Int. Journal of Refractory Metals and Hard Materials 50 (2015) 23–30 Contents lists available at ScienceDirect Int. Journal of Refractory Metals and Hard Materials j ourna l homepage: www.e lsev ie r .com/ locate / IJRMHM Review Refractory metals revolutionizing the lighting technology: A historical review☆ P. Schade a, H.M. Ortner b,⁎, I. Smid c a HTM Consulting Berlin, Germany b Darmstadt University of Technology, Germany c Pennsylvania State University, USA ☆ This paper had been presented in part at the “9th Int ⁎ Corresponding author. E-mail addresses:
[email protected] (P. Schade), Hug http://dx.doi.org/10.1016/j.ijrmhm.2014.11.002 0263-4368/© 2014 Published by Elsevier Ltd. a b s t r a c t a r t i c l e i n f o Article history: Received 13 August 2014 Received in revised form 11 November 2014 Accepted 16 November 2014 Available online 18 November 2014 Keywords: Refractory metals Lighting technology History Coolidge-process Non-sag tungsten Incandescent lamps LED-production From a historical point of view, the development of the PM processing steps and tools “for making tungsten duc- tile” byWilliam D. Coolidge in 1909 marks the breakthrough for the use of tungsten filaments in the lighting in- dustry and the beginning of the industrial era of Powder Metallurgy. However, other refractory metals, osmium and tantalum, had been considered useful lamp filaments prior to tungsten. Other technological concepts as, e.g. the gas mantle lamp of Auer von Welsbach, are still in limited use today. The significance of the Coolidge process is shortly outlined for the production of ductile tungsten. In addition some further important accompanying discoveries and inventions will be mentioned: cemented carbides and tungsten heavy metals. 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. It is in our opinion, the most ingenious “dispersoid” ever developed by mankind: non-effective at lower temperatures and thus not in- terfering with the wire drawing and coiling processes but very effective as dispersoid at the operating tempera- tures of incandescent lamps as mentioned above. Considering geometrical dimensions, themicrostructural features of the finestwires with diameters in the single micrometer domain and the corresponding fabrication of diamond dies, necessary for the deformation of wires, also represent precursors of today's nanotechnology and micromachining. Some further important points for the fabrication of incandescent lamps are mentioned: the introduction of the Langmuir sheath and the significance of molybdenum for the lighting industry. Finally the ban of incandescent lamps by the European Union is outlined and a brief overview of modern means of lighting is given. Compact fluorescent lamps and high intensity discharge lamps and the use of refractory metals in the LED-production end this brief overview on modern lighting and no device is without a refractory metal. © 2014 Published by Elsevier Ltd. Contents 1. Material selection process for incandescent lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 1.1. First selection: carbon filaments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 1.2. The “gas mantle lamp” of Carl Auer von Welsbach 1883 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 1.3. The first metallic filament lamp used osmium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 1.4. The tantalum lamp was the first metal filament lamp which really reached the commercial stage . . . . . . . . . . . . . . . . . . . . . 25 2. Finally, the race was won by tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.1. Ductile tungsten by the Coolidge process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2. The birth of NS (non-sag) tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3. The casual invention of hard metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4. The scientific background of doped tungsten wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.5. The evolution of potassium bubbles in NS-tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3. The introduction of the “Langmuir sheath” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 . Conference on Tungsten, Refractory and Hardmaterials” at Orlando, May 18–22, 2014.
