Application of automated image analysis to the study of mineral matter in raw and processed coals

Automated Image Analysis (AIA) and Scanning Electron Microscopy (SEM) was developed and applied to the characteriza- tion of mineral matter in two series of processed coals. Fundamental factors for the application of image analysis to the characterization of minerals in coal which were addressed include development of chemistry definitions for classification of minerals in coal, sampling design, characterization of mineral matter mass distributions by size and type for both raw and processed coals, and AIA overestimation of pyritic sulfur;Two methods of developing a chemistry definition file were described. A priori class definition and autoclassification were discussed as complementary procedures for writing an initial definition file, and guidelines were suggested for evaluating and revising chemistry files;A formula was developed for designing AIA analyses so that adequate sample area and particles may be analyzed to produce reliable and reproducible results. An example was also presented of using actual particle count to determine precision on a category- by-category basis;AIA was applied to the characterization of mineral matter in samples of 200 mesh Illinois No. 6 (Illinois), Pittsburgh (West Virginia), Adaville No. 11 (Wyoming), and Dietz No. 1&2 (Montana) coals before and after float-sink cleaning, and to the characteriza- tion of samples of Illinois No. 6 and Pittsburgh No. 8 coals before and after cleaning with the TRW Gavimelt molten caustic (NaOH-KOH) process. AIA provided a method to identify statistically significant differences in mineral matter features among the coals studied, such as the distribution among mineral phases (e.g., predominant mineral phases being different from one coal to another), the difference in size distribution (i.e., especially after cleaning), and the ability to monitor the formation of new phases associated with chemical cleaning;Factors leading to AIA overestimation of pyritic sulfur by as much as 50% were evaluated. Area inflation due to threshold setting was shown to cause overestimation of less than 10%. Preferential settling of pyrite particles was shown to be a minor effect (6%) for 200 mesh coals and can be avoided by proper sample preparation. Porosity in;large particles of pyrite was found to be the most significant reason for overestimation of pyrite; *DOE report IS-T-1204. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy

Combined effects of transition metal (Ni and Rh) substitution and annealing/quenching on physical properties of CaFe2_{2}As2_{2}

We performed systematic studies of the combined effects of annealing/quenching temperature ({\itshape T}$_{A/Q}$) and T = Ni, Rh substitution ({\itshape x}) on the physical properties of Ca(Fe$_{1-x}$T$_{x}$)$_{2}$As$_{2}$. We constructed two-dimensional, {\itshape T}$_{A/Q}$-{\itshape x} phase diagrams for the low-temperature states for both substitutions to map out the relations between ground states and compared them with that of Co-substitution. Ni-substitution, which brings one more extra electron per substituted atom and suppresses the {\itshape c}-lattice parameter at roughly the same rate as Co-substitution, leads to a similar parameter range of antiferromagnetic/orthorhombic in the {\itshape T}$_{A/Q}$-{\itshape x} space as that found for Co-substitution, but has the parameter range for superconductivity shrunk (roughly by a factor of two). This result is similar to what is found when Co- and Ni-substituted BaFe$_{2}$As$_{2}$ are compared. On the other hand, Rh-substitution, which brings the same amount of extra electrons as does Co-substitution, but suppresses the {\itshape c}-lattice parameter more rapidly, has a different phase diagram. The collapsed tetragonal phase exists much more pervasively, to the exclusion of the normal, paramagnetic, tetragonal phase. The range of antiferromagnetic/orthorhombic phase space is noticeably reduced, and the superconducting region is substantially suppressed, essentially truncated by the collapsed tetragonal phase. In addition, we found that whereas for Co-substitution there was no difference between phase diagrams for samples annealed for one or seven days, for Ni- and Rh- substitutions a second, reversible, effect of annealing was revealed by seven-day anneals

Anisotropic magnetization and resistivity of single crystalline RNi1-xBi2+-y (R = La-Nd, Sm, Gd-Dy)

