Search NASA⌕ Search

Engineering topics

Miller, R. I.

Publications and source records attributed to Miller, R. I..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Theoretical aspects of undercooling, nucleation and growth of metals in space

A mathematical model which can predict solid material properties from known melt properties and solidification environment is being developed to aid in selecting materials systems which could benefit scientifically or commercially from experimentation in space. In its present, preliminary form, the model calculates grain size and dendrite morphology from the undercooling parameter and the parameter which characterizes heterogeneous nucleation. Calculations with the present model are compared to experimental data, and needed improvements in the model are described. Some theoretical problems relating to undercooling and environmentally-induced solute segregation are also discussed.

Miller, R. I.↗
NASA NTRS1976

Undercooling of materials during solidification in space

A theoretical research program was undertaken on the under cooling and solidification of materials under variable external field conditions. A catalog of theories and models of nucleation of solid phases in the melt is provided, as is a discussion of the relation of undercooling to intermolecular potentials, the dependence of growth rate on undercooling, the influence of undercooling on liquid-solid interface stability and solid structure, the direct effects of external fields on melts, the relation of solid physical properties to structure and the role of nucleants in solidification. Results of the theoretical analysis of several experiments related to the space processing applications program are given, and recommendations for future experiments and further theoretical developments along with procedures for correlation of theory and experiment are specified.

Miller, R. I.↗
NASA NTRS1975

External field effects on diffusion and solidification derived from the free-volume model

Expressions for the diffusion coefficient and the solidification rate from the free-volume model of liquids developed by Turnbull and Cohen have been used to estimate the effects which microgravity and magnetic fields will have on these quantities. The mathematical formalism describing changes of the diffusion coefficient and the solidification rate is the same for both the microgravity and magnetic field cases, but the difference between the magnitudes of the two effects is quite large. The change in the two parameters is found to be less than .0001% for the microgravity case and on the order of 0.1 to 1.1% for the magnetic field case for four representative materials. The diffusion coefficient and the solidification rate are found to increase under the influence of an applied magnetic field, and this is in agreement with experimental observations.

Miller, R. I.↗
NASA NTRS1974

Further analysis of field effects on liquids and solidification. Study of the liquid-solid transition for materials processing in space

Expressions for diffusion coefficient, D, and solidification rate, Uc, from the free volume model of liquids developed by Turnbull and Cohen have been used to estimate the effects which microgravity and magnetic fields will have on these quantities. The mathematical formalism describing changes in D and Uc is the same for both the microgravity and magnetic field cases, but the difference between the magnitudes of the two effects is quite large. The change in D and Uc is found to be less than 0.0001% for the microgravity case and on the order of 0.1 to 1.1% for the magnetic field case for four representative materials. D and Uc are found to increase under the influence of an applied magnetic field, and this is in agreement with experimental observations.

Seiler, R. F.↗
NASA NTRS1974

Further analysis of field effects on liquids and solidification

Numerical calculations of the magnitude of external field effects on liquids are presented to describe how external fields can influence the substructure of the field. Quantitative estimates of magnetic and gravitational effects are reported on melts of metals and semiconductors. The results are condensed in tables which contain the input data for calculation of the field effects on diffusion coefficient, solidification rate and for calculation of field forces on individual molecules in the melt.

Seiler, R. F.↗
NASA NTRS1974

External field effects on solidification - Macroscopic and microscopic models

Comparison of theoretical effects of magnetic and gravitational fields on melts of metals and semiconductors provides two kinds of insight into space processing materials problems. First, the task of comparing effects of the two fields yields mathematical models which may be used to estimate material properties and responses to processing under various field conditions. Then, by using these models, microgravity effects can be inferred from magnetic field data already obtained in laboratories on earth. Magnetohydrodynamics (static and oscillating fields) and the Free Volume Model are used to study macroscopic effects, and Sekerka's interface stability theory is used to show how external fields can affect microsegregation. It is demonstrated that the mathematical formalism is essentially the same for describing macroscopic effects of both microgravity and magnetic fields.

Miller, R. I.↗
NASA NTRS1974

Thermodynamic properties derived from the free volume model of liquids

An equation of state and expressions for the isothermal compressibility, thermal expansion coefficient, heat capacity, and entropy of liquids have been derived from the free volume model partition function suggested by Turnbull. The simple definition of the free volume is used, and it is assumed that the specific volume is directly related to the cube of the intermolecular separation by a proportionality factor which is found to be a function of temperature and pressure as well as specific volume. When values of the proportionality factor are calculated from experimental data for real liquids, it is found to be approximately constant over ranges of temperature and pressure which correspond to the dense liquid phase. This result provides a single-parameter method for calculating dense liquid thermodynamic properties and is consistent with the fact that the free volume model is designed to describe liquids near the solidification point.

Miller, R. I.↗
NASA NTRS1973

Study of liquid-solid transition for materials processing in space. Analysis of field effects on dense liquid materials

The behavior of dense liquids near the solidification point under the influence of magnetic fields or near-zero gravity conditions were analyzed within the framework of existing liquid state models and classical field theory. The dynamic body forces which are an effect of the magnetic field on liquids are discussed. It is concluded that both magnetic fields and low gravity conditions produce an increase in the diffusion coefficient which result in an increased growth rate of crystals, and time varying magnetic fields induce eddy currents in the liquid which produce body forces and tend to disrupt convection.

Miller, R. I.↗