Magnetic viscosity, temperature dependence

Finally, the data published by Gee (30) permit one to evaluate the sharpness of a transition involving floor temperature. Gee studied the temperature dependence of the viscosity of liquid sulfur and observed its sudden, steep increase at a critical temperature followed by its decrease at still higher temperatures. He developed the first, relatively complete theory of equilibrium polymerization of liquid sulfur (30) from which he estimated the chain length of the polymeric sulfur at various temperatures. His results have been recently confirmed by experimental measurements of magnetic susceptibility of the liquid sulphur (50) and its electron spin resonance (57). 

Further studies reported by Johnson, Geanangel and Shore 17> have led to the isolation of KB5H8 as a microcrystalline white powder of limited thermal stability. The temperature dependence of the LiBsHg UB nmr spectrum (Fig. 3) was ascribed to boron nuclear quadrupolar relaxation effects caused by the increased viscosity of the solution at lower temperatures 17>. The spectmm of the potassium salt showed the same temperature dependence as LiBsHs but at temperatures 60 to 70 degrees lower 17>. The magnetic equivalence of the basal boron atoms... 

The physical properties of the slag and bullion at the feed temperature of 1150 ° C are listed in Table I. Relationships for the temperature dependence of density and viscosity were developed from the plant data provided by Cominco Ltd. Because of a lack of information, the magnetic permeabilities of both slag and bullion were taken to be equal to the value for free space. 

In this equation, a is the conductivity, A is a constant proportional to the number of carrier ions, B is a constant, and To is the temperature at which the configurational entropy of the polymer becomes zero and is close to the glass transition temperature (Tg). The VTF equation fits conductivity rather well over a broad temperature range extending from Tg to about Tg +100 K. Equation [3.2] is an adaptation of the William-Landel-Ferry WLF relationship developed to explain the temperature dependence of such polymer properties as viscosity, dielectric relaxation time and magnetic relaxation rate. The fact that this equation can be applied to conductivity implies that, as with these other properties, ionic... 

Sampaio et al. developed a model for interpreting magnetic viscosity S H, T) experiments at low temperature performed on small particles of Ba-ferrite. Their model, taking into account both particle size and switching field distribution, describes the experimental low-temperature dependence, S(H, T) T, and predicts the observed scaling behavior on field and on temperature. 

Figure G.3. Temperature dependence of the magnetic viscosity S for fine CrOj particles. (Reproduced with permission from Ref. 370.)...

For an isolated spin-1 system, it is convenient to define sum and difference magnetizations [Eqs. (2.84)-(2.85)] in the J-B experiment. The decay of the difference (quadrupolar order) proceeds exponentially at a rate T q, while the sum (Zeeman order) recovers exponentially towards equilibrium at a different rate. The J-B experiment allows simulataneous determination of these rates from which Ji uJo) and J2 2ujo) can be separated. Table 5.1 briefly summarizes thermotropic liquid crystals in which spectral density measurements were reported. Figure 5.4 illustrates the temperature and frequency dependences of spectral densities of motion (in s by including the interaction strength Kq factor) for 5CB-di5. The result is fairly typical for rod-like thermotropic liquid crystals. The spectral densities increase with decreasing temperature in the nematic phase of 5CB. The frequency dependence of Ji uJo) and J2(2a o) indicate that molecular reorientation is likely not in the fast motion regime. However, the observed temperature dependence of the relaxation rates is opposite to what is expected for simple liquids. This must be due to the anisotropic properties (e.g., viscosity) of liquid crystals. 

The proper choice of a solvent for a particular application depends on several factors, among which its physical properties are of prime importance. The solvent should first of all be liquid under the temperature and pressure conditions at which it is employed. Its thermodynamic properties, such as the density and vapour pressure, and their temperature and pressure coefficients, as well as the heat capacity and surface tension, and transport properties, such as viscosity, diffusion coefficient, and thermal conductivity also need to be considered. Electrical, optical and magnetic properties, such as the dipole moment, dielectric constant, refractive index, magnetic susceptibility, and electrical conductance are relevant too. Furthermore, molecular characteristics, such as the size, surface area and volume, as well as orientational relaxation times have appreciable bearing on the applicability of a solvent or on the interpretation of solvent effects. These properties are discussed and presented in this Chapter. 

Self diffusion coefficients can be obtained from the rate of diffusion of isotopically labeled solvent molecules as well as from nuclear magnetic resonance band widths. The self-diffusion coefficient of water at 25°C is D= 2.27 x 10-5 cm2 s 1, and that of heavy water, D20, is 1.87 x 10-5 cm2 s 1. Values for many solvents at 25 °C, in 10-5 cm2 s 1, are shown in Table 3.9. The diffusion coefficient for all solvents depends strongly on the temperature, similarly to the viscosity, following an Arrhenius-type expression D=Ad exp( AEq/RT). In fact, for solvents that can be described as being globular (see above), the Stokes-Einstein expression holds ... 

A magnetic field has no appreciable influence on the viscosity of a paramagnetic solution. 5 The dependence of viscosity on temperature for potassium chloride solutions follows equation (13), 9.VIII E. 

Relaxation Times, Paramagnetic Effects, and N.Q.R. Studies.—A study of the relaxation times of phosphoryl compounds at two magnetic fields, and of the dependence of spin rotation and dipolar interactions upon viscosity and temperature, led to the approximate separation of dipole-dipole, anisotropy, and spin-rotation interactions, and indicated that second-order paramagnetic shielding was dominant. The P relaxation times 7i and Tz were determined for several lipid-water phases. Comparisons of changes of Tg which occur at the transition temperature for dipalmitoyl-lecithin indicated that the relaxation times reflect the mobility of the lipid head-group. ... 

Figure 1. Viscosity dependence upon temperature and magnetic field orientation of p-azoxyanisole (from Porter and Johnson, Ref. 8)...

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