous nucleation caus- It is suggested that the formation of concen- ed by impurities. A high degree of undercool- tric welds, where the stresses are expected to in- ing on solidification is an important crease as the diameter of the circular weld phenomenon in the production of amorphous, increases, is a convenient way of evaluating the single crystal and metastable phase structures. weldability of a material. Clearly microgravity research is providing an An English language translation of this paper understanding of material behaviour which will appears in Automatic Welding, 1986, 39, (2), lead ultimately to new Earth- and Space-based 13-1 5; this cover-to-cover translation of the products and processes. I.R.M.

Platinum Metals Rev., 1987, 31, (2) 62