100 years of doped tungsten wire

Int. Journal of Refractory Metals and Hard Materials 28 (2010) 648–660

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M

Review

100 years of doped tungsten wire P. Schade HTM Consulting Berlin, Germany

a r t i c l e

i n f o

Article history: Received 4 May 2010 Accepted 5 May 2010 Keywords: Tungsten History Coolidge process Wire properties Potassium bubbles effects Light sources

a b s t r a c t From a historical point of view, the development of the PM processing steps and tools “for making tungsten ductile” by William D. Coolidge in 1909 marks the breakthrough for the usage of tungsten filaments in the lighting industry and the beginning of the industrial era of modern Powder Metallurgy. Some important technological developments before introducing the Coolidge process will be described briefly (Just and Hanaman procedures, Kuzel process, Pintsch method) together with the corresponding implications for today's modern technologies and materials (Sol-Gel, CVD, MIM, ODS alloys, W-RE welding electrodes). With regard to the Coolidge process some always recurring misunderstandings, especially of the doping process, will be corrected. In addition, some accompanying discoveries and inventions (Tungsten Heavy Metals, Gradient Materials, Cemented Carbides) will be mentioned too. Finally, the scientific importance of the potassium bubbles as the strongest pinning points at highest temperatures against the movement of dislocations and grain boundaries will be highlighted shortly. Considering geometrical dimensions, the microstructural features of the finest wires and the corresponding fabrication of diamond dies, necessary for the deformation of wires, also represent precursors of today's nanotechnology and micromachining. © 2010 Elsevier Ltd. All rights reserved.

Contents 1. Introduction . . . . . . . . . . . 2. Early metallurgical attempts. . . . 3. The Coolidge process . . . . . . . 4. The invention of hardmetals. . . . 5. The scientific background of doped 6. Conclusions and outlook . . . . . Acknowledgements . . . . . . . . . . References . . . . . . . . . . . . . .

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1. Introduction The invention of electrical lamps with carbon threads by T. A. Edison and J. W. Swan in 1879 promised a new era in electrical lighting. The history of tungsten wire processing is coupled strongly with the further development of this industrial branch. From the beginning, the different carbon filaments used in all the early lamps had low light outputs, short lives due to poor vacua because of the limited technical possibilities, and showed blackening of the bulb wall as well as pronounced brittleness.Therefore, the

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search for better incandescent materials started soon resulting in the “gas mantle lamp” of Carl Auer von Welsbach 1883 [1] and the socalled Nernst lamp in 1898 [2]. In tribute to the 150th anniversary of the birth of C. Auer von Welsbach (1858–1929), the Austrian Mint has released a bi-metallic Silver/Niobium coin entitled “Fascination Light” in 2008. The value side of the coin shows a nostalgic gas lantern lighter in front of the Town Hall of Vienna, and the back-side a halfportrait of Carl Auer von Welsbach as well as a series of light sources (Fig. 1). Today, a lot of historical gas lanterns are burning all over the world, for example more than 40,000 in the old quarters of Berlin, including also a museum of nostalgic gas lanterns in the so-called “Tiergarten” quarter, and in a number of further German towns, but also in London and Boston (Beacon Hill quarter), in Prague (Charles

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meeting with C. W. Röntgen in 1899, he developed also an improved version of an X-ray tube in 1913, the so-called Coolidge tube. The present review will focus on the historical and scientific aspects of tungsten wire processing. In the first part of the paper the early metallurgical attempts to make tungsten filaments for the lighting industry and their impact on today's modern materials and technologies are discussed. In the second part the Coolidge process and the modern scientific background of the nanosized potassium bubbles is described. Finally, the third part describes new challenges of the materials science of tungsten and gives an outlook for the perspectives of tungsten wire considering new technical and political developments of light sources. 2. Early metallurgical attempts

Fig. 1. Auer von Welsbach coin, Austria, 2008.

Bridge) and last but not least in Althofen (Austria), the birth place of C. Auer von Welsbach (Fig. 2). The further progress, and in particular the introduction of tungsten as the filament material, is from the today's point of view already a modern interdisciplinary development involving a combination of scientific insight, increasing materials understanding and process technology knowledge, intuition, and above all also commercial demand, with the pioneering work taking place in the first decade of the last century. Since the beginning of the 20th century, tungsten has illuminated the world more and more, and the use of tungsten filaments in light bulbs has become very familiar to us, in particular in domestic lighting. The process that is now generally used by companies throughout the world to make ductile tungsten wire corresponding to the different customer needs is generically referred to as the Coolidge process, in honour of its developer, William D. Coolidge. In addition to the importance of the tungsten technology for the light sources industry, the different processing steps have also formed the basis for progress and success of the today's modern powder metallurgy for a multitude of further materials. Possibly, due to the stay of Coolidge in Leipzig and a

Fig. 2. Auer gas mantle lamps, Berlin (Tiergarten).

