Helium lambda-line

Helium Purification and Liquefaction. HeHum, which is the lowest-boiling gas, has only 1 degree K difference between its normal boiling point (4.2 K) and its critical temperature (5.2 K), and has no classical triple point (26,27). It exhibits a phase transition at its lambda line (miming from 2.18 K at 5.03 kPa (0.73 psia) to 1.76 K at 3.01 MPa (437 psia)) below which it exhibits superfluid properties (27). 

Liquid helium-4 can exist in two different liquid phases liquid helium I, the normal liquid, and liquid helium II, the superfluid, since under certain conditions the latter fluid ac4s as if it had no viscosity. The phase transition between the two hquid phases is identified as the lambda line and where this transition intersects the vapor-pressure curve is designated as the lambda point. Thus, there is no triple point for this fluia as for other fluids. In fact, sohd helium can only exist under a pressure of 2.5 MPa or more. 

The triple-point temperature of air is the solidification temperature of the liquid (see Reference 2 for details). The boiling-point temperature for air is the bubble-point temperature (i.e., the temperature at which boiling begins as the pressure of the liquid is lowered). The dew-point (vapor) properties of air at 101.325 kPa are calculated at a temperature of 81.72 K the liquid and vapor properties of these two state points are not in equilibrium. The triple-point properties of helium are given at the temperature of the lambda line (change from normal-to-superfluid helium) for the saturated-liquid state. 

Stance see Fig. 2.2. The most striking properties, however, are those exhibited by liquid helium at temperatures below 2.17 K. As the liquid is cooled below this temperature, instead of solidifying, it changes to a new liquid phase. The phase diagram of helium thus takes on an additional transition line separating the two phases into liquid He I at temperatures above the line and liquid He II at lower temperatures. The low-temperature liquid phase, called liquid helium II, has properties exhibited by no other liquid. Helium II expands on cooling its conductivity for heat is enormous and neither its heat conduction nor viscosity obeys normal rules (see below). The phase transition between the two liquid phases is identified as the lambda line, and the intersection of the latter with the vapor-pressure curve is known as the lambda point. The transition between the two forms of liquid helium, I and II, is called the X... 

Not all the physical properties of helium-4 undergo such dramatic changes at the lambda transition. For instance, there is no latent heat involved in crossing the lambda line and no discontinuous change in volume. The lambda transition is usually considered a transition of the second order, i.e., the Gibbs energy has a discontinuity in the second derivative. 

The mysteries of the helium phase diagram further deepen at the strange A-line that divides the two liquid phases. In certain respects, this coexistence curve (dashed line) exhibits characteristics of a line of critical points, with divergences of heat capacity and other properties that are normally associated with critical-point limits (so-called second-order transitions, in Ehrenfest s classification). Sidebar 7.5 explains some aspects of the Ehrenfest classification of phase transitions and the distinctive features of A-transitions (such as the characteristic lambda-shaped heat-capacity curve that gives the transition its name) that defy classification as either first-order or second-order. Such anomalies suggest that microscopic understanding of phase behavior remains woefully incomplete, even for the simplest imaginable atomic components. 

The helium spectrum of the WN5 star HD 50896 (EZ Canis Majoris, WR6) is studied. Our aim is to establish a technique which allows the determination of the parameters of a Wolf-Rayet star from a systematic analysis of its spectral lines. Since the method of "iteration with approximate Lambda operators" became available for application to expanding atmospheres (Hamann, 1986, 1987), we are now able to compare observed... 

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