Multiple Cycle Tests Research Articles

Roles of double salt formation and NaNO3 in Na2CO3-promoted MgO absorbent for intermediate temperature CO2 removal

Absorption and desorption of carbon dioxide on Na2CO3-promoted MgO have been studied at temperatures compatible with warm gas cleanup (300–470°C) from a pre-combustion syngas. The absorbents are synthesized through the formation and activation of the precipitate resulting from the addition of sodium carbonate to an aqueous solution of magnesium nitrate. The absorbent, which comprises MgO, Na2CO3 and residual NaNO3 after activation, forms the double salt Na2Mg(CO3)2 on exposure to CO2. The thermodynamic properties of the double salt, obtained through computational calculation, predict that the preferred temperature range for absorption of CO2 with the double salt is significantly higher compared with MgO. Faster CO2 uptake can be achieved as a result of this higher temperature absorption window. Absorption tests indicate that the double salt absorbent as prepared has a capacity toward CO2 of 15wt.% (3.4mmol CO2/g absorbent) and can be easily regenerated through both pressure swing and temperature swing absorption in multiple-cycle tests. Thermodynamic calculations also predict an important effect of CO2 partial pressure on the absorption capacity in the warm temperature range. The impurity phase, NaNO3, is identified as a key component in facilitating CO2 absorption by these materials. The reason for reported difficulties in reproducing the performance of these materials can be traced to specific details of the synthesis method, which are reviewed in some detail.

Vertically aligned ZnO nanorods and graphene hybrid architectures for high-sensitive flexible gas sensors

We present the fabrication and characterization of new type of flexible gas sensors, composed mainly of a bottom ZnO conductive layer on metal foil, vertically aligned ZnO nanorod channel, and graphene-based top conductive electrode. Multiple cycling tests demonstrated the ZnO nanorods (NRs) and graphene (Gr) hybrid architectures accommodated the flexural deformation without mechanical or electrical failure for bending radius below 0.8 cm under the repeated bending and releasing up to 100 times. In addition, the hybrid architectures fabricated on glass substrate showed good optical transmittance larger than ∼70% for visible light, indicating potential application in transparent devices. Furthermore, our gas sensors demonstrated the ppm level detection of ethanol gas vapor with the sensitivity (resistance in air/resistance in target gas) as high as ∼9 for 10 ppm ethanol.

Mechanical behavior of calcified plaques: a summary of compression and stress-relaxation experiments.

This paper summarizes the results from mechanical testing of atherosclerotic plaques performed in the Cardiovascular Mechanics Laboratory and the Laboratory for Implantable Materials at UMBC. The motivation for our work is that balloon angioplasty, stenting, and roto-ablation are mechanical processes that are designed to permanently alter the shape of an occluded arterial lumen. The mechanisms of permanent plaque deformation are not known. Therefore, to study the mechanical behavior of plaques, we performed mechanical tests on atherosclerotic lesions with different compositions and investigated differences in the materials' mechanical responses. Atherosclerotic plaque specimens were subjected to two main types of loading: multiple cyclic compression and stress-relaxation. The multiple-cycle test protocol was two fifteen-cycle loading phases that were separated by a 10-15 minute unloaded "rest" period. The compressive stress-relaxation test protocol was a series of three consecutive loadings (called phases I, II, and III). Each phase consisted of a 25% compression that was achieved in less than 1 second, a 10 minute relaxation period, and a 10 minute unloaded "rest" period between loadings. In the multiple cycle compressive loading, plaques exhibited three distinct types of behavior, which corresponded to the plaque compositions. Calcified plaques showed behaviors distinct from other plaque types and healthy vessels. In contrast to the cyclic compression results, plaque types could not be distinguished solely on the basis of stress relaxation behavior. Calcified and fibrous plaques had similar behavior, and therefore histology was used for definite identification. Calcified plaques have unique mechanical properties, and therefore interventions like angioplasty, roto-ablation, and stenting may require protocols specific for calcified lesions. The optimum protocols for calcified plaques may be quite different from plaques with other compositions. It is essential to learn more about the mechanical behavior of all plaque types to increase the success rate of occlusive atherosclerosis treatments.

Effect of microstructural coarsening on Hertzian contact damage in silicon nitride

The critical role of grain size in determining the nature of damage accumulation in silicon-nitride ceramics is evaluated using Hertzian contact testing. Single-cycle tests are conducted on materials of two grain sizes, 0.5 Μm (fine) and 2.0 Μm (coarse). Damage patterns for these two materials are compared and contrasted using a special bonded-interface specimen to investigate the subsurface regions. Optical and thermal wave-imaging techniques provide complementary pictures of the damage patterns: whereas the optical image maps elements of both deformation and fracture, the thermal wave image maps only the fracture. Taken together, these two imaging methods disclose a fundamental transition in the mechanical response in the two silicon nitrides, from cone-crack-dominated in the fine material to distributed-microcrack-dominated in the coarse material. Scanning electron microscopy (SEM) confirms the incidence of microfracture in the latter case. Thermal-wave measurements also allow a quantitative evaluation of the microfracture damage. Multiple-cycle tests on the coarse material show a build up of subsurface damage with increasing number of cycles, indicating mechanical fatigue. The results are discussed in terms of a shear-fault model, in which subsurface microcracks initiate from intrinsic planes of shear weakness in the microstructure. Implications concerning the microstructural design of silicon nitride ceramics for strength and wear applications are briefly considered, with reference to countervailing resisting and driving forces in the long-crack and short-crack toughness characteristics.

Effects of pore fluid chemistry on stable sliding of Berea sandstone

Single-cycle and multiple-cycle frictional-sliding experiments were employed to evaluate the effects of pore fluid environments on yield strength, frictional-sliding dynamics, and gouge production and morphology. Circular right cylinders cored from Berea sandstone sawcut at 35° to the axes were saturated in water, an inorganic brine, and various anionic, cationic, and nonionic aqueous surface-active agents. Samples were deformed under an effective confining pressure of 50 MPa and an axial strain rate of 6×10−5 sec−1 until a 2% axial strain beyond yield (defined as the onset of sliding) was achieved. All samples were displaced by stable sliding. In the single-cycle tests the unsaturated and water-saturated samples displayed small stress peaks at yield. During stable sliding samples saturated with DTAB and SDS displayed slight increases in differential stress and statistically significant higher frictional coefficients than other environments (including water) but were very similar in behavior to dry, unsaturated samples. In the multiple-cycle tests, samples were loaded to 2% strain beyond yield and unloaded to a differential stress of approximately 5–10 MPa a total of four times. These results further suggest that DTAB exerts a ‘strengthening’ effect on the sandstone relative to water which, to a limiting value, increased with displacement. The DTAB and SDS environments also produced a coarser grain-size distribution in the gouge relative to gouge produced in the other environments. Investigation of the gouge by scanning electron microscope revealed that these larger ‘grains’ were composed of dense, apparently cemented aggregates of ultrafine, platy quartz particles.