After the Auer gas mantle lamp and the Nernst lamp using ceramics as incandescent elements the so-called “squirted” Osmium lamps followed. The first metallic filament lamp was developed by C. Auer v. Welsbach in1898 [3]. Some years later, new lamps with drawn Tantalum wires were introduced by W. von Bolton and O. Feuerlein in 1902 [4]. It is very remarkable that the drawing technology at that time already developed wires with diameters down to about 50 μm for a high melting metal. The Tantalum lamp was the first metal filament lamp which really reached the commercial stage, substituting largely the German Osmium lamps. Also General Electric acquired the exclusive rights to manufacture the Tantalum lamp in the United States [5]. However, the high evaporation rate of early osmium filaments and the brittleness of tantalum wires when using alternating currents induced strong disadvantages. It is interesting to read that W. D. Coolidge, starting his career at GE, was working intensively about the brittleness phenomena of the imported German tantalum wires during his first year at GE in 1905. Nevertheless, it is surprising that, as late as 1912, the known steam-ship “Titanic” was outfitted entirely with Tantalum filament bulbs. But already in 1903, A. Just's and F. Hanaman's [6] first tungsten filaments marked a new era of modern lighting by increasing the efficacy of carbon lamps from about 3.2 lumen per Watt (lm/W) to about 7.9 lm/W. Also the efficacy of the first metallic filament lamps (osmium and tantalum) with about 6.3 lm/W was clearly surpassed. The reason why tungsten was not earlier investigated was, possibly, a false melting point information concerning tungsten [7] in comparison to osmium and tantalum and, especially, the description that tungsten appeared to be inherently brittle. As late as 1912, M. Pirani and A. R. Meyer of the Siemens and Halske Metal Laboratory (some years later part of the newly formed OSRAM study group in Berlin) had determined the melting point of tungsten with a value of 2965 °C (about 450 °C below the true value of 3420 °C) [8]. The first patent of A. Just and F. Hanaman [6] presumably goes back to earlier investigations of A. de Lodyguine [9] as well as to the C. of Auer von Welsbachs squirted osmium filaments [3]. They used the heating of a carbon-filament in a mixture of a tungstenoxychlorideand H2-atmosphere resulting in tungsten deposits on the carbon surface as well as the following formation of brittle tungsten carbide in the core (Fig. 3). The transformation of tungsten carbide to tungsten at high burning temperatures in the lamp as well as the intentional annealing of the filaments in moist H2 resulted finally in still brittle, but pure tungsten tubes. Here, we can find the origin of modern CVD processes, the basis for the today's standard annealing technology of black “graphitized” tungsten wire as well as the background of the modern tungsten halogen lamps, introduced about 50 years later by E. Fridrich and E. Zubler [10]. The next milestone was reached only 1 year later in 1904, again by A. Just and F. Hanaman [11]. According to their new patent, recognizing already the role of carbon in increasing the brittleness of tungsten metal, compounds of tungsten were mixed with carbonless binder, avoiding the earlier binder mixture of sugar and gum, to build a paste. The paste

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Fig. 3. First tungsten coated carbon lamp, 1905. Fig. 4. Squirted tungsten lamp, 1910.

was then extruded through fine diamond dies, wound into wire loops, cut into “hairpins” and then heated to a red heat in a suitable atmosphere to remove the binder. Each “hairpin” with straight segments was then mounted in clips, and heated by the passage of an electric current in hydrogen atmosphere. At very high temperatures the fine tungsten particles sintered together and formed a solid metallic tungsten filament skeleton (Fig. 4). These filaments, although elastic, were quite brittle, but they could be formed to shape a hairpin or loop at a red heat [11]. The method is similar to the squirted osmium-procedure of Auer von Welsbach [3]. However, the patent was granted in 1904 [11] and rights were bought by the German Auer-Gesellschaft (DGA). In Hungary, too, tungsten filament lamps were produced in Budapest at the Egger Company, the later TUNGSRAM Company according to the Just and Hanaman patents. In 1906 the new Tungsten lamp according to the Just and Hanaman patent was demonstrated in Berlin to a delegation from the General Electric Company of New York. Soon afterwards the DGA and GE signed a patent licence agreement for tungsten filament lamps and a know-how exchange agreement for lamps with extruded (“squirted”) metal filaments. The agreement gave GE the exclusive patent rights for America. From 1905 until about 1911, the majority of tungsten filaments were practically produced by this method, although a number of different alternatives were established within the research laboratories of the lighting companies. In the U. S., William D. Coolidge introduced an amalgamated tungsten filament consisting of mercury, cadmium, and finally additional bismuth as a binder for tungsten powder in 1906 [12]. The mixture was also squirted as usual through a die, after which the binder was removed by applying high temperatures. The bonding of the tungsten particles was performed by passing an electrical current through the filament. However, the filament was still too brittle. Coolidge discovered in the following that less brittle filaments made by his amalgam process could be flattened by pressing them between heated steel blocks or by passing them through a mill with heated steel rolls.

This can be already considered as the basics for the later successful thermo-mechanical treatment of sintered tungsten ingots. After 3 years of concentrated R&D, Coolidge and his colleagues finally succeeded in producing ductile amalgamated tungsten that could be drawn through diamond dies down to 0.249 mm [12]. It is very interesting to note that at the same time between 1906 and 1910 also patent licence agreements [5] were made between the German Auer-Company and the General Electric Company in the U.S., leading to direct support of an Auer team for the installation of the first equipments for the manufacture of tungsten according to the Just and Hanaman patents in Schenectady, NY. As a sign of gratitude for help given in setting up the American tungsten lamp factory, the colleagues of GE gave the DGA, some years later, the rights to use their swaging and drawing process and the associated patents of Coolidge. This procedure was repeated in the late 1970's when specialists of the German Osram GmbH had supported the installation and introduction of the Kocks mill operation for tungsten deformation at GE in Cleveland, Ohio. Early in the last century a lot of further developments were made which were not very successful in the lighting industry but impacted some important later developments in material's technology: - 1905: H. Kuzel “Squirting process of tungsten powder paste with sugar and gum as binder as well as decarburization in an H2/H2O atmosphere”, [13], - 1905: H. Kuzel “Tungsten colloid process without binder”, [14], - 1906: Siemens and Halske “Filling of Cu tubes with fine tungsten powder, heating as well as deformation by rolling or drawing”, [15], - 1907: AEG “Development of tungsten–nickel (2–4%) for drawing tungsten filaments including the following evaporation of nickel by annealing in vacuum”, see [16], - 1907: J. Pintsch “Development of single crystal process from squirted thoriated tungsten by zone/gradient annealing”, [17]. Some examples of the early lamps, all with straight filaments due to the brittle nature of tungsten are depicted in Figs. 4 to 5. As an exotic example, Fig. 6 shows also a picture of the centennial carbon

