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Pressure Induced Amorphization in Silicon Caused by the Impact of Electrosprayed Nanodroplets

Journal paper
M. Gamero-Castano, A. Torrents, L. Valdevit, J-G. Zheng
Physical Review Letters 105 (2010) 145701
Publication year: 2010

Abstract

This Letter describes the shock-induced amorphization of single-crystal Si bombarded by nanodroplets. At impact velocities of several kilometers per second, the projectiles trigger strong compression pulses lasting tens of picoseconds. The phase transition, confirmed via transmission electron microscopy and electron backscatter diffraction, takes place when the projectile’s stagnation pressure is approximately 15 GPa. We speculate that the amorphization results either from the decompression of the beta-Sn phase or during the compression of the diamond phase.

MEMS resonant load cells for micro-mechanical test frames: Feasibility study and optimal design

Journal paper
A. Torrents, K. Azgin, S. W. Godfrey, E. S. Topalli, T. Akin, L. Valdevit
Journal of Micromechanics and Microengineering, 20 (2010) 125004 (17pp)
Publication year: 2010

Abstract

This paper presents the design, optimization and manufacturing of a novel micro-fabricated load cell based on a double-ended tuning fork. The device geometry and operating voltages are optimized for maximum force resolution and range, subject to a number of manufacturing and electromechanical constraints. All optimizations are enabled by analytical modeling (verified by selected finite elements analyses) coupled with an efficient C++ code based on the particle swarm optimization algorithm. This assessment indicates that force resolutions of ∼0.5–10 nN are feasible in vacuum (∼1–50 mTorr), with force ranges as large as 1 N. Importantly, the optimal design for vacuum operation is independent of the desired range, ensuring versatility. Experimental verifications on a sub-optimal device fabricated using silicon-on-glass technology demonstrate a resolution of ∼23 nN at a vacuum level of ∼50 mTorr. The device demonstrated in this article will be integrated in a hybrid micro-mechanical test frame for unprecedented combinations of force resolution and range, displacement resolution and range, optical (or SEM) access to the sample, versatility and cost.

Influence of Configuration on Materials Selection for Actively-Cooled Combustors

Journal paper
N. Vermaak, L. Valdevit, A. G. Evans, F. W. Zok and R. M. McMeeking
AIAA Journal of Propulsion and Power 26 (2010) 295-302
Publication year: 2010

Abstract

The influence of combustor size and shape on material feasibility is explored using (structural and fuel) weight, as well as fuel economy as metrics. A materials selection methodology developed for actively cooled rectangular panels has been embellished to include cylindrical/annular configurations. The procedure incorporates an analytical model for temperature and stress distributions subject to thermomechanical loads representative of hypersonic flight conditions. The model has been numerically verified using finite element simulations. By combining the model with optimization routines, materials robustness maps have been produced, depicting the range of thermal loads and fuel flow rates that satisfy all design constraints. A wide selection of high-temperature materials has been investigated. Comparisons of cylindrical and rectangular combustors are made for the leading candidates. It is established that the cylindrical designs allow both lighter optimal structures as well as greater robustness and fuel economy.

Materials Property Profiles for Actively Cooled Panels: An Illustration for Scramjet Applications

Journal paper
N. Vermaak, L. Valdevit, A.G. Evans
Metallurgical and Materials Transactions A 40A (2009) 877-890
Publication year: 2009

Abstract

A scheme for identifying and visualizing the material properties that limit the performance of candidate materials for actively cooled aerospace propulsion components is presented and illustrated for combustor panels for Mach 7 hypersonic vehicles. The method provides a framework for exploring the nonlinear interactions between design and materials optimization. By probing the active constraints along the border of feasible design space, the limiting properties have been elucidated for a representative group of candidate materials. Property vectors that enhance design options have also been determined. For one of the promising candidate alloys (the Ni-based superalloy, INCONEL X-750), the possibilities of reclaiming design space and lowering optimal combustor panel weight by tailoring its strength properties are assessed.

