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58-424: A Wehnelt cap has the shape of a topless, hollow cylinder. The bottom side of the cylinder has an aperture (through hole) located at its center, with a diameter that typically ranges from 200 to 1200 μm. The bottom face of the cylinder is often made from platinum or tantalum foil. A Wehnelt acts as a control grid and it also serves as a convergent electrostatic lens . An electron emitter is positioned directly above

116-642: A froth flotation process to recover molybdenite from ores; flotation remains the primary isolation process. During World War I , demand for molybdenum spiked; it was used both in armor plating and as a substitute for tungsten in high-speed steels . Some British tanks were protected by 75 mm (3 in) manganese steel plating, but this proved to be ineffective. The manganese steel plates were replaced with much lighter 25 mm (1.0 in) molybdenum steel plates allowing for higher speed, greater maneuverability, and better protection. The Germans also used molybdenum-doped steel for heavy artillery, like in

174-429: A pterin -based molybdenum cofactor (Moco) in their active site: sulfite oxidase , xanthine oxidoreductase , aldehyde oxidase , and mitochondrial amidoxime reductase . People severely deficient in molybdenum have poorly functioning sulfite oxidase and are prone to toxic reactions to sulfites in foods. The human body contains about 0.07 mg of molybdenum per kilogram of body weight, with higher concentrations in

232-649: A cation in aqueous solution, although the Mo cation is known to form under carefully controlled conditions. Gaseous molybdenum consists of the diatomic species Mo 2 . That molecule is a singlet , with two unpaired electrons in bonding orbitals, in addition to 5 conventional bonds. The result is a sextuple bond . There are 39 known isotopes of molybdenum, ranging in atomic mass from 81 to 119, as well as 13 metastable nuclear isomers . Seven isotopes occur naturally, with atomic masses of 92, 94, 95, 96, 97, 98, and 100. Of these naturally occurring isotopes, only molybdenum-100

290-648: A commodity. The Knaben mine in southern Norway, opened in 1885, was the first dedicated molybdenum mine. Closed in 1973 but reopened in 2007, it now produces 100,000 kilograms (98 long tons; 110 short tons) of molybdenum disulfide per year. Large mines in Colorado (such as the Henderson mine and the Climax mine ) and in British Columbia yield molybdenite as their primary product, while many porphyry copper deposits such as

348-411: A cylindrical cathode) and a cylindrical anode. The grid is usually made of a very thin wire that can resist high temperatures and is not prone to emitting electrons itself. Molybdenum alloy with a gold plating is frequently used. It is wound on soft copper sideposts, which are swaged over the grid windings to hold them in place. A 1950s variation is the frame grid, which winds very fine wire onto

406-447: A flame-resistant coating for other metals. Although its melting point is 2,623 °C (4,753 °F), molybdenum rapidly oxidizes at temperatures above 760 °C (1,400 °F) making it better-suited for use in vacuum environments. TZM (Mo (~99%), Ti (~0.5%), Zr (~0.08%) and some C) is a corrosion-resisting molybdenum superalloy that resists molten fluoride salts at temperatures above 1,300 °C (2,370 °F). It has about twice

464-524: A greater supply of molybdenum in the soil, people living in those areas have about 16 times greater risk for esophageal squamous cell carcinoma . Molybdenum deficiency has also been reported as a consequence of non-molybdenum supplemented total parenteral nutrition (complete intravenous feeding) for long periods of time. It results in high blood levels of sulfite and urate , in much the same way as molybdenum cofactor deficiency . Since pure molybdenum deficiency from this cause occurs primarily in adults,

522-511: A net electric field (due to both anode attraction and Wehnelt repulsion) that allows emission from only that area of the tip. The Wehnelt bias voltage determines the tip's emission area, which in turn determines both the beam current and effective size of the beam's electron source. As the Wehnelt's negative bias voltage increases, the tip's emitting area (and along with it, the beam diameter and beam current) will decrease until it becomes so small that