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have used the squirting process with the main disadvantage of limited ductility for the further handling of filaments and lamps. Furthermore, the Siemens and Halske patent [15] can be considered as a precursor of the deformation processes of today's modern superconductor wires. The AEG development of a tungsten–nickel pseudo-alloy was the early precursor of the Heavy Metal Alloys introduced 1935 in production [16]. The Pintsch process [16] formed the basis for the later single crystal technology and today's gradient anneals (see Table 1). Furthermore, the Pintsch process, and the development of the Pintsch single crystal filaments from thoriated tungsten were the main reason for invalidating the Coolidge patent in 1927 (see next paragraph). 3. The Coolidge process

Fig. 5. Sirius colloid lamp, 1910.

light bulb of the Fire Department in Livermore, CA, with very pronounced deformations of the filament due to creep processes. In addition, Fig. 7 schematically shows the principles of the “squirting process” [18] as well as of the “gradient anneal” according to J. Pintsch [19]. Squirted thoriated tungsten wires have been treated in a temperature gradient at an annealing temperature of about 2200 °C inducing the growth of long single crystalline fibres. From a historical point of view, the so-called “squirting” technology of C. Auer von Welsbach [3] as well as of A. Just and F. Hanaman [11], the colloid process of H. Kuzel [14] and the amalgam process of W. D. Coolidge [12], formed the basis of the today's modern MIM (Metal Injection Moulding) technology. All these technologies

William D. Coolidge (1873–1975), Fig. 8, began his career at the General Electric's Research Laboratory in September 1905. It is interesting to read that Coolidge's first assignment was to investigate why the German tantalum lamp filaments quickly broke when operated on alternating current, [see 20], possibly, due to the limited technical possibilities for lamp vacua as well as to residual gases from the bulb. At the same time, Coolidge worked also, due to the experience about the deformation of amalgamated tungsten, with pure tungsten powders and brittle sintered tungsten bars. The big breakthrough took place in 1909, when Coolidge and his team of the GE were successful in producing ductile tungsten filaments by suitable mechanical deformation at elevated temperatures and corresponding intermediate heat treatments. In Coolidge's lab note book entry for July 16, 1909 one may find [21]: “Swaged tungsten (3/8"’ square) down to 128 mils; then to 78 mils; then to 53.5 mils — then bent it cold”. Furthermore, it must have been a yet more exciting day on September 27, 1910 when Coolidge recorded in his Notebook [16]: “Reeling tungsten wire, to handle long lengths”. Today, the ability to handle tungsten wire of high spool weights (for example 4.5 kg to about 18 kg in comparison to the early 0.250 to

Fig. 6. Centennial carbon light bulb, Fire Department House, Livermore, CA and old fashioned first Mazda Coolidge lamp, 1911 (right).

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Fig. 7. Squirting process of Just and Hanaman (left), see [17] and the gradient anneal according to Pintsch (right), see [18].

0.300 kg) and coiling filaments without breakage at very high strain rates (up to 40,000 rpm) are the backbone of the incandescent lamp industry. Coolidge began filing patents in 1909, first on dies and die supports, and was awarded later the patent for ductile tungsten [22] on December 30, 1913. Fig. 9 shows some drawings of the patent. Ductilization was initially the most important part of the Coolidge patent. GE granted licences to several companies. However, Coolidge's 1913 patent was challenged by the Independent Lamp and Wire Co., Weehawken, New Jersey. During a 1927 litigation in the Delaware US district court, Judge Morris ruled Coolidge's ductilization claims invalid because it was not an invention as defined by patent law, see [23]. Table 1 Important metallurgical inventions of the first decades of the 20th century. Year

Inventors

1898 C. Auer von Welsbach 1902 W. v. Bolton/ O. Feuerlein 1903 A. Just and F. Hanaman 1904 A. Just and F. Hanaman 1905 H. Kuzel 1907 J. Pintsch 1907 AEG 1909 W. D. Coolidge 1911 C. G. Fink

1913 I. Langmuir 1913 W. D. Coolidge 1922 A. Pacz 1922 H. Baumhauer 1923 K. Schröter 1931 P. Tury/ T. Milner 1936 W. Geiss

Items

Impact

Squirted osmium wire

Modern MIM-technology

Drawn tantalum wire

Drawing of refractory metals CVD-technology/halogen cycle process (lamps) MIM-technology

Tungsten coated carbon threads Squirted tungsten wire Tungsten colloid process Gradient anneal of thoriated tungsten wire Tungsten–nickel process PM tungsten processing

MIM-technology Ductile single crystal wires

Iron–Nickel–Copper Composite Material (lead-in wire) Langmuire sheath and thermoemission Coolidge X-ray tube

Lighting industry, electrical devices

Potassium silicate doping of tungsten oxide Infiltration of tungsten carbide with iron Cemented carbides (tungsten carbide with cobalt) AKS-doping of tungsten

First hard metal dies

Coiled coils of tungsten

Tungsten heavy metals Ductile tungsten wire

Tungsten filament coiling, gas filling of lamps Medicine “microalloying”

Liquid-phase sintering Modern industry branches

Fig. 8. William D. Coolidge, 1927.

