STUDY OF THE LONG-TERM STABILITY OF THE EFFECTIVE CONCENTRATION OF IONIZED SPACE CHARGES (N-EFF) OF NEUTRON-IRRADIATED SILICON DETECTORS FABRICATED BY VARIOUS THERMAL-OXIDATION PROCESSES

Experimental study of the reverse annealing of the effective concentration of ionized space charges (N-eff, also called effective doping or impurity concentration) of neutron irradiated high resistivity silicon detectors fabricated on wafers with various thermal oxides has been conducted at room temperature (RT) and elevated temperature (ET). Various thermal oxidations with temperatures ranging from 975 degrees C to 1200 degrees C with and without trichlorethane (TCA), which result in different concentrations of oxygen and carbon impurities, have been used. It has been found that, the RT annealing of the N-eff is hindered initially (t < 42 days after the radiation) for detectors made on the oxides with high carbon concentrations, and there was no carbon effect on the long term (t > 42 days after the radiation) N-eff reverse annealing. No apparent effect of oxygen on the stability of N-eff has been observed at RT. At elevated temperature (80 degrees C), no significant difference in annealing behavior has been found for detectors fabricated on silicon wafers with various thermal oxides. It is apparent that for the initial stages (first and/or second) of N-eff reverse annealing, there may tie no dependence on the oxygen and carbon concentrations in the ranges studied.

Improved neutron radiation hardness for Si detectors: Application of low resistivity starting material and or manipulation of N-eff by selective filling of radiation-induced traps at low temperatures

Radiation-induced electrical changes in both space charge region (SCR) of Si detectors and bulk material (BM) have been studied for samples of diodes and resistors made on Si materials with different initial resistivities. The space charge sign inversion fluence (Phi(inv)) has been found to increase linearly with the initial doping concentration (the reciprocal of the resistivity), which gives improved radiation hardness to Si detectors fabricated from low resistivity material. The resistivity of the BM, on the other hand, has been observed to increase with the neutron fluence and approach a saturation value in the order of hundreds k Omega cm at high fluences, independent of the initial resistivity and material type. However, the fluence (Phi(s)), at which the resistivity saturation starts, increases with the initial doping concentrations and the value of Phi(s) is in the same order of that of Phi(inv) for all resistivity samples. Improved radiation hardness can also be achieved by the manipulation of the space charge concentration (N-eff) in SCR, by selective filling and/or freezing at cryogenic temperatures the charge state of radiation-induced traps, to values that will give a much smaller full depletion voltage. Models have been proposed to explain the experimental data.

Facile synthesis of PtRu/C electrocatalyst with high activity and high loading for passive direct methanol fuel cell by synergetic effect of ultrasonic radiation and mechanical stirring

The PtRu/C electrocatalyst with high loading (PtRu of 60 wt%) was prepared by synergetic effect of ultrasonic radiation and mechanical stirring. Physicochemical characterizations show that the size of PtRu particles of as-prepared PtRu/C catalyst is only several nanometers (2-4 nm), and the PtRu nanoparticles were homogeneously dispersed on carbon surface. Electrochemistry and single passive direct methanol fuel cell (DMFC) tests indicate that the as-prepared PtRu/C electrocatalyst possessed larger electrochemical active surface (EAS) area and enhanced electrocatalytic activity for methanol oxidation reaction (MOR). The enhancement could be attributed to the synergetic effect of ultrasound radiation and mechanical stirring, which can avoid excess concentration of partial solution and provide a uniform environment for the nucleation and growth of metal particles simultaneously hindering the agglomeration of PtRu particles on carbon surface.

Crystallization behaviors of n-nonadecane in confined space: Observation of metastable phase induced by surface freezing

Crystallization and phase transition behaviors of n-nonadecane in microcapsules with a diameter of about 5 mu m were studied with the combination of differential scanning calorimetry ( DSC) and synchrotron radiation X-ray diffraction ( XRD). As evident from the DSC measurement, a surface freezing monolayer, which is formed in the microcapsules before the bulk crystallization, induces a novel metastable rotator phase ( RII), which has not been reported anywhere else. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the RII phase and induces its appearance, while the lower free energy in the confined geometry turns the transient RII phase to a " long- lived" metastable phase.