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[email protected] Unlabelled image http://dx.doi.org/10.1016/j.ijrmhm.2014.11.002 Unlabelled image http://www.sciencedirect.com/science/journal/02634368 www.elsevier.com/locate/IJRMHM 24 P. Schade et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 23–30 4. Tungsten halogen lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5. Molybdenum for lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6. The ban of the incandescent lamp by the European Union . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 7. The development of energy saving lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.1. Fluorescent tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 7.2. The best alternative: the compact fluorescent lamp (CFL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 8. High intensity discharge lamps (HID) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 8.1. Thoriated tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 8.2. Getter materials for lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 9. The LED-lighting revolution — not without refractory metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 9.1. Driverless electronics substitute LED-switching power supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 10. Conclusion and outlook— tungsten still an element of lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Preamble Refractory metals The Americans call them “exotic” The British “less common” The European Union “sensitive” We stay with the truth and call them “refractory”. Walter M. Schwarzkopf, 1972 “The refractorymetals comprise selectedmetals withmelting points around 2000 °C and higher, with similar powder metallurgical pro- duction and application. This includes the followingmetals and their refractory alloys: Tungsten, Molybdenum, Chromium, Tantalum, Niobium and Rhenium” [1]. 1. Material selection process for incandescent lamps 1.1. First selection: carbon filaments 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. Fig. 1. a) Centennial carbon filament light bulb, Fire Department House, Livermore, CA, USA, with very pronounced deformations of the filament due to creep processes. b) Old fashioned first Mazda Coolidge lamp, 1911 (right hand side). The brand name Mazda re- fers to the newly introduced tungsten filament lamp and was used in the USA by both General Electric and Westinghouse. Mazda lamps produced before May 1911 utilized sintered tungsten filaments, whereas later drawn tungsten filaments were used. As can been seen straight uncoiled tungsten wires were used. Courtesy of Istvan Meszaros, GE Hungary [2] However, from the beginning, the various carbon filaments used in all the early lamps had low light outputs and short lives due to poor vacu- ums because of the limited technical possibilities. Consequently, there was a blackening of the bulb wall as well as pronounced brittleness. Fig. 1a shows a carbon filament light bulb of the Fire Department house in Livermore, CA, USA [2]. It is burning formore than onehundred years already. Fig. 1b shows a “Mazda Coolidge Lamp”, 1911. It was one of the first lamps with a drawn ductile tungsten lighting wire. At the beginning of the 20th century the search started for a more satisfying filament material than carbon. However, the next great step forward was not a “filament lamp” but the invention of a quite different design, the “gas mantle lamp”. 1.2. The “gas mantle lamp” of Carl Auer von Welsbach 1883 The heart of these lamps is usually a conical or hemispherical struc- ture consisting of 99.1% ThO2 and 0.9% CeO2, which yields a bright light in a Bunsen- or a propane air-flame [3]. This “Gasglühlicht” (lighting by glowing in gas) improved the already known lighting by gas and the “Auer-Gühstrumpf” (glowing stocking by Auer) had been patented in 1891. The Auer light proved to be much more economic and brighter than the carbon filament lamp of Edison. The birth of the “Auer Glühstrumpf” was also the birth of modern lighting technology [4]. Today, still a lot of historical gas lanterns are burning all over the world, e.g. more than 40.000 in the old quarters of Berlin and in a num- ber of further German towns, but also in London and Boston (Beacon Hill quarter), in Prague and many other towns worldwide (Fig. 2). On March 10, 1906, Auer von Welsbach applied for the trade name OSRAM for “Electrical incandescent and discharge lamps” at the Fig. 2. Shows Auer gas mantle lamps burning in the “Tiergarten” in Berlin. image of Fig.€1 image of Fig.€2 Fig. 4.William D. Coolidge, 1927. 25P. Schade et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 23–30 “Kaiserlichen Patentamt” in Berlin. Up to this time, the only source of artificial lighting had been glowing carbon in various forms: the pine torch, the candle, the kerosene lamp, the coal gas for lighting and the carbon filament lamp [4]. In recognition of this first real breakthrough in lighting technology, a memorial coinwas dedicated to Carl Auer vonWelsbachby theAustrian Government which is shown in Fig. 3a,b. It is a bi-metallic coin: the core metal is niobiumwhereas silver is used for the counterpart as surround- ing metal. A special feature is the use of colored niobium inserts. The green color is generated by a thin oxide layer produced by anodic oxida- tion [5]. 1.3. The first metallic filament lamp used osmium It was again developed by Auer von Welsbach in 1898 [6]. The sig- nificance of osmium at that time is evident from generating the brand name OSRAM which is made up of osmium and wolfram (the German name for tungsten) [7]. The choice of osmium was understandable but unfortunate. It has a very high melting point of ca. 3000 °C. However, its easily volatile tetroxide is highly toxic. Osmium is extremely expensive [8]. 