Beryllium played an important role in the discovery of the neutron. The nuclear reactions that occurred when fast alpha-particles collided with light nuclei were extensively studied. The (α,p) [alpha in, proton out] reaction was an example, as in N14(α,p)O17. The reaction with Be9 produced a very penetrating radiation, very unlike a fast proton, that was initially believed to be a gamma ray (photon). However, in 1932 Chadwick showed that it was an uncharged massive particle that could eject protons by collision, which a gamma could never do. The reaction was, in fact, Be9(α,n)C12. This solved the outstanding problem of the constitution of the nucleus, since everything was consistent with an assembly of Z protons and A - Z neutrons, all with half-integral spin.

The thermal neutron absorption cross section of Be9 is only 10 mbarns, a rather small value, so beryllium makes a good neutron moderator in fission reactors. It is lighter than C (9 vs. 12), so a neutron can lose more energy in one collision with beryllium that with carbon. Neutron absorption reactions are Be9(n,α)He6 (winding up as Li6), and Be9(n,2n)Be8 (winding up as 2α). Beryllium is not only a moderator, but also a source of neutrons. Beryllium was considered a promising material for high-temperature nuclear reactors (carbon, of course, cannot be used). Beryllium was used as a neutron reflector to reduce the size of reactor cores. It is used in nuclear weapons for the same purpose.

Metallurgy

Beryllium ores contain no more than 5% Be, because it is so light. The first step is to decompose the beryl, and separate the beryllium. This can be done with hydrofluoric acid or fluorides, producing a soluble fluoberyllate such as BeF2·2KF. Sulphates or chlorides can also be formed. Beryllium can then be precipitated as the hydroxide Be(OH)2, which on heating gives BeO. BeO, beryllia, is a very useful ceramic with a melting point of 2570°C and great resistance to thermal shock.

The metal can be produced by electrolysis at temperatures just below its melting point, or about 1300°C. Unlike most metallic halides, BeCl2 is poorly conducting when fused, so it is usually mixed with NaCl. Barium and sodium fluorides, in which BeO or BeF2 are dissolved, can also be electrolyzed. The metal is obtained in fine flakes or globules, and considerable processing is necessary to remove the slag. BeO can be reduced by carbon, but the product is the carbide, Be2C. Currently, the preferred method is reducing BeF2 by magnesium metal. In general, the metallurgy of beryllium is very difficult.

Beryllium and beryllium oxide in any forms are quite expensive, and this fact limits their use. The current price (2004) for the powder metal is $375 per pound, and for copper master alloy, $160 per pound of Be content.

Properties and Uses

Beryllium crystallizes in the hexagonal close packed structure. It is definitely a metal, but a hard and brittle one. Its electron configuration is 1s22s2, so its compounds can be expected to be electron-poor and somewhat exotic. It is a rather small ion, of radius 0.31Å. Its ionic valence is clearly 2, and this is shown in numerous compounds. The first ionizing potential is 9.28V, greater even than that of magnesium. In the halides, the electron transfer is not by any means complete, and these compounds do not ionize easily. The compounds of beryllium are colorless. Aside from these properties, beryllium behaves similarly to aluminium. It is not much like magnesium, calcium or barium, the other elements called the alkaline earths, except in valence. In fact, it is not very alkaline at all, and its oxide and hydroxide are not even soluble. It is not mentioned in qualtitative analysis texts, since it is encountered very rarely. It probably is separated with the aluminium and must then be distinguished from it by the precipitation of a basic carbonate by adding ammonium carbonate. An excess of reagent dissolves the precipitate. Like aluminum, it forms a protective oxide layer on exposure to air, which makes the surface very hard. Beryllium resists atmospheric corrosion at high temperatures better than titanium or zirconium. Above 600°C, the oxide is first formed, and then the nitride, Be3N2 at 1000°C. Metallic beryllium should not be used at temperatures over 600°C.

The specific gravity is 1.85, only slightly greater than that of magnesium (1.74) and considerably less than that of aluminium (2.7). Its hardness is 6.5, melting point 1285°C, boiling point 2780°C. It would be an exellent light, strong structural material for high temperatures if it were not for its brittleness and extreme difficulty of working. Its electrical resistivity is 18.5 μΩ-cm, a relatively low value, making it useful for electrical leads. The thermal expansion coefficient is 12.3 x 10-6 °C-1, heat capacity 0.425 cal/g/°C and heat conductivity 0.3847 cal/s/cm2/°C/cm. The latent heat of fusion is 341 cal/g.

As a sintered powder (1000°C, 500 psi), the ultimate strength is 45,000 psi, yield point 25,000 psi, Young's modulus 44 x 106 psi, and Poisson's ratio 0.024 (? this seems a very small value). The high Young's modulus and low density make the speed of sound in beryllium quite large (12,500 m/s). The elongation of a tensile specimen on fracture is only around 2%, so the ductility is low. Impact strength is also low. Hot-pressed beryllium can be succesfully machined and drilled. Beryllium can also be vacuum cast, but it is very difficult to machine the castings. Beryllium can be forged hot if encased in steel, and it can be welded, but must be protected from the air.

The alloy 97.75 Cu, 2.25 Be has six times the strength of copper. It is nonsparking (chips do not oxidize readily in air), nonmagnetic, and does not exhibit fatigue failure. Similar alloys may have from 2% to 3% beryllium. This material makes excellent springs, and is a good electrical conductor, since the resistivity of the copper is not raised excessively by the beryllium. Beryllium for alloying is supplied as a 4% alloy with copper, called "master alloy." Since little beryllium is used, its high cost is a minor factor. Be-Al alloys, with up to 65% Be, are also being studied. Phosphor bronze is a substitute for beryllium copper, but is not as serviceable. The Chemical Society's internet periodic table says beryllium is used "to increase the ability to conduct electricity" in copper and nickel, but this is erroneous. Beryllium improves the mechanical properties of the metals, but does not increase the resistivity as much as other alloying elements.

Beryllium and its compounds are very poisonous, especially as dusts. When inhaled, they can produce beryllosis, which is like silicosis, and destroys the lungs. Although some people are little affected, others can develop a sensititivity to beryllium called chronic beryllium disease that scars the lungs. Carcinoma can also result from beryllium poisoning. It is chilling to think that glucinium was named because of the sweet taste of its compounds; its poisonous nature was probably verified at the same time. BeO was used as a phosphor in fluorescent lamps, which made broken discarded fluorescent lamps a great hazard. I understand that BeO phosphor is no longer used. This is much more of a hazard than mercury in refuse. Atomic weapons workers are also subject to beryllium poisoning, although great care has been taken to eliminate the hazard. It is easy to blame beryllium for any lung problems that occur with these workers whatever the cause, to the great delight of lawyers. Beryllium copper and similar uses of the metal, or of beryllia, are not hazardous. There is very little beryllium in the environment, and no evidence that trace amounts are dangerous. The EPA limit is 0.01 μg/m3

in air, the OSHA limit 2 μg/m3 for an 8-hour shift. Beryllium dust from burning coal is a negligible hazard, because of the very small amounts involved.

Because of its low atomic number, beryllium is nearly transparent to X-rays and can be used as windows for X-ray tubes. Currently, the greatest demand for beryllium comes from the telecommunications equipment industry.

References