This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Simultaneous Thermal Analyzer combining Differential Scanning Calorimetry and ThermoGravimetrical Analysis (DSC/TGA) will be coupled with Perkin-Elmer mass spectrometer and FTIR to study new materials and thermal signatures of biological specimens. The materials research will focus on novel aluminum-based alloys with open-framework silicon clathrates. DSC studies of these materials provide a unique opportunity to determine phase diagrams of aluminum alloyed with two different crystallographic forms of silicon. Open silicon framework is broken at the clathrate surface resulting in a large number of Si dangling bonds, which, in turn, form strong connections with the aluminum matrix, resulting in better aluminum alloys. Furthermore, since large varieties of substituted clathrates are possible, DSC studies may result in synthesis of "designer alloys", and open new routes to tailor properties of these materials. Combined DSC/TGA-mass spectroscopy of biological samples will concentrate on thermal transitions exhibited by spores and bacteria. Main goal of these studies will be to develop new methods for fast identification of selected biological species in addition to recently developed luminescence-based approach. DSC will be also used to measure the effect of growth medium and environmental conditions on the ultrastructure of selected environmental bacterial samples. The acquisition of the DSC/TGA will make significant impact on undergraduate research and education in rural area of northeastern Alabama by expanding educational and multidisciplinary research opportunities for a diverse student population of Jacksonville State University, which is 60% female and 29% African American.
Layman Summary: Demand for better materials and fast identification of biological samples is increasing. Thermal analysis plays an important role in these areas. In this method, thermal response of the sample, and/or sample mass is recorded as a function of temperature. Such thermal signature is often unique for many biological specimens. In conjunction with other methods, such as optical fluorescence, thermal data allow for a fast identification of unknown spores or bacteria. Thermal analysis is also indispensible in search for and understanding of new materials. In this project, thermal methods will be used to determine stability of new class of aluminum alloys. These novel alloys are built around silicon "scaffolding" forming a silicon-reinforced aluminum. In fact, these alloys structurally are quite similar to the reinforced concrete. Since silicon scaffolding can be modified by incorporating other atoms, such as copper or nickel, these alloys can be tailored for a particular application. Hence, detailed studies of these materials could improve aluminum alloys which are widely used from aerospace industry to manufacturing household items. The acquisition of thermal analyzer will make significant impact on undergraduate research and education in rural area of northeastern Alabama by expanding educational and multidisciplinary research opportunities for a diverse student population of Jacksonville State University, which is 60% female and 29% African American.
The NSF award ID 0958826 was used to purchase simultaneous thermal analyzer SDT-Q600 from TA Instruments. The SDT-600 apparatus has been used to study various alloys, including alloys of silicon with gold and aluminum. Silicon forms very simple eutectic alloy with gold [1]. The Si-Au alloy containing 3.2% of Si melts at t = 363 oC, well below melting temperatures of gold (1064 oC) and silicon (1414 oC). This means that addition of only 3.2% of silicon to gold lowers the melting temperature of gold from 1064 to 363 oC. Similarly, addition of 12.6 % of Si to aluminum lower the melting temperature of aluminum from 660 to 577 oC. In this work, we have investigated alloys of two silicon allotropes with gold and aluminum. The most important result of this award is discovery of spontaneous transformation of silicon allotrope, so called type II Si136 clathrate [2] into crystalline silicon during formation of Si136-gold and Si136-aluminum alloys. We have shown that two different crystal types of silicon form exactly the same alloys with gold and aluminum. Crystal structures of silicon and the Si136 clathrate are shown in Fig. 1. The Si136 allotrope is an open-framework structure, made by stacking together 28-atom silicon cages in a cubic, face-centered cell. The Si136 clathrate is isomorphic with crystalline silicon. Formally, the Si136 allotrope is obtained by replacing every silicon atom in "normal" silicon by a 28-atom silicon cage. Since crystalline silicon contains eight atoms in the unit cell, the Si136 clathrate has eight 28-atom cages in the unit cell. In this structure, each 28-atom cage is surrounded by four other cages, forming characteristic zig-zag structure, shown in Fig. 1. "Empty" space between 28-atom cages is filled by 16 smaller, 20-atom cages. Therefore, the Si136 allotrope crystallizes in face-centered, cubic lattice. Similarly as in crystalline silicon, every atom in Si136 allotrope has four nearest-neighbors at a distance of 2.35 Å. The Si136 allotrope is very stable at normal conditions, but it transforms back into "normal" silicon at temperatures higher than 600 oC. Spontaneous transformation of Si136 clathrate is shown in Fig. 2, which displays the Differential Scanning Calorimetry (DSC) heat flux data for pure Si136 and three Si136-gold mixtures, containing 2.9 %, 5.5 %, and 12.9% of Si136, respectively. The DSC data confirm that pure Si136 clathrate is stable up to temperature of approx. 600 oC. Above this temperature, the Si136 allotrope spontaneously transforms into crystalline silicon, releasing heat. This transformation is indicated by a large, upward, exothermic peak in the DSC heat-flux data. However, when the Si136 allotrope is alloyed with gold, the DSC data display two peaks – a small upward (exothermic) peak between 345 and 350 oC, which is followed by much larger downward (endothermic) peak at ~ 360 oC. First exothermic peak is due to heat released during transformation of the Si136 allotrope into more stable, crystalline silicon. The area under the peak is proportional to the amount of Si136 allotrope in the mixture. Larger second peak indicates melting of Si-Au alloy. These data show that Si136 first transforms into ordinary silicon and then forms alloy with gold. When Si136 clathrate is heated with gold, the structural transition of Si136 into crystalline silicon takes place at temperatures at least ten degrees lower than the melting temperature of the eutectic Si-Au alloy (363 oC). We observed such transitions for all Si136-Au mixtures containing up to 90% of Si136. This is very interesting example of phase transformation triggered by inert gold. This phase transition is very similar to a domino effect. It starts at a certain number of points of contact between gold and Si136 grains, and then spontaneously propagates into bulk of Si136 particles. Only gold and aluminum can trigger this process. Similar results have been obtained for aluminum-Si136 alloys. During heating of Si136-aluminum mixtures, the Si136 clathrate transforms into crystalline silicon at temperatures between 550 and 565 oC. Then the eutectic melting occurs at ~ 577 oC and ordinary silicon-aluminum alloy is formed. Therefore, we have shown that gold and aluminum trigger spontaneous phase transformation of Si136 allotrope and that these two different crystalline types of silicon form identical alloys with gold and aluminum. References Desk Handbook: Phase Diagrams for Binary Alloys, H. Okamoto, Ed., ASM International, Materials Park, Ohio (2010). Low-density framework form of crystalline silicon with a wide optical band gap, J. Gryko, P. F. McMillan, R. F. Marzke, G. K. Ramachandran, D. Patton, S. K. Deb, and O. F. Sankey, Phys. Rev. B 62, R7707 (2000); J. Gryko, US Patent 6,423,286.