1.4. The tantalum lamp was the first metal filament lamp which really reached the commercial stage Lamps with drawn tantalum wires had been introduced by W. von Bolton and O. Feuerlein in 1902 (Siemens & Halske AG). They substituted largely the osmium lamps. It is surprising that as late as 1912, the famous steam ship “Titanic”was outfitted entirely with Ta-filament bulbs [2]. 2. Finally, the race was won by tungsten It was soon realized that tungsten would be the ideal metal for incandescent lamps especially due to the fact that tungsten exhibits the highest melting point of all metals. Already in 1903, A. Just's and F. Hanaman's first tungsten filaments marked a new era of modern lighting by surpassing the efficacy of carbon lamps of about 3.2 lumen per Watt [lm/W] to about 7.9 lm/W for tungsten, thereby also surpassing the effica- cy of Os and Ta of about 6.3 lm/W [9]. The great problem for the use of tungsten was initially that the high sintering temperatures could not be reached and quite a series of co-sintering processes of e.g. amalgamated tungsten powders had been developed. All these operations unfortunate- ly led to brittle tungsten [2]. 2.1. Ductile tungsten by the Coolidge process The great breakthrough was ultimately reached when William Coolidge (1873–1975), Fig. 4, and his team at General Electric Company Fig. 3. “Fascination Light” coin of the Austrian Mint made of 9 g of silver and 6.5 g of niobium (2008). Left) Obverse side: Auer vonWelsbach with the development of lighting from first light bulbs with filaments to modern fluorescent “Neon-lamps”. Right) Reverse side: gas lantern lighter, value of coin € 25.- in the US were successful in producing ductile tungsten filaments by suitable mechanical deformation at elevated temperatures and corre- sponding intermediate heat treatments (British patent: 23,499/1909). The method since then is called the Coolidge process for making tung- sten ductile, and the main features of the process are still valid for today's technology [10,11]. The most unexpected result was that on working (swaging) sintered tungsten bars down to smaller and smaller sizes they become increasingly ductile. At low diameters the wire could even be drawn through diamond drawing dies at moderate temperature [2]. Commercializing the ductile wire process started in 1910/11 and rapid further development work was carried out in close coop- eration between the General Electric Comp. and the German light- ing industry. Within a short time tungsten light bulbs spread all over the world equipped with ductile tungsten wires. Today ability to handle tungsten wires and coil filaments without breakages is the backbone of the incandescent lamp industry. In this regard it is fascinating to read that already in such pioneer-times more than 100000 light bulbs were manufactured per day in Berlin/ Germany [7]. 2.2. The birth of NS (non-sag) tungsten In the process 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 observed as early as January 1910 [10,11] that when his tungsten oxide had been heated in special clay Battersea-type crucibles, the filaments would have longer lives [11] due to increasing 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 Tung- sten 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 pro- cess, although the reason why these crucibles created this benefit image of Fig.€3 image of Fig.€4 26 P. Schade et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 23–30 was not clear for many years until progress in chemical analysis down to the part per million range (ppm) became available [12–15]. Hence, the next great step was an intentional doping method with the expressed aim of obtaining tungsten wire of non-sagging and non- offsetting quality. The so-called AKS doping was introduced by P. Tury and T. Millner at Tungsram in 1931 (AKS = aluminium–potassium (in German: kalium)-silicate) [12]. This new doped tungsten quality was the prerequisite for the introduction of the today's coiled coils of tung- sten wires by W. Geiss in 1936 [17]. Systematic intentional doping of tungsten oxide powder with K,Na-silicate was patented already in 1922 [16]. “This process was the first powder metallurgical process used in the industry and initiated developments in many other areas, e.g. furnace technology and sintering technology. The tungsten wire was crucial for the development of the lighting industry; on the other hand the process became very important for the growth of the whole powder technology” [18]. 2.3. The casual invention of hard metals It should be mentioned that a very important invention was made when a substitute was searched for the very expensive dia- mond drawing dies for the drawing of tungsten wires. Experiments were almost given up with WC when the researchers at OSRAM started to admix Fe, Ni and finally Co to make the very brittle WC a bit more ductile. Co turned out to be the optimal choice and it still is today. The whole process of this invention and the further development of hard metals are described in detail in [14]. More- over, 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 [2]. 