Carbon hard”, definition

F ig. 29. Schematic graph showing the definition of the parameter, R, used to empirically estimate the fraction of single graphene layers in hard carbon samples. 

As mentioned earlier, the hardness of a water sample is often reported as parts per million of calcium carbonate. Using our definition of parts per million for liquid solutions (Table 5.4), we have... 

Layer or layers of minerals (especially calcium carbonate) deposited, by the throwing down, or precipitation, onto a heat-transfer surface, reducing its U value. Scales are often hard and dense and difficult to remove. The scale can be crystalline in nature (a solid body having a characteristic internal structure, with symmetrically arranged plane surfaces and definite angles), or amorphous (lacking any characteristic crystalline shape). 

The authors suggest that this is more than a purely physical fixation of oxygen, being in all probability the outcome of a physicochemical attraction between oxygen and carbon. Physical, inasmuch as it seems hardly possible to assign any definite molecular formula to the complex formed, which, indeed, shows progressive variation in composition chemical, in that no isolation of the complex can bo effected by physical means. 

The discussion in the Introduction led to the convincing assumption that the strain-dependent behavior of filled rubbers is due to the break-down of filler networks within the rubber matrix. This conviction will be enhanced in the following sections. However, in contrast to this mechanism, sometimes alternative models have been proposed. Gui et al. theorized that the strain amplitude effect was due to deformation, flow and alignment of the rubber molecules attached to the filler particle [41 ]. Another concept has been developed by Smith [42]. He has indicated that a shell of hard rubber (bound rubber) of definite thickness surrounds the filler and the non-linearity in dynamic mechanical behavior is related to the desorption and reabsorption of the hard absorbed shell around the carbon black. In a similar way, recently Maier and Goritz suggested a Langmuir-type polymer chain adsorption on the filler surface to explain the Payne-effect [43]. 

The decrease in the size of soap bubbles caused by air diffusion has been observed by some researchers [480]. Manegold et. al. [480] have determined gas permeability of bubble films with a diameter of 5 cm produced from a 2% Nekal solution and filled with hydrogen and carbon dioxide. These experiments can hardly be used to draw any definite conclusions on the effect of adsorption layer on the rate of diffusion transfer. 

Commercial ammonium carbonate is amixture of ammonium hydrogen carbonate and ammonium carbamate, NH2-CO-ONH4, and is probably a definite compound of the two salts.7 It is formed by distillation of a mixture of ammonium chloride with carbonate of potassium, sodium, or calcium, and was formerly manufactured by the dry distillation of animal excrement, horn, and other substances. It is now obtained as a by-product of the gas-manufacture, and after sublimation condenses in hard lumps. 

Ferric hydroxide is readily soluble in mineral acids yielding the corresponding ferric salt, and in ferric chloride solution yielding oxy compounds which are acidic in their behaviour, decomposing carbonates, and can therefore hardly be termed basic chlorides.3 It dissolves in aqueous oxalic acid to an extent directly proportional to the concentration of the acid, no definite basic oxalate being formed at 25° C. from solution.4 It does not combine with carbon dioxide when freshly precipitated and suspended in water.5... 

Alloys of chromium and silicon are readily obtained by heating chromium sesquioxide with excess of silicon at 11 white heat, or with silicon carbide, or silicon carbide and carbon, in the electric furnace or by strongly heating chromium sesquioxide, silica, and aluminium. From these alloys several definite silicides have been isolated, which are usually grey in colour, hard and brittle, and very resistant to acids, except hydrofluoric acid, which readily, decomposes them. The silicides, CrjSi, CrjSi, CrgSia, and CrSij, have been obtained in a state of comparative purity by special methods of preparation. ... 

The aforementioned definition of carbidic carbon is not a very precise one, as shown by the temperature-programmed reduction work of McCarty and Wise (54). This inaccurate definition reflects the nature of the problem the overlayers observed are heterogeneous and the reactivities reported are hard to compare. 

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