From: Robina Suwol
Date: 19 Dec 2005
Time: 19:07:14
Remote Name: 69.149.40.254
The word "Tungsten" was probably first used by A. F. Cronstedt in 1755, who
applied it to the mineral subsequently known as "scheelite," which is the
natural form of calcium tungstate. C. C. Leonhard named this mineral scheelite
in 1821 in recognition of the discovery made by K. W. Scheele, in 1781, that the
mineral was a compound of lime with a previously unknown acid, which he called "Tungstic
Acid," a name by which it is still known. Before Scheele made his discovery, the
mineral was generally regarded as containing tin. The word tungsten denotes a
substance of high density and is derived from the Swedish language, "tung,"
meaning heavy, and "sten," meaning stone.
In 1783 the Spanish brothers, J. J. and F. d'Elhujar, published the
results of their investigations on wolframite carried out with the Swede, T.
Bergmann, while they were working in his laboratory. They showed that this
mineral contained the same tungstic acid, previously found in scheelite, but
combined with iron and manganese, instead of calcium. They were also the first
to record the preparation of elementary tungsten, which they made by reducing
tungstic oxide with charcoal, and to which they gave the name "Wolfram." The
origin of the word wolfram is obscure. Mennicke attributes it to the alchemists,
who called the metal "spuma lupi," which means wolf spume or foam. Another
suggestion is that the word is of German origin from wolf, meaning a beast of
prey, and rabin or ram, which has several meanings, including dirt and soot. The
word may also be derived from the Swedish word "Wolf rig," which means eating.
All these meanings are assumed to be associated with the early difficulties of
extracting tin from cassiterite when it was contaminated with wolframite; the
two minerals are frequently found together, and the wolfram was thought to eat
the tin as a wolf eats sheep. The common termination used in mineralogy, to give
the name "wolframite" to the mineral, was used in 1820 by A. Breithaupt in his
book, Kurze Charalderistik des Mineral Systems.
The metal is known as tungsten in some countries and as wolfram in others,
including Sweden, the country of origin of the name tungsten. The chemical
symbol W, which is universally used to denote tungsten, suggests that wolfram
was formerly the more generally accepted name for the element. In Britain the
mineral wolframite is also known as wolfram.
For many years tungsten remained one of the rare elements, and it was not
until 1847, when Oxland took out a patent for the manufacture of sodium
tungstate, tungstic acid, and tungsten from cassiterite (tinstone), that the
element became of any industrial importance. Oxland's second patent, taken out
in 1857, described the manufacture of the iron-tungsten alloys that form the
basis of modem high-speed steels. The metal itself, however, found no
application until nearly fifty years later, when it was first employed in the
manufacture of filaments for electric incandescent lamps. From 1878, when Swan
demonstrated his eight and sixteen-candle power carbon lamps at Newcastle,
search was made for a more satisfactory filament material than carbon. The early
carbon lamp had an efficiency of about 1.0 lumen per watt, which was improved
during the next 20 years by modifications in methods of preparing the carbon, to
about 2.5 lumens per watt. A further improvement was made in 1898 to about 3.0
lumens per watt by heating the filaments electrically in an atmosphere of
petroleum vapor, which caused the deposition of carbon in the pores of the
filament and gave it a bright metallic appearance. At the same time A. Von
Welsbach produced the first successful metal filament was by using osmium;
attempts had previously been made to use platinum, but its relatively low
melting point of 1774°C. prevented its successful development. Lamps using
osmium filaments had an efficiency of about 6.0 lumens per watt. Since osmium is
the rarest of the platinum metals it could never have been used on a large
scale. Tantalum, with a melting point of 2996°C., compared with osmium, 2700°C.,
was extensively used as a drawn wire from 1903 to 1911, following work by Von
Bolton of Siemens and Halske. Lamps with tantalum filaments had an efficiency of
about 7.0 lumens per watt. Developments in the use of tungsten started about
1904, and it has been used exclusively since about 1911. The modern tungsten
filament lamp used for general lighting purposes, which employs drawn wire, has
an efficiency of about 12 lumens per watt, while lamps of high wattage have
efficiencies up to about 22 lumens per watt. The modem fluorescent lamp,
although it employs tungsten cathodes, does not depend upon tungsten for its
much higher efficiency, which is of the order of 50 lumens per watt.
In 1904 the Siemens-Halske Co. tried to apply the drawing process they had
developed for tantalum to the production of filaments of the more refractory
metals, tungsten, thorium, etc. The brittleness and lack of ductility of
tungsten prevented their attaining success by this method, although later, in
1913-1914, it was demonstrated that fused tungsten could be rolled and drawn at
very high temperatures, using very small reduction steps. By striking an arc
between a tungsten rod and a partially sintered
tungsten pellet in a graphite crucible, coated on the inside with tungsten metal
powder and containing an atmosphere of hydrogen, small pieces of fused tungsten,
about 10 mm. diameter and 20-30 mm. long, were produced, which could be worked
with difficulty. It was found that working properties could be improved to some
extent by the addition of thorium oxide, which reduces
the tendency to develop a columnar type of structure during cooling of the fused
mass. This process was never used commercially. In the same year Just and
Hannaman patented a process for producing tungsten filaments by mixing the
finely divided metal powder with an organic binder, extruding through dies, and
heating in suitable gases to remove the binder, leaving a pure
tungsten filament. During 1906-7, the well known extrusion process, which was
the method by which the majority of tungsten filaments were made for the next
four or five years, was developed.