2.4. The scientific background of doped tungsten wires Systematic intentional dopingof tungstenoxide powderwas patented already 1922 [16]. 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 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-Spectroscopy (AES) [13]. Doped tungsten is unique in that it is a “composite” between two non-alloyable constituents, tungsten and potassium. A minute concen- tration 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 nano-sized 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 (Fig. 6) and mainly as barriers against subgrain and grain boundary migration. After recrystallization the bubbles stabilize a creep-resistant overlap- ping 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 onmaterial induced failure mechanisms of lamps. The very rich liter- ature on this topic can be found in [2] and [13,18,20]. [13] are Special Is- sues of the Int. J. of Refractory Metals and Hard Materials dedicated to the chemistry of NS-tungsten. 2.5. The evolution of potassium bubbles in NS-tungsten Since the discovery of bubbles in the sixties of the 20th century, many papers have been published about the formation of potassium filled bubbles and their effects in NS-tungsten. The best respective over- view based on the available literature is found in [2]. Here, only a short overview of important steps of the present knowledge on the evolution of the bubble effect is given: In general, there is an agreement in the literature that the following items are established facts: – Already during the doping process, and due to the complex chemis- try 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 and 50 nm. – In the following hydrogen reduction step these particles will be in- corporated into the powder particles by CVT-reactions, leading to overgrowth processes and neck formation of tungsten particles within the powder bed. – During sintering the dopant particles decay, silicon and aluminium as well as oxygen are soluble in the tungsten matrix and can diffuse away through open pore channels as well as via grain boundaries during the second sintering stage. Insoluble potassium forms in the known bubbles, which are stabilized by the equilibrium between the Laplace pressure of the bubble and the high internal potassium vapor pressure at high temperatures. – The following hot deformation steps of the sintered ingots by rolling, swaging anddrawing induce, in dependence on the type of deforma- tion aswell as temperature and velocity, amarked refinement of the microstructure. The bubbles will be co-deformed with the tungsten matrix, corresponding to overall shape changes, into narrow and long ellipsoids or tubes. – At intermediate anneals the bubble tubes spheroidize into single bubbles or break up into rows of bubbles, the behavior of which is determined by a critical aspect ratio of the bubble tubes of 8.89 corresponding to the Rayleighmechanism for the breakup of a cylin- drical fluid into spheres. – As soon as the bubbles have formed by the breakup of the potassium ellipsoids they will grow to an equilibrium size at the elevated tem- perature. – 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 electrical resistance leading to increased evaporation and local thinning of the filament) which induce the failure of tungsten filaments. 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 [11] 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. Bryant and B.P. Bewlay [19]. It should bementioned that the potassium filled bubbles are – in our opinion – the strongest pinning points at highest temperatures against the movement of dislocations (Fig. 5) and grain boundaries and, hence, the most ingenious “dispersoid” ever developed by mankind: non-effective at lower temperatures and thus not interfering with the wire drawing and coiling processes but very effective as dispersoid at the operating temperatures of incandescent lamps as mentioned above. Commercializing the ductile wire process started in 1911 and rapid further development-work was carried out in close cooperation be- tween the General Electric Company and the German lighting industry. Within a short time tungsten light bulbs spread all over the world equipped with the ductile tungsten wires. Today, the ability to handle Fig. 5. Interaction of dislocation networks with potassium bubbles. 27P. Schade et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 23–30 tungsten wires and coil filaments without breakages is the backbone of the incandescent lamp industry [2,17,23,24]. 