580-413: A process of purine catabolism , is catalyzed by xanthine oxidase , a molybdenum-containing enzyme. The activity of xanthine oxidase is directly proportional to the amount of molybdenum in the body. An extremely high concentration of molybdenum reverses the trend and can inhibit purine catabolism and other processes. Molybdenum concentration also affects protein synthesis , metabolism , and growth. Mo

638-419: A rigid stamped metal frame. This allows the holding of very close tolerances, so the grid can be placed closer to the filament (or cathode). By placing the control grid closer to the filament/cathode relative to the anode, a greater amplification results. This degree of amplification is referred to in valve data sheets as the amplification factor , or "mu". It also results in higher transconductance , which

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696-500: A surface or as a solid lubricant. Even when molybdena was distinguishable from graphite, it was still confused with the common lead ore PbS (now called galena ); the name comes from Ancient Greek Μόλυβδος molybdos , meaning lead . (The Greek word itself has been proposed as a loanword from Anatolian Luvian and Lydian languages). Although (reportedly) molybdenum was deliberately alloyed with steel in one 14th-century Japanese sword (mfd. c.  1330 ), that art

754-401: A very parallel beam (for diffraction). Although a smaller source can be imaged to a smaller spot, or a more parallel beam, one obvious trade off is a smaller total beam current. Control grid The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode , tetrode and pentode , used to control the flow of electrons from the cathode to

812-455: Is a chemical element ; it has symbol Mo (from Neo-Latin molybdaenum ) and atomic number 42. The name derived from Ancient Greek Μόλυβδος molybdos , meaning lead , since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered (in the sense of differentiating it as a new entity from the mineral salts of other metals) in 1778 by Carl Wilhelm Scheele . The metal

870-853: Is a fission product . It is a parent radioisotope to the short-lived gamma-emitting daughter radioisotope technetium-99m , a nuclear isomer used in various imaging applications in medicine. In 2008, the Delft University of Technology applied for a patent on the molybdenum-98-based production of molybdenum-99. Molybdenum forms chemical compounds in oxidation states −4 and from −2 to +6. Higher oxidation states are more relevant to its terrestrial occurrence and its biological roles, mid-level oxidation states are often associated with metal clusters , and very low oxidation states are typically associated with organomolybdenum compounds . The chemistry of molybdenum and tungsten show strong similarities. The relative rarity of molybdenum(III), for example, contrasts with

928-488: Is a component in most nitrogenases . Among molybdoenzymes, nitrogenases are unique in lacking the molybdopterin. Nitrogenases catalyze the production of ammonia from atmospheric nitrogen: The biosynthesis of the FeMoco active site is highly complex. Molybdate is transported in the body as MoO 4 . Molybdenum is an essential trace dietary element . Four mammalian Mo-dependent enzymes are known, all of them harboring

986-419: Is a measure of the anode current change versus grid voltage change. The noise figure of a valve is inversely proportional to its transconductance; higher transconductance generally means lower noise figure. Lower noise can be very important when designing a radio or television receiver. A valve can contain more than one control grid. The hexode contains two such grids, one for a received signal and one for

1044-399: Is also used in steel alloys for its high corrosion resistance and weldability . Molybdenum contributes corrosion resistance to type-300 stainless steels (specifically type-316) and especially so in the so-called superaustenitic stainless steels (such as alloy AL-6XN , 254SMO and 1925hMo). Molybdenum increases lattice strain, thus increasing the energy required to dissolve iron atoms from

1102-526: Is an essential element for all higher eukaryote organisms, including humans. A species of sponge , Theonella conica , is known for hyperaccumulation of molybdenum. In its pure form, molybdenum is a silvery-grey metal with a Mohs hardness of 5.5 and a standard atomic weight of 95.95 g/mol. It has a melting point of 2,623 °C (4,753 °F), sixth highest of the naturally occurring elements; only tantalum , osmium , rhenium , tungsten , and carbon have higher melting points. It has one of

