Sigurds Arajs

Electrical conduction in thin chromium films

We have prepared thin (50–1500 A) polycrystalline films of Cr by vacuum (10−8 Torr) evaporation methods using the deposition rate on Corning glass 7059 and its temperature as experimental parameters. Electrical resistivity (ρ) of these films has been studied in situ as a function of temperature (T), film thickness (t), and the above preparation variables. Our films exhibit well‐definied anomalies at the Neel temperature which is strongly dominated by internal stresses. The ρ vs T curves are briefly compared with those of other investigators.

Magnetic filtration of submicron hematite particles

Magnetic filtration of submicron (65.5 nm) hematite particles passed in aqueous solution through a bed of stainless‐steel spheres has been investigated under conditions of constant flow rate (∼3 cm/min) and fixed solution pH(=11). Measurements were taken as a function of field strength for magnetic fields up to several kOe in both the longitudinal and transverse orientation, and for collecting spheres in three different sizes ranging from about 50 to 200 μm. Filtration efficiency increases with field strength up to a saturation value which depends upon specific system characteristics (size and composition of the collecting elements, field orientation, etc.). The recovery of the filtered particles also was studied and found to vary with applied field and collector characteristics. The results are interpreted on the basis of current theories of magnetic filtration.

Magnetic filtration of submicroscopic particles through a packed bed of spheres

A small‐scale experimental facility for studying the magnetic filtration of submicroscopic particles through a bed of packed spheres is described. The experimental parameters are the particle size and concentration, solution pH, flow rate, magnetic field, filter characteristics, and time. Some results for aqueous solutions containing submicron α‐Fe2O3 spherical particles are presented, and discussed from the viewpoint of recent theoretical studies.

Magnetic filtration of particles in laminar flow through a bed of spheres

Abstract We present a theory for the capture of magnetic particles by a bed of spheres in HGMS. The laminar approximation to fluid flow is used in conjunction with the free surface condition (Happels model) to obtain the fluid velocity profile near a representative collector sphere in the assemblage. An effective medium treatment is used to model the magnetic field, thus taking into account the effects of neighboring spheres. results are reported for the capture of both paramagnetic and diamagnetic particulates by a filter operating either in the longitudinal or transverse design mode.

Effects of electric and magnetic fields on the convective heat transfer in gaseous O2 and N2O

New studies of convective heat transfer in gaseous paramagnetic O2 and diamagnetic N2O have been carried in the presence of electric and magnetic fields. The corresponding heat transfer coefficients indicate a complicated development of heat convections which appear to be strongly interacting. These new results are presently not fully understood in terms of existing theories.

HEAT TRANSFER FROM A WIRE TO HEXANE IN A NONUNIFORM ELECTRIC FIELD

The influence of a nonuniform electric field (ac or dc) on heat transfer from an electrically heated platinum wire immersed in n‐hexane was investigated. The platinum wire was concentrically mounted in a closed copper cylinder filled with the test liquid. The copper cylinder was placed horizontally in a constant temperature bath. Heat transfer coefficients for natural convection were obtained to serve as a reference for the electric field measurements. The electric field results showed an increase in the heat transfer rate from the wire over free convection for both ac and dc fields. The electric Nusselt numbers in ac fields were a factor of 4 higher than corresponding results in dc fields. This behavior differed from results reported in the literature for n‐hexane, which was further demonstrated by replotting some of the raw data using the same physical and electrical properties employed to reduce the present experimental data.

Diffusive capture of ultrafine particles by spheres

Abstract We present a theory for the capture of ultrafine particles by spheres for the case where magnetically driven diffusion is dominant. A variational principle is used to study the particle concentration field and capture efficiency of a representative collecting sphere as a function of magnetic field strength and flow rate for small Peclet numbers. The study is intended to complement earlier work on cylindrical collectors and our own recent calculations for spherical collectors in the realm of large Peclet numbers.

Hysteretic behavior of cobalt powders

Spherical cobalt powders with mean diameters D between 0.5 and 8 μm and standard deviations σ, scaling approximately as 0.25D, have been prepared by a nonaqueous solution technique. Characterization of these powders has involved x‐ray diffraction analysis, scanning electron microscopy, and differential thermal analysis. SEM reveals additional levels of substructure with size domains ranging from 0.2 to 0.3 μm and below. ‘‘As‐prepared’’ powders revealed a microcrystalline structure through XRD analysis. High‐temperature annealing (923 K) in flowing N2 resulted in a crystallographic change from microcrystalline to a preferred cubic crystal structure as evident through XRD. Room‐temperature magnetization studies have been made by a vibrating sample magnetometer as a function of magnetic fields up to 20 kOe. Particular attention has been given to the coercivity and reduced remanence. The dependence of these quantities on the crystallinity of the powders will be briefly discussed.

Magnetic properties of the Sm-Gd alloy system

Preliminary results are reported for a magnetic study of the Sm‐Gd alloy system. For Gd‐rich alloys, the results show a ferromagnetic transition with Curie temperature decreasing to approximately 255 K for 30% Sm. For the Sm‐rich end of the alloy range, the Sm‐type antiferromagnetic transitions are seen. The ferromagnetic influence of Gd clearly exists down to Gd concentrations of 20%, where a ferromagnetic‐like transition is seen with a Curie temperature of 180 K. In one sample of 60% Sm–40% Gd, we have seen evidence of different magnetic regions, one of which is a Gd ferromagnet with a Tc of approximately 290 K.

Experimental studies in magnetic separation of ultrafine hematite

Magnetic filtration of submicron hematite particles passed in aqueous solution through a ball matrix filter has been investigated under conditions of constant flow rate (3 cm/min) and controlled solution pH. The particulates were spherical, with nominal diameters of 0.06 and 0.12 microns. Filtering efficiency in larger fields was better for the smaller particulates, but in fields below several hundred Oe this trend reversed. A reduction from pH=11 to pH=10 resulted in a 10% decrease in removal of 0.06-micron particulates at high fields, and a commensurate increase in low fields. >