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Pandey, R. K.

Publications and source records attributed to Pandey, R. K..

9 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Analysis of Phase Separation in Czochralski Grown Single Crystal Ilmenite

Ilmenite (FeTiOs) is a wide bandgap semiconductor with an energy gap of 2.58 eV. Ilmenite has properties suited for radiation tolerant applications, as well as a variety of other electronic applications. Single crystal ilmenite has been grown from the melt using the Czochralski method. Growth conditions have a profound effect on the microstructure of the samples. Here we present data from a variety of analytical techniques which indicate that some grown crystals exhibit distinct phase separation during growth. This phase separation is apparent for both post-growth annealed and unannealed samples. Under optical microscopy, there appear two distinct areas forming a matrix with an array of dots on order of 5 pm diameter. While appearing bright in the optical micrograph, atomic force microscope (AFM) shows the dots to be shallow pits on the surface. Magnetic force microscope (MFM) shows the dots to be magnetic. Phase identification via electron microprobe analysis (EMPA) indicates two major phases in the unannealed samples and four in the annealed samples, where the dots appear to be almost pure iron. This is consistent with micrographs taken with a scanning probe microscope used in the magnetic force mode. Samples that do not exhibit the phase separation have little or no discernible magnetic structure detectable by the MFM.

Wilkins, R.↗
NASA NTRS1995

Growth and analysis of highly oriented (11n) BCSCO films for device research

Films of BCSCO superconductor of the type Bi2CaSr2Cu2Ox have been grown by liquid phase epitaxy method (LPE), using a partially closed growth chamber. The films were grown on (001) and (110) NdGaO3 substrates by slow cooling process in an optimized temperature range below the peritectic melting point (880 C) of Bi2CaSr2Cu2O8. Optimization of parameters, such as seed rotation, soak of initial growth temperature and growth period results in the formation of 2122 phase BCSCO films. The films grown at rotation rates of less than 30 and more than 70 rpm are observed to be associated with the second phase of Sr-Ca-Cu-O system. Higher growth temperatures (is greater than 860 C) also encourage to the formation of this phase. X-Ray Diffraction (XRD) measurements show that the films grown on (110) NdGaO3 have a preferred (11 n)-orientation. It is pertinent to mention here that in our earlier results published elsewhere we obtained c-axis oriented Bi2CaSr2Cu2O8 phase films on (001) NdGaO3 substrate. Critical current density is found to be higher for the films grown on (110) than (001) NdGaO3 substrate orientation. The best values of zero resistance (T(sub co)) and critical current density obtained are 87 K and 105 A/sq cm, respectively.

Raina, K. K.↗
NASA NTRS1995

Growth and Analysis of Highly Oriented (11n) BCSCO Films for Device Research

Films of BCSCO superconductor of the type Bi2CaSr2Cu2O(x), have been grown by liquid phase epitaxy method (LPE), using a partially closed growth chamber. The films were grown on (001) and (110) NdGaO3 substrates by slow cooling process in an optimized temperature range below the peritectic melting point (880 C) of Bi2CaSr2Cu2O8. Optimization of parameters, such as seed rotation, soak of initial growth temperature and growth period results in the formation of 2122 phase BCSCO films. The films grown at rotation rates of less than 30 and more than 70 rpm are observed to be associated with the second phase of Sr-Ca-Cu-O system. Higher growth temperatures (greater than 860 C) also encourage to the formation of this phase. XRD measurements show that the films grown on (110) NdGaO3 have a preferred (11n)-orientation. It is pertinent to mention here that in our earlier results published elsewhere we obtained c-axis oriented Bi2CaSr2Cu2O8 phase films on (001) NdGaO3 substrate. Critical current density is found to be higher for the films grown on (110) than (001) NdGaO3 substrate orientation. The best values, zero resistance (T(sab co)) and critical current density obtained are 87 K and 10(exp 5) A/sq cm respectively.

