Manjula M. Ibrahim

Magnetism and spin dynamics of nanoscale FeOOH particles

The nature of magnetism and electron paramagnetic resonance (EPR) spin dynamics in 30 A particles of FeOOH have been investigated in the range 5–350 K. X‐ray diffraction shows that the particles are neither α‐Fe2O3 nor α‐FeOOH based and they convert to Fe3O4 (α‐Fe2O3) on heating to 800 K in vacuum (air). Magnetization M vs T data show a blocking temperature TB in the range 40 K<TB<100 K so that for T≥100 K, scaling of M with H/T for H up to 50 kOe verifies superparamagnetism. In EPR, both the linewidth ΔH and the g value decreases upon increasing T so that near 300 K, g≂2.05 (characteristic of Fe3+). A model of motional narrowing is used to explain the ΔH vs T behavior. The dramatic changes in M, ΔH, g value, and the recoilless fraction of Mossbauer spectroscopy observed near 50 K are all related to the thermal motion of the superparamagnetic nanoscale particles.

The linear temperature dependence of the paramagnetic resonance linewidth in the manganate perovskites and

The temperature dependence of the static magnetic susceptibility (5 to 700 K) and that of the electron paramagnetic resonance linewidth (300 to 700 K) measured at 9.25 GHz is reported for bulk polycrystalline samples of and with negative giant magnetoresistance. For both systems, and the product increase linearly with temperature above . Following the analysis of Huber and Seehra for , it is argued that this linear temperature dependence of is most probably due to contributions from spin - phonon interaction. This result supports the recent suggestions of the importance of lattice effects in the magnetism of these oxides.

Determination of particle size distribution in an Fe 2 O 3 -based catalyst using magnetometry and x-ray diffraction

In this paper, the techniques of SQUID magnetometry and line broadening in x-ray diffraction are employed for determining an important parameter for catalysts, viz. the particle size distribution. Magnetization versus temperature (5 K–400 K) and magnetization versus field (up to 55 kOe) data are reported for an α–Fe 2 O 3 based catalyst. After determining the region of superparamagnetism, the distribution function f(r) is determined assuming a log normal distribution and Langevin paramagnetism of superparamagnetic particles. The distribution is found to be fairly symmetric with center near 65 A and range of 35 to 115 A. From line-broadening of Bragg peaks in x-ray diffraction, particle radii varying between 75 A and 110 A are obtained. These results are compared with the reported Mossbauer measurements of Huffman et al. on the same sample.

Observations of magnetization reversal and magnetic clusters in copper ferrite films

The observation of a magnetization reversal and a superparamagnetic behavior are reported for films of copper ferrite sputtered in an oxygen atmosphere. X‐ray diffraction studies indicate the presence of 14‐nm nanoscale crystals of CuFe2O4 in the films. Data on the temperature dependence of the film magnetization M for zero‐field‐cooled (ZFC) and field‐cooled (FC) show that ZFC and FC curves separate at a temperature Tf and that Tf decreases with increasing static field strength. For T<Tf, M vs T data for low fields indicate the presence of a compensation point in virgin samples. For T≳Tf, a superparamagnetic character is evident form the high‐field magnetization data. An average cluster size of 16 nm obtained from such data is in excellent agreement with the results of x‐ray diffraction studies.

Thermal and catalytic degradation of commingled plastics

Abstract Thermogravimetry, in situ electron spin resonance (ESR) spectroscopy and in situ and ex situ X-ray diffraction (XRD) are employed to investigate the thermal and catalytic degradation of a sample of commingled plastics (CP), XRD studies show CP to contain about 90% polyethylene (PE) and 10% polypropylene (PPE) and a smaller amount of TiO 2 . Analysis of the weight loss data in argon to 550°C yields an activation energy E ≈ 38 kcal mol −1 for the thermal decomposition of CP. In XRD studies, a melting point of 135°C is inferred whereas the onset of irreversible degradation begins only around 360°C. The in situ ESR experiments of CP and CP loaded with 10% Al 2 O 3 (nanoscale) and 10% sulfur, both under 500 psig of H 2 , show that for CP alone an ESR signal indicative of degradation is first seen near 380°C, whereas for the loaded CP this degradation temperature is reduced to 280°C. This enhanced catalytic degradation detected by ESR is believed to be due to elemental sulfur.

