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To explore the fundamental flash sintering mechanisms in yttria-stabilized zirconia (3YSZ), a detailed microstructural characterization based on transmission electron microscopy (TEM) has been conducted on 3YSZ flash sintered in air and Ar atmospheres. The study combines the surface morphology characterization via scanning electron microscopy (SEM) and the detailed grain structure analysis based on TEM. The results show that large clusters are comprised of very fine nanocrystalline subgrains, possibly caused by global grain boundary sliding and grain rotation which occur during the flash sintering process. Dislocation arrays were found near grain boundary triple junctions, suggesting the high mass transport rate during the rapid densification process in flash sintering. Yttrium-segregation was observed at grain boundaries near triple junctions as the result of the electrochemical forces.

This work investigates the feasibility of a binderless, extrusion-based additive manufacturing approach to fabricate alumina ( A l 2 O 3 ) parts from nanopowder. Traditional manufacture of ceramics with subtractive methods is limited due to their inherent hardness and brittleness, inevitably leading to ceramic parts with less-than-optimal geometries for the specific application. With an additive manufacturing approach, ceramic parts with complex 3D geometries, including overhangs or hollow enclosures, become possible. These complex ceramic parts are highly valuable in heat exchanger, condenser, biomedical implant, chemical reactant vessel, and electrical isolation applications. This research employed direct coagulation of alumina nanopowder slurries with the polyvalent salt tri-ammonium citrate providing the solidification mechanism in an extrusion-based printing process. The viscosity of the slurries was adjusted from ∼35 Pa-s to ∼1000 Pa-s by adjusting pH from ∼9 to ∼4, resulting in a paste that is suitable for extrusion, which retains near-net geometry. It was shown that the direct coagulation approach can be used to create a suspension with tuneable flow characteristics and coagulation rate, and a mechanism describing the process was proposed. The direct coagulation printing (DCP) method is described in detail, including how slurry is extruded, solidified, and printed in complex geometries, and sintered to full density. Parts were printed with a sintered resolution of 450 μm and green densities as high as 65%. After sintering at 1550 °C for up to 2 h, parts were shown to be fully dense (>97%) with an average grain size of ∼2 μm. Mechanical properties were characterized with a comparison to different materials and methods from literature, showing hardness and flexural modulus up to ∼1800 HV and 400 GPa, respectively.

A functionally graded material (FGM) in terms of grain size gradation is fabricated using zinc oxide (ZnO) with spark plasma sintering and an additive manufacturing technique by diffusion bonding layers of material sintered at different temperatures to achieve a thermoelectric generator (TEG) material that can dissipate heat well and retain high energy conversion efficiency for longer-lasting and comparably efficient TEGs. This FGM is compared to a previously made FGM with continuous grain size gradation. Uniform and graded grain size conditions are modeled for thermoelectric output by using thermoelectric properties of the uniform grain size as well as the varying properties seen in the FGMs. The actual thermoelectric output of the samples is measured and compared to the simulations. The grain size has a large effect on the efficiency and efficiency range. The samples are thermally cycled with a fast heating rate to test the thermal stress robustness and degradation, and the resistance at the highest temperature is measured to indicate degradation from thermal stress. The measured efficiency after cycling shows that the FGMs survive longer lifetime than that with uniform small grains.

Titanium alloys are widely used for dental, medical, automotive, aeronautic and aerospace applications due to their chemical, physical, biological and mechanical properties. Orthopedic implants of the as-cast and heat-treated Ti–10Si–5B alloy present good bone integration capacity as well as Tiss+Ti6Si2B alloys exhibit higher oxidation resistance than Tiss+Ti5Si3 alloys. In this sense, Ti-8Si-4B, Ti-10Si-5B, Ti-14Si-7B, Ti-20Si-10B, and Ti-22Si-11B alloys (atomic percentage - at.-%) with Ti6Si2B-based microstructures were produced by high-energy ball milling and further spark plasma sintering (SPS) at 1100 °C for 6 min under an uniaxial pressure of 191 MPa. Thus, the corrosion resistance of these SPSed Ti-Si-B alloys was evaluated using potentiodynamic polarization method in different media: artificial saliva (AFNOR NF S90-701) and simulated body fluid (SBF). The SPSed Ti-Si-B alloys were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). Porosity tests based on Arquimedes’ principle were also performed to evaluate their porosity and density. Despite the short sintering time, our results have confirmed that fine and dense SPSed Ti-Si-B samples with microstructures based on solid solution Ti (Tiss) and Ti6Si2B phases were produced from mechanically alloyed Ti-Si-B powders. The Ti-22Si-11B alloy containing higher amounts of Ti6Si2B presented lower values of passivation current density (2.69 and 0.08 μA.cm-2) and higher corrosion potential (−0.232 and −0.258 V) when compared to Ti-8Si-4B alloy (2.58 and 0.25 μA.cm-2; −0.396 and −0.339 V) in artificial saliva and SBF media, respectively. Moreover, it demonstrates superior corrosion resistant than Ti-7Zr-20Si-10B, Ti-13Nb-13Zr, Ti-6Al-4V and pure Ti.