Feasibility of metallic structural heat pipes as sharp leading edges for hypersonic vehicles

Journal paper
C. Steeves, M.Y. He, S.D. Kasen, L. Valdevit, H.N.G. Wadley and A.G. Evans
Journal of Applied Mechanics 76 (2009) 031014 (9p)
Publication year: 2009

Abstract

The influence of combustor size and shape on material feasibility is explored using (structural and fuel) weight, as well as fuel economy as metrics. A materials selection methodology developed for actively cooled rectangular panels has been embellished to include cylindrical/annular configurations. The procedure incorporates an analytical model for temperature and stress distributions subject to thermomechanical loads representative of hypersonic flight conditions. The model has been numerically verified using finite element simulations. By combining the model with optimization routines, materials robustness maps have been produced, depicting the range of thermal loads and fuel flow rates that satisfy all design constraints. A wide selection of high-temperature materials has been investigated. Comparisons of cylindrical and rectangular combustors are made for the leading candidates. It is established that the cylindrical designs allow both lighter optimal structures as well as greater robustness and fuel economy.

Organic substrates for flip chip design: a thermo-mechanical model that accounts for heterogeneity and anisotropy

Journal paper
L. Valdevit, V. Khanna, A. Sharma, S. Sri-Jayantha, D. Questad, K. Sikka
Microelectronics Reliability, 48 (2008), 245-260
Publication year: 2008

Abstract

We present a thermo-mechanical characterization of organic substrates that accounts for heterogeneity both in the in-plane and out-of-plane directions. Systematic observation of the board files of a number of substrates of commercial interest reveals primarily three recurrent topological arrangements of copper and polymer; for each arrangement, the in-plane effective thermo-elastic properties are calculated via appropriate composite materials models. The averaging process in the out-of-plane direction (i.e. the stacking effect) is performed using standard laminated plate theory. The model is successfully applied to various regions of three organic substrates of interest (mainly differing in core thickness): the analytically calculated effective Young’s moduli (E) and coefficients of thermal expansion (CTE) are shown to be typically within 10% of the experimental measurements. An important attribute of this model is its ability to provide substrate description at various levels of complexity: a few effective properties are outputted that can be useful for further purely analytical investigations; at the same time, the model provides the full stiffness matrix for each region of the substrate, to be used for more detailed finite elements simulations of higher-level structures (e.g. silicon die/underfill/substrate/cooling solution assemblies). Preliminary application of this model to the warp analysis of a flip-chip is presented in the end.

A materials selection protocol for lightweight actively cooled panels

Journal paper
L. Valdevit, N. Vermaak, F. W. Zok, A. G. Evans
Journal of Applied Mechanics 75 (2008) 061022 (15p)
Publication year: 2008

Abstract

This article provides a materials selection methodology applicable to lightweight actively cooled panels, particularly suitable for the most demanding aerospace applications. The key ingredient is the development of a code that can be used to establish the capabilities and deficiencies of existing panel designs and direct the development of advanced materials. The code is illustrated for a fuel-cooled combustor liner of a hypersonic vehicle, optimized for minimum weight subject to four primary design constraints (on stress, temperatures, and pressure drop). Failure maps are presented for a number of candidate high-temperature metallic alloys and ceramic composites, allowing direct comparison of their thermostructural performance. Results for a Mach 7 vehicle under steady-state flight conditions and stoichiometric fuel combustion reveal that, while C–SiC satisfies the design requirements at minimum weight, the Nb alloy Cb752 and the Ni alloy Inconel X-750 are also viable candidates, albeit at about twice the weight. Under the most severe heat loads (arising from heat spikes in the combustor), only Cb752 remains viable. This result, combined with robustness benefits and fabrication facility, emphasizes the potential of this alloy for scramjets. 

