Kinetic modifications of C4 PEPC are qualitatively convergent, but larger in Panicum than in Flaveria

C4 photosynthesis results from a set of anatomical features and biochemical components that act together to concentrate CO2 within the leaf and boost productivity. This complex trait evolved independently many times, resulting in various realizations of the phenotype, but in all C4 plants the primary fixation of atmospheric carbon is catalyzed by phosphoenolpyruvate carboxylase. Comparisons of C4 and non-C4 PEPC from a few closely related species suggested that the enzyme was modified to meet the demands of the C4 cycle. However, very few C4 groups have been investigated, hampering general conclusions. To test the hypothesis that distant C4 lineages underwent convergent biochemical changes, we compare the kinetic variation between C4 and non-C4 PEPC from a previously assessed young lineage (Flaveria, Asteraceae) with those from an older lineage found within the distantly related grass family (Panicum). Despite the evolutionary distance, the kinetic changes between the non-C4 and C4 PEPC are qualitatively similar, with a decrease in sensitivity for inhibitors, an increased specificity (kcat/Km) for bicarbonate, and a decreased specificity (kcat/Km) for PEP. The differences are more pronounced in the older lineage Panicum, which might indicate that optimization of PEPC for the C4 context increases with evolutionary time

Practical limitations to indentation testing of thin films

A method that is becoming increasingly common for measuring the mechanical behavior of thin films is low-load indentation testing. However, there can be complications in interpreting the results as many factors can affect hardness and moduli measurements such as surface roughness and determination of the indentation contact area. To further the understanding, the mechanical properties of thin (50 nm) films of AlN on sapphire substrates were evaluated using a scanning force microscopy (SFM) based pico-indentation device to allow imaging of the surface and indentations. The primary emphasis was the types of problems or limitations involved in testing very thin, as deposited films in which properties are desired over indentation depths less than 50 nm

Fracture of hard thin films using nanoindentation and nanoscratch techniques: A materials and mechanics approach

Thin films are used in many applications where special properties are needed to insure performance and reliability. Of particular interest are thin tantalum nitride films. They are used extensively in microelectronic applications because of their long term stability and low thermal coefficient of resistance. They are sputter deposited which produces films with a high structural defect content and high compressive residual stresses both of which can alter the physical and mechanical properties of microelectronic thin films. Although these films are strong heat generators, they exhibit no changes in structure or composition of the interface with aluminum oxide substrates that degrade performance or reliability. However, the use of high power density components is driving a move to replace aluminum oxide with aluminum nitride for greater heat transfer. 6 This replacement substrate creates concern as residual stresses and long-term operation could induce detrimental changes along the thin film interface not observed in aluminum oxide devices. As a result, the authors employed nanoindentation and continuous nanoscratch testing to determine the effects of the intrinsic compressive residual stresses on the properties and fracture resistance of the thin tantalum nitride films. These techniques sample small volumes of material while preserving the production configuration of a free surface. Although nanoscratch tests lack a rigorous derivation of stress distributions and strain energy release rates, good approximations for strain energy release rates can be obtained using mechanics-based models for blister formation where residual stresses dominate interfacial fracture behavior. When combined with scanning and transmission electron microscopy, the results define structure-property relationships and resistance to fracture of these hard films

Microwave heating, isothermal sintering, and mechanical properties of powder metallurgy titanium and titanium alloys

