Arc techniques

For the synthesis of materials, the reactants are placed in the copper crucible. An arc is struck by allowing the cathode to touch the anode. The current is raised slowly while the cathode is simultaneously withdrawn so as to maintain the arc. The arc is then positioned so that it bathes the sample in the crucible. The current is increased until the reactants melt When the arc is turned off, the product solidifies in the form of a button. Because of the enormous temperature gradient between the melt and the water-cooled crucible, a thin solid layer of the sample usually separates the melt from the copper hearth in this sense, the sample forms its own crucible and hence contamination with copper does not take place. Contamination of the sample by tungsten vaporizing from the cathode can be avoided by using water-cooled cathodes. The arc method has been successfully used for the synthesis of various oxides of Ti, V and Nb. A number of lower-valence rare-earth oxides, LnO, 5 have been prepared by arc fusion of LnjOj 

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Quality Control. The spectrometer is the most suitable instmment for determining most low level residual impurities. ASTM E414 is the standard method for the measurement of impurities in copper by the briquette dc-arc technique (65). In this method, the sample in the form of chips, drillings, or powder is briquetted and excited in a d-c arc opposite a high purity copper rod. Impurities in the ranges noted can be measured ... 

Chlorimet 2 has 63 percent Ni and 32 percent Mo and is somewhat similar to Hastelloy B-2. It is available only in cast form, mainly as valves and pumps. This is a tough alloy, very resistant to mechanical and thermal snock. It can be machined with carbide-tipped tools and welded with metal-arc techniques. 

M. and Lefrant, S., Bundles of single wall nanotubes produced by the electric arc technique. In Electron Microscopy ICEM-14, Vol. 3, ed. H. 

There is a potentially dangerous reaction of carbon tetrachloride with dimethylformamide in presence of iron. The same occurs with 1,2,3,4,5,6-hexachlorocyclohexane, but not with dichloromethane or 1,2-dichloroethane under the same conditions [1], A quantitative study of the reaction by DSC and ARC techniques shows that in a 1 1 wt. mixture with carbon tetrachloride in absence of iron, an exothermic reaction sets in below 100°C. Under adiabatic conditions, the heat release (207.6 J/g) would take a runaway reaction to over 240°C. In presence of 3% of iron powder, the same mixture shows 2 exotherms, one at 56°C (108 J/g) and the second at 94°C (275 J/g), a final adiabatic temperature exceeding 285°C being possible [2], Dimethylacetamide behaves similarly but more so. 

Exothermic decomposition of 2-nitroaniline in chemical processes was studied by DSC and ARC techniques. The stability in reaction mixtures was markedly less than for the pure, isolated compound. 

Following an explosion dining drying of 2-nitrobenzyl bromide, the thermal stability of all six isomeric nitrobenzyl halides was investigated by TGA, DSC and ARC techniques. All the halides decomposed exothermally with abundant gas evolution, and the 2-nitro chloride is less thermally stable than the 3- or 4-isomers. See 2-Nitrobenzyl bromide... 

The Ba.C) N, nanotubes and fullerene-like structures have been synthesized by various laboratories in recent years. The most popular method is the plasma arc technique. The first report on the synthesis of BN nanotubes was by Zettl and coworkers (55). Since BN is an insulator, a composite anode was prepared from a tungsten rod with an empty bore in the center, which was stuffed with a pressed hexagonal BN powder. For the cathode, a water-cooled Cu rod was used. The collected gray soot contained limited amount of multi wall BN nanotubes. It is possible that in this case, the tungsten serves also as a catalyst. By perfecting this method, macroscopic amounts of double-wall BN nanotubes of a uniform diameter (2 nm)... 

In the Technical Safety Laboratory of Eastman Kodak, the role played by calorimetric methods (such as High Pressue DSC and ARC techniques) in assessing the stability of chemicals and processes, though a central one, is but part of a range of techniques used for this overall purpose. The relations between these methods is discussed and presented in the form of logic flow diagrams. 

