Liquid Invar

The Invar effect, which was discovered by Guillaume [14] at the end of the 19th century, refers to the anomalously low coefficient of thermal expansion of certain alloys [23, 36], FeesNiss being the archetype. Different compositions of these alloys also show related effects, such as a constant modulus of elasticity with tranperature [12], 

Although it has long been reahsed [34] that these effects were of magnetic origin, the actual mechanism through which this occurs has been highly debated [22, 26, 30], While a full description of this complex problan is beyond the scope of this work, the essential competing theories are discussed below. 

The two main types of model [26] for explaining the Invar effect are those [34] based on a high local magnetic moment-low local magnetic moment transition (HM/LM), and those involving a frustrated local magnetic order [23,27], 

Farmer, Structural Studies of Liquids and Glasses Using Aerodynamic Levitation, Springer Theses, DOI 10.1007/978-3-319-06575-5 6, 

Although the model of Rancourt and Dang [27] does not require noncoUine-arity, it should be noted that several authors [22, 30] consider this an important result of the frustrated local magnetic order. 

---

In situ aerodynamic levitation and neutron diffraction with isotopic snbslitution was used to determine the atomic structure of liquid Invar (Fe6sNi35) at -1800 K. Although the introduction of Ni is substitutional, a small degree of chemical was apparent in the Bhatia-Thomton (1970) partial structure factors. Significant magnetic correlations were detected -1300 K above the Invar Curie temperature. 

Low temperatures involve problems of differential thermal expansion. With the outer wall at ambient temperature and the inner wall at the liquid boiling point, relative movement must be accommodated. Some systems for accomplishing this are patented. The Gaz Transport of Franee reduces dimensional change by using a thin inner liner of Invar. Another patented French system accommodates this change by means of the flexibility of thin metal which is creased. The creases run in two directions, and the form of the crossings of the creases is a feature of the system. 

A piece of Invar (density = 8.00 g/ml) weighs 15.4726 g in air and 13.9213 g when suspended in liquid nitrogen at a temperature of - 196°C. What is the density of liquid nitrogen at that temperature (Invar has a very small coefficient of thermal expansion, and its change in density with temperature may be neglected in this problem.)... 

PMS liquids are corrosion-inert substances. Under normal conditions and heated to 100-150 °C they do not cause corrosion and for a long period of time do not change in airflow when in contact with aluminum and magnesium alloys, bronzes, carbon and doped steels, as well as titanium alloys. PMS liquids do not change their properties under 100 °C in air for 200 hours in contact with the above-listed alloys as well as with beryllium, bismuth, cadmium, Invar alloy, brass, copper, mel-chior, solder, lead, silver. The stability of the properties of PMS liquids in these conditions is usually accompanied by the absence of metal and alloy corrosion, although the colour of the metal surface may slightly change. 

In the Premier Mill the rotor is shaped like the frustrum of a cone, similar to that in Fig. 20-53. Surfaces are smooth, and adjustment of the clearance can be made from 25 pm (0.001 in) upward. A small impeller helps to feed material into the rotor gap. The mill is jacketed for temperature control. Direct-connected liquid-type mills are available with 15- to 38-cm (6- to 15-in) rotors. These mills operate at 3600 r/min at capacities up to 2 m% (500 gal/h). They are powered with up to 28 kW (40 hp). Working parts are made of Invar alloy, which does not expand enough to change the grinding gap if heating occurs. The rotor is faced with Stellite or silicon carbide for wear resistance. For pilot-plant operations, the Premier Mill is available with 7.5- and 10-cm (3- and 4-in) rotors. These mills are belt-driven and operate at 7200 to 17,000 r/min with capacities of 0.02 to 2 m /h (5 to 50 gal/h). 

Studies were made to determine stress levels developed in applications of Invar pipe in various vacuum insulated piping configurations where the Invar liquid line was restrained within the vacuum casing configuration without the use of flexible sections. These studies indicated that developed stresses at liquid hydrogen temperatures were within acceptable limits. 

The use of Invar alloy for liquid piping eliminates a need for flexible bellows sections. As a result, we could anticipate favorable costs and reliability. 

As a significant theme of this thesis is the application of aerodynanuc levitation to glass science. Chap. 2 contains a discussion of the characteristics of glass and a summary of theoretical attempts to describe the glassy state. Other pertinent theoretical descriptions, such as the theory of liquid semiconductors and the Invar problem, wiU be found in the relevant experimental chapters. 

Although there have been a number of different attempts at experimentally determining the mechanism behind the Invar effect, the problem is yet to be conclusively resolved [21, 22]. A consideration of how the models extend into the liquid state, particularly with respect to magnetic correlations, may allow for certain possibilities to be excluded. 

In this thesis I have shown how levitation techniques can be combined with nentron diffraction, XAS, and computer simulations, in order to determine the strnctures of a diverse range of amorphous materials. The structural information reported relates to several distinct areas of condensed matter the novel glass systems of BaTi20s (BTO) and rare earth doped BTO the controversial issue of isocompositional first order liquid-liquid phase transitions and polyamorphism in yttria aluminate liquids and the extension of Invar into the liqnid state to provide further constraints on crystalline models. 

In addition to these main themes, Tom also demonstrated how the aerodynamic levitation method could be successfully applied to wide angle neutron diffraction experiments as applied to the chemical and magnetic ordering in liquid Fe-Ni invar alloys at high temperatures. 

---