Especially, experimental evidence had been introduced by the defendant, that thoriated single crystal tungsten wire of the Pintsch type, which had not been subjected to the Coolidge process, was in fact ductile (i.e. drawable) at room temperature. The court adopted the following legal position: “Ductility is inherent in tungsten. ‘Coolidge tungsten’ is not a new metal; it is a discovery but not a patentable invention. Ductility in tungsten can not be produced by any means, no matter how difficult or ingenious: ductility is there all the time.” Before that, the House of Lords in England had rendered a similar decision on the British Coolidge patent. However, the general ruling, that the discovery of a property is not a manufacture and therefore not patentable, does not diminish the important technological significance of the Coolidge process. In the progress of the development of the tungsten technology, Coolidge realized very soon that the source of the tungsten oxide played an important role in determining the final properties of the wires and filaments. He realized as early as January 1910 [22] that when his tungsten oxide had been heated in special clay Battersea-type crucibles, the filaments would have longer lives [19] due to increased resistance to grain-off-setting and sagging of the filaments. Although Coolidge was unable to determine the exact nature of the substance that had been absorbed into the tungsten oxide (he speculated on alumina and silica), the influencing positive effect of minor amounts of foreign elements was clearly recognized. This was the “birth” of the today's modern doping process, although the reason why these crucibles created this benefit was not clear for many years until progress in chemical analysis down to the parts per million-range (ppm) became available. In addition, Coolidge also experimented with other refractory materials as additives, as the following quote from his patent indicates [22]:

High temperature strength High performance lamps

“I have succeeded in preventing offsetting in drawn tungsten filaments without using the Battersea crucible process by mixing

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Later, in 1910, to quote from F. Blau of Deutsche Gasglühlicht AG, which was now producing tungsten lamps under licence from the General Electric Company, after further interviews with W. D. Coolidge, see Liebhavsky [21]: “Now even, after I know the procedure quite exactly, I can say that I have never met with any other substance whose physical properties are so gradually, fundamentally changed by a process of mechanical working as is the case with tungsten.”

Fig. 9. Drawings of the Coolidge patent, 1913.

with the tungsten powder certain refractory materials, such as the oxides of thorium, zirconium, yttrium, erbium, didymium or ytterbium. I have used with especial success thorium nitrate, which gives thoria when decomposed.” The importance of these early findings can be seen in the today's modern ODS alloys (Oxide Dispersion Strengthened) and also in the thoria free tungsten electrodes for discharge lamps and Rare Earth (R.E.) tungsten welding electrodes. When on a visit to Berlin in 1909 W. D. Coolidge demonstrated his first ductile tungsten wire in the form of a small spool to F. Blau, technical director of the formerly Auer-Gesellschaft (one of the largest tungsten lamp producers in Germany at that time) who had also been investigating the problem of brittleness. Coolidge at first was confronted with disbelief which was followed by enthusiasm [16,24]: “I (Coolidge) remember this circumstance very well because of the excitement and surprise and incredulity which he (Blau) manifested at the time. He asked me over and over again what it was. I told him it was pure tungsten wire, only to have his question repeated again and again.” In the meantime, the spool was also tested in the chemical Laboratory of F. Blau, confirming only some time later that it really consisted of tungsten.

The main characteristic steps of the Coolidge process are the following (technology terms in square brackets were not originally developed and introduced by Coolidge): selection of precursor material APT (ammonium-paratungstate), respectively, yellow oxide (WO3), [tungstic acid (H2WO4), blue oxide (W20O58) ]–[KS-doping, AKS-doping]–H2-reduction to metal powder–[HF washing]–pressing and presintering–direct sintering–[rolling]–swaging–drawing–[coiling]. Schematically, the process is illustrated in Fig. 10. The specific properties of the new tungsten wires, especially the excellent bend ductility, and the fundamental work of I. Langmuir [25] concerning convection, conduction and radiation of heat in gases, leading to the formulation of the known “Langmuir sheath” of hot stationary gas surrounding a glowing filament, have initiated the use of an inert gas filling and the adapted development of tungsten coils. Already 1913 marks the year of introducing gas filled lamps with further increased lamp efficiency. Some years later, an intentional doping method with the expressed aim of getting tungsten wire of non-sagging and nonoffsetting quality corresponding to the specific Battersea quality of Coolidge had been developed in the US by A. Pacz of GE in 1917 [26]. It was based on Coolidge's method [22], but the new starting material was now tungstic acid doped with potassium- and sodium-silicate. Pacz called this metal “218”, which is even today in use at GE, although the precursor material has been changed to “tungsten blue oxide” in the mean time. The following story is told that the wire resulted after the 218th experiment. However, another explanation is that the title No. 218 was derived simply from the serial number of the lot of raw tungstic acid used in the first production of the new material by Pacz. Only just more than 10 years later, the 3rd doping element, aluminium, corresponding to the today's modern AKS-doping, was introduced by P. Tury and T. Millner at TUNGSRAM in 1931 [27], resulting in the so-called GK-material (gross kristallin) with coarse crystalline microstructure after the recrystallization anneal. It is interesting that the Hungarian patent [27] is only acquired by GE and granted in the US after a visit of I. Langmuir at Budapest in the 1930s when the Hungarian researchers had demonstrated the high ductility of the GKtungsten wire after high-temperature anneals. Although there were more problems occurring during the drawing process, the resulting higher recrystallization temperatures and the increased high-temperature creep strength were very advantageous. Nevertheless, this new material quality was the prerequisite for the introduction of the today's coiled coils (Fig. 11) by W. Geiss in 1936 [28]. In addition to the first developments of W. D. Coolidge's “ductile” tungsten process (1909), there were further important steps such as C. G. Fink's DUMET lead-in wire consisting of an iron–nickel core with copper cladding, the first composite material with gradient properties adapted to the properties of bulb glasses and substituting platinum wire (1911), the development of the Coolidge X-ray tube (1913) as well as A. Pacz's doping procedure (1917). 4. The invention of hardmetals The next important milestone in the chronology of tungsten is the year 1923. It marks the invention of cemented carbides or hardmetals, combining tungsten carbide (WC) and cobalt powder by mixing,