This article presents a detailed assessment of microwave (MW) heating, isothermal sintering, and the resulting tensile properties of commercially pure Ti (CP-Ti), Ti-6Al-4V, and Ti-10V-2Fe-3Al (wt pct), by comparison with those fabricated by conventional vacuum sintering. The potential of MW sintering for titanium fabrication is evaluated accordingly. Pure MW radiation is capable of heating titanium powder to ≥1573 K (1300 C), but the heating response is erratic and difficult to reproduce. In contrast, the use of SiC MW susceptors ensures rapid, consistent, and controllable MW heating of titanium powder. MW sintering can consolidate CP-Ti and Ti alloys compacted from -100 mesh hydride-dehydride (HDH) Ti powder to ~95.0 pct theoretical density (TD) at 1573 K (1300 C), but no accelerated isothermal sintering has been observed over conventional practice. Significant interstitial contamination occurred from the Al2O3-SiC insulation-susceptor package, despite the high vacuum used (≤4.0 × 10-3 Pa). This leads to erratic mechanical properties including poor tensile ductility. The use of Ti sponge as impurity (O, N, C, and Si) absorbers can effectively eliminate this problem and ensure good-to-excellent tensile properties for MW-sintered CP-Ti, Ti-10V-2Fe-3Al, and Ti-6Al-4V. The mechanisms behind various observations are discussed. The prime benefit of MW sintering of Ti powder is rapid heating. MW sintering of Ti powder is suitable for the fabrication of small titanium parts or titanium preforms for subsequent thermomechanical processing

Evaluation of the current status of hydrogen embrittlement and stress-corrosion cracking in steels

A review of recent studies on hydrogen embrittlement and stress-corrosion cracking in steels shows there are several critical areas where data is either ambiguous, contradictory, or non-existent. A relationship exists between impurity segregation and hydrogen embrittlement effects but it is not known if the impurities sensitize a preferred crack path for hydrogen-induced failure or if impurity and hydrogen effects are additive. Furthermore, grain boundary impurities may enhance susceptibility through interactions with some environments. Some studies show that an increase in grain size increases susceptibility; at least one study shows an opposite effect. Recent work also shows that fracture initiates at different locations for external and internal hydrogen environments. How this influences susceptibility is unknown

Modeling the segregation of hydrogen to lattice defects in nickel

In order to better understand the effect of hydrogen on the fracture behavior of nickel, this study uses the embedded atom method (EAM) to model the segregation of hydrogen to lattice defects in nickel. The dislocations modeled include an edge, a screw, and a Lomer dislocation in the locked configuration, i.e. the Lomer-Cottrell Cock (LCL). Several coincident site lattice boundaries are also investigated, these being the {Sigma}3(112) and {Sigma}11(113) tilt boundaries. It will be shown that the trap site energies in the vicinity of both the edge and screw dislocations is only about 0.1 eV while for the LCL and all of the grain boundaries the maximum trap site energy in the vicinity of the defect is on order 0.3 eV. Using a Monte-Carlo method to a impose a hydrogen environment produces much stronger segregation of hydrogen to the deeper traps. When compared to recent experimental studies showing that a binding energy between 0.3-0.4 eV is required for trap site controlled fracture in IN903, it can be concluded that the embrittlement process is most probably associated with trapping of hydrogen to the Lomer-Cottrell Locks

Interface effects on the adhesion of thin aluminum films

Differences in the adhesion and fracture toughness of aluminum films on sapphire due to the presence of controlled contaminants are being investigated. Adhesion is evaluated by use of nanoindentation and continuous scratch tests. A comparison was made of the properties of textured thin films of aluminum (178 to 1890 nm) that were vapor deposited onto (0001) oriented sapphire substrates. A very thin (10 {angstrom}) layer of carbon was deposited at the interface of selected samples prior to the vapor deposition of the aluminum. Spalling was observed during continuous scratch testing in specimens with carbon at the interface but not in specimens without carbon at the interface

Interface Effects on the Adhesion of Thin Aluminum Films

Differences in the adhesion and fracture toughness of aluminum films on sapphire due to the presence of controlled contaminants are being investigated. Adhesion is evaluated by use of nanoindentation and continuous scratch tests. A comparison was made of the properties of textured thin films of aluminum (178 to 1890 nm) that were vapor deposited onto (0001) oriented sapphire substrates. A very thin (10 {angstrom}) layer of carbon was deposited at the interface of selected samples prior to the vapor deposition of the aluminum. Spalling was observed during continuous scratch testing in specimens with carbon at the interface but not in specimens without carbon at the interface