Crystal hardness

Fig. 10. Linear plot of crystal hardness as a function of square root of crystal thickness for various metarials Paraffins (O) PE (M = 170.000) (A) PE (M, = 2x 106) ( )...

The hardnesses of these crystals remains high up to the 600-700 C range and then decline rapidly (Chen, Xuan, and Otani, 2003). Also, it should be noted that the crystal hardnesses are anisotropic with the (110) surfaces more difficult to indent than the (100) surfaces (Li and Bradt, 1991). 

Red cubic crystal hard and refractory density 1.90 g/cm decomposes... 

Silvery-white lustrous metal when pure or dark gray amorphous powder orthorhombic crystals hardness 2.3 Mohs density 6.25 g/cm melts at 452°C vaporizes at 990°C modulus of elasticity 6.0x10 psi thermal neutron absorption cross section 4.7 0.1 barns insoluble in water, carbon disulfide, and benzene also insoluble in HCl soluble in sulfuric acid, nitric acid, and aqua regia also soluble in caustic potash and in solutions of alkali metal cyanides. 

Other important properties of Cr02 when used as a magnetic pigment are its black color, electrical conductivity (2.5-400 O-1 cm-1 [5.27]) and relatively high crystal hardness (Mohs hardness 8-9 [5.20]). Therefore, coating formulations based on Cr02 require less or even no additives such as carbon black (good conductivity, black color) or refractory oxides such as alumina. 

Fig. 18. Variation of crystal hardness, measured at room temperature, as a function of unit cell density for the various VF2/F3E compositions...

Figure 19 illustrates the dependence of the crystal hardness upon crystal density for various compositions. This plot permits to distinguish between the two... 

Fig. 19. Dependence of crystal hardness upon unit cell density through the Curie transition...

Since the vek computation was only rough, as Fersman himself admitted, the hardness values calculated from the formulae (3.3) and (3.4) were not sufficiently accurate and failed to consider all crystallochemical factors. Next, Sobolev in 1946 established a relationship between hardness and coordination number, attempting to extend the applications of Goldschmidt s formula to complex minerals, including silicate minerals. It was discovered by that time that crystal hardness with both ionic and covalent bonds, depends on ... 

Investigations into the influence of composition on hardness play an especially important role in mixed crystal synthesis processes. Subramanian and Bansigir (1980) synthesized a series of mixed optical crystals KCl-KBr (Fig. 5.17). They demonstrated a definite influence of mole composition on crystal hardness and the absence of a strict correlation between hardness and dislocation density in the crystals obtained in the process. The maximum number of dislocations involved pure crystals of both compounds, each exhibiting minimum hardness. 

Comparison of results of crystal hardness anisotropy determination by Vickers and scratch methods... 

Hinckley believes the differences between the two types of clays is largely a function of the environment of deposition and post-depositional leaching. The poorly crystallized hard clays were believed to have been deposited in a marine environment where face to face flocculation occurred. The well-crystallized soft clays were presumably flocculated edge to face in a fresh-water environment. The more porous soft clays were more thoroughly leached and re-crystallized than the less porous hard clays. Like other kaolinite studies this one does not provide any specific information about the actual chemical composition of the kaolinite minerals. 

Lolingite, FeAs2, occurs in Saxony and Norway in rhombic crystals. Hardness 5 to 5 5 density 6-8 to 8 7. Greyish m colour, it gives a greyish black streak. When heated in an open tube a white sublimate of arsenious oxide or white arsenic S5 is obtained in the absence of air metallic arsenic is volatilised to a dark sublimate. 

Crystalline phase volume by itself is generally not sufficient to determine material properties some measure of the dispersion of that crystalline material is also needed. A product that contained all the mass in a few large crystals would have completely different material properties than the same product containing a dispersion of numerous small crystals. Hardness is related to the size of crystals in a system, with more numerous small crystals typically giving a harder product. The product with numerous small crystals has many more interparticle contacts under applied force than a product with the same crystalline phase volume but fewer larger crystals. 

The third proposed mechanism is chemical bonding between molybdenum disulphide and the material of the surface. Both chemical and hardness effects will be discussed in more detail later, but at this stage it is important to note that both crystal hardness and chemical reactivity are greatest at crystallite edges, and relatively low on the basal planes. 

Effects of additives, impurities, and pH on the crystai habit, crystai size distribution, crystai purity, and crystal hardness. 

Among other things, it became established that the nature of the structure adopted by a given compound on crystallization would then exert a profound effect on the solid-state properties of that system. For a given material, the heat capacity, conductivity, volume, density, viscosity, surface tension, diffusivity, crystal hardness, crystal shape and color, refractive index, electrolytic conductivity, melting or sublimation properties, latent heat of fusion, heat of solution, solubility, dissolution rate, enthalpy of transitions, phase diagrams, stability, hygroscopicity, and rates of reactions were all affected by the nature of the crystal structure. 

Relationship between crystal hardness and melting temperature... 

Equation (4.13) shows that the reciprocal value of the crystal hardness is related to the melting temperature of the crystals. 

Table 4.1. Experimental values for the density, p, crystallinity measured by density, wdp), crystallinity measured by DSC, Wc(DSC), long period, L, crystal thickness, Ic, microhardness, H, crystal hardness. He, melting temperature, T, surface free energy, Oe, and the number of chain folds per molecule, n, for PEO samples with different molecular weights. (From Baltd Calleja Santa Cruz, 1996.)...

Figure 4.16. (a) Variation of hardness, H, and crystal hardness. He, of melt-crystallized PEO as a function of molecular weight, (b) The long period L, lamellar thickness Ic and extended-chain length (dashed line) of the above PEO samples as a function of molecular weight. (After Balta Calleja Santa Cruz, 1996.)... 

Table 6.1. Microhardness perpendicular to the chain orientation (strain) direction H , indentation anisotropy at room temperature (25°C) AH, X-ray crystallinity Wc(WAXS), percentage of a and modifications (according to Tashiro et al, 1980) and crystal hardness of homo-PBT He stretched at various strains, s. 

Bowden and Yoffe [40] have developed the well known concept of hoX spots and that the initiation of explosion stems from hot spots created by thermal factors and crystal hardness and shape. Small bubbles of air included in... 

Cancrinite crystals were used as aggregate in Portland type III cement mixtures to bind the crystals. Hard products were obtained only when the cancrinite was prepared from fired clays. It appears that any unreacted clay in the cement mixture will cause the cement to crumble when air dried. This may be due to shrinkage of the clay upon drying. 

The strength and range of the electrostatic attractions make ionic crystals hard, high-melting, brittle solids that are electrical insnlators. Melting an ionic crystal, however, disrupts the lattice and sets the ions free to move, so ionic liquids are good electrical conductors. 

The 40-pcr cent acid. solution obtained from the absorber may be decolorized by the use of decolorizing carbon containing no add soluble ash, and maleic acid recovered by crystallization. Hard rubber lined pumps and aluminum lines arc used and crystallization is carried out in enameled cast iron vats. Maleic acid is sold in the crystalline fonn although it is commercially obtainable in solution. 

Compare the crystal hardness and melting temperatures of molecular solids and ionic solids. 

Hard. brittle, garnet-type crystals. Hardness about equal to that ot quartz Melts above I 040 . The melt is easily amenable to doping and crystal nucleation by epitaxy. Crystals may be oriented along the simple axes (100), (110), and (ill) using the natural (110) face of the crystal. 

The entire operation lasts about eight hours and yields well-crystallized, hard LiH about 99.6% pure. It is bluish in spots due to contamination with a slight excess of Li. 

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