3. The introduction of the “Langmuir sheath” The specific properties of the new tungsten wires, especially the ex- cellent bend ductility, and the fundamental work of I. Langmuir [21,22] concerning convection, conduction and radiation of heat in gases, lead- ing 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 [17]. The gas which originally had been used by Langmuir in 1912, was nitrogen [21]. At that time argon was not available in the USA and, hence, Ar was introduced by I. Langmuir and L. Tonks at GE only in 1918 [23]. In Germany, Friedrich, Mey and Jacoby introduced nitrogen in 1913 at AEG [24]. In 1930, Brody introduced the Kr-filling at Tungsram in Budapest [25]. Fig. 6 shows the dependence of lamp life-time on the cold filling pressure for N2, Kr, Xe and Ar [18]. A mixture of 85–95% argon and 5–15% nitrogen is mostly used as filling gas nowadays. 4. Tungsten halogen lamps Byusing the temperature dependent chemical properties of tungsten– halogen and tungsten–oxygen–halogen compounds, the detrimentalwall blackening of the lamp envelope by the evaporation and condensation of tungsten can be avoided, so that higher filament temperatures (up to Fig. 6. Dependence of the lamp life time on the gas-type and its cold filling pressure [18]. 3000 °C) and higher filling pressures lead to higher efficacies of up to 27 lm/W and to higher life times. Tungsten halogen lamps use reversible chemical reactions described by the following equations: Wþ X⇔WX gð Þ Wþ Xþ O⇔WOX gð Þ: X is a halogen (usually iodine or bromine). The reactions proceed to the right at low temperatures to react which evaporated tungsten, forming volatile compounds that do not condense on the wall of the bulb. In contact with the tungsten filament at elevated temperatures, the halides and oxyhalides decompose, redepositing tungsten on the filament, Fig. 7, and releasing halogen for the cycle again. Careful control of the cycle is needed to ensure that clean-up is sufficiently rapid to prevent wall-blackening, without being so rapid as to erode the tung- sten filament legs at lower temperatures before normal burnout of the lamp occurs [27]. Today, further distinct improvements have been reached by the application of modern multiple IR-reflecting SiO2/TiO2 coatings of the outer lamp bulb. This leads to energy savings of about 25 to 30% [7,26]. 5. Molybdenum for lighting Incandescent lighting was the first molybdenum metal application [28]. Its use, dating back to 1908, was in the form of wire filament sup- ports in incandescent lamps. Lighting applications continue to account for large quantities of Mo metal production today as both lamp compo- nents andwireswere used asmandrels to coil tungstenfilaments. At the time of writing this is changing dramatically as the incandescent lamp is being replaced by halogen lamps, IRC-halogen lamps, compact fluores- cent lamps (CFL) and light emitting diode lamps (LEDs). Lighting technology is far more advanced than it was a century ago [2], but mo- lybdenum is still used for support wire and glass feed-throughs in halo- gen lamps and as mandrel wire around which tungsten filaments are coiled during manufacturing. The feed-through employs Mo-foil because of its low coefficient of thermal expansion (CTE), good electrical conductivity and strong bond- ingwith silica glass.Molybdenum's excellent high temperature strength and mechanical stability as well as resistance to corrosion by the halo- gen gases inside the lamp envelope (a critical feature for the ap- plication) make it ideal also for reflectors in halogen lamps for cars. These properties make it a material of choice for similar components of high-intensity lamps [28,29]. Fig. 7. Tungsten redeposition and crystal growth during the life-time of a halogen lamp. image of Fig.€5 image of Fig.€6 image of Fig.€7 Fig. 9. Coiled coil of an automotive T5 lamp as compared in size to an eye of a needle: wire diameter 15.5 μm; inner coil of turns per mm: 33; total number of turns of the filaments: 265; weight of filament: 0.416 mg (magnification ca. ×20) [18]. 28 P. Schade et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 23–30 6. The ban of the incandescent lamp by the European Union Today, about 19% of the global electric power produced worldwide serves for light production of several billion lamps [7]. Most of this energy is consumed 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. It is long known that the incandescent lamps produce much more heat than light: only 5 to 10% of the consumed electrical current is converted to light. The rest is converted to heat. This is the onset of the ban of the incandescent lamp (Fig. 8) [30]. On the 1st of March 2007, the ELC (European Lamp Companies Federation) announced an industry commitment to support a govern- ment shift to more efficient lighting products for the domestic market [31]. In the meantime also the European Parliament has confirmed the Road Map (see Fig. 9) 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 emission and savings of about 63000 GWh of electricity per year [32]. This is still very impressive but it should be mentioned that this ban caused massive resentments by many politicians as well as by a large part of European