1160-495: Is an essential element in most organisms; a 2008 research paper speculated that a scarcity of molybdenum in the Earth's early oceans may have strongly influenced the evolution of eukaryotic life (which includes all plants and animals). At least 50 molybdenum-containing enzymes have been identified, mostly in bacteria. Those enzymes include aldehyde oxidase , sulfite oxidase and xanthine oxidase . With one exception, Mo in proteins

1218-433: Is bound by molybdopterin to give the molybdenum cofactor. The only known exception is nitrogenase , which uses the FeMoco cofactor, which has the formula Fe 7 MoS 9 C. In terms of function, molybdoenzymes catalyze the oxidation and sometimes reduction of certain small molecules in the process of regulating nitrogen , sulfur , and carbon . In some animals, and in humans, the oxidation of xanthine to uric acid ,

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1276-399: Is often exploited in R.F. amplifiers where an alteration of the grid bias changes the mutual conductance and hence the gain of the device. This variation usually appears in the pentode form of the valve, where it is then called a variable-mu pentode or remote-cutoff pentode. One of the principal limitations of the triode valve is that there is considerable capacitance between the grid and

1334-557: Is reduced by the aluminothermic reaction with addition of iron to produce ferromolybdenum . A common form of ferromolybdenum contains 60% molybdenum. Molybdenum had a value of approximately $ 30,000 per tonne as of August 2009. It maintained a price at or near $ 10,000 per tonne from 1997 through 2003, and reached a peak of $ 103,000 per tonne in June 2005. In 2008, the London Metal Exchange announced that molybdenum would be traded as

1392-400: Is separated at this stage by treatment with hydrogen sulfide . Ammonium molybdate converts to ammonium dimolybdate , which is isolated as a solid. Heating this solid gives molybdenum trioxide: Crude trioxide can be further purified by sublimation at 1,100 °C (2,010 °F). Metallic molybdenum is produced by reduction of the oxide with hydrogen: The molybdenum for steel production

1450-482: Is unstable. Molybdenum-98 is the most abundant isotope, comprising 24.14% of all molybdenum. Molybdenum-100 has a half-life of about 10   y and undergoes double beta decay into ruthenium-100. All unstable isotopes of molybdenum decay into isotopes of niobium , technetium , and ruthenium . Of the synthetic radioisotopes , the most stable is Mo, with a half-life of 4,839 years. The most common isotopic molybdenum application involves molybdenum-99 , which

1508-527: Is used as the valve body of torpedo engines, rocket nozzles and gas pipelines, where it can withstand extreme thermal and mechanical stresses. It is also used as radiation shields in nuclear applications. Other molybdenum-based alloys that do not contain iron have only limited applications. For example, because of its resistance to molten zinc, both pure molybdenum and molybdenum- tungsten alloys (70%/30%) are used for piping, stirrers and pump impellers that come into contact with molten zinc. Molybdenum

1566-561: Is used for the spectroscopic detection of phosphorus. The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides: The accessibility of these oxidation states depends quite strongly on the halide counterion: although molybdenum(VI) fluoride is stable, molybdenum does not form a stable hexachloride, pentabromide, or tetraiodide. Like chromium and some other transition metals, molybdenum forms quadruple bonds , such as in Mo 2 (CH 3 COO) 4 and [Mo 2 Cl 8 ] . The Lewis acid properties of

1624-437: Is used in steel alloys, including high-strength alloys and superalloys . Most molybdenum compounds have low solubility in water. Heating molybdenum-bearing minerals under oxygen and water affords molybdate ion MoO 4 , which forms quite soluble salts. Industrially, molybdenum compounds (about 14% of world production of the element) are used as pigments and catalysts . Molybdenum-bearing enzymes are by far

1682-409: Is usually a tungsten filament or Lanthanum hexaboride (LaB 6 ) hot cathode with a V-shaped (or otherwise pointed) tip. This bias voltage creates a repulsive electrostatic field that suppresses emission of electrons from most areas of the cathode. The emitter tip is positioned near the Wehnelt aperture so that, when appropriate bias voltage is applied to the Wehnelt, a small region of the tip has