Magnetic susceptibilities of Co- and Ni-doped MgO

The temperature dependence (1.7-300 K) of the DC magnetic susceptibilities chi for the systems MpMg1-pO (M=Co,Ni) with p<pc (where pc approximately=0.14 is the percolation threshold) are reported. Two sets of samples were studied for both dopants: two lightly doped samples with p<0.01 and two more heavily doped samples with p=0.06 to 0.12. For the lightly doped samples, results are analysed in terms of contributions from single magnetic ions, exchange-coupled pairs and any contributions from the spin-orbit coupling. For Co2+ in MgO, a contribution from spin-orbit coupling leads to a temperature-dependent magnetic moment ( mu T=3.84 mu B at 5 K to 5.15 mu B at 300 K) and an apparent Curie-Weiss variation, whereas for Ni2+ in MgO, the paramagnetic chi P follows the Curie-law variation. From this analysis, concentrations of the dopants are determined. Further, the temperature independence of chi PB/ chi PA for two doping levels A and B at low concentrations is used to show that any contributions from exchange-coupled pairs are negligible. The more heavily doped samples show non-linear variations of chi P-1 against T, presumably due to contributions from magnetic clusters of different sizes.

Comparison of the liquefaction yields of coals with their composition, free radical density and thermal parameters

Abstract Pyrolysis behaviour of four coals, a vitrinite, and a bituminite sample has been investigated between 25°C and 600°C by in situ electron spin resonance (ESR) spectroscopy, thermogravimetry/differential thermogravimetry (TG/DTG) and differential scanning calorimetry (DSC) and the results are compared with the percentage conversion of these samples to pyridine-soluble liquefaction products. The three distinct temperature stages of pyrolysis reported earlier by Seehra et al. in ESR spectroscopy are observed in these samples also. Linear regression analysis is used to ascertain correlation between percentage conversion and compositional and thermal parameters. Among the compositional parameters, the best correlation is obtained with the atomic H/C ratio (correlation coefficient, r = 0.82) and among the thermal parameters, the rate of mass loss measured in DTG gives the best correlation ( r =0.92), followed by the rate of thermally generated free radicals in stage 3 ( r =0.72). The significance of these results is discussed.

Effects of hydrogen pressure and temperature on free radicals produced in coal-tire coprocessing

Abstract In-situ ESR (Electron spin resonance) spectroscopy of free radicals is used to investigate the coprocessing of Blind Canyon coal with Michelin tire rubber for temperatures up to 480°C and under H 2 and argon pressures up to 1000 psig. Comparing the free radical intensities of the coal and the tire alone with those obtained for the 1:1 coal-tire mixtures, determined under 500 and 1000 psig of H 2 and under 1000 psig of inert argon, the following observations are made. First, a strong synergism between the coal and the tire polymer is indicated, as the intensity of the ESR-active free radicals in the coal-tire mixture under H 2 pressure are enhanced severalfold beginning at ≈ 200°C and peaking near 350°C. Second, increasing the H 2 pressure from 500 to 1000 psig reduces the free radical intensity, most likely because of enhanced hydrogenation. Third, the tire polymer gives a weak ESR-active signal whose characteristics change dramatically between 140°C and 200°C, indicating a phase change in the tire material. The synergistic effect observed here is consistent with the reported improved yields in direct liquefaction experiments using coal-tire mixtures.

Magnetic specific heat and critical magnetic susceptibility of the diluted antiferromagnet CopMg1−pO

For CopMg1−pO, a prototype randomly diluted fcc antiferromagnet, measurements of the specific heat versus temperature show that the size of the λ anomaly at TN decreases as p decreases. These results show that with increasing dilution, a progressively larger fraction of the magnetic entropy is removed by short‐range order processes. The critical susceptibility χc at TN increases as p decreases, supporting the idea that near Mg sites, the balance of the two antiferromagnetic sublattices is destroyed. After correcting for single ion contribution, χc(p)/χc(1) is found to vary as (p−pc)−τ for p<0.47 (pc=0.136). The experimental value of τ≊0.7 is in reasonable agreement with the numerical estimate of τ=0.5±0.2 by Harris et al. for cubic systems.

Free radical monitoring of the coprocessing of coal with chemical components of waste tires

The effects of a Michelin tire tread and its various chemical components (butadiene rubber, aromatic oil, carbon, sulfur, ZnO) on the free radical intensities N of Blind Canyon coal using in-situ electron spin resonance spectroscopy are reported from ambient to 500°C. These experiments show that the tire tread and its components lower the temperature of thermal cracking of the coal and promote enhanced cracking as evidenced by increased magnitudes of N for temperatures below 440°C. These results support the reported improved liquefaction of the coal with waste tire polymers.