Elevated temperature erosion studies on some materials for high temperature applications

The surface degradation of materials due to high temperature erosion or combined erosion-corrosion is a serious problem in many industrial and aeronautical applications. As such, it has become an important design consideration in many situations. The materials investigated in the present studies are stainless steels, Ti-6Al-4V, alumina ceramics, with and without silicate glassy phase, and zirconia. These are some of the potential materials for use in the high temperature erosive-corrosive environments;The erosion or erosion-corrosion experiments were performed in a high temperature sand-blast type of test rig. The variables studied included the temperature, material composition, heat treatment condition, impingement velocity and angle, erodent concentration, etc. The morphological features of the eroded or eroded-corroded surfaces, substrate deformation, and oxide characteristics were studied by optical and scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, thermogravimetric analysis. The scratch test, single ball impact, and indentation tests were used to understand the behavior of oxide film in particle impacts;Based on these investigations, the understanding of the mechanisms involved in the mechanical or combined mechanical and chemical actions in erosion was developed. The analysis focused on the interactions among factors such as the temperature-dependent properties of materials, microstructure including the role of multiple phases in ceramics, high temperature erosion characteristics, cracking fracture of ceramics, oxidation rate, and the composition, structure, and properties of oxide scales. The enhanced corrosion rate from erosion damage, the effect of corrosion on material mass loss, and enhanced erosion rate at high temperatures were interpreted in terms of the above factors;This dissertation is composed of five major parts which present various aspects of high temperature erosion and erosion-corrosion based on the investigations on the three kinds of materials for high temperature applications;PARTS I and II are the investigations of high temperature erosion-corrosion behavior of ferritic, austenitic, martensitic and precipitation hardening stainless steels. PART III deals with SEM studies of the material damage in alumina ceramics by single and multiple particle impacts. PART IV extends the above studies on alumina ceramics to high temperature environment. PART V covers the high temperature erosion-corrosion behavior of Ti-6Al-4V. In contrast to other materials studied in this thesis, the titanium alloy undergoes rapid oxidation at elevated temperatures

Radiation Effects in Apatite and High Entropy Alloy under Energetic Ions and Electrons

Radiation effects in apatite and high entropy alloy under energetic ions and electrons are studied in this doctoral dissertation to develop advanced crystalline ceramic waste forms and nuclear structural materials. Apatite is proposed as a ceramic waste form for the immobilization of radionuclides, but its performance is strongly affected by the irradiation of the incorporated radionuclides. It is thus important to understand the radiation effects in apatite structure and the underlying physics. Effects of chemical composition, grain size, interfacial structure, as well as radiation conditions on the microstructural evolution, phase transformation and damage mechanisms of apatite under alpha-decay and beta-decay events, simulated by 1 MeV Kr ions and 200 keV electrons respectively, are investigated. Composition effect on silicate apatite shows the better radiation tolerance under higher cerium content. Size effect on hydroxyapatite exhibits the reduction of radiation stability with the decrease of grain size due to excess surface energy in nanoparticles. A further study addresses densified nanocrystalline hydroxyapatite exhibits higher radiation tolerance than the same sized hydroxyapatite nanoparticle as a result of lower interface energy. Effect of radiation conditions on the recrystallization behaviors of pre-amorphized hydroxyapatite is also studied. In-situ TEM observation reveals a rapid recrystallization process and a notable size effect, which smaller sized sample nucleates and fully recrystallizes under lower electron fluence. Recrystallization mechanism is attributed to ionization process as a result of breaking and reforming of dangling bonds. The radiation effect study is further extended to include high entropy alloys intended as structural materials in advanced nuclear reactors. Two types of high entropy alloys are selected as model alloys to investigate the irradiation-induced behaviors under 1 MeV Krions. Study on nanocrystalline AlxCoCrFeNi alloys shows a notable ion-irradiation-induced grain growth, whose mechanisms are attributed to a disorder-driven mechanism for the initial fast increase of grain size and defect-stimulated mechanism for the later slow grain size increase, elucidated by the thermal spike model. Study on HfNbTaTiVZr alloy reveals a crystal-to-amorphous phase transformation with critical amorphization dose of 2 displacements per atom (dpa) at 298 K, while the amorphization is suppressed when the temperature increases to 423 K