The process consisted in mixing very fine black tungsten powder with
dextrin or starch in order to form a plastic mass, which was forced under
hydraulic pressure through a fine diamond die. The thread produced in this way
was sufficiently strong to be wound on cards and dried. The filament was then
cut into "hairpins," which were heated in an inert gas to a red heat to drive
out moisture and the lighter hydrocarbons. Each "hairpin" was then mounted in
clips, and raised to bright incandescence by the passage of an electric current,
whilst being surrounded by a gas, such as hydrogen, chosen to react with the
binding material, so that pure tungsten only remained. At the highest
temperature the fine particles of tungsten sintered together and formed a solid
homogeneous metallic filament. These filaments, although elastic, were quite
brittle, but could be formed to shape at a red heat.
Just and Hannaman also developed another process at the same time. This was
known as the "coating" process, and showed remarkable ingenuity. A carbon
filament as small as 0.02 mm. in diameter was employed as the base, and this was
coated with tungsten by raising it to incandescence in an atmosphere of hydrogen
and tungsten hexachloride. The coated filament was then raised to bright
incandescence in hydrogen at a pressure of about 20 mm. of mercury. The carbon
core dissolved in the tungsten, forming tungsten carbide, the change being so
complete that the resulting filament was tubular in cross-section, no carbon
remaining in the core. The filament so obtained presented a glittering white
appearance and was very fragile. The next step
consisted in heating the filament in hydrogen containing steam, which oxidized
the carbon and left a compact filament of pure tungsten. The filaments thus
obtained were similar to those made by the extrusion process, except that they
were tubular in cross-section.
Many other processes for the production of tungsten filaments appeared in
the following years, but the product obtained was in all cases of the same type,
namely, an elastic but brittle tungsten filament. Amongst the more important may
be mentioned the colloidal method of Kuzel, first developed in 1904. By this
method a gelatinous pasty mass of metallic tungsten was prepared by striking an
arc between tungsten electrodes under water. The material contained no binding
medium, but was itself sufficiently
plastic to be extruded into fine threads. On heating these to a high temperature
in hydrogen by means of an electric current, the colloidal mass was converted
into crystalline metal and the filaments were in all respects similar to those
produced by the ordinary extrusion process. The method was largely used on the
European Continent, and to some extent in the United States.
Another method successfully developed in America in 1906 was the amalgam
process. Finely divided tungsten powder was mechanically mixed with twice its
weight of cadmium-mercury amalgam, from which filaments were formed by
extrusion. The filaments were strong and exceedingly ductile. The amalgam was
subsequently removed by volatilization at a high temperature and
a pure tungsten filament was obtained. A method which achieved considerable
success, and was used between 1908 and 1910 by the Siemens and Halske Co., was
that of mixing tungsten metal powder with 6-10 percent of nickel, as nickel
oxide, pressing the powder into ingots, and sintering in hydrogen at 1575° C.
The ingots were first rolled to rod of 1 mm. diameter at 350° C.,
and then, with frequent anneals at 1500-1600° C., drawn cold to wire as fine as
0.03 mm. The drawn wire was quite ductile. The nickel was removed by heating the
finished filaments in vacuo at 1500° C. A full account of this process has been
given by M. Pirani. Other processes were also developed, such as drawing wires
from tantalum tubes packed with tungsten powder. It was not, however, until 1909
that Coolidge, in America, was successful in making ductile tungsten from the
metal powder by suitable heat treatment and mechanical working.
In all previous processes some binding agent, either organic or metallic,
had been employed to give the necessary plasticity, and was subsequently removed
by chemical or thermal treatment. The filaments that resulted were pure tungsten
as far as analysis could show, and yet the metal was in all cases completely
brittle. Even these brittle filaments, however, could be bent and worked to some
extent at a relatively low temperature, and even at temperatures below that at
which oxidation takes place.The problem of making ductile
tungsten did not, therefore, appear to be one of purifying the material,
although it was realized that pure metal was probably essential if a ductile
product was to be obtained. Rather, the problem was caused by the grain
structure of the tungsten itself. By using a sufficiently high temperature
initially, it was found that as the metal was subjected to mechanical work its
ductility increased, until finally it became so ductile that it could be rolled,
or drawn into wire, at room
temperature.
Although only a small percentage of the ore which comes on the market is
used for the manufacture of lamp filaments and similar products, the great
importance which tungsten has assumed scientifically and technically is the
outcome of work directed to its production for this purpose. The knowledge
gained has also been of inestimable value to workers in the newer fields of
powder metallurgy, particularly in the manufacture of hard carbides.
Consideration of the stages that have been passed in the development of modern
processes gives some understanding of the difficulties that have been overcome.
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