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Fig. 10. PM processing steps of tungsten. Courtesy of E. Kimmel, Towanda, PA.

pressing and liquid-phase sintering by K. Schröter [29], chief engineer at the OSRAM study group in Berlin, Germany. The corresponding patent was submitted on March 30, 1923 which was granted on Oct. 30, 1925. Karl Schröter is stated as the only inventor, but he has never let anyone doubt that the success was the result of the whole OSRAM study group, headed in this time by Franz Skaupy [16]. But, at nearly the same time, a promising technique for the production of wire drawing dies was developed and practised in another part of Osram (wire department in Berlin-Charlottenburg) for internal use by H. Baumhauer, namely the infiltration of a sintered tungsten carbide skeleton with iron [24]. However, the infiltration process was not commercially exploited because it was overtaken by the process that finally was the most successful, namely the liquid-phase sintering of tungsten carbide and cobalt powders, leading to today's cemented carbides or hardmetals.

Initially, the primary interest of OSRAM to look for hard alternatives to the abrasive steel and expensive diamond dies for the drawing process used in the production of tungsten wire was solved. Field tests with the new die material in the wire departments of OSRAM Berlin in 1923 were very successful and diamond dies were replaced quickly down to 0.3 mm, the OSRAM Management decided, because of noncompatibility with regard to the lighting business, to sell the licence rights. Consequently, the patent rights were bought by the Friedrich Krupp AG in Essen, Germany at the end of 1925. However, patent rights were acquired also by the General Electric Co. in the US for the production of “Carboloy” grades introduced in1928 [16]. Already at the Leipzig Spring Fair in 1927 the new material named WIDIA (referring to the German Words “Wie Diamant” — i.e. like Diamond) started a worldwide success story. This development was to have a revolutionary impact — not only on tungsten. Today, the

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Fig. 11. Tungsten coiled coil. Fig. 12. Tungsten crystal growth in halogen lamps (M = 600:1).

cemented carbide production forms the main usage of tungsten powder. But the hardmetal branch needed also more and more other carbide forming metals as tantalum, titanium and vanadium. Finally, modern coating technologies followed in the sixties of the last century. Moreover, the next new material from the first decade of the tungsten wire chronology, “tungsten heavy metal”, based on tungsten with additions of iron, nickel, cobalt or copper, started with production about 1935 [16]. Furthermore, some remarkable discoveries of the lighting industry for incandescent lamps were made in the second half of the last century. After more than 20 years, a new great breakthrough took place in 1959, when E. Fridrich and E. G. Zubler [10] were successful in the application of the first regenerative halogen gas cycle to high performance incandescent lamps. This resulted in higher requirements with regard to tungsten purity and creep strength of the tungsten filaments. Fig. 12 shows the redeposition and the pronounced crystal growth of tungsten within the temperature gradient of the coil of a halogen lamp. In this time, also the revival of thoriated tungsten in 1953 [16] as well as the discovery of the ductilization effect of rhenium for tungsten and molybdenum in 1956 [30] took place, together with new and special applications. The 1960's and 1970's were the beginning of extensive research concerning the microstructure of doped tungsten, leading to the discovery of potassium filled bubbles, their formation and evolution as well as their effects on recrystallization and high temperature creep (see next paragraph). Finally, in the 1980's the demands on the creep strength and exact control of the bubble distribution increase yet more due the successful development of IR-reflecting (infrared) coatings for the bulbs of halogen lamps, reflecting the radiated heat back to the coil. For a most economical energy saving, this means keeping an exact position of the coil within the focus of the backradiation, i.e. very high creep strength, during the life time. 5. The scientific background of doped tungsten wires Systematic intentional doping of tungsten oxide powder was patented already 1922 [25]. However, an understanding of the key dopant element potassium and of its role in the formation and stabilization of the creep-resistant recrystallized interlocking microstructure was obtained only after 1964 [31,32] when modern microstructural and chemical analysis of nanometer sized aggregates could be performed with the new tools scanning and transmission electron microscopes and with new surface-analytical instruments, especially Auger-Electron-Spectrometry (AES). Doped tungsten is unique in that it is a “composite” between two non-alloyable constituents, tungsten and potassium. A minute concentration of the latter in the range of few ten parts per million