Radiation Shielding of Lunar Regolith/Polyethylene Composites and Lunar Regolith/Water Mixtures

Space radiation is a complex mixed field of ionizing radiation that can pose hazardous risks to sophisticated electronics and humans. Mission planning for lunar exploration and long duration habitat construction will face tremendous challenges of shielding against various types of space radiation in an attempt to minimize the detrimental effects it may have on materials, electronics, and humans. In late 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) discovered that water content in lunar regolith found in certain areas on the moon can be up to 5.6 +/-2.8 weight percent (wt%) [A. Colaprete, et. al., Science, Vol. 330, 463 (2010). ]. In this work, shielding studies were performed utilizing ultra high molecular weight polyethylene (UHMWPE) and aluminum, both being standard space shielding materials, simulated lunar regolith/ polyethylene composites, and simulated lunar regolith mixed with UHMWPE particles and water. Based on the LCROSS findings, radiation shielding experiments were conducted to test for shielding efficiency of regolith/UHMWPE/water mixtures with various percentages of water to compare relative shielding characteristics of these materials. One set of radiation studies were performed using the proton synchrotron at the Loma Linda Medical University where high energy protons similar to those found on the surface of the moon can be generated. A similar experimental protocol was also used at a high energy spalation neutron source at Los Alamos Neutron Science Center (LANSCE). These experiments studied the shielding efficiency against secondary neutrons, another major component of space radiation field. In both the proton and neutron studies, shielding efficiency was determined by utilizing a tissue equivalent proportional counter (TEPC) behind various thicknesses of shielding composite panels or mixture materials. Preliminary results from these studies indicated that adding 2 wt% water to regolith particles could increase shielding of the regolith materials by about 6%. The findings may be utilized to extend the possibilities of potential candidate materials for lunar habitat structures, will potentially impact the design criteria of future human bases on the moon, and provide some guidelines for future space mission planning with respect to radiation exposure and risks posed on astronauts

Slag Characterization and Removal Using Pulse Detonation Technology During Coal Gasification

The main activity in the first quarter of 1998 was concentrated on understanding the detonation code, so that it can be linked with the in-house CFD code NPARC for simulation. The objective is to obtain the velocity and pressure distribution inside the detonation tube and compare with the experimental data that we have obtained from the experiments. Once the code is validated, the simulation will be extended to obtain the pressure and velocity fields in the large chamber, i.e., outside the exit of the detonation tube where the slag samples are attached

Potential Use of In Situ Material Composites such as Regolith/Polyethylene for Shielding Space Radiation

NASA has an extensive program for studying materials and methods for the shielding of astronauts to reduce the effects of space radiation when on the surfaces of the Moon and Mars, especially in the use of in situ materials native to the destination reducing the expense of materials transport. The most studied material from the Moon is Lunar regolith and has been shown to be as efficient as aluminum for shielding purposes (1). The addition of hydrogenous materials such as polyethylene should increase shielding effectiveness and provide mechanical properties necessary of structural materials (2). The neutron radiation shielding effectiveness of polyethylene/regolith stimulant (JSC-1A) composites were studied using confluent human fibroblast cell cultures exposed to a beam of high-energy spallation neutrons at the 30deg-left beam line (ICE house) at the Los Alamos Neutron Science Center. At this angle, the radiation spectrum mimics the energy spectrum of secondary neutrons generated in the upper atmosphere and encountered when aboard spacecraft and high-altitude aircraft. Cell samples were exposed in series either directly to the neutron beam, within a habitat created using regolith composite blocks, or behind 25 g/sq cm of loose regolith bulk material. In another experiment, cells were also exposed in series directly to the neutron beam in T-25 flasks completely filled with either media or water up to a depth of 20 cm to test shielding effectiveness versus depth and investigate the possible influence of secondary particle generation. All samples were sent directly back to JSC for sub-culturing and micronucleus analysis. This presentation is of work performed in collaboration with the NASA sponsored Center for Radiation Engineering and Science for Space Exploration (CRESSE) at Prairie View A&M

High-Resolution 3D Structure Determination of Kaliotoxin by Solid-State NMR Spectroscopy

