Nanoindentation Under Dynamic Conditions

Nanoindentation has emerged as a leading technique for the investigation of mechanical properties on small volumes of material. Extensive progress has been made in the last 20 years in refining the nstrumentation of nanoindentation systems and in analysis of the resulting data. Recent development has enabled investigation of materials under several dynamic conditions. The palladium-hydrogen system has a large miscibility gap, where the palladium lattice rapidly expands to form a hydrogen-rich β phase upon hydrogenation. Nanoindentation was used to investigate the mechanical effects of these transformations on foils of palladium. Study of palladium foils, which had been cycled through hydrogenation and dehydrogenation, allowed the extent of the transformed region to be determined. Unstable palladium foils, which had been hydrogenated and were subject to dynamic hydrogen loss, displayed significant hardening in the regions which were not expected to have transformed. The reason for this remains unclear. Impact indentation, where the indenter encounters the sample at relatively high speeds, can be used to probe the strain rate dependence of materials. By combining impact indentation and elevated temperature indentation, the strain rate dependence of the superelasticity of nickel-titanium was probed over a range of temperatures. Similar trends in elastic energy ratios with temperature were observed with the largest elastic proportions occurring at the Austenite finish transformation temperature. Multiple impact and scratch indentation are two modes of indentation which are thought to approximate erosive and abrasive wear mechanisms, respectively. These were utilised to investigate the wear resistance of several novel coatings formed by plasma electrolytic oxidation (PEO) of Ti-6Al4-V. Multiple impact indentation results appear to subjectively rank the erosive wear performance of both ductile and brittle materials. Comparison of normalised performance of coating systems on aluminium in abrasive wear to scratch hardness showed similar degrees of resistance.This material is based upon work supported under a National Science Foundation Graduate Research Fellowship. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Additional support for this work was provided by funding from the Atomic Weapons Establishment

Engineering of Substrate Surface for the synthesis of Ultra-Thin Composite Pd and Pd-Cu Membranes for H2 Separation

This work describes a novel technique to prepare ultra-thin composite Pd-porous metal membranes for H2 separation. This novel technique consists of the gradual smoothing of the Porous Metal (PM) support\u27s surface with several layers of pre-activated alumina particles of different sizes. The deposition of coarse, fine and ultra-fine alumina particles resulted in the narrowing of the PM\u27 surface pore size distribution. The excellent surface smoothness achieved after the grading of the PM \u27s surface support allowed for the preparation of gas tight Pd layers as thin as 5.6?m. The Pd layers were extremely uniform due to the presence of the grade layer and strongly attached to the support. Composite Pd membranes prepared on graded supports showed H2 permeance as high as 50 m3/(m2 h bar0.5) at 500ºC and ideal selectivities (H2/He) as high as 27000. Moreover, the H2 permeance and ideal selectivity were stable over 1100 hours at 500ºC in H2 atmosphere. Composite Pd-Cu membranes showed H2 permeance as high as 30 m3/(m2 h bar0.5) at 450ºC and ideal selectivities (H2/He) as high as 900. The H2 permeance and ideal selectivity of Pd-Cu membranes were stable over 500 hours at 450ºC in H2 atmosphere. The outstanding long-term H2 permeance and ideal selectivity stability of all composite Pd and Pd-Cu membranes represented a breakthrough in composite Pd membrane synthesis. The thermal stresses arising from the mismatch in the coefficient of thermal expansion between the Pd film and the support were determined by means of x-ray diffraction. The results indicated that the release of stresses began to occur at temperatures close to 400ºC. Also, the release of stresses took place with a visible sintering of Pd clusters within the thin Pd film. The stresses due to the absorption of H2 were also studied and modeled. It was estimated that the maximum compressive stress under which these composite Pd membranes were characterized was equal to 260 MPa

Study of fuel cells using storable rocket propellants Final report, 28 Jan. 1964 - 29 Jan. 1965

Fuel cells using storable rocket propellants for reactant

An Investigation of the Cause of Leak Formation in Palladium Composite Membranes.

