Desiderio Kovar

Negative stiffness honeycombs for recoverable shock isolation

Purpose – The purpose of this paper is to study the behavior of negative stiffness beams when arranged in a honeycomb configuration and to compare the energy absorption capacity of these negative stiffness honeycombs with regular honeycombs of equivalent relative densities. Design/methodology/approach – A negative stiffness honeycomb is fabricated in nylon 11 using selective laser sintering. Its force-displacement behavior is simulated with finite element analysis and experimentally evaluated under quasi-static displacement loading. Similarly, a hexagonal honeycomb of equivalent relative density is also fabricated and tested. The energy absorbed for both specimens is computed from the resulting force-displacement curves. The beam geometry of the negative stiffness honeycomb is optimized for maximum energy absorption per unit mass of material. Findings – Negative stiffness honeycombs exhibit relatively large positive stiffness, followed by a region of plateau stress as the cell walls buckle, similar to reg...

Crack stability and strength variability in alumina ceramics with rising toughness-curve behavior

Aluminas with four distinct microstructures have been fabricated to investigate the influence of grain size and grain morphology on strength variability. The four microstructures comprise two grain size scales and are characterized as either equiaxed with a narrow size distribution or elongate with a higher aspect ratio and a broader size distribution. Indentation-strength tests indicate that only the coarse-grain, elongate microstructure exhibits a strong rising toughness-curve (T-curve or R-curve). Furthermore, in situ measurements demonstrate that the coarse-grain, elongate microstructure is the only one that displays significant stable crack extension from annealed indentation flaws free of contact-induced residual stress. Strength tests on polished specimens indicate that the highest mean strength is achieved in the fine-grain, equiaxed material with little or no T-curve. The lowest strength variability, however, is exhibited by the coarse-grain, elongate alumina and is rationalized in terms of the strong rising T-curve and its associated influence on crack stability. The study suggests that maximum reliability is achieved when the T-curve is sufficiently strong to stabilize the propagation of natural flaws en route to failure.

Annealing of nanostructured silver films produced by supersonic deposition of nanoparticles

Silver nanostructured films were produced by supersonic jet deposition of nanoparticles generated by laser ablation of microparticle aerosols (LAMA). The nano- and microscale morphologies of films were investigated and the electrical conductivities of films were measured in the as-deposited state and after annealing at temperatures from 100 to 400 °C. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used to analyze the film morphologies and the grain sizes within the films. These analyses showed that the as-deposited grain size was 14–24 nm and the relative film densities ranged from 60% to 80%, depending on processing conditions. As a result of the extremely fine grain size and the lack of organics present on the surface of the grains within the films, the annealing temperatures required to achieve reasonable conductivities (20%−50% of bulk) were reduced dramatically compared to films produced from suspensions. For example, the grain size in the films began to increase at annealing tem...

Supersonic jet deposition of silver nanoparticle aerosols: Correlations of impact conditions and film morphologies

We describe experiments and modeling for the deposition of silver lines and films via the impaction of a silver nanoparticle aerosol delivered through a supersonic jet. The aerosol gas dynamics of the jet flow field, nanoparticle acceleration in the jet, and deposition by impaction onto the substrate were modeled for both a flat-plate nozzle and for a conical nozzle designed to obtain higher impaction velocities. We modeled nanoparticle dynamics for He, Ar, and N2 gasses, all initially at room temperature and 1 atm pressure, flowing through a 250 μm orifice into vacuum with a pressure ratio of ~5000. Experiments were conducted to deposit silver nanoparticle aerosols under the same conditions as were modeled. The silver nanoparticles were generated by laser ablation of a flowing microparticle aerosol entrained in either He or Ar that produced nanoparticles 5−10 and 15−20 nm in diameter, respectively. Deposition was made onto an unheated substrate in vacuum. The morphology of the deposited films was determi...

High-temperature deformation of Al2O3/Y-TZP particulate composites

Abstract Al 2 O 3 /Y-TZP particulate composites with compositions of between 20 and 80 vol.% Y-TZP were produced by tapecasting, lamination, and sintering. The processing methods employed resulted in fine grain sizes with only small variations between the composites produced. The resulting particulate composites were tested in compression at a temperature of 1350 °C over strain rates from 10 −5 to 3.16×10 −4 s −1 . Microstructural changes during testing were minor. Stress exponents were measured to the range from approximately two to three, which are consistent with published data on similar materials from tensile experiments. Models of composite creep behavior are compared to the experimental data over the full range of compositions studied. A constrained isostrain model is found to provide better predictive capabilities than either an unconstrained model, an isostress model, or a rheological model. Furthermore, the constrained isostrain model provides the most reasonable predictions for creep rates of 100% Al 2 O 3 and 100% Y-TZP materials.

