Joseph J. Ritter

Enhanced magnetocaloric effect in Gd3Ga5−xFexO12

The working refrigerant material in the majority of magnetic refrigerators has been Gd3Ga5O12 (GGG) which has an upper temperature limit near 15 K. In this paper we report on the field‐induced adiabatic magnetic entropy change, ΔSm(H,T), of a series of iron‐substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 which have the potential to increase the working temperature range or to reduce the field requirements of cryogenic magnetic refrigeration. Depending on Fe concentration, x, the entropy change of these materials at applied fields of 0.9 and 5.0 T is much greater than that of GGG, especially at temperatures above 15 K. At low Fe concentrations, the results are consistent with formation of magnetically ordered clusters of spins at low temperatures. Room temperature electron paramagnetic resonance measurements show that Fe3+ ions mediate exchange interactions which are responsible for clustering at low temperatures.

Iron magnetic moments in iron/silica gel nanocomposites

Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe have been prepared by low‐temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X‐ray diffraction, electron microscopy, Mossbauer effect, and magnetization measurements have been used to show that these bulk materials are paramagnetic composites at room temperature and remain in that state to 10 K. In this condition the Fe is present in nanometer‐sized regions and exists in ionic form (both Fe3+ and Fe2+ ). It possesses a large magnetic moment which decreases linearly from 3.9 μB/ Fe atom to 2.8 μB /Fe atom as the Fe content increased from 11% to 40%. For this composition increase, a negative Curie‐Weiss temperature was found which increased in magnitude linearly from −13 to −46 K. It is suggested that many of the iron atoms in the as‐cured nanocomposites interact antiferromagnetically, and that the magnitude of the effect increases with the Fe concentration. After ...

CO2 tea laser-induced photochemical enrichment of boron isotopes

Abstract Mixtures of BCl 3 and H 2 S are irradiated with 10.55 μm radiation (P(16) line of the 001–100 band of CO 2 ) from a TEA laser. After several hours of irradiation it is found that the maximum 10 B to 11 B ratio of recovered gaseous boron containing material (primarily unreacted BCl 3 ) is 0.413 ± 0.004. The corresponding ratio of the BCl 3 starting material is 0.242 ± 0.002. The 10 B concentration has therefore been increased from 19.5% to 29.2%. Further, by irradiating similar mixtures with 10.18 μm radiation (R(30) line of the same CO 2 vibrational band) this ratio changes to 0.169 ± 0.002, the 10 B concentration being lowered to 14.4%. All experiments are performed in a small static system and chemical procedures for recovering milligram quantities of BCl 3 selectively enriched in either isotope are described.

Change in magnetic state of Fe+silica gel nanocomposites due to low temperature treatment in ammonia

Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe were prepared by room temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). Previous electron microscopy, x‐ray diffraction, and Mossbauer effect data showed these bulk materials are comprised of nanometer‐sized regions of iron compounds embedded in a silica gel matrix. They were also all paramagnetic below 300 K. Here the effect on the magnetic state of these nanocomposites following a low temperature (T<400 °C) treatment in 1 atm of ammonia (after a prior anneal in 1 atm of hydrogen) is presented, along with the dependence on the H2 pretreatment. In all cases the room temperature Mossbauer spectra for the material in the NH3‐treated and H2‐pretreated conditions were similar. However, when treated in H2 at 770 °C (2 h) the Mossbauer spectra also contained a significant component having a large isomer shift (∼1.3 mm/s) and quadrupole splitting (∼3.2 mm/s). This mate...

An Hermetically Sealed Inert Atmosphere Cell for X-Ray Powder Diffraction

The construction and use of a captive inert atmosphere cell for the preservation of atmospherically sensitive samples during X-ray diffraction is described.