(ppm), called dopant, is distributed in the tungsten wire as longitudinal rows of nanosized bubbles (about 5 to 50 nm), filled with liquid or gaseous potassium. They interact with all lattice defects and act as pinning points for dislocations as well as dislocation networks (see Fig. 13) and mainly as barriers against subgrain and grain boundary migration. After recrystallization the bubbles stabilize a creep-resistant overlapping grain structure. Nowadays, potassium bubbles form the strongest high temperature barriers known. At very high temperatures above 3000 °C, however, induced by the additional presence of temperature gradients as well as mechanical stresses and impurities, movement and exaggerated growth of bubbles can occur, resulting in instabilities of the microstructure. In so far, the tungsten technology is exemplary because it teaches an important materials science lesson on the mutual interaction between processing, microstructure and properties as well as on materials induced failure mechanisms of lamps. Since the discovery of bubbles in the sixties of the 20th century, many papers have been published about bubble effects in tungsten. The following topics have been included: - the formation of bubble forming compounds during the doping process as well as their incorporation via CVT (Chemical Vapour Transport) processes during the reduction [33–36], - the detection of potassium as the bubble forming element [37–39], - the evolution of bubbles during sintering [34,35,40–42], - their deformation during processing [37, 41, 43 – 46], - their formation from the narrow deformed cylinders or ellipsoids [34,42,44–48], - their size and their growth [40,43,49–51], - the observation that, under certain conditions (stresses, impurities), they can grow to extremely large sizes with diameters greater than 10 μm, leading to the failure of high performance halogen lamp filaments [40,51,52]. An overview of important steps of the evolution of the bubble effect knowledge is given in Table 2. In general, there is agreement in the literature that the following items are established facts: - Already during the doping process, and due to the complex chemistry of the W–K–Si–Al–O–NH3 system, amorphous and/or crystalline KAlSi3O8, KAlSi2O6 and Al2O3 particles form, the size of which ranges between 5 to 50 nm. - In the following hydrogen reduction step these particles will be incorporated into the powder particles by CVT-reactions, leading to overgrowth processes and neck formation of tungsten particles within the powder bed.

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evaporation and local thinning of the filament) which induce the failure of tungsten filaments [52].

Fig. 13. Interaction of dislocation networks with potassium bubbles (M = 56,000:1).

- During sintering the dopant particles decay, silicon and aluminium as well as oxygen are soluble in the tungsten matrix and can diffuse away through the open pore channels as well as via grain boundaries during the 2nd sintering stage. The insoluble potassium forms the known bubbles, which are stabilized by the equilibrium between the Laplace pressure of the bubble and the high internal potassium vapour pressure at high temperatures. - The following hot deformation steps of the sintered ingots by rolling, swaging and drawing induce, in dependence on the type of deformation as well as temperature and velocity, a marked refinement of the microstructure. The bubbles will be co-deformed with the tungsten matrix, corresponding to the overall shape changes, into narrow and long ellipsoids or tubes. - At intermediate anneals the bubble tubes spheroidize into single bubbles or breakup into rows of bubbles, the behaviour of which is determined by a critical aspect ratio of the bubble tubes of 8.89 corresponding to the Rayleigh mechanism for the breakup of a cylindrical fluid into spheres [47,48]. - As soon as the bubbles have formed by the breakup of the potassium ellipsoids they will grow to an equilibrium size at the elevated temperature. The equilibrium radius can be expressed accurately by equating the pressure in the bubble produced by the potassium vapour and the opposing pressure produced by the surface tension of the solid tungsten (La Place pressure). - Mechanical stresses, temperature gradients and/or impurities can generate exaggerated bubble growth, leading to cavities up to 10 μm in diameter, and in the following to “hot spots” (cavity induced effects on the electric resistance leading to increased

Besides many valuable contributions, presented since 1968 at the traditional Plansee Seminars, especially, the book “The Metallurgy of Doped/Non-Sag Tungsten” edited by E. Pink and L. Bartha [53] in 1989, provides an excellent compendium of the knowledge on this topic. A newer short overview concerning the physical and metallurgical background of the Coolidge process is given by C. L. Briant and B. P. Bewlay [54]. Nevertheless, there are still some deficiencies of scientific knowledge, new demands with regard to a better control of process steps and new challenges for a deeper microscopic understanding of the main important materials properties coupled with multiscale modelling, including the development of corresponding microstructural evolution equations as well as a sufficient description of materials properties and their implementation, for the prognostics of the materials response. This concerns, especially, the following topics: - deformation mechanisms during wire processing, - dynamic recovery and dynamic recrystallization during deformation, - high-temperature–low-stress creep - fracture mechanisms, and - phenomena of ductile to brittle transition. The next two examples illustrate progress and deficits of the metallurgical knowledge concerning the microstructural evolution of the fibre structure during wire drawing as well as the fracture behaviour. New results of quantitative evaluation of TEM and SEM micrographs in dependence on the true strain of wire drawing have shown that the subgrain and grain width obeys whether the axisymmetric elongation corresponding to the decrease of wire diameter, nor the plane-strain flow by double slip (Fig. 14). Both predictions fail for true strains greater than 1.5, suggesting subgrain losses by dynamic recovery processes. Recent EBSD investigations show, in addition, that during the rolling and swaging process of tungsten ingots, due to the deformation temperatures and high reduction in area, dynamic recrystallization processes occur. The fibre size of heavily drawn tungsten wires, ranging down to about 100 nm, due to the Hall–Petch effect, contributes about 33% to the yield and tensile strengths. This is illustrated in Fig. 15a, showing single fibre rupturing after necking down to a chisel edge type with little interference from neighbouring fibres. Although this microstructure, in combination with high dislocation densities in the range of 2 × 1015 m− 2, the high strength of up to 5.2 GPa and the simultaneously high ductility explains, there is no understanding of the mechanism concerning the relatively independent fibre