1740-1147: The Bingham Canyon Mine in Utah and the Chuquicamata mine in northern Chile produce molybdenum as a byproduct of copper-mining. About 86% of molybdenum produced is used in metallurgy , with the rest used in chemical applications. The estimated global use is structural steel 35%, stainless steel 25%, chemicals 14%, tool & high-speed steels 9%, cast iron 6%, molybdenum elemental metal 6%, and superalloys 5%. Molybdenum can withstand extreme temperatures without significantly expanding or softening, making it useful in environments of intense heat, including military armor, aircraft parts, electrical contacts, industrial motors, and supports for filaments in light bulbs . Most high-strength steel alloys (for example, 41xx steels ) contain 0.25% to 8% molybdenum. Even in these small portions, more than 43,000 tonnes of molybdenum are used each year in stainless steels , tool steels , cast irons, and high-temperature superalloys . Molybdenum

1798-572: The anode (plate) electrode. The control grid usually consists of a cylindrical screen or helix of fine wire surrounding the cathode, and is surrounded in turn by the anode. The control grid was invented by Lee De Forest , who in 1906 added a grid to the Fleming valve ( thermionic diode ) to create the first amplifying vacuum tube, the Audion ( triode ). In a valve, the hot cathode emits negatively charged electrons , which are attracted to and captured by

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1856-460: The 1920s had figures which are strictly comparable, so there was no advance in this area. However, early screen-grid tetrodes of the 1920s, have C ag of only 1 or 2 fF, around a thousand times less. 'Modern' pentodes have comparable values of C ag . Triodes were used in VHF amplifiers in 'grounded-grid' configuration, a circuit arrangement which prevents Miller feedback. Molybdenum Molybdenum

1914-500: The Earth's crust with an average of 1.5 parts per million and the 25th most abundant element in the oceans, with an average of 10 parts per billion; it is the 42nd most abundant element in the Universe. The Soviet Luna 24 mission discovered a molybdenum-bearing grain (1 × 0.6 μm) in a pyroxene fragment taken from Mare Crisium on the Moon . The comparative rarity of molybdenum in

1972-478: The Earth's crust is offset by its concentration in a number of water-insoluble ores, often combined with sulfur in the same way as copper, with which it is often found. Though molybdenum is found in such minerals as wulfenite (PbMoO 4 ) and powellite (CaMoO 4 ), the main commercial source is molybdenite (Mo S 2 ). Molybdenum is mined as a principal ore and is also recovered as a byproduct of copper and tungsten mining. The world's production of molybdenum

2030-406: The Wehnelt aperture, and an anode is located below the Wehnelt. The anode is biased to a high positive voltage (typically +1 kV to +30 kV) relative to the emitter so as to accelerate electrons from the emitter towards the anode, thus creating an electron beam that passes through the Wehnelt aperture. The Wehnelt is biased to a negative voltage (typically −200V to −300V) relative to the emitter, which

2088-471: The alloys on a large scale were hampered with inconsistent results, a tendency toward brittleness, and recrystallization. In 1906, William D. Coolidge filed a patent for rendering molybdenum ductile , leading to applications as a heating element for high-temperature furnaces and as a support for tungsten-filament light bulbs; oxide formation and degradation require that molybdenum be physically sealed or held in an inert gas. In 1913, Frank E. Elmore developed

2146-470: The anode (C ag ). A phenomenon known as the Miller Effect causes the input capacitance of an amplifier to be the product of C ag and amplification factor of the valve. This, and the instability of an amplifier with tuned input and output when C ag is large can severely limit the upper operating frequency. These effects can be overcome by the addition of a screen grid , however in the later years of

2204-439: The anode current. A given change in grid voltage causes a proportional change in plate current, so if a time-varying voltage is applied to the grid, the plate current waveform will be a copy of the applied grid voltage. A relatively small variation in voltage on the control grid causes a significantly large variation in anode current. The presence of a resistor in the anode circuit causes a large variation in voltage to appear at

2262-407: The anode, which is given a positive voltage by a power supply. The control grid between the cathode and anode functions as a "gate" to control the current of electrons reaching the anode. A more negative voltage on the grid will repel the electrons back toward the cathode so fewer get through to the anode. A less negative, or positive, voltage on the grid will allow more electrons through, increasing