We present a detailed study of RNi1-xBi2+-y (R = La-Nd, Sm, Gd-Dy) single crystals by measurements of stoichiometry and temperature dependent magnetic susceptibility, magnetization, and electrical resistivity. This series forms with partial Ni occupancy as well as a variable Bi occupancy. For R = Ce-Nd, Sm, Gd-Dy, the RNi1-xBi2+-y compounds show local-moment like behavior and order antiferromagnetically at low temperatures. Determination of anisotropies as well as antiferromagnetic ordering temperatures for RNi1-xBi2+-y (R = Ce-Nd, Sm, Gd-Dy) have been made. Although crystalline samples from this family exhibit minority, second phase superconductivity at low temperatures associated with Ni-Bi and Bi contamination, no evidence of bulk superconductivity has been observed

Lack of superconductivity in the phase diagram of single-crystalline Eu(Fe1-xCox)2As2 grown by transition metal arsenide flux

The interplay of magnetism and superconductivity (SC) has been a focus of interest in condensed matter physics for decades. EuFe2As2 has been identified as a potential platform to investigate interactions between structural, magnetic, electronic effects as well as coexistence of magnetism and SC with similar transition temperatures. However, there are obvious inconsistencies in the reported phase diagrams of Eu(Fe1-xCox)2As2 crystals grown by different methods. For transition metal arsenide (TMA)-flux-grown crystals, even the existence of SC is open for dispute. Here we re-examine the phase diagram of single-crystalline Eu(Fe1-xCox)2As2 grown by TMA flux. We found that the lattice parameter c shrinks linearly with Co doping, almost twice as fast as that of the tin-flux-grown crystals. With Co doping, the spin-density-wave (SDW) order of Fe sublattice is quickly suppressed, being detected only up to x = 0.08. The magnetic ordering temperature of the Eu2+ sublattice (TEu) shows a systematic evolution with Co doping, first going down and reaching a minimum at x = 0.08, then increasing continuously up to x = 0.24. Over the whole composition range investigated, no signature of SC is observed.Comment: 24 pages, 7 figure

Tuning of charge density wave transitions in LaAuxSb2 by pressure and Au stoichiometry

Two charge density wave transition can be detected in LaAuSb2 at similar to 110 and similar to 90K by careful electrical transport measurements. Whereas control of the Au site occupancy in LaAuxSb2 (for 0.9 less than or similar to x less than or similar to 1.0) can suppress each of these transitions by similar to 80K, the application of hydrostatic pressure can completely suppress the lower transition by similar to 7.5kbar and the upper transition by similar to 17kbar. Clear anomalies in the resistance as well as the magnetoresistance are observed to coincide with the pressures at which the charge density wave transitions are driven to zero

Single crystal growth and superconductivity of Ca(Fe1-xCox)2As2

We report the single crystal growth of Ca(Fe1-xCox)2As2 (0 <= x <= 0.082) from Sn flux. The temperature-composition phase diagram is mapped out based on the magnetic susceptibility and electrical transport measurements. Phase diagram of Ca(Fe1-xCox)2As2 is qualitatively different from those of Sr and Ba, it could be due to both the charge doping and structural tuning effects associated with Co substitution.Comment: 5 pages, 4 figure

Energy Gap Evolution Across the Superconductivity Dome in Single Crystals of (Ba1−x_{1-x}Kx_x)Fe2_2As2_2

The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping ($x\approx$0.4) to becoming nodal at $x >$0.8. Although this marked evolution was identified in several independent experiments, there are no details of the gap evolution to date because of the lack of high-quality single crystals covering the entire K-doping range of the superconducting dome. We conducted a systematic study of the London penetration depth, $\lambda (T)$, across the full phase diagram for different concentrations of point-like defects introduced by 2.5 MeV electron irradiation. Fitting the low-temperature variation with the power law, $\Delta \lambda \sim T^{n}$, we find that the exponent $n$ is the highest and $T_c$ suppression rate with disorder is the smallest at optimal doping, and they evolve with doping being away from optimal, which is consistent with increasing gap anisotropy, including an abrupt change around $x\simeq 0.8$, indicating the onset of nodal behavior. Our analysis using a self-consistent $t$-matrix approach suggests the ubiquitous and robust nature of s$_{\pm}$ pairing in IBSs and argues against a previously suggested transition to a $d-$wave state near $x=1$ in this system