Table 2 Evolution of the knowledge of potassium bubble effects in tungsten. 1964–1972

Discovery of bubbles

1972–1974 1974–1977

Detection of potassium as bubble forming dopant element Recrystallization and bubbles

1977

1977–1981

Model of potassium inclusion during neck formation between powder particles Detection of potassium-alumina-silicate particles by electron diffraction Reduction mechanism of tungsten oxides and CVT-mechanism of potassium entrapment High-temperature–low-stress creep

1980–1995

Evolution of bubbles during deformation and anneals

1997–2008

Mechanisms of potassium bubble growth Modelling of potassium evolution as well as SANSa measurements

1980 1982–1992

a

SANS = Small Angle Neutron Scattering.

A. Wronski and A. Fourdeux; R. C. Koo; J. L. Walter; D. M. Moon; G. Das and S. V. Radcliffe D. B. Snow; H. G. Sell, D.F. Stein, R., Stickler, A. Joshi and E. Berkey O. Horacsek; K.C. Thompson-Russell; H. Warlimont, G. Necker and H. Schultz; P. Schade; D. B. Snow, P. Schade, J. Staudt, D. Radloff and H.J. Lunk P. Schade R. Haubner, E. Lassner, B. Lux, W. D. Schubert, B. Zeiler; H.-J. Lunk and P. Schade; S. Yamazaki et al. D. M. Moon and R. Stickler; J. W. Pugh; P.K. Wright; O. Horacsek; P. Schade, G. Zilberstein M. Vukcevich; J. L. Walter; C. L. Briant, B. Bewlay; O. Horacsek, L. Bartha, I. Gaal; P. Schade P. Schade, O. Horacsek, Cs. Toth and A. Nagy; I. Gaal, P. Schade, P. Harmat and L. Bartha

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deformation. In addition, Fig. 15d shows some peculiarities as cavity growth, transcrystalline and intercrystalline fractures as well as obviously stable potassium bubbles which are not completely understandable at the present time. 6. Conclusions and outlook The development of the Coolidge process had a great impact on today's modern PM industry and, especially, on the global lighting industry. Because of its importance for requirements of greater energy efficiency of lamps, the Coolidge process is still the subject of research today. Higher energy efficiency means higher filament temperatures with the necessity for higher geometric stability. This places greater demands with regard to wire performance and especially the high temperature creep strength, which must be optimized by corresponding thermo-mechanical processing for the formation of the appropriate microstructure. However, independent of the progress in understanding the microstructural evolution of doped tungsten wires, there are still questions to answer and further challenges to meet. One current subject of scientific debate is whether the bubble rows into which the potassium bubbles are aligned during the thermo-mechanical deformation process, are immobile at high temperatures. Also, the difference between experimental and calculated results concerning the deformation induced aspect ratio of potassium bubble ellipsoids

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needs further explanation. Furthermore, the current understanding of the dynamic recovery, recrystallization, and grain growth that occur during processing are just beginning to be understood, as are all the mechanisms by which the wire achieves its high temperature strength and creep resistance. In particular, bubble growth mechanisms are of special interest because of the impact on failure mechanisms in high performance halogen lamps. Moreover, to have an increasingly more complete understanding of the microstructural evolution throughout the production process and the corresponding effects on the properties of tungsten, there is a strong need to couple modern modelling efforts on atomistic basics with the microstructural observations as well as the macroscopic materials response. A newer example is given in Fig. 16, showing the atomistic modelling of grain boundary fracture in tungsten. Finally, some concluding remarks concerning an actual topic of the discussion in the community: Further substitution and, finally, ban of incandescent lamps. Today, about 19% of the global electric power produced worldwide serves for light production from several billion lamps [55]. Most of this energy consumed is by discharge lamps (about 70%), only the minor part (about 30%) by incandescent lamps. 80% of the light consumption market refers to professional lighting (industry, commerce, public) and only the remaining 20% to private consumers. On 1 March 2007, the ELC (European Lamp Companies Federation) announced an industry commitment to support a government shift to

Fig. 14. Longitudinal and transversal fibre morphology of doped tungsten wire (above) fibre with function of true strain, modelling of fibre aspect ratio, slip system and subgrain curling due to plane-strain deformation during wire drawing. Modelling courtesy of J. Ocenasek, Prague, 2005.