High-resolution solid-state NMR spectroscopy can provide structural information of proteins that cannot be studied by X-ray crystallography or solution NMR spectroscopy. Here we demonstrate that it is possible to determine a protein structure by solid-state NMR to a resolution comparable to that by solution NMR. Using an iterative assignment and structure calculation protocol, a large number of distance restraints was extracted from 1H/1H mixing experiments recorded on a single uniformly labeled sample under magic angle spinning conditions. The calculated structure has a coordinate precision of 0.6 Å and 1.3 Å for the backbone and side chain heavy atoms, respectively, and deviates from the structure observed in solution. The approach is expected to be applicable to larger systems enabling the determination of high-resolution structures of amyloid or membrane proteins

Transient mTOR Inhibition Facilitates Continuous Growth of Liver Tumors by Modulating the Maintenance of CD133+ Cell Populations

The mammalian target of the rapamycin (mTOR) pathway, which drives cell proliferation, is frequently hyperactivated in a variety of malignancies. Therefore, the inhibition of the mTOR pathway has been considered as an appropriate approach for cancer therapy. In this study, we examined the roles of mTOR in the maintenance and differentiation of cancer stem-like cells (CSCs), the conversion of conventional cancer cells to CSCs and continuous tumor growth in vivo. In H-Ras-transformed mouse liver tumor cells, we found that pharmacological inhibition of mTOR with rapamycin greatly increased not only the CD133+ populations both in vitro and in vivo but also the expression of stem cell-like genes. Enhancing mTOR activity by over-expressing Rheb significantly decreased CD133 expression, whereas knockdown of the mTOR yielded an opposite effect. In addition, mTOR inhibition severely blocked the differentiation of CD133+ to CD133- liver tumor cells. Strikingly, single-cell culture experiments revealed that CD133- liver tumor cells were capable of converting to CD133+ cells and the inhibition of mTOR signaling substantially promoted this conversion. In serial implantation of tumor xenografts in nude BALB/c mice, the residual tumor cells that were exposed to rapamycin in vivo displayed higher CD133 expression and had increased secondary tumorigenicity compared with the control group. Moreover, rapamycin treatment also enhanced the level of stem cell-associated genes and CD133 expression in certain human liver tumor cell lines, such as Huh7, PLC/PRC/7 and Hep3B. The mTOR pathway is significantly involved in the generation and the differentiation of tumorigenic liver CSCs. These results may be valuable for the design of more rational strategies to control clinical malignant HCC using mTOR inhibitors

Elevated temperature erosion studies on some materials for high temperature applications

The surface degradation of materials due to high temperature erosion or combined erosion-corrosion is a serious problem in many industrial and aeronautical applications. As such, it has become an important design consideration in many situations. The materials investigated in the present studies are stainless steels, Ti-6Al-4V, alumina ceramics, with and without silicate glassy phase, and zirconia. These are some of the potential materials for use in the high temperature erosive-corrosive environments;The erosion or erosion-corrosion experiments were performed in a high temperature sand-blast type of test rig. The variables studied included the temperature, material composition, heat treatment condition, impingement velocity and angle, erodent concentration, etc. The morphological features of the eroded or eroded-corroded surfaces, substrate deformation, and oxide characteristics were studied by optical and scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, thermogravimetric analysis. The scratch test, single ball impact, and indentation tests were used to understand the behavior of oxide film in particle impacts;Based on these investigations, the understanding of the mechanisms involved in the mechanical or combined mechanical and chemical actions in erosion was developed. The analysis focused on the interactions among factors such as the temperature-dependent properties of materials, microstructure including the role of multiple phases in ceramics, high temperature erosion characteristics, cracking fracture of ceramics, oxidation rate, and the composition, structure, and properties of oxide scales. The enhanced corrosion rate from erosion damage, the effect of corrosion on material mass loss, and enhanced erosion rate at high temperatures were interpreted in terms of the above factors;This dissertation is composed of five major parts which present various aspects of high temperature erosion and erosion-corrosion based on the investigations on the three kinds of materials for high temperature applications;PARTS I and II are the investigations of high temperature erosion-corrosion behavior of ferritic, austenitic, martensitic and precipitation hardening stainless steels. PART III deals with SEM studies of the material damage in alumina ceramics by single and multiple particle impacts. PART IV extends the above studies on alumina ceramics to high temperature environment. PART V covers the high temperature erosion-corrosion behavior of Ti-6Al-4V. In contrast to other materials studied in this thesis, the titanium alloy undergoes rapid oxidation at elevated temperatures.</p