2320-413: The anode. The variation in anode voltage can be much larger than the variation in grid voltage which caused it, and thus the tube can amplify, functioning as an amplifier . The grid in the first triode valve consisted of a zig-zag piece of wire placed between the filament and the anode. This quickly evolved into a helix or cylindrical screen of fine wire placed between a single strand filament (or later,

2378-421: The beam is "pinched" off. In normal operation, the bias is typically set slightly more positive than the pinch bias, and determined by a balance between desired beam quality and beam current. The Wehnelt bias controls beam focusing as well as the effective size of the electron source, which is essential for creating an electron beam that is to be focussed into a very small spot (for scanning electron microscopy) or

Wehnelt cylinder - Misplaced Pages Continue

2436-484: The butyrate and perfluorobutyrate dimers, Mo 2 (O 2 CR) 4 and Rh 2 (O 2 CR) 4 , have been reported. The oxidation state 0 and lower are possible with carbon monoxide as ligand, such as in molybdenum hexacarbonyl , Mo(CO) 6 . Molybdenite —the principal ore from which molybdenum is now extracted—was previously known as molybdena. Molybdena was confused with and often utilized as though it were graphite . Like graphite, molybdenite can be used to blacken

2494-585: The liver and kidneys and lower in the vertebrae. Molybdenum is also present within human tooth enamel and may help prevent its decay. Acute toxicity has not been seen in humans, and the toxicity depends strongly on the chemical state. Studies on rats show a median lethal dose (LD 50 ) as low as 180 mg/kg for some Mo compounds. Although human toxicity data is unavailable, animal studies have shown that chronic ingestion of more than 10 mg/day of molybdenum can cause diarrhea, growth retardation, infertility , low birth weight, and gout ; it can also affect

2552-583: The lowest coefficients of thermal expansion among commercially used metals. Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale. It does not visibly react with oxygen or water at room temperature, but is attacked by halogens and hydrogen peroxide. Weak oxidation of molybdenum starts at 300 °C (572 °F); bulk oxidation occurs at temperatures above 600 °C, resulting in molybdenum trioxide . Like many heavier transition metals, molybdenum shows little inclination to form

2610-471: The lungs, kidneys, and liver. Sodium tungstate is a competitive inhibitor of molybdenum. Dietary tungsten reduces the concentration of molybdenum in tissues. Low soil concentration of molybdenum in a geographical band from northern China to Iran results in a general dietary molybdenum deficiency and is associated with increased rates of esophageal cancer . Compared to the United States, which has

2668-453: The mineral in which it resided, and from which it might be isolated. Peter Jacob Hjelm successfully isolated molybdenum using carbon and linseed oil in 1781. For the next century, molybdenum had no industrial use. It was relatively scarce, the pure metal was difficult to extract, and the necessary techniques of metallurgy were immature. Early molybdenum steel alloys showed great promise of increased hardness, but efforts to manufacture

2726-560: The most common bacterial catalysts for breaking the chemical bond in atmospheric molecular nitrogen in the process of biological nitrogen fixation . At least 50 molybdenum enzymes are now known in bacteria, plants, and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation. Most nitrogenases contain an iron–molybdenum cofactor FeMoco , which is believed to contain either Mo(III) or Mo(IV). By contrast Mo(VI) and Mo(IV) are complexed with molybdopterin in all other molybdenum-bearing enzymes. Molybdenum

2784-414: The pervasiveness of the chromium(III) compounds. The highest oxidation state is seen in molybdenum(VI) oxide (MoO 3 ), whereas the normal sulfur compound is molybdenum disulfide MoS 2 . From the perspective of commerce, the most important compounds are molybdenum disulfide ( MoS 2 ) and molybdenum trioxide ( MoO 3 ). The black disulfide is the main mineral. It is roasted in air to give