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Fig. 15. Fracture morphologies of doped tungsten wires; a) ductile single fibre rupture; b) ductile dimple fracture ,T = 1700 °C; c) transcrystalline cleavage with river lines after 1700 °C anneal; d) creep failure with stress induced bubble growth, T = 3,200 °C (M = 10,000:1).

more efficient lighting products for the domestic market [56]. In the meantime also the European Parliament has been confirmed the Road Map (see Fig. 17) and a respective phase-out of the least efficient lamps from the European market by 2015, leading to an estimated amount of 23 Mt of reduction of CO2 emissions and savings of about 63,000 GWh of electricity per year [57]. Already over the last three decades, incandescent lamps have been gradually replaced by more and more efficient discharge lamps (CFL, fluorescent tubes, HP mercury vapour, metal halide, LP-sodium or HP sodium, short arc lamps) and IRC- (infra red coated) halogen lamps. This evolutionary substitution by more efficient light sources, however, has not at all lowered the consumption of tungsten in the field of lighting, because all these lamps contain tungsten, either as coiled filaments or as an electrode material in a multitude of various shapes. Current discussions on modern lighting, energy savings, minimizing CO2 emissions and global warming as well as the above mentioned subsequent product replacements will have no negative effect on tungsten demand in the near future. Modern 18 W-CFL lamp coils (2 times14 mg), indeed, use the same amount of tungsten in relation to an incandescent 100 W-lamp (28 mg). But in spite of the significantly higher lifespan, at current times more lamps will be necessary than ever because more light will be produced. A negative tendency might be expected in the long term by an increase of LED's, not only for niches as traffic lights, car rearlights, instrument panels, car radios, and others, but also to domestic

lighting. Up to now, more tungsten is being used for lighting than ever, due to the still growing global market and the fact that tungsten is used in most of the alternative devices as coils and electrode material [56]. In addition, tungsten is increasingly finding applications in areas where high luminous fluxes are needed for the respective uses (photo

Fig. 16. Atomistic modelling of grain boundary fracture in tungsten. Courtesy of P. Gumbsch, Freiburg.

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Fig. 17. Replacement scheme of incandescent lamps. (ELC Background document, Brussels, 2007).

lithography, semiconductor technology, IMAX projection) or also in the field of environmental industry (water cleaning) as well as modern laser technology. Acknowledgements The author would like to thank L. Bartha and I. Gaal, both Budapest, W. D. Schubert, Vienna, G. Leichtfried, W. Knabl, A. Hoffmann and H. M. Ortner, all Reutte, P. Gumbsch and M. Ebling, both Freiburg, M. Pfau, Berlin, D. Schember, APMI Princeton, B. Altmann and B. Eberhard, both Schwabmünchen, H. Kolaska, Bottrop, K. J. A. Brookes, London, E. Kimmel, Towanda, J. Ocenasek, Prague, J. Seehawer, Munich, E. Vogt, Berlin and the colleagues of the Stadt Museum Fürstenwalde, Deutsche Technik Museum Berlin and OSRAM Lichtschau Munich as well as the Pintsch BAMAG History-Team, Dinslaken, for many helpful suggestions, references and picture material. References [1] Auer von Welsbach C. Leuchtkörper für Incandescentbrenner. DE 1885;39:162. [2] Nernst W. Verfahren zur Erzeugung von elektrischem Glühlicht. DRP 1897;104:872. [3] Auer von Welsbach C. Aus Osmium bestehende Fäden für elektrische Glühlampen und Verfahren zu ihrer Herstellung. DRP 1898;138:135. [4] von Bolton W, Feuerlein O. Die Tantallampe, eine neue Glühlampe der Firma Siemens & Halske AG. Elektrotechn Z 1905;26:4, 105. [5] 100 years of OSRAM—light has a name. Anniversary of the Trade Mark. OSRAM GmbH. Munich: Corporate Communications; 2006. [6] Just A, Hanaman F. Verfahren zur Herstellung von Glühkörpern aus Wolfram oder Molybdän für elektrische Glühlampen. DRP 1903;154:262. [7] von Wartenberg H. Der Schmelzpunkt des reinen Wolframs. Ber Deutsch Chem Ges 1907;40:12, 3287–91. [8] Pirani M, Meyer AR. Neubestimmung des Wolframschmelzpunktes. Verh DeutschPhysikal Ges 1912:426. [9] de Lodyguine A. Illuminant for incandescent lamps. US Patent 575,002 (1893). [10] Fridrich E, Zubler EG. An iodine incandescent lamp with virtually 100 percent lumen maintenance. Ill Eng 1959;54:12, 734. [11] Just A, Hanaman F. Verfahren zur Herstellung von Glühkörpern aus Wolfram oder Molybdän. DRP 1904;182:766. [12] Johnson PK. Tungsten filaments—the first modern PM Product. IJ of Powder Met 2008;44:4, 43. [13] Kuzel H. Verfahren zur Herstellung von Leuchtkörpern für elektrische Glühlampen. DRP 1905;182:683. [14] Kuzel H. Verfahren zur Herstellung von Glühkörpern aus den Metallen Cr, Mn, Mo, U, W, V, Ta, Nb, Ti, Th, Zr für elektrische Glühlampen. DRP 1905;194:348. [15] Siemens, Halske. Verfahren zur Herstellung von Körpern aus Wolframmetall oder Legierungen desselben durch Ziehen oder Walzen. DRP 1906;194:682. [16] Lassner E, Schubert WD. The history of tungsten. Part II. The growth of the tungsten tree: evolution in chemistry and technology. ITIA Newsletter, London; December 2005. p. 2–11. [17] Pintsch J. Metalldrähte, – fäden oder – bänder und Verfahren zu ihrer Herstellung. DRP 1913;291:994. [18] Smithells CJ. Tungsten: its metallurgy, properties, and application. London: Chapman & Hall Ltd.; 1952. [19] Agte C, Vacek J. Wolfram und Molybdän. Berlin: Akademie Verlag; 1959.

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