Stephen Thurgate

Microcalorimeter x-ray detectors offer the specific advantage of being high-resolution energy-dispersive detectors. Furthermore, they can be designed to cover almost any energy range, from soft x-rays to γ rays. Many of the current energy values of L, M, and N x-ray lines in the soft x-ray range (below 1.2 keV) have not been established through a chain of calibration. Based on our experience, we propose here a method of measuring the energies of these lines that should establish peak positions to a few tenths of an electron volt. It would involve the calibration of a microcalorimeter detector with diagram line energy values determined by a grating x-ray spectrometer calibrated by a plane grating monochromator using synchrotron radiation. We present L-line spectra from Cu, Co, and Ni obtained with a microcalorimeter detector to demonstrate the feasibility of obtaining high-resolution spectra in the energy range below 1 keV.
•Microcalorimeter x-ray detectors are energy dispersive and have high resolution.•Microcalorimeter x-ray detectors can be calibrated accurately by standard atomic transitions.•Many x-ray emission lines have not been accurately measured.•Microcalorimeter x-ray detectors can detect multiple x-ray lines over a broad spectrum.•For low energies, a grating monochromator and grating spectrometer can be used to calibrate a microcalorimeter detector.

Microcalorimeter X-ray detectors employing a transition-edge sensor are capable of very high energy resolution, but achieving this in practice depends on an understanding of the readout process. The combination of thermal cooling and thermoelectric feedback present in most circumstances produces a pulse whose area and amplitude both vary nonlinearly with photon energy. The resolution is further affected by the presence of pileup pulses in the pulse or baseline that need to be detected and rejected down to an arbitrarily small level. We describe here a method that includes both the rejection of pileup pulses and the correction of the nonlinearities. We have implemented the process in a system that accepts pulses from multiple detectors and analyzes them in real time. An initial brief X-ray spectrum from a standard sample supplies sufficient data for pulse characterization and energy calibration. The system is then also enabled to combine pulses from multiple detectors into an energy-calibrated histogram in real time.

Olivine (LiCo 1/3Mn 1/3Ni 1/3PO 4) powders were synthesized at 550-600°C for 6 h in air by a sol-gel method using multiple chelating agents and used as a cathode material for rechargeable batteries. Range of chelating agents like a weak organic acid (citric acid - CA), emulsifier (triethanolamine - TEA) and non-ionic surfactant (polyvinylpyrrolidone - PVP) in sol-gel wet chemical synthesis were used. The dependence of the physicochemical properties of the olivine powders such as particle size, morphology, structural bonding and crystallinity on the chelating agent was extensively investigated. Among the chelating agents used, unique cycling behavior (75 mAh/g after 25 cycles) is observed for the PVP assisted olivine. This is due to volumetric change in trapped organic layer for first few cycles. The trapped organic species in the electrode-electrolyte interface enhances the rate of lithium ion diffusion with better capacity retention. In contrast, CA and TEA showed a gradual capacity fade of 30 and 38 mAh/g respectively after multiple cycles. The combination of all the three mixed chelating agents showed an excellent electrochemical behavior of 100 mAh/g after multiple cycles and the synergistic effect of these agents are discussed.

Manganese dioxide (MnO2) appears to be an effective cathode material for a battery system. No studies on lithium insertion in aqueous media are known to the best of our knowledge. However, in one of our previous papers we reported that lithium could be intercalated into a MnO2 host compound using an aqueous LiOH electrolyte; however simple chemistry suggests that it should not. It is found that a battery with LiOH electrolyte functions quite differently from the cell that uses Li2SO4. This paper describes the surface modifications that accompany the electrochemical behavior of MnO2 during redox (discharge) processes in the lithium hydroxide and sulfate media. XPS and SIMS techniques were used to study the resultant surface of the MnO2 cathode and the spectra reveal that the formation of an insoluble layer of Li2CO3 precedes the process of reduction. SEM was used to study the microstructure of the MnO2 cathode.