Magnetic Properties Of Iron/Silica Gel Nanocomposites

Homogeneous gelled composites of iron and silica containing 5–30 wt. % Fe have been prepared by low temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X-ray diffraction data, characterized by the presence of a diffuse scattering peak centered at 20≈24 degrees and the absence of any strong Bragg scattering from the iron-containing regions, indicates that these bulk materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. Scanning electron microscopy observations show that this matrix is characterized by the presence of many interconnected pores and that the size of these pores is related to the particle size of the Fe-containing regions. The paramagnetic nature of these materials at room temperature, as well as the small size of the iron-containing regions, is indicated by the appearance in many of the samples of only a high intensity central doublet in the 57 Fe M6ssbauer spectra. The Mossbauer effect data demonstrates that the form of the iron can be changed by a subsequent treatment in an atmosphere of ammonia or hydrogen at elevated temperatures: for a 10 wt. % Fe sample treated with ammonia, only a central doublet was observed but with a much larger quadrupole splitting and isomer shift. Both of these subsequently treated materials became superparamagnetic at room temperature. In addition, magnetic susceptibility measurements indicate that the hydrogen treated material becomes a spin glass at low temperatures.

Microwave spectrum, dipole moment, and structure of ethynyldifluoroborane☆

Abstract The microwave spectrum of ten isotopic species of ethynyldifluoroborane (HCCBF 2 ) has been measured. Rotational constants of the ground state and the lowest excited vibrational state have been obtained. Structural parameters obtained are: r CH = 1.058 ± 0.003 A , r CC = 1.206 ± 0.003 A , r CB = 1.513 ± 0.005 A , r BF = 1.323 ± 0.005 A , and ∠FBF = 116.5 ± 1.0°. The dipole moment is 1.87 6 ± 0.010 D .

Magnetic properties of colloidal silica: Potassium silicate gel/iron nanocomposites

Homogeneous composites of Fe and silica containing 5–40 wt % Fe have been prepared by infiltration of ferric nitrate solution into a colloidal silica: potassium silicate gel at room temperature. Previous electron microscopy, small angle neutron scattering, and nitrogen sorption (BET) measurements on such gels have shown they are comprised of an interconnected silica matrix, having pores ranging from 2–800 nm in size. Mossbauer effect and magnetization data show the Fe is in a paramagnetic environment with weak antiferromagnetic interactions. Curie–Weiss analysis indicated negative intercepts of the temperature axis for all the samples and that the magnetic moment per Fe atom (μFe) decreased as the Fe content increased at a rate of ∼0.0058 μB/% Fe. For most of the samples μFe≊2.1 μB. By contrast, in silica gel/Fe nanocomposites formed by the polymerization of an aqueous solution of tetraethoxysilane and iron nitrate, the Fe possessed magnetic moments varying from 3.9 to 2.9 μB. It is suggested that the dif...

Nanocomposites For magnetic Refrigeration

Upon the application of an external magnetic field, the magnetic spins in a material partially align with the field, thereby reducing the magnetic entropy of the spin system. When performed adiabatically, the specimens temperature will rise. This temperature rise, δT, related to the entropy change by the heat capacity, is known as the magnetocaloric effect. Upon cycling the magnetic field, this effect can be used for transferring heat from one thermal reservoir to another, forming the basis for a magnetic refrigerator. Recently, NIST scientists predicted composite magnetic materials containing nanometer-size magnetic species could possess enhanced magnetocaloric effects [1-2], especially at high temperatures or low magnetic fields. Magnetic nanocomposites may be prepared in many different ways, and recent magnetocaloric effect data measured on Fe-doped gadolinium gallium garnets are presented to show both the effect of processing and a methodology for optimizing δT.

Fe Mössbauer effect in YxPr1−xBa2(Cu0.98Fe0.02)3O7

57Fe Mossbauer effect and magnetic‐susceptibility measurements were performed on Y0.2Pr0.8Ba2(Cu0.98Fe0.02)3O7−δ and Y0.8Pr0.2Ba2(Cu0.98Fe0.02)3O7−δ, where δ≊0. The insulating 80% Pr compound showed susceptibility anomalies and a large hyperfine field distribution at low temperatures in addition to a hyperfine field spectrum for Fe on the Cu(2) sites. The superconducting 20% Pr compound displayed none of these effects at any temperature. After examining the alternatives, it is postulated that magnetic ordering of the Pr ions and an enhanced rare‐earth transition‐metal interaction due to f‐electron admixture is responsible for these observations.