Development of neutron scattering kernels for cold neutron reflector materials

The newest neutron scattering applications are highly intensity-limited techniques that demand reducing the neutron losses between source and detectors. In addition, the nuclear industry demands more accurate data and procedures for the design and optimization of advanced fission reactors, especially for the treatment of fuel and moderator materials. To meet these demands, it is necessary to improve the existing calculation tools, through the generation of better models that describe the interaction of neutrons with the systems of interest. The Neutron Physics Department at Centro Atomico Bariloche (CNEA, Argentina) has been developing over the time new models for the interaction of slow neutrons with materials, to produce scattering kernels and cross section data in the thermal and cold neutron energy region. Besides the studies carried out on neutron moderators, we have recently begun looking at materials that could serve as efficient neutron reflectors over those energy ranges. In this work we present the results of transmission and scattering experiments on diamond nanopowder and magnesium hydride, carried out simultaneously at the VESUVIO spectrometer (ISIS, UK), and compare them with newly generated cross-section libraries.

KINETICS OF THE MICROARC OXIDATION COATING GROWTH IN THE NO-BATH PROCESS

The authors consider theoretical foundations of the coating formation when using the no-bath method of microarc oxidation (MAO) as applied to a point counter electrode. The relationships between the growth rate of the coating thickness and the electrical, geometric and chemical parameters of the MAO process have been mathematically determined. An algorithm for calculating the productivity of the MAO process, due to the growth rate of the coating thickness has been developed. The off ered methodology for the experimental selection of modes of the no-bath MAO of aluminum alloys with an electrically neutral nozzle was verifi ed to obtain a coating on a small area of a part with a fl at surface. In the experiment, with a constant “Nozzle-Workpiece” distance equal to 10 mm, the distance “Electrode-Workpiece” took the values of 5, 10, 20, 30 mm. As a “fl at” surface, the authors took the end face of a 50 mm rod made of the D16T alloy based on aluminum, and a rod made of the 08Kh18H10T steel with a diameter of 6 mm served as an electrode. Use was made of the composition applied in the bath method which included an electrolyte: 8 g of KOH, 30…35 g of Na2SiO3, 1 g of artifi cial diamond nanopowder per 6 liters of distilled water. The duration of the MAO process was 120 min. It has been experimentally established that an increase in the “Electrode-Workpiece” distance decreases the potential for coating formation, which decreases the current in the electrochemical circuit. The analysis of the MAO coating thickness has revealed that 5…15 mm is the optimal distance from the electrode and nozzle to the workpiece, which ensures a stable coating thickness of more than 100 microns on an area equal to or greater than the cross-sectional area of the nozzle supplying electrolyte. The expediency of using the developed mathematical model and the methodology for selecting the process modes with the no-bath method of microarc oxidation has been experimentally confi rmed. The experimental studies have established that the proposed scheme of the MAO process provides an increase in the productivity (the thickness growth rate) of coating deposition by 20% and a decrease in energy consumption by 25%.

On the disperse acicular ferrite formation in the structure of cold-resistant joints under temperatures up to –70°С in MMA welding of 10KhSND steel. Part 1

The article presents an analysis of the metallurgical techniques that provide high quality electrodes for manual arc welding of low-carbon low-alloyed cold-resistant steels. It is shown that it is possible to improve technological and operational properties of welded joints at very low climatic temperatures up to –70°C implementing micro-alloying of the weld metal with nitrogen, titanium, cerium oxide and diamond nanopowder produced by detonation synthesis. The composition introduced into the electrode coating modifier mixture is identified. The cumulative effect of its components on the weld impact strength under temperature testing within the range from –20 up to –70°C was established. The matrix of the weld metal is composed mainly of disperse acicular ferrite, hardened by nanoparticles allegedly nitrides and carbonitrides of titanium and aluminum. It is shown that the centers for the crystallization of acicular ferrite are micro-sized non-metallic inclusions formed on ultrafine titanium nitrides. It was revealed that the toughness of the weld metal at low climatic temperatures is higher than toughness of joints welded by massively imported Japanese KOBELCO electrodes LB-52U. The results of the study make it possible to increase the cold resistance of welded structures for petrochemical plants and other facilities located in the Extreme North of the Russian Federation.Part 2 of the article will be devoted to the study of the welding and technological properties of coated electrodes.

Use of Niobium Oxide as a Binder in the Production of Diamond Nanostructured Composites

The sintering of nanodiamond powders is of interest for both applied engineering of tool materials and fundamental materials science of nanodisperse covalent-type ceramic materials. It is a accept as a general notion that the driving force for sintering of monophase particles is determined by the level of the surface energy. In the case of diamond nanopowder, this level must be significantly higher which makes sintering a difficult process. This difficulty of sintering is connected with the low diffusive mobility of carbon causing the formation of a graphite structure onto surface of the diamond crystals. From this point of view the use of niobium oxide as a binder could be a solution. In an attempt to inhibit the diamonds graphitization process, Nb2O5 and small amounts of amorphous carbon were introduced in the reaction zone. Sintering process was conducted at 6.0 GPa of pressure and 1100-1400oC for a processing time of 30 seconds. At the end of the process, the samples were cleaned, and prepared to be characterized by X-ray diffraction, scanning electron microscopy, density and porosity. From these results it was proposed a densification mechanism based on the consolidation of the particle by diffusion and coalescence of clusters.

Purification and Functionalization of Diamond Nanopowders

Purification of diamond nanopowder (DNP) was conducted in a less-destructive mild polyphosphoric acid (PPA)/phosphorous pentoxide (P2O5). The wide-angle X-ray diffraction (XRD) showed that the intensity of the characteristic diamond d-spacing (111) at 2.07 Å from purified DNP (PDNP) was fairly increased compared to pristine DNP, indicating that significant amount of carbonaceous impurities were removed. Chemical modification of pristine DNP and PDNP with 4-ethylbenzoic acid was carried out to afford 4-ethylbenzoyl-functionalized DNP (EBA-g-DNP) and PDNP (EBA-g-PDNP). The morphologies of EBA-g-DNP and EBA-g-PDNP from scanning electron microscopy (SEM) were further affirmed the feasibility of chemical modification. The results suggested that the reaction condition was indeed viable for the one-pot purification and functionalization of DNP. The resultant functionalized DNP could be useful for nanoscale additives. Hence, EBA-g-DNP and EBA-g-PDNP was brominated by using N-bromosuccinimide (NBS). The resultant N-brominated DNP and PDNP could be used as initiator for the atom transfer radical polymerization (ATRP) to introduce many polymers onto the surface of functionalized DNP and PDNP.