For instance, in rutile, theory predicts that point defects should cause no internal friction peak if stress is applied in the c direction. One peak should occur for isolated defects when specimens are strained in the a direction and another peak should occur in this direction for paired defects. Also, one peak, should occur for isolated defects when specimens are strained in the [110] direction; paired defects would not cause a peak in this direction. In rutile specimens doped with Ni or Cr, no peaks occurred for those specimens oriented in the c direction, in accordance with theory. With Ni as the dopant, two peaks were found for specimens oriented in the a direction and only one peak for specimens oriented in the [110] direction. With Cr as the dopant, only one peak was found in both the a and [110] directions. From this, it may be concluded that Ni causes both isolated and paired defects, while Cr causes only isolated defects.

Crystal Structure Determined by X-Ray Diffraction. The study of the crystal structures of boron compounds revealed two new interesting structures in different fields of boron chemistry. The determination of the structures of the isomorphous pair SrB,O, and PbB,O, resulted in a type of boron grouping not previously reported in anhydrous borates. All boron atoms are intetrahedral coordination and the structure contains some oxygen atoms shared by three tetrahedra. This structure and the previously reported BaB407, where the boron coordination is equally divided into triangular and tetrahedral, show that concepts of predicting the ratio of triangular to tetrahedral boron as a function of composition are not valid.

Another interesting study was the first complete determination of a halogen substituted boron hydride. The structure of iodopentaborane (BHI) was determined at -25 °C. The iodine is attached to the apex of the pentaborane group, thus confirming nuclear magnetic resonance experiments.

Rate of Vaporization of High-Temperature Materials Investigated. A program for the determination of the rate of vaporization of the highmelting-point metals useful in the development of thermionic generators of electric power is being conducted at the Bureau with the support of the National Aeronautics and Space Administration. A microbalance operating in the very-high-vacuum region which permits measurements at temperatures up to 3000 °C is used in this work.

The rate of vaporization, vapor pressure and heat of sublimation of tungsten, the most refractory of the elements, were determined during the year. The heat of sublimation from the (100) face of single-crystal tungsten was found to be the same as that of the polycrystalline material, within experimental error.

High-Temperature Mass Spectrometry. High-temperature mass spectrometry is being used to investigate the vaporization of solid materials and the chemical reactions of their vapors. In addition to the chemical composition of the vapor species, Bureau scientists are determining their partial pressures and heats of sublimation, the equilibrium constants and heats of their reactions, and the heats of formation of any new vapor species formed.

A complete study was made of the BeO-BeF2 system. The ternary gas species Be2OF2 was identified and its thermodynamic properties determined. In addition, the heat of sublimation of BeF2 was measured. A study of the BeO-Al2O3 system, still in progress, showed the existence of the ternary gas species BeOAL.

This research is being supported by the Advanced Research Projects Agency under its "Thermodynamics of the Light Elements" program.

Liquid-Liquid Phase Separation in Inorganic Oxides Studied. The morphology of new phases, which form when a pressure or tempera. ture change is imposed upon a homogeneous system, is generally a function of foreign surfaces which assist the nucleation process. A notable exception is certain liquid-liquid phase separations, which apparently occur without the aid of foreign surfaces resulting in liquid-liquid dispersions of extremely small particle size. These dispersions, by rapidly coalescing into larger units, elude further scientific examination. By choosing a highly viscous system of molten oxides, which could quickly be cooled into glasses, however, it was possible to arrest various stages of the coalescence-process The two phases are clearly visible, both forming an interconnected threedimensional network.

Successive stages of the rearrangement process following the liquid-liquid separation were investigated by electron microscopy and by nitrogen adsorption on one of the isolated phases. The kinetics of the process was also determined. A statistical theory explaining the network character of the phases was suggested and various theoretical models for rearrangements by diffusive transport were calculated and compared with the experimental data. The results suggest an interface controlled bulk diffusion process. The process was found to be Arrhenius having an activation energy of 1.67 × 105 joules/mole (40 kcal/mole).

Boron Compounds Studied by Nuclear Resonance. The chemistry of substituted derivatives of the lower boron hydrides was investigated as part of a program dealing with synthesis, properties, and reactions of inorganic compounds. The first known example of a partially fluorinated boron hydride species, difluoroborane (HBF.), was prepared and characterized. Synthesis of this compound indicates that derivatives of this type may not be as inherently unstable as was supposed. Chemical and spectroscopic properties of the new compound were studied to provide a better understand. ing of the effect of substitution on the reactivity of boron hydride derivatives.

Measurements of high-resolution nuclear magnetic resonance (NMR) spectra at low temperature were carried out for a number of boron compounds. These studies provide information about the structures of these compounds and may lead to a better understanding of the chemical bonding in them.