The design space of Superalloy-based actively cooled combustor walls for Mach 7-12 hypersonic vehicles

Conference Proceedings
L. Valdevit, N. Vermaak, F. W. Zok and A. G. Evans
ASME IMECE 2007-41348, Seattle, USA, Nov 2007
Publication year: 2007

Abstract

The walls of combustion chambers used for air-breathing hypersonic vehicles are subject to substantial thermo-mechanical loads, and require active cooling by the fuel in conjunction with advanced material systems. Solutions based on metallics are preferable to ceramic matrix composites due to their lower cost and greater structural robustness. Previous work suggested that a number of metallic materials (e.g. Nickel, Copper and Niobium alloys) could be used to fabricate actively cooled sandwich structures that withstand the thermo-mechanical loads for a Mach 7, hydrocarbon-powered vehicle (albeit with different weight efficiencies). However, this conclusion changes when the Mach number is increased. This work explores the feasibility of the Nickel superalloy MARM246 for a wide range of Mach numbers (7–12). Since hydrocarbon fuels are limited to Mach 7–8, Hydrogen is used as the coolant of choice. A previously derived analytical model (appropriately modified for gaseous coolant) is used to explore the design space. The relative importance of each design constraint is assessed, resulting in the distillation of essential guidelines for optimal design.

Metallic structural heat pipes as sharp leading edges for Mach 7 vehicles

Conference Proceedings
C. Steeves, L. Valdevit, M. He and A. G. Evans
ASME IMECE 2007-42397, Seattle, USA, Nov 11-15, 2007
Publication year: 2007

Abstract

Sharp leading edges on hypersonic vehicles experience very large heating loads and consequent high temperatures. One strategy for for accommodating these effects is to provide very high effectively thermal conductivity which allows heat to be transferred from the hot leading edge to large cool surfaces for radiation into space. Heat pipes integrated within metallic leading edges provide this function, as well as being easy to manufacture and highly robust compared to other material choices. This paper will examine the feasibility of metallic leading edge heat pipes for hypersonic vehicles in Mach 7 flight. Using temperatures and heat fluxes calculated elsewhere, analytic approximations of the temperature distributions and stresses in a prototypical system are analyzed. The analysis is supplemented and confirmed by finite element calculations. Feasibility of the system is assessed by simple calculations on the operational limits of heat pipes.

Design and implementation of actively cooled panels for scramjets

Conference Proceedings
N. Vermaak, L. Valdevit, F. W. Zok and A. G. Evans
ASME IMECE 2007-41347, Seattle, USA, Nov. 11-15, 2007
Publication year: 2007

Abstract

The operating conditions of scramjet engines demand designs that include active cooling by the fuel and the use of lightweight materials that withstand extreme heat fluxes and structural loads. An optimization tool has previously been introduced to direct the development of advanced materials that outperform existing high temperature alloys and compete with ceramic matrix composites. This analysis presents verification and accretion of the analytical design tool through a combination of numerical and experimental techniques. Selected computational fluid dynamics (CFD) analyses have been performed to verify critical thermal assumptions. A high-power CO2 laser provides heat fluxes representative of hypersonic flight conditions.

Structural performance of near-optimal sandwich panels with corrugated cores

Journal paper
L. Valdevit, Z. Wei, C. Mercer, F. W. Zok, A. G. Evans
International Journal of Solids and Structures, 43 (2006), 4888-4905
Publication year: 2006

Abstract

An experimental and computational study of the bending response of steel sandwich panels with corrugated cores in both transverse and longitudinal loading orientations has been performed. Panel designs were chosen on the basis of failure mechanism maps, constructed using analytic models for failure initiation. The assessment affirms that the analytic models provide accurate predictions when failure initiation is controlled by yielding. However, discrepancies arise when failure initiation is governed by other mechanisms. One difficulty is related to the sensitivity of the buckling loads to the rotational constraints of the nodes, as well as to fabrication imperfections. The second relates to the compressive stresses beneath the loading platen. To address these deficiencies, existing models for core failure have been expanded. The new results have been validated by experimental measurements and finite element simulations. Limit loads have also been examined and found to be sensitive to the failure mechanism. When face yielding predominates, appreciable hardening follows the initial non-linearity, rendering robustness. Conversely, for designs controlled by buckling (either elastic or plastic) failure initiation is immediately followed by softening. The implication is that, when robustness is a key requirement, designs within the face failure domain are preferred.