Stabilization of silver nanoparticles in nonanoic acid: A temperature activated conformation reaction observed with surface enhanced Raman spectroscopy

Silver nanoparticles were synthesized by ultraviolet (λ=248 nm) laser ablation of an aerosol of micron-sized source particles entrained in nitrogen. As a result of thermionic electron emission and photoionization, nanoparticles produced in this manner were highly charged. The resulting aerosol was primarily composed of nanometer sized particles. The charged nanoparticles were deflected by an electric field that was perpendicular to the aerosol flow. Deflected nanoparticles were deposited directly into n-nonanoic acid flowing along the negative collection electrode. Suspensions of nanoparticles collected in this manner were dark gray in color and were found to be flocculated. When the suspensions were heated to temperatures above 75 °C, a color change from gray to clear was observed. Ultraviolet/visible extinction spectroscopy was performed on each suspension following annealing at different temperatures and times. By modeling the absorption decrease as a first order reaction, a good fit for the data was f...

Highly Conductive Nanoparticulate Films Achieved at Low Sintering Temperatures

Nanoparticulate Ag films have been produced by the laser ablation of microparticle aerosol (LAMA) deposition process. LAMA enables the production of thick, nanoparticulate films that are free of organics and offers the ability to control the degree of agglomeration and initial film density. The films were subsequently annealed at a range of temperatures from 100°C to 250°C to densify the films and increase conductivity. We show that, by reducing the degree of agglomeration in the films, sintering of LAMA-produced films occurs at low temperatures and results in near fully dense Ag films that exhibit an order of magnitude higher conductivity when compared to thick films produced by other techniques that are sintered at similar temperatures. Good agreement is observed between experiments and a sintering model that suggests that surface diffusion is dominant at temperatures below 150°C, and a combination of surface and grain boundary diffusion are responsible for sintering at slightly higher temperatures.

Laser ablation of nanoparticles and nanoparticulate, thick Fe1.92Tb0.3Dy0.7 films

Two laser processes, flat plate ablation (FPA) and laser ablation of microparticle aerosols (LAMA), capable of producing nanoparticles and nanoparticulate thick films of Terfenol-D (Fe 1.92 Tb 0.3 Dy 0.7 ) were investigated. The influence of processing parameters on the sizes, compositions, and morphologies of the nanoparticles produced using these processes were studied by transmission electron microscopy. The nanoparticles were used to deposit nanoparticulate films by supersonic impaction with thicknesses ranging from 4 to 50 μm, depending on processing conditions. The microstructures and properties of the films were studied using scanning electron microscopy and magnetometry. The LAMA process produced nanoparticles with a mean size and standard deviation (SD) of 8 to 10 nm ± 5 nm, depending on the type of gas used during synthesis. In contrast, nanoparticles produced using the FPA process exhibited a much broader size distribution varying from 5 to 150 nm and a much greater variation in compositions compared to the LAMA process. Films produced using LAMA also had lower levels of porosity compared to those produced using FPA as a result of the smaller, more uniform nanoparticles from which they were produced and the resulting higher impaction velocities. Compared to the FPA-produced films, the LAMA-produced films exhibited greater resistance to oxidation, higher magnetizations (13–15 emu/g versus 9–11 emu/g, depending on processing conditions) and lower coercivities (versus 41–59 Oe versus 80–110 Oe).

Pressure-Assisted Sintering of Nanocrystalline Silver Lines Produced by Laser Ablation of Microparticle Aerosols

The Laser Ablation of Microparticle Aerosol (LAMA) process provides a method for generating high yields of aerosolized nanoparticles (NPs) that are then accelerated through a nozzle and impacted onto substrates. The process allows nanostructured films and patterned lines to be directly written onto substrates at room temperature. Using the LAMA process, we have demonstrated the ability to produce thick patterned deposits with densities ranging from 60–70% in the as-deposited state. After deposition, the lines were densified using pressure-assisted sintering and the microstructures and strengths of the films were investigated. Pressure-enhanced sintering resulted in densification and strong bonding at processing temperatures of 100–175°C and pressures of 100–600 MPa. Mechanisms for sintering at low temperatures are discussed.Copyright

Nanoparticles of Er-doped glass produced by laser ablation of microparticles

We report the production of Er impurity-doped glass nanoparticles (NPs) by laser ablation of doped glass microparticles entrained in a flowing argon aerosol. The NP composition for this process is similar to the starting feedstock material, so the manufacture of impurity-doped NP requires only an impurity-doped feedstock. In experiments, NPs with a relatively large mean size of 20 nm were produced to purposely not confine the fluorescence of the impurity; however, other valuable properties of the NPs such as low-temperature sintering were retained. We measured the resulting changes in stoichiometry using energy dispersive spectroscopy in both scanning electron microscopy and transmission electron microscopy. We measured the spectra from both sparsely deposited regions consisting of individual NPs and clusters of NPs, and densely deposited regions where the deposits formed nanostructured films. The spectra measured from sparse and dense deposits were similar; for samples stored for six months in atmospheric conditions, the fluorescence measured from isolated NPs was quenched, but not the fluorescence measured from the densely deposited nanostructured films. In samples in which the fluorescence was measured within weeks of deposition, the fluorescence lifetimes were found to be only 0.5 ms shorter than those of the starting microparticles, indicating that the nanostructure did not significantly influence the defect or impurity quenching of the Er ions.