A series of 11 nitrobenzaldehydes was examined by TGA, DSC and ARC techniques. Only 5-hydroxy-2-nitrobenzaldehyde decomposed exothermally in an unsealed container, but all did so in sealed capsules, under dynamic, isothermal or adiabatic conditions, with evolution of much gas. Initial decomposition temperatures in °C (compound, ARC value, and DSC value at lOVmin, respectively, followed by ARC energy of decomposition in kJ/g) were - 2-nitro-, 176, 220, 1.44 3-nitro-, 166, 218, 1.94 4-nitro-, 226, 260, 1.27 2-chloro-5-nitro-, 156, 226, 697 2-chloro-6-nitro-, 146, 220,. 832 4-chloro-3-nitro-, 116, 165, 1.42 5-chloro-2-nitro-, 240, 3-hydroxy-4-nitro-, 200, 4-hydroxy-3-nitro-, 200, 5-hydroxy-2-nitro-, 175, 3-methoxy-4-nitrobenzaldehyde, 245°C, -. 4-Nitrobenzaldehyde showed by... 

C. Journet, D.K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fischer, Large-scale production of single-waUed carbon nanotubes by electric-arc technique, Nature 388 (1997) 756-758. 

In their further study on the thermal reaction between carbonaceous anodes and electrolytes, Richard and Dahn improved the ARC technique and miniaturized it for small sample amounts (<1.0 g) while retaining the sensitivity. °- The self-heating was measured for MCMB in various electrolytes with different initial heating temperatures, and the SHR profile, characterized by a common initial peak, was obtained for nearly all of the tests, as shown in Eigure 35. The authors speculated that the SHR profile represented in Figure 35 featured characteristics of three distinct reaction processes. The initial peak. 

A direct result of the work by Maleki et al. is the gradual shift of safety research from carbonaceous anode to cathode materials. In the following years, Dahn and co-workers used an improved ARC technique to conduct a series of systematic investigations on the reaction between electrolyte and cathode materials based on LiCo022 2 spinel... 

Joumet C, Maser WK, Bernier P et al (1997) Large-scale production of single-walled carbon nanotubes by the electric-arc technique. Nature 388 756-758... 

Exothermic decomposition of 2-nitroaniline in chemical processes was studied by DSC and ARC techniques. The stability in reaction mixtures was markedly less than for the pure, isolated compound [1]. Later calorimetric study of pyrolysis of 2-nitro-aniline in presence of ammonium chloride showed apparent activation energy greater than 200 kJ/mol but reaction enthalpy above 2.1 kJ/g. Maximum rate of pressure rise was 200 bar/min and temperature 400°C/min [2]. 

Thermal stabilities of the six o-, m- and p-nitrobenzyl bromides and chlorides were studied comparatively by TGA, DSC and ARC techniques. In open sample containers, only o-nitrobenzyl bromide decomposed exothermally, but in sealed capsules all the compounds decomposed exothermally after melting, with abundant gas evolution. 

Multi-walled nanotubes are made of concentric tubes whose number can be as high as 50. Prepared also by the electric arc technique, they can be purified in air at = 600°C. High Resolution transmission Electron Microscopy images reveals clear images and in our case, we could for example statistically determine an average number of 18 tubes with inner diameters between 1 and 1.5 nm and outer diameters between 5 and 15 nm. 

The first production of macroscopic quantities of endohedral metallofullerenes were also reported by the Rice group (Chai et al., 1991). They used high-temperature laser vaporization of La203/graphite composite rods and the corresponding contact arc technique to produce various sizes of La-metallofullerenes. Contrary to expectation, only the La Cs2 fullerene survived in a solvent and was extractable by toluene even though La Cgo and La C7o were also seen in the mass spectra of the sublimed film from soot. In other words, the major La-metallofullerene with air stability is La Cg2, and La C6o and La C7o are somehow unstable in air and in solvents. 

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