citizens [30]. This is mainly because this ban was decided exclusively by the European Commissionwithout the consent of the European Parliament. It was obviously decided in a quick process and by pressure exerted by an industrial lobby of the two biggest lamp producers of Europe, most likely due to the fact that profits tended against zero for incandescent lamps due to the enormous economic pressure from Asia. By this ban, the way is free for the introduction of novel but essen- tially more expensive means of lighting which should now be shortly outlined. It should bementioned, however, that the ban of incandescent lamps is only of concern to Europe whereas other hugemarkets like the whole of Asia as well as South America and Africa are not concerned. Recommendations are announced everywhere to change over to energy saving lamps but the ban of incandescent lamps is only of concern to the countries of the EuropeanUnion. In theUSA a similar pressure is exerted on lighting devices. These efficiency-requests by the USA are shown in Table 1 [33]. They can only be reached by IR-halogen lamps. By the way, it is remarkable that Fidel Castro was the first to start with “the ban” of incandescent lamps! 7. The development of energy saving lamps 7.1. Fluorescent tubes “Fluorescent tubes are the workhorse in office and industrial lighting. As in the case of the CFL lamps (see below) they generateUV-radiation Fig. 8. Replacement scheme of incandescent lamps (ELC Background Document, Brussels, 2007). by a gas discharge (using a small amount ofmercury— less than 5mg per tube)which then transforms to visible radiation by the use offluo- rescent powders, which are coated on the inside of the tube. The nec- essary electron emission is produced by emitter coated tungsten coils. The emittermaterial consists of amixture of alkaline earth oxides” [7]. 7.2. The best alternative: the compact fluorescent lamp (CFL) Themost eminent and popular lamps today are compact fluorescent lamps (CFL), the so-called “energy savers”. They belong to the group of lowpressure discharge lamps. Theywere specifically designed to direct- ly replace incandescent lamps. They use up to 5 times less electricity than standard incandescent lamps (max. light efficacy: 65 lm/W). The rated lifespan of CFL-lamps is between 5 and 15 years. They resist 500000 switching cycles, and are equipped with quick start circuitry. Some lamps are also available in dimmable version. They were introduced in 1985, but have been developed further since then in terms of light quality, durability and substitutability. CFL lamps consist of a bulb, an electronic ballast (integrated or non-integrated) and either screw or bayonet fitting. They are produced for both, AC and DC inputs. The use of high quality fluorescent lamps (triphosphor lamps) with efficacies of up to 100 lm/W can save 70% energy in combination with electronic control gear [7]. 8. High intensity discharge lamps (HID) “In HID lamps the power dissipation of an electric current passing through a gaseous medium at a pressure greater than or equal to 1 atm is converted into radiation. Much higher radiating temperatures can be achieved than in any incandescent lamp. Appropriate selection of the gaseousmedium results in favorable spectral distribution of radi- ated power, with a much smaller fraction of IR rays. Therefore, these Table 1 Situation in the USA: Energy Independence and Security Act of 2007. – Phase 1: all general purpose bulbs must be 30% more efficient by 2014. – Phase 2: by 2020, all general purpose lamps must produce 45 lm/W. Traditional wattage New max wattage Lumens Implementation date 100 72 1490-2600 January 1, 2011 75 55 1050-1489 January 1, 2012 60 45 750-1049 January 1, 2013 40 29 510-749 January 1, 2015 image of Fig.€8 image of Fig.€9 29P. Schade et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 23–30 light sources are very bright, and are up to 10 times as efficient as incan- descent lamps. The following are in use: – Mercury high pressure lamps – Sodium high pressure lamps – Metal-halide lamps – Xenon and xenon–mercury short arc lamps Each system comprises an inner discharge tube (arc tube) contain- ing the high pressure gas or vapor enclosed in a hermetically sealed outer envelope. The outer jacket is required for thermal insulation, pro- tection of the arc tube seals from oxidation, and absorption of any short wavelengthUV rays thatmay be emitted from the arc tube. Arc tubes for mercury and metal halide lamps are quartz; arc tubes for high pressure sodium lamps are fabricated from translucent polycrystalline alumina (PCA) to withstand corrosion by hot molten and gaseous sodium. The energy coupling takes place via tungsten electrodes. HID lamps are for high output, high luminous efficiency and long op- erating times. Their main applications are lighting for roads, outdoor areas, and halls, shopping areas, flood lighting, plant irradiation, photog- raphy, medical technology and automobiles. Recent advancements of metal halide discharge lamps with ceramic arc tubes and high pressure sodium lamps with efficacies of up to 150 lm/W and improved electronic ballast characteristics are capable of a 60% reduction in energy consumption and 30% in maintenance costs” [7]. Table 2 gives an overview on lamp efficacies and burning lifetimes of various standard lamp types” (courtesy of Schubert, Lassner and Schade [9]). 