The electrochemistry of olivine-type iron phosphate (FePO 4) as a battery cathode material, in aqueous lithium hydroxide (LiOH), has been investigated. The material forms intercalated LiFePO 4 reversibly on electroreduction/oxidation. The formation of Fe 3O 4 phase, in addition to the regeneration of FePO 4 during reverse oxidation of LiFePO 4, also occurs. In this regard, the mechanism of FePO 4 discharge/charge in aqueous LiOH differs from that in non-aqueous solvents.

The electrochemical behavior of olivine-type lithium manganese phosphate as a cathode material was investigated in a saturated aqueous lithium hydroxide electrolyte. The crystal structure and surface characterization of the olivine type and the products which are formed on its oxidation and subsequent reduction were studied. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and secondary ion mass spectrometry were used for these investigations. was found to be reversibly delithiated/lithiated on electro-oxidation/reduction

The electrochemical behavior of titanium dioxide (TiO2) in aqueous lithium hydroxide (LiOH) electrolyte has been investigated. Cyclic voltammetry shows that electroreduction results in the formation of a number of products. X-ray diffraction of the electroreduced TiO2 shows that Li x TiO2, Ti2O3, Ti2O and TiO are formed. The formation of Li x TiO2 is confirmed through X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies of the electroreduced TiO2. The formation of Li x TiO2 is electro reversible. In this respect, the electrochemical behavior of TiO2 in concentrated aqueous lithium hydroxide electrolyte is similar to that for lithium perchlorate (LiClO4) non-aqueous media.

The redox behavior and surface characterization of LiFePO4 in aqueous lithium hydroxide (LiOH) electrolyte have been investigated. The objective of this work is to investigate the electrochemical behavior of LiFePO4 in an aqueous lithium hydroxide electrolyte and its comparison with that in non-aqueous lithium ion electrolytes. Cyclic voltammetry results show that LiFePO4 undergoes partially reversible oxidation/reduction. The products formed on electrooxidation and subsequent reduction of LiFePO4 were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). It is found that lithium extraction from LiFePO4 occurs during oxidation. The products formed on subsequent reduction consist of LiFePO4 and Fe3O4.

The electrochemical behavior of γ-MnO2 in lithium hydroxide (LiOH) and potassium hydroxide (KOH) aqueous media has been studied using slow-scan cyclic voltammetry (25 μV s−1) in conjunction with X-ray analysis (XRD) and scanning electron microscopy (SEM). The reduction of γ-MnO2 in aqueous LiOH results in intercalation of Li+ forming a new phase of lithium intercalated MnO2 (LixMnO2). The process is found to be reversible. In this regard, the reduction of γ-MnO2 in LiOH is quite different from that in aqueous KOH, which is irreversible and no lithium intercalation occurs. This difference in behavior is explained in terms of the relative ionic sizes of Li + and K+. The LixMnO2 lattice is stable only for Li+ because Li+ and Mn4+ are of approximately the same size whereas KxMnO2 is not stable because K+ has almost double the size.

The use of X-ray photoelectron spectroscopy (XPS) is expanding rapidly with the advent of more convenient, high-resolution laboratory-based instruments. The increase in the numbers and distribution of synchrotrons has also fuelled this burgeoning interest. Powell (C. J. Powell, J. Vac. Sci. Technol. A 21 (S42) 2003) has shown that the number of papers published on or using XPS has doubled in the past 10 years. He reports that there are in excess of 3000 papers published using XPS data each year across a wide range of applications, making it the most important technique for the determination of the composition of nano-scale surface films. Yet XPS and the associated technique of Auger-electron spectroscopy (AES) are not yet developed to the stage where the data and its interpretation are unambiguous. Auger photoelectron coincidence spectroscopy (APECS) provides a means of simplifying spectra, allowing for the decomposition of these into components that have a common origin. The spectra so produced provide a unique opportunity to test theory. In this paper, we will review recent progress in developing better APECS spectrometers and illustrate the power of APECS with new data acquired by these.