Raina, K. K.↗
NASA NTRS1994

Ilmenite as Dual-Use Material

This paper addresses the subject of dual-use space technology transfer of a novel, non-traditional material termed ilmenite, found in a large percentage in the moon rocks brought back by NASA's APOLLO missions. The paper is somewhat premature in the sense that though the material as a mineral has been known for a long time, very little is known about pure single crystal ilmenite and hence few applications have been demonstrated. Yet, in another sense, it is very timely due to the fact that ilmenite promises to be a very interesting competition to silicon, silicon carbide and other compound semiconductors, especially those that are employed in high power, high temperature and large data storage/retrieval applications. It seems to be an excellent example of a small investment-high return situation. While some of the applications of this material - for production of oxygen, for instance - have been well-known, electronic applications have received relatively little attention. One reason for this was the fact that growth of single crystal ilmenite requires precise process conditions and parameters. We believe for the first time these have been determined in the Center for Electronic Materials, Texas A&M University. The work being done at Texas A&M University and Prairie View A&M University (supported by Battelle Pacific Northwest Laboratories and the Center for Space Power) indicates the excellent potential this material has in space as well as in terrestrial applications. To mention a few: as a wide band gap semiconductor it has applications in high temperature, high power situations, especially when heat dissipation is a problem such as may occur in the Space Station; the possibility of this material radiating in the blue region, it has immense applications in optoelectronics; as a material with a high density of highly directional d-bands, it lends itself to novel processing conditions and perhaps even to 'tunability' of physical parameters; as a potential scintillating material, it has possible applications as a sensor in waste management; as an oxygen sensor it has possible applications in automotive electronics; and as a radiation resistant material, it has obvious applications in the space environment. Results - experimental and theoretical - obtained so far in our laboratories will be reported with particular emphasis on the transfer of technology involving this fascinating material.

Kumar, A. A.↗
NASA NTRS1994

Improved method for calculating strain energy release rate based on beam theory

The Timoshenko beam theory was used to model cracked beams and to calculate the total strain-energy release rate. The root rotations of the beam segments at the crack tip were estimated based on an approximate two-dimensional elasticity solution. By including the strain energy released due to the root rotations of the beams during crack extension, the strain-energy release rate obtained using beam theory agrees very well with the two-dimensional finite element solution. Numerical examples were given for various beam geometries and loading conditions. Comparisons with existing beam models were also given.

Sun, C. T.↗
NASA NTRS1993

A method for calculating strain energy release rate based on beam theory

The Timoshenko beam theory was used to model cracked beams and to calculate the total strain energy release rate. The root rotation of the beam segments at the crack tip were estimated based on an approximate 2D elasticity solution. By including the strain energy released due to the root rotations of the beams during crack extension, the strain energy release rate obtained using beam theory agrees very well with the 2D finite element solution. Numerical examples were given for various beam geometries and loading conditions. Comparisons with existing beam models were also given.

Sun, C. T.↗
NASA NTRS1992

Thin film growth of the 2122-phase of BCSCO superconductor with high degree of crystalline perfection

Thin films of the 80 K-phase of BiCaSrCu-oxide superconductor having the composition of Bi2Ca1.05Sr2.1Cu2.19O(x) and high degree of crystalline perfection have been grown on c-axis oriented twin free single crystal substrates of NdGaO3. This has been achieved by carefully establishing the growth conditions of the LPE experiments. The temperature regime of 850 to 830 C and quenching of the specimens on the termination of the growth period are found to be pertinent for the growth of quasi-single crystalline superconducting BCSCO films on NdGaO3 substrates. The TEM analysis reveals a single crystalline nature of these films which exhibit 100 percent reflectivity in infrared regions at liquid nitrogen temperature.

Raina, K. K.↗
NASA NTRS1991

Paramagnetic resonance and susceptibility of ilmenite, FeTiO3 crystal

Large high-purity single crystals of FeTiO3 with ilmenite structure have been grown from a stoichiometric melt of Fe2O3 and TiO2 under an inert atmosphere using the modified Czochralski technique. Susceptibility and X-band paramagnetic resonance studies have been performed. Susceptibility measurements indicate a Neel temperature of about 59 K. The paramagnetic resonance spectrum for magnetic field perpendicular to the crystal c axis consists of a portion of a single, very intense approximately Lorentzian absorption line with its peak at about 600 G and half width at half maximum almost 1200 G. The absorption extends to zero magnetic field. For magnetic field approximately parallel to the c axis, the paramagnetic absorption is much smaller and may be considered a superposition of two approximately Lorentzian line shapes. The magnetic resonance measurements indicate a weak temperature dependence and large angular anisotropy.

Mcdonald, P. F.↗