Viscosity of Glasses Measured. A high-temperature rotating-cylinder viscometer was modified so that the viscosity of molten glasses can be measured over the range 101 to 101o poise. The method, based on room temperature calibration with standard viscosity oils, involves three different

Sodium borosilicate glass, heated below immiscibility temperature, separates into two immiscible glassy phases. Electron micrograph shows preshadowed carbon-replicas of fracture surface. (See page 152)

techniques in measuring the torque on the inner cylinder. Over the range 101 to 10 poise, the outer cylinder is rotated at a constant speed and the torque is measured on the inner cylinder with an electromagnetic torque head by a null balance method. Over the range 101.5 to 107.5 poise, an aperiodic method is used, wherein the inner cylinder is rotated through an angle and time as it decays to its zero position. Finally, over the range of 106.5 to 1010 poise the inner cylinder is driven and timed through a specified angle by means of the electromagnetic torque head. These techniques overlap each other so that a continuous viscosity curve can be established for the glass. With these new determinations the results of viscosity measurements by the fiber elongation method on Standard Glass No. 710 were verified in the range of 10 to 1010 poise. Furthermore, precise viscosity measurements in the range of 106 to 10 poise can now be made on many glasses where none were possible before. This method also lends itself to viscosity measurements on glasses that cannot be drawn into rods (such as dehydrated B2O3 glass) and measured by the fiber elongation method.

Mechanical Properties of Glass Studied at Elevated Temperatures. Increased use of glass at elevated temperatures indicates a need of reliable information, not previously available, on the strength of glass under these conditions. The effect of temperature on the strength of several commercially available glasses was studied to supply this information.

The results indicated that the strength of glass in relation to temperature was apparently dependent upon composition. The modulus-of-rupture

values for annealed glass show that after a short-term exposure to elevated temperatures the glasses with the lower silica content decrease in strength while the glasses with the higher silica content continually increase in strength from room temperature upwards. Heating for long periods of time did not adversely affect the strength of annealed glass, and at the higher temperatures tended to strengthen the glass.

The modulus-of-rupture values obtained for the semi-tempered and tempered specimens gave results as expected: generally, the longer and higher the specimens were heated, the lower were the modulus-of-rupture values.

Young's modulus was also affected by temperatures. The same glasses that decrease in strength decrease in Young's modulus, while the glasses that continually increase in strength initially increase in Young's modulus.

Configurational Entropy is a Criterion for Glass Transformation. A new analysis of existing experimental data clarified the understanding of the conditions under which equilibrium supercooled liquids change into glasses. Previous theories suggested that this change occurs when the free volume fraction of a material decreases to some critical value. It is now indicated that an equally determining property is the configurational entropy of the material. This indication comes independently from both molecular relaxation data and calorimetric data for supercooled liquids. The relaxation data may be in the form of viscosity data, dielectric data, dynamic mechanical properties, or creep or stress relaxation data. Both types of evaluation indicate that when the equilibrium supercooled liquid changes into a glass, the configurational entropy has decreased to about 0.8 entropy units per polymeric chain unit. This requires a polymeric interpretation of the molecular structure of the glass forming material. The fact that configurational entropy is now indicated to be a critical property in glass transformation has stimulated re-examination of previous theories and their possible relation to configurational entropy considerations. Methane Crystals Formed. Experiments on low-temperature crystallization yielded single grains of methane as large as 8 mm × 5 mm × 5 mm in a polycrystalline block. Thermal-etching techniques make possible nondestructive three-dimensional observation of the grains for spectroscopic, solid state, and surface physics studies at low temperatures. Methane is the simplest molecular solid, next to the solidified rare gases, but in contrast to these, undergoes solid-solid phase-transitions. The experiments are aimed at elucidating the nucleation mechanism for such phase changes.

Metal Oxide Melting Point Standards Redetermined. A recent critical compilation showed that the melting points for many of the more common and commercially important oxides are not known with any degree of accuracy or reliability. In order to help rectify this situation, a program was undertaken to redetermine the melting points of selected oxides and to establish calibration standards.

Because many oxides are susceptible to either reduction or oxidation upon heating, it is necessary to study their melting behavior under carefully controlled environmental conditions. An induction furnace was constructed

Four sides of a methane crystal at 87 °K seen through liquid argon. The heavy lines are grain boundaries on the back faces of the crystal and the lighter ones are grain boundaries on the front faces. These crystals provide excellent sources for spectroscopic, solid state, and surface physics studies at low temperatures. (See page 154)

which permits the heating of oxide samples to 3000 °C in vacuum or inert atmosphere and to 2400 °C in an oxidizing environment. Either pure tungsten or iridium crucibles serve as both the heating element (susceptor) and as the blackbody for temperature measurement. Considerable attention was given to the design of the susceptor-blackbody in order that a high degree of accuracy of temperature measurement can be achieved. Theoretical calculations as well as experimental testing showed that the blackbody has an effective emittance of 0.999 at 2000 °C. This small deviation fom a perfect radiator, which has an emittance of 1.000, will cause an error in measurement at 2000 °C of less than 0.25 °C.

Because of various influencing factors, only a limited number of oxides are considered suitable for use as melting point standards. At present, the more promising materials include Al2O3, Y2O3, and Sc2O3. The melting point of high-purity single crystals of Al2O, (sapphire) in vacuo is now being actively investigated.

Standard X-Ray Powder Diffraction Patterns. NBS, in cooperation with the American Society for Testing and Materials and the American Crystallographic Association, is actively engaged in a continuing program for the further refinement and extension of the use of x-ray powder diffraction patterns as a means of identifying and characterizing crystalline phases. Cooperative work is in progress on the determination of intercomparative intensities to extend the accuracy of quantitative analysis by this method, and in the preparation and measurement of tungsten and silver samples for use as internal calibration standards for spacings. The Bureau initiated a program for the calculation of patterns of materials not readily available or too unstable for experimental study. During the year experimental work was com