2842-403: The rest of Asia (mostly Armenia, Russia, Iran and Mongolia) produce the remainder. In molybdenite processing, the ore is first roasted in air at a temperature of 700 °C (1,292 °F). The process gives gaseous sulfur dioxide and the molybdenum(VI) oxide : The resulting oxide is then usually extracted with aqueous ammonia to give ammonium molybdate: Copper, an impurity in molybdenite,

2900-427: The signal from a local oscillator. The valve's inherent non-linearity causes not only both original signals to appear in the anode circuit, but also the sum and difference of those signals. This can be exploited as a frequency-changer in superheterodyne receivers. A variation of the control grid is to produce the helix with a variable pitch. This gives the resultant valve a distinct non-linear characteristic. This

2958-409: The strength of pure Mo, and is more ductile and more weldable, yet in tests it resisted corrosion of a standard eutectic salt ( FLiBe ) and salt vapors used in molten salt reactors for 1100 hours with so little corrosion that it was difficult to measure. Due to its excellent mechanical properties under high temperature and high pressure, TZM alloys are extensively applied in the military industry. It

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3016-420: The super-heavy howitzer Big Bertha , because traditional steel melts at the temperatures produced by the propellant of the one ton shell. After the war, demand plummeted until metallurgical advances allowed extensive development of peacetime applications. In World War II , molybdenum again saw strategic importance as a substitute for tungsten in steel alloys. Molybdenum is the 54th most abundant element in

3074-453: The surface. Molybdenum is also used to enhance the corrosion resistance of ferritic (for example grade 444) and martensitic (for example 1.4122 and 1.4418) stainless steels. Because of its lower density and more stable price, molybdenum is sometimes used in place of tungsten. An example is the 'M' series of high-speed steels such as M2, M4 and M42 as substitution for the 'T' steel series, which contain tungsten. Molybdenum can also be used as

3132-678: The trioxide: The trioxide, which is volatile at high temperatures, is the precursor to virtually all other Mo compounds as well as alloys. Molybdenum has several oxidation states , the most stable being +4 and +6 (bolded in the table at left). Molybdenum(VI) oxide is soluble in strong alkaline water, forming molybdates (MoO 4 ). Molybdates are weaker oxidants than chromates . They tend to form structurally complex oxyanions by condensation at lower pH values, such as [Mo 7 O 24 ] and [Mo 8 O 26 ] . Polymolybdates can incorporate other ions, forming polyoxometalates . The dark-blue phosphorus -containing heteropolymolybdate P[Mo 12 O 40 ]

3190-450: The tube era, constructional techniques were developed that rendered this 'parasitic capacitance' so low that triodes operating in the upper very high frequency (VHF) bands became possible. The Mullard EC91 operated at up to 250 MHz. The anode-grid capacitance of the EC91 is quoted in manufacturer's literature as 2.5 pF, which is higher than many other triodes of the era, while many triodes of

3248-629: Was 250,000 tonnes in 2011, the largest producers being China (94,000 t), the United States (64,000 t), Chile (38,000 t), Peru (18,000 t) and Mexico (12,000 t). The total reserves are estimated at 10 million tonnes, and are mostly concentrated in China (4.3 Mt), the US (2.7 Mt) and Chile (1.2 Mt). By continent, 93% of world molybdenum production is about evenly shared between North America, South America (mainly in Chile), and China. Europe and

3306-413: Was first isolated in 1781 by Peter Jacob Hjelm . Molybdenum does not occur naturally as a free metal on Earth; in its minerals, it is found only in oxidized states . The free element, a silvery metal with a grey cast, has the sixth-highest melting point of any element. It readily forms hard, stable carbides in alloys , and for this reason most of the world production of the element (about 80%)

3364-478: Was never employed widely and was later lost. In the West in 1754, Bengt Andersson Qvist examined a sample of molybdenite and determined that it did not contain lead and thus was not galena. By 1778 Swedish chemist Carl Wilhelm Scheele stated firmly that molybdena was (indeed) neither galena nor graphite. Instead, Scheele correctly proposed that molybdena was an ore of a distinct new element, named molybdenum for

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