Robert J. Naumann

Growth kinetics of tetragonal lysozyme crystals

Abstract A method has been devised for immobilizing protein crystals in small volumes under defined conditions in order to determine growth rates on various faces. Using this method, we have investigated the growth kinetics of the [110] face of tetragonal hens egg white lysozyme crystals at varying degrees of bulk supersaturation. The growth rate data were analyzed using a simple convective-diffusive model to determine an empirical relationship between growth rate and local supersaturation at the interface. This empirical relationship describes the surface kinetics of the growth process, which together with the convective-diffusive model can be used to predict various details of the growth process as a function of crystal size, bulk supersaturation, and gravity level. It was shown that transport is dominated by convection in normal gravity for all crystal sizes larger than a few microns whereas transport is diffusion limited in a microgravity environment for sizes up to a few millimeters. Convection can become significant even in a microgravity environment for crystals approaching cm sizes. Growth of lysozyme will always be limited by surface kinetics in normal gravity because convective transport is sufficient to supply solute to the growth interface as fast as it can be incorporated into the lattice. In microgravity the transport can become sufficiently slow as the crystal becomes larger so that growth is limited by transport rather than surface kinetics. The implications of this are discussed.

Preliminary investigations into solutal flow about growing tetragonal lysozyme crystals

Abstract A series of preliminary experiments were done to investigate solutal flow about growing lysozyme crystals and its effects. Density gradient driven flow as observed using a schlieren optical system. Crystals used ranged from 0.3 to 1.72 mm across the (110) face, and protein concentrations were from 3.7 to 23.7 mg/ml ( C sat = 1.2 mg/ml at 18°C). The convective plume velocities were found to be from 10 to 50 μm/s, which correlated with those predicted to occur based upon a diffusive-convective model. When micro-crystals of lysozyme ( .5 mm) crystals in the terminal phases of a typical crystal growth procedure.

Effect of variable thermal conductivity on isotherms in Bridgman growth

Abstract A change in thermal conductivity associated with melting or solidification can have a profound influence on the isotherms near the solidification interface if the material is being directionally solidified in an ampoule whose walls carry a substantial portion of the heat. This analysis was prompted by a recent discovery that the thermal conductivity of Hg1−xCdxTe increased dramatically as the material is heated above the solidus curve. An illustrative example is shown in which the sample is approximated as an infinite cylinder with constant but different thermal properties in the solid and melt. The boundary conditions are fixed on the surface by a conductive ampoule in a two-zone Bridgman furnace with an adiabatic region separating the two zones. The effect of the adiabatic zone in this case is to intensify the curvature of the interface rather than to lessen it.

Materials sciences in space

A summary is presented of recent theoretical and experimental examinations of materials processing methods in microgravity conditions. The discussion covers Skylab and Spacelab flights, rocket and parabolic flights, and drop tube experiments. Attention is given to crystal growth, fluid physics, metallurgical and electrophoresis experiments.

Homogeneous nucleation kinetics

Abstract Homogeneous nucleation kinetics are rederived in a manner fundamentally similar to the approach of classical nucleation theory with the following modifications and improvements. First, the cluster is a parent phase cluster and does not require energization to the parent state. Second, the thermodynamic potential used to describe parent phase stability is a continuous function along the pathway of phase decomposition. Third, the kinetics of clustering corresponds directly to the diffusional flux of monomers through the cluster distribution and are formally similar to classical theory with the resulting kinetic equation modified by two terms in the preexponential factor. These terms correct for the influence of a supersaturation dependent clustering within the parent phase and for the influence of an asymmetrical cluster concentration as a function of cluster size at the critical cluster size. Fourth, the supersaturation dependence of the nucleation rate is of the same form as that given by classical nucleation theory. This supersaturation dependence must however be interpreted in terms of a size dependent surface tension. Finally, there are two scaling laws which describe supersaturation to either constant nucleation rate or to the thermodynamically determined physical spinodal.

Equation of State for Porous Metals under Strong Shock Compression

An equation of state is developed for metals in the high‐pressure high‐temperature region that can be extended in a thermodynamically consistent manner to the vapor phase. The constants are obtained from observed thermodynamic properties and are specified for Al, Fe, Cu, Ni, Pb, and W. This equation of state was checked against experimental data by computing the Hugoniot curves for normal and porous samples for cases where experimental data are available. Also, the velocity of sound and release isentropes were computed for various shock conditions.

Thermal Considerations in Continuous Flow Electrophoresis

Abstract Two-dimensional heat flow in a continuous flow electrophoretic chamber is analyzed assuming Poiseuille flow and finite conductivity of the chamber walls. The thermal field can be characterized in terms of several dimensionless parameters which allow the solution to be applied to a wide variety of operating conditions. Since most electrophoretic chambers have a high aspect ratio, heat flow through the edge walls is not a major effect and the two-dimensional model is adequate. A major advantage of using this simplified approach is that analytical solutions can be obtained which provide insights that are difficult to get from three-dimensional numerical approaches. For example, a criterion is developed for determining the maximum power that can be used in machines operated in upflow or downflow configuration. Also it is shown that the actual structure of the flowfield has no effect on the fully developed thermal field. The model is compared with experimental measurements, and the implications of the...

Recent information gained from satellite orientation measurement

Abstract Much attention has been given to the motion of the center of mass of a satellite, but comparatively little attention has been given to the motion of the body about its center of mass. Knowledge of body motion about the mass center and the actual orientation at particular times is necessary to evaluate certain information gained from satellites. This paper points out certain anomalies in temperature measurements from the Army Explorer IV which suggested that external torques may be acting on the vehicle. This led to radio frequency measurements which confirmed the fact that the angular momentum vector is precessing. This paper also outlines a method whereby the orientation of a vehicle may be determined at any desired time.

One-dimensional thermal modeling of vertical bridgman-type crystal growth

Abstract Articles by Naumann [J. Crystal Growth 58 (1982) 554] and Jasinski et al. [J. Crystal Growth 61 (1983) 339], examined the axial temperature distribution of the charge during vertical Bridgman-type crystal growth. A comparison of their results demonstrates that Naumanns model is applicable for K (ratio of crucible to charge thermal conductivity) ⩾ 1 or for small Biot numbers. The Jasinski model provides a correction for larger Biot numbers and is also applicable for small values of K.

Observed Torque-Producing Forces Acting on Satellites

In addition to the forces that influence the motion of the center of mass of a satellite, various forces are also present which influence the motion about the center of mass. These torque–producing forces may result in a change in satellite orientation that affects the thermal balance, operation of solar cells, various scientific measurements, and drag forces. This latter effect results in a coupling between the equations describing the orientation with those describing the orbital motion. Therefore, strictly speaking, the orientation problem is part of the orbital problem.