8.1. Thoriated tungsten Besides pure tungsten, tungsten–thoria “alloys” are used as electrode material in discharge lamps because of their low work function, high melting point and low vapor pressure. These properties result in lower temperature load, minor electrode burnback, lower sputtering damage and hence a longer life-time of the lamps. Again, as for K-doped tungsten, the combination of tungsten and thoria has some unique properties. The lowwork function over the life time of the lamp –maintained by a diffu- sional supply process of thorium to the electrode tip – is only possible due to the low solubility of thorium in tungsten. There is also an indication that evaporated thorium stabilizes the arc setting on the electrodes. To date, no othermaterial shows as promising a behavior for electrode appli- cation in discharge lamps [18]. 8.2. Getter materials for lamps Due to the high affinity of refractorymetals to oxygen and hydrogen, some high melting metals, especially zirconium, are used for gettering gaseous impurities, especially oxygen. Zirconium intermetallics are used for this purpose on lamps with an external bulb. For high pressure Table 2 Comparison of standard lamp types, lamp efficacy and burning life. Lamp type Max. efficacy [lm/W] Burning life [hours] Edison lamp/1878/carbon filament 1.4 40 Incandescent lamps 15 1000 Halogen lamps 27 2000 Fluorescent lamps 104 16,000 Compact fluorescent lamps 65 12,000 Gas discharge lamps Mercury high-pressure lamps 100 12,000 Metal-halide lamps 107 12,000 Sodium high-pressure lamps 150 20,000 Sodium low-pressure lamps 200 10,000 xenon lamps containing mercury, zirconium and tantalum are used as internal getters in the discharge vessel [18]. 9. The LED-lighting revolution — not without refractory metals “Already for over 30 years, LEDs have been used in various areas of application, whether for industrial systems, hi-fi equipment, car lights or advertising. LED technical development continues to stride ahead. In the course of recent years, thewhite LEDs' luminous effica- cy has increased to a startling 130 lumens per watt andmore. This is a trend that will continue into the future. In addition, the physical effect of electroluminescence was discovered more than 100 years ago.” [34] At first glance one would not think that LED-design and production were connected to refractory metals at all. However, this is not so: “Every LED is born in a crucible. In this crucible, aluminium oxide is melted and structured into a monocrystalline sapphire crystal. De- pending on the method used, this process takes between two and three weeks. The art of growing single crystals: The material is first heated to over 2000 °C and is then allowed to cool slowly in a con- trolled environment. Only few metals can be used to produce these crucibles. This is because they must meet two requirements: on the one hand, they must be resistant to the high temperatures involved and, on the other, they must not contaminate the melted sapphire. Molybdenum or tungsten is used as crucible materials. The resulting sapphire monocrystal is cut into thin wafer slices be- fore being polished ready for use as a substrate for the subsequent semiconductor structure of the LED”. [35] Originally, LEDs emitted a cold, bluish light. Today's LEDs emit a warm light from 2300 K onwards. They can cover a wide range of color temperatures up to approximately 7000 K. Special electronic sys- tems (e.g. Dali andKNX) allow the adaption of light conditions. Essential advantages of LEDs over conventional lighting devices are: – Their long life time of 50000 h andmore (as already mentioned ear- lier) – A saving of electrical energy of up to 85% – The total absence of dangerous substances such as mercury [36] In 2015, LEDs will reach a higher market share than classical energy saving lamps according to the Fraunhofer-Institute for Applied Solid State Physics. Further, the price for LEDs is sinking and reaches the price for energy saving lamps, surpassing the latter in life time. The light-spectrum of LEDs can be approximated to the spectrum of an in- candescent lamp by special coatings [30,34]. 