Crucible material

Boron nitride is one of the most outstanding corrosion-resistant materials. It is inert to gasoline, benzene, alcohol, acetone, chlorinated hydrocarbons and other organic solvents. It is not wetted by molten aluminum, copper, cadmium, iron, antimony, bismuth, silicon, germanium, nor by many molten salts and glasses. It is used extensively as crucible material, particularly for molten metals, glasses and ceramic processing. 

Elements dissolved in boron influence its crystal structure. Dissolved impurities also influenee the physical and chemical properties of boron, especially the electrical properties, because boron is a semiconductor. Preparation of solid solutions in jS-rh boron requires a careful choice of crucible material. To avoid contamination, boron nitride or a cold, coinage-metal crucible should be used or the levitation or floating-zone melting techniques applied. 

Most metcils can be used as crucible materials, but only the precious metals seem to possess the non-reactivily required for melts at the higher temperatures. Altunina is less shock- resistant than silica but can be used at higher temperatures. Shock-resistance relates to how fast one can heat the crucible and its contents up to the melting point of the material without cracking the crucible. Zr02 and BN are most useful for metal... 

The high temperatures attainable in a plasma furnace, with no restrictions with regard to factors such as the furnace atmosphere or the crucible material, has made processing by thermal decomposition a practicable and useful method. A good example is the thermal decomposition of low-cost zircon (ZrSi04) to produce zirconium dioxide (Zr02). The fur-... 

Crucibles must be suitable for the required experimental conditions with respect to their material, capacity and shape. As in the techniques of the chemical laboratory, crucible materials are selected to avoid the possibility of reaction between crucible and sample material. The main materials used are the precious metals, oxide ceramics, quartz and graphite. The size of the crucible is determined by the volume to be weighed. 

Table 2 shows a compilation of different crucible materials, the working temperatures, atmospheres and some important physical data. Metal crucibles are used more for the investigation of clays, oxides, ceramics, glasses, inorganic materials as... 

Crucible material Usable in Usable in air to vacuum to °C °C Fusion point [°C Density [g cm s I Gas tight- ness Resistance against temperature changes Chemical properties... 

Hlukhyy and Pottgen (2004) prepared Pd-Mg compounds (Pd2Mg5) and Ir-Mg-In alloys. Pd2Mg5 was synthesized from cold pressed pellets prepared from Pd powder and Mg small pieces. The pellets were sealed under an argon pressure of 800 mbar in niobium tubes. The niobium tubes placed in the water-cooled sample chamber of an induction furnace were first heated at about 1000K for half a minute and then cooled down to 700 K within 30 minutes and quenched by switching off the furnace. The samples could easily be separated from the niobium tubes, and no reaction with the crucible material was observed. 

Another difficulty arises from the chemical properties of the actinide metals. They are chemically reactive, rapidly corroded by moist air, pyrophoric, and, when in the molten state, dissolve common crucible materials. The radioactivity of short-lived isotopes of Am and Cm makes their long-term storage difficult small amounts can be stored successfully under ultrahigh vacuum. Large amounts of isotopes such 238pu with a Ti/2 of only 87.7 years are best stored under a pure inert... 

Beryllium oxide shows excellent thermal conductivity, resistance to thermal shock, and high electrical resistance. Also, it is unreactive to most chemicals. Because of these properties the compound has several applications. It is used to make refractory crucible materials and precision resistor cores as a reflector in nuclear power reactors in microwave energy windows and as an additive to glass, ceramics and plastics. 

As many physical properties of the actinide metals depend significantly on the sample purity, refining of the metals is mandatory. The choice of the refining methods is determined by the chemical reactivity of the actinide metal in the presence of the constituents of air, by high temperature reactions with crucible materials, by the specific radioactivity and the availability of the actinide elements. 

Gold and gold-based alloys are used for corrosion-resistant equipment. Gold—platinum alloys, 75 Au-25 Pt or 84 Au-15 Pt-1 Rh, are used as crucible material for many molten salts (98). Spinnerets for rayon manufacture are based on the Au—Pt system which exhibits a broad miscibility gap in the solid state so that the alloys can be age-hardened. Spinneret alloys contain 30—40% or more platinum modified by small additions of usually rhodium (99). Either gold or gold—platinum alloys are used in rupture disks for service with corrosive gases (100). 

The skutterudites do not melt congruently and involve pnicogens (P, As, Sb) that generally have high vapor pressures at the formation temperatures of the compounds. The high melting temperatures of Fe, Ru and Os coupled with the reactivity of the lanthanide metals with convenient crucible materials (e.g., Si02) makes the synthesis of many of these compounds difficult. As a result, variations in the reported properties of a particular lanthanide skutterudite compound can often be traced to differences in sample composition and quality. 

The vapour phases in equilibrium with M2Mo04 and M2W04 (M = Li or Na) have been investigated by mass spectrometry. The sodium salts were reduced by the crucible material, but the lithium ones evaporated mainly as the molecular species.511 An electron diffraction study of Ba2W04 in the gas phase has shown that each W042- ion co-ordinates as a bidentate ligand to one Ba2+ ion.512... 

The actinides are reactive metals, typified by the reactions of thorium and uranium shown in Figures 10.1 and 10.2. These binary compounds frequently have useful properties. Thus, choosing examples from thorium chemistry, Th2S3 is a high-temperature crucible material, ThN is a superconductor, and Th3P4 and Th3As4 are semiconductors. 

In the case of impure starting compounds (hydroxide and oxide layers are frequent impurities), corrosion problems appear leading, for example, to the formation of phosphide oxides. Such corrosion problems may be prevented by the use of iimer containers made from Mo, Nb, or Ta. Silica as a crucible material is also problematic, since silicon might replace some of the phosphorus atoms, and this is hardly detectable by X rays. 

Figure 3.5. Illustration of an atomizer for the production of metallic powders. The molten metal/alloy is sprayed into a cooling tower under the flow of an atomizing gas. The particulates are allowed to cool as they descend downward, and are collected in a hopper at the bottom of the tower. Reproduced with permission from Crucible Materials Corporation (http //www.cruciblecompaction.com/...

Dry ashing and wet digestion have some disadvantages that detract from accuracy and precision in the resuits. Such disadvantages include long manipulation times, an increased consumption of chemicais, increased viscosity and losses of elements by volatilization and fume-hood emissions in addition to the risk of sample contamination. Also, reactions between some analytes and crucible materials can give rise to combustion residues. 

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