In this research it was shown that the electroless plated palladium deposited as large number of randomly oriented grains, which were separated by grain boundaries (GB). The nano-scale dimensions of these grain boundaries allowed the diffusion of helium through the palladium membrane. This implied that in a dense palladium membrane, the grain boundary network was so convoluted that helium flux could be neglected. The transmission electron microscope (TEM) images of the palladium at room temperature showed grains of about 50 nm in size and nuclei of about 5 nm in size. The TEM images of a pre-annealed Pd sample at 500ºC in hydrogen atmosphere for 48 hours, showed big grains of 100 to 200 nm in size and most of the grain boundary intersections had dihedral angles very close to 120°. However, the pre-annealed Pd sample at 500ºC in helium atmosphere for 48 hours, showed grains of the size of 70 to 100 nm and many of the grain boundary intersections did not show dihedral angles of 120°. This proved that high temperature annealing not only caused significant grain growth and grain boundary (straightening) migration, but also the grain boundary migration was faster in hydrogen than in helium atmosphere. Also, the hydrogen and helium characterization of the palladium membranes showed that the leak formed faster in hydrogen than in helium. Thus, combining the TEM observations with the membrane characterization results, it is possible to conclude that grain boundary migration is one of the most probable reasons for leak formation in palladium composite membranes. The TEM images of the pre-annealed Pd sample also showed that the grain boundaries can achieve an equilibrium configuration within 48 hours of annealing at 500°C in hydrogen. This research helped in better understanding of the role of grain boundary migration on the leak formation in the composite palladium membranes and this information can be useful for the production of leak resistant stable membranes in the future

The formation of carbon nanofibers and thin films from the catalytic decomposition of ethylene by palladium

It has been demonstrated that palladium can be an exceptional catalyst toward the deposition of solid carbon from ethylene in two distinct forms: nanofibers and thin films. Four forms of palladium were tested: sputtered film, foil, sub-micron powder, and nanopowder. The deposition of carbon can be achieved by a very simple method. In this method ethylene and oxygen or hydrogen are flowed through a single-zone, horizontal tube furnace at atmospheric pressure and temperatures typically from 550-700°C. The addition of a secondary gas such as oxygen or hydrogen is vital in driving the deposition. Although both gases improve deposition, the manner in which they do differs. Ethylene-oxygen mixtures are preferred at lower temperatures (i.e. 550°C) than ethylene-hydrogen mixtures (i.e. 700°C). Pd sub-micron was the most prolific form of palladium at producing solid carbon in a combustion environment, whereas nanopowder was in ethylene-hydrogen mixtures. Palladium, of any form, did not catalyze appreciable carbon deposition at any temperature in ethylene alone. These findings suggest that radical species may be imperative to inciting carbon deposition. Independent of the previous finding, it is suggested different mechanisms of growth exist for fibers and thin films. This difference in mechanism is attributed to carbon acting to self-catalyze further deposition. The resulting carbon deposition rate and morphology were found to be a function of temperature, position in the reactor, duration of the reaction, gaseous environment, and form of palladium. These factors were all interconnected, and had to be considered collectively to predict the efficacy of the reaction toward solid carbon production. Crystallinity was found to increase with temperature, and ethylene-hydrogen mixtures produced more crystalline structures than were formed in a combustion environment, however the carbon produced under any conditions tested here was never fully graphitic, and instead was turbostratic or nearly amorphous. Based on the findings of the general catalysis study, the promise of application for the carbon nanofibers was anticipated and demonstrated through the formation of fibrous carbon foams. These foams can be generated using a small quantity of palladium (\u3c5% carbon output by mass), and both the macro- and microscale properties will define the overall properties, and therefore the projected use. These fibrous carbon foams can be combined with other materials to form composites which can be integrated during the formation of the foam. Because the foam process does not require high temperatures, a variety of materials with low melting temperatures can be safely incorporated. Also discussed is the potential of carbon nanofibers as an improved method of polymer reinforcement by tailoring morphology through reaction parameters