Optimal active cooling performance of metallic sandwich panels with prismatic cores

Journal paper
L. Valdevit, A. Pantano, H. A. Stone, A. G. Evans
Int. Journal of Heat and Mass Transfer, 49 (2006), 3819-3830
Publication year: 2006

Abstract

All-metallic sandwich panels with prismatic cores are being currently investigated for combined structural and active cooling performance. We present a new approach to active cooling performance, and use it to optimize the panel geometry for four different systems: aluminum-air, aluminum-water, aluminum-gasoline and titanium-gasoline. The results show that some geometric parameters can be fixed without much detriment in thermal performance. Moreover, while optimal core densities are typically 25–50%, near-optimal results can be obtained with densities as low as 10%. These findings provide considerable geometric flexibility when attempting combined thermal and structural optimization.

Design of actively cooled panels for scramjets

Conference Proceedings
L. Valdevit, N. Vermaak, K. Hsu, F. W. Zok and A. G. Evans
14th AIAA/AHI International Space Planes and Hypersonic Systems and Technologies Conference, Canberra, Australia, Nov 6-9, 2006. AIAA Paper 2006-8069
Publication year: 2006

Abstract

The operating conditions of scramjet engines demand designs that include active cooling by the fuel and the use of lightweight materials that withstand extreme heat fluxes under oxidizing conditions. The goal of this analysis is to provide an optimization tool that can be used to direct the development of advanced materials that outperform existing high temperature alloys and compete with ceramic matrix composites. For this purpose an actively cooled plate has been optimized for minimum weight under three primary constraints. (i) Resistance to pressure loads arising from fuel injection and combustion, as well as thermal loads associated with the combustion temperature. (ii) A temperature distribution in the structure during operation that does not exceed material limits, subject to a reasonable pressure drop. (iii) A maximum temperature in the fuel (JP-7) low enough to prohibit coking. It is shown that all design requirements typical of Mach 5-7 hypersonic vehicles can be met by a small subset of material systems. Those made using C/SiC composites are the lightest. Others made using Nb alloys and (thermal barrier coated) superalloys are somewhat heavier, but might prevail in a design selection because of their structural robustness, facility of fabrication and cost-effectiveness.

Structurally optimized sandwich panels with prismatic cores

Journal paper
L. Valdevit, J. W. Hutchinson, A. G. Evans
International Journal of Solids and Structures, 41 (2004), 5105-5124
Publication year: 2004

Abstract

Multifunctional sandwich panels with corrugated and prismatic diamond cores have been analyzed and their behavior compared with panels designed using truss and honeycomb cores. Failure mechanism maps have been devised that account for interactions between core and face members during buckling. The optimal dimensions and the minimum weight have been evaluated. The load capacities predicted for near-optimal designs have been validated by conducting selected finite element calculations. Designs that use diamond prismatic cores (with corrugation order 4) are slightly more weight efficient than trusses, when optimized for a specific loading direction. Honeycomb cores, while somewhat more weight efficient, especially at lower load capacities, are not amenable to the fluid flows needed for cooling. We conclude that the diamond prismatic topology is the most weight efficient among designs amenable to simultaneous load bearing and active cooling.

Active cooling by metallic sandwich structures with periodic cores

Journal paper
T. J. Lu, L. Valdevit, A. G. Evans
Progress in Materials Science, 50 (2004), 789-815
Publication year: 2004

Abstract

We review the thermal characteristics of all-metallic sandwich structures with two dimensional prismatic and truss cores. Results are presented based on measurements in conjunction with analytical modeling and numerical simulation. The periodic nature of these core structures allows derivation of the macroscopic quantities of interest—namely, the overall Nusselt number and friction factor—by means of correlations derived at the unit cell level. A fin analogy model is used to bridge length scales. Various measurements and simulations are used to examine the robustness of this approach and the limitations discussed. Topological preferences are addressed in terms scaling relations obtained with three dimensionless parameters—friction factor, Nusselt number and Reynolds number—expressed both at the panel and the cell levels. Countervailing influences of topology on the Nusselt number and friction factor are found. Case studies are presented to illustrate that the topology preference is highly application dependent.