9.1. Driverless electronics substitute LED-switching power supplies Up to recently, LEDs required costly switching modules which con- verted alternating current to direct current. Frequently, these modules limited the life time of the LED. Euro-Lighting recently introduced a driverless AC-technique with which the LEDs produce a flickerless light [35]. The IC which is required needs only few building parts and can be implemented with little requirements for space and minor cost. 10. Conclusion and outlook — tungsten still an element of lighting “Since the dawn of the 20th century, tungstenmetal has been a syn- onym for the incandescent bulbfilament, and the symbol of the bulb has become iconic for sudden inspiration. However, after more than 100 years of use in the different fields of lighting, tungsten filament bulbs have been, and are currently being, replaced by more energy efficient 30 P. Schade et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 23–30 solutions. It started with fluorescent tubes in the late nineteen thirties, with halogen lamps at the end of the fifties/beginning of the sixties, and finally by compact fluorescent lamps in the middle of the nineteen eighties. Only domestic lighting is still a stronghold of filament lamps, simply because of their low initial cost and convenience” [7]. However, in Europe they are systematically banned by a strict Road Map (see Fig. 9). Other countries are following (see Table 1). “Replacement of filament lamps by discharge lamps (CFL, fluores- cent, HP mercury vapor, metal halide, LP-sodium or HP sodium, short arc) has not at all lowered tungsten consumption in the field of lighting, because all these lamps contain tungsten in a certain form: either as filament, or as electrode material (coiled filament, coiled coil, triple coil, rod-like or massive) in the form of tungsten, porous tungsten, emitter-coated tungsten or thoriated tungsten. In particular high inten- sity discharge (HID) lamps have increased tungsten consumption sig- nificantly. The replacement of incandescent lamps will have no negative effect on tungsten demand of the lighting industry in the near future, as much as the same amount of tungsten will be used in an old filament bulb and a modern CFL lamp. A negative tendency might be expected in the long term as the lifespan of such new lamps is significantly longer (up to 15 times!). However, more lampswill be necessary than ever (asmore lightwill be produced), and even the number of filament bulbsmight still slightly increase in the next few years (and then come down because of the phasing out of energy-guzzling devices). In addition, tungsten is increasingly finding applications in areas where high luminous fluxes are needed for respective uses (photo li- thography, semiconductor technology, IMAX-projection) or in form of low-pressure cold cathode discharge lamps (CCFL) for scanners, flat screens, laptops, or television. Electrode weights are in the order of mg (CFL: 10–20 mg) or several g (HID-lamps) but can go up to kg in high performance short-arc lamps for cinema projections (up to 15 kW). Lower demand in the long term has to be expected from the devel- opment of electrode-free discharge lamps (induction lamps) and the further development of LED devices which are bringing about a revolu- tion in the lighting market. Nevertheless, about 20 billionm of lampwire is still drawn, a length which corresponds to about 50 times of the earth–moon distance. Light- ing still consumes today between 4 and 5% of the total tungsten produc- tion” [7]. The amount of research invested in the whole area of lighting technology is enormous and there are very few places on earth in very remote areas where electrical lighting does not play an essential role for work and leisure time of mankind. It is said that one picture says more than 1000 words. We would, therefore, like to close with a final figure which demonstrates what wonderful things can be fabricatedwith an originally fairly brittlemate- rial: tungsten! Acknowledgment The authors would like to thank E. Lassner and W.D. Schubert for their approval to use essential passages from references [7,9,32] as citations in this paper. 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Material selection process for incandescent lamps 1.1. First selection: carbon filaments 1.2. The “gas mantle lamp” of Carl Auer von Welsbach 1883 1.3. The first metallic filament lamp used osmium 1.4. The tantalum lamp was the first metal filament lamp which really reached the commercial stage 2. Finally, the race was won by tungsten 2.1. Ductile tungsten by the Coolidge process 2.2. The birth of NS (non-sag) tungsten 2.3. The casual invention of hard metals 2.4. The scientific background of doped tungsten wires 2.5. The evolution of potassium bubbles in NS-tungsten 3. The introduction of the “Langmuir sheath” 4. Tungsten halogen lamps 5. Molybdenum for lighting 6. The ban of the incandescent lamp by the European Union 7. The development of energy saving lamps 7.1. Fluorescent tubes 7.2. The best alternative: the compact fluorescent lamp (CFL) 8. High intensity discharge lamps (HID) 8.1. Thoriated tungsten 8.2. Getter materials for lamps 9. The LED-lighting revolution — not without refractory metals 9.1. Driverless electronics substitute LED-switching power supplies 10. Conclusion and outlook — tungsten still an element of lighting Acknowledgment References