Structure and Chemistry of Model Catalysts in Ultrahigh Vacuum

The study of catalysis is a key area of focus not only in the industrial sector but also in the nature and biological systems. The market for catalysis is a multi-billion dollar industry. Many of the materials and products we use on a daily basis are formed through a catalytic process. The quest to understanding and improving catalytic mechanisms is ongoing. Many model catalysts use transition metals as a support for chemical reactions to take place due to their selectivity and activity. Palladium, gold, and copper metals are studied in this work and show the ability to be catalytically reactive. It is important to understand the characteristics and properties of these surfaces. A well-known example of catalysis is the conversion of carbon monoxide (CO), a very harmful gas to carbon dioxide (CO2) which is less harmful. This reaction is mainly seen in the automotive industry. This reaction is investigated in this work on a Au(111) single crystal, which is normally inert but becomes reactivity with the adsorption of oxygen on the surface. Temperature Programmed Desorption (TPD) is used to understand some of the chemistry and effects with and without the addition of H2O. The oxidation of CO is shown to be enhanced by the addition of water, but warrants further analysis too fully understand the different mechanisms and reaction pathways existing. The field of nano-electronics is rapidly growing as technology continues to challenge scientists to create innovative ideas. The trend to produce smaller electronic products is increasing as consumer demands persist. It has been shown previously that 1,4-phenlyene diisocyanobenzene (1,4-PDI) on Au(111) react to form one-dimensional oligomer chains comprising alternating gold and 1,4-PDI units on the Au(111) surface. A similar compound 1,3-phenlyene diisocyanobenzene (1,3-PDI) was studied in order to investigate whether the oligomerization found for 1,4-PDI is a general phenomenon and to ultimately explore the effect of molecular geometry on electron transport using a range of surface-sensitive techniques. Sulfur-containing molecules, in particular those with sulfur-sulfur linkages, are used as lubricant additives for ferrous surfaces[1-14] so that dialkyl disulfides have been used as simple model compounds to explore the surface and tribological chemistry on iron [15,16] where they react at the high temperatures attained at the interface during rubbing to deposit a ferrous sulfide film. However, the tribological chemistry can depend critically on the nature of the substrate so that a good lubricant additive for one type of surface may not be applicable to another. In particular, the lubrication of sliding copper-copper interfaces in electrical motors [17-20] provides a particular challenge. To study this system surface sensitive techniques Low energy electron diffraction (LEED) and TPD surface analysis was employed. LEED experiments suggest that tribological experiments can be conducted on copper foils rather than copper single crystals and produce comparable results. The ability to produce ideal model catalysts is very important in the Surface science field. To enhance catalytic performance of these catalysts, various strategies can be used in the preparation process. One approach in this quest is to produce an alloy surface that increases the activity of the surface. The process of developing and understanding the chemistry of AuPd alloys was probed in detail using TPD, LEED and Density Functional Theory (DFT)

Direct Experimental Evidence of Metal-Mediated Etching of Suspended Graphene

Atomic resolution high angle annular dark field imaging of suspended, single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al and Au atoms had been deposited was carried out in an aberration corrected scanning transmission electron microscope. In combination with electron energy loss spectroscopy, employed to identify individual impurity atoms, it was shown that nano-scale holes were etched into graphene, initiated at sites where single atoms of all the metal species except for gold come into close contact with the graphene. The e-beam scanning process is instrumental in promoting metal atoms from clusters formed during the original metal deposition process onto the clean graphene surface, where they initiate the hole-forming process. Our observations are discussed in the light of calculations in the literature, predicting a much lowered vacancy formation in graphene when metal ad-atoms are present. The requirement and importance of oxygen atoms in this process, although not predicted by such previous calculations, is also discussed, following our observations of hole formation in pristine graphene in the presence of Si-impurity atoms, supported by new calculations which predict a dramatic decrease of the vacancy formation energy, when SiOx molecules are present.Comment: final version accepted in ACS Nano + supplementary info. 22+6 pages, 4+5 figure