Ultra-low-density digitally architected carbon with a strutted tube-in-tube structure

Porous materials with engineered stretching-dominated lattice designs, which offer attractive mechanical properties with ultra-light weight and large surface area for wide-ranging applications, have recently achieved near-ideal linear scaling between stiffness and density. Here, rather than optimizing the microlattice topology, we explore a different approach to strengthen low-density structural materials by designing tube-in-tube beam structures. We develop a process to transform fully dense, three-dimensional printed polymeric beams into graphitic carbon hollow tube-in-tube sandwich morphologies, where, similar to grass stems, the inner and outer tubes are connected through a network of struts. Compression tests and computational modelling show that this change in beam morphology dramatically slows down the decrease in stiffness with decreasing density. In situ pillar compression experiments further demonstrate large deformation recovery after 30-50% compression and high specific damping merit index. Our strutted tube-in-tube design opens up the space and realizes highly desirable high modulus-low density and high modulus-high damping material structures

Structural relaxation process in pure amorphous silicon

Amorphous silicon (a-Si) is a material of major scientific and technological interest. It has been a very active topic of investigation for several decades, frequently crossing the border between physics and materials science. Research has been motivated partly by the fact that a-Si is an excellent model of a covalently bonded continuous random (CRN) network; thus a detailed understanding of the structure and properties of this material may aid the understanding of many other disordered materials. Notwithstanding many years of research and widely accessible experimental techniques, the structural properties of as-implanted a-Si and its relaxation mechanism remain poorly understood and are indeed subject of lively debate. This thesis addresses this issue by looking at the transition from unrelaxed to relaxed states which may provide insight into the nature of the structural relaxation process. In all three forms of a-Si studied: ion-implanted, pressure-induced, and re-irradiated relaxed a-Si, analysis of Raman spectra indicated a local ordering of the material approaching a continuous random network in the fully relaxed material due to thermal annealing. The bond-angle distortion is found to reduce at temperatures above 250 degree Celsius, whereas previous studies show reduction below this temperature. Electrical conductivity measurements of ion-implanted a-Si showed a decrease in the number of dangling bonds upon annealing. This phenomenon is observed during low temperature annealing up to 250 degree Celsius, before the bond-angle ordering commenced. Differential scanning calorimetry measurements show a heat release during low temperature annealing that is predominately due to defect annealing from the structure, but the heat release continues up to annealing temperature where the bond-angle distortion is reduced. Indentation tests showed that the transition in the deformation mechanism in ion-implanted a-Si mechanism from plastic to phase transformation related to the reduction of defects in the structure. This transition occurred at annealing temperatures before the bond-angle distortion reached its minimum value. The effect of in-diffused hydrogen upon annealing was also extensively investigated in this current study. It is observed that in-diffused hydrogen does not contribute significantly to the short-range ordering and hence structural relaxation. Finally, the results from all studies, namely Raman spectroscopy, electrical measurements, calorimetry, and indentation are brought together to develop a new model for structural relaxation in a-Si. Broadly, it is found that there are two main steps in structural relaxation: defect removal at low temperatures up to 250 degree Celsius, which is accompanied by significant heat release, and a reduction in bond-angle distortion at higher temperatures, where the amount of heat release is smaller. In terms of existing models, the current findings are consistent with elements of the previous defect/lattice strain models but differ in that a more distinct separation of defect removal and reduction in strain (bond-angle distortion) is found in the present study

The indentation hardness of silicon measured by instrumented indentation: What does it mean?

The indentation hardness of three different pure forms of silicon was investigated by two different methods. The hardness was probed by direct imaging of the residual impressions and by instrumented indentation using the Oliver–Pharr method. The forms of silicon used were a defective form of amorphous silicon, an amorphous form close to a continuous random network, and a crystalline silicon. The first form deforms via plastic flow and the latter two via phase transition. Two different unloading rates, fast and slow, were used to vary the phase transition behavior. This influenced the relative hardness as measured by instrumented indentation, which is not a reliable method to quantify hardness values in phase transforming materials. Thus, for our phase transforming silicon system, the relative hardness between samples can only be determined correctly by direct imaging, provided that the image accurately reveals the extent of the phase transformed volume.The authors thank the Australian Research Council for funding and J.E.B. gratefully acknowledges the ARC for a QEII fellowship

Raman study on the phase transformations of the meta-stable phases of Si induced by indentation

Raman scattering is used to investigate the metastable Si phases formed by indentation. The indent strain, phase distribution and the kinetics of the phase transformations are examined

Tantalum Suboxide Films with Tunable Composition and Electrical Resistivity Deposited by Reactive Magnetron Sputtering

Tantalum-based films with tailored composition, density, and electrical resistivity are of interest for next generation hohlraums for magnetized indirect-drive inertial confinement fusion. Here, we use reactive direct-current magnetron sputtering to deposit tantalum suboxide films with O content in the range of 46–71 at.%. In contrast to a common approach involving varying reactive gas contents, compositional control is achieved kinetically by changing the total chamber pressure and the deposition rate, while keeping the working gas mix of Ar-5%O2 constant. The resultant films are X-ray amorphous with electrical resistivity varying by over seven orders of magnitude. The dominant conduction mechanism changes from metallic to activated tunneling above ∼55 at.% of O, which is characterized by a sharp increase in resistivity and a decrease in the carrier density at low temperatures