Flash sintering has attracted significant attention as its remarkably rapid densification process at low sintering furnace temperature leads to the retention of fine grains and enhanced dielectric properties. However, high-temperature mechanical behaviors of flash-sintered ceramics remain poorly understood. Here, we present high-temperature (up to 600 °C) in situ compression studies on flash-sintered yttria-stabilized zirconia (YSZ). Below 400 °C, the YSZ exhibits high ultimate compressive strength exceeding 3.5 GPa and high inelastic strain (~8%) due primarily to phase transformation toughening. At higher temperatures, crack nucleation and propagation are significantly retarded, and prominent plasticity arises mainly from dislocation activity. The high dislocation density induced in flash-sintered ceramics may have general implications for improving the plasticity of sintered ceramic materials., Flash sintering allows for rapid ceramic processing, but the mechanical behavior of such ceramics remains poorly understood. Here, the authors compress micropillars of yttria stabilized zirconia to show flash sintering promotes outstanding plasticity.

To gain fundamental insight into the relationship between length scales and mechanical behaviour, Ni-Fe multilayered materials with a 5-μm-layer thickness and a modulated grain size distribution have been synthesized by pulsed electrodeposition. Microstructural studies by SEM and TEM reveal the alternating growth of well-defined layers with either nano (d = 16 nm) or coarse grains (d ≥ 500 nm). Room temperature tensile tests have been performed to investigate the mechanical response and understand the underlying deformation mechanisms. Tensile test results and fractographic studies demonstrate that the overall room temperature mechanical behaviour of the multilayered material, i.e. strength and ductility, is governed primarily by the layers containing nanocrystalline grains. The measured properties have been discussed in the context of modulated grain structure of the multilayered sample and contribution of each grain size regime to the overall strength and ductility.

The presence of agglomerates during nanopowder sintering can be problematic and can limit achievable final densities. Typically, the practical solution is to use high pressures to overcome agglomerate breakdown strengths to reach higher packing fractions. The strength of agglomerates is often difficult to determine and makes processing parameters challenging to optimize. In this work, we used in situ transmission electron microscopy nanoindentation experiments to assess the mechanical properties of individual MgAl 2 O 4 agglomerates under constant indenter head displacement rates. Electron microscopy revealed highly porous agglomerates with pores on both the micron and nanometric length scales. Individual agglomerate strength, at fracture, was calculated from compression tests with deformation behavior correlating well with previously reported modeling results. Macroscopic powder properties were also investigated using green-pressed pellets consolidated at pressures up to 910 MPa. The unexpectedly high strength is indicative of the role agglomerates play in MgAl 2 O 4 nanopowder densification.

In situ transmission electron microscopy (TEM) was used to investigate mechanisms of dielectric breakdown during the electric-field-assisted consolidation of nickel nanoparticles which possess an ultrathin surface oxide layer. A constant positive bias was applied directly to isolated agglomerates of nickel nanoparticles during TEM observation. The evolution of the resulting leakage currents across the dielectric oxide films separating adjacent particles was analyzed. The formation of local conductive pathways between individual particles was observed with an electrical signature similar to that of time-dependent dielectric breakdown. Individual current increments with time occurred as a result of inter-particle neck formation initiated by dielectric breakdown and subsequent oxygen migration due to Joule heating. Neck formation and growth as well as particle rearrangements led to accelerated current increase through the particle agglomerate that gave rise to rapid densification. A quantitative analysis of the injected charges into the particle agglomerate suggests that neck formation has occurred once a critical charge density was obtained on the particle surfaces, i.e. commensurate with an increase in the local electrical field strength across individual dielectric oxide layers that separated the metallic particles.

Ultrafine-grained (UFG) Ni and multi-walled carbon nanotube (CNT) composite powders were prepared using non-covalent functionalization of CNTs to promote cohesion between the metal powders and CNTs. Following consolidation using spark plasma sintering, the resultant Ni–CNT composites had densities >97 % with well-dispersed CNT reinforcements. Tensile testing revealed comparable fracture strengths between the reinforced and unreinforced UFG Ni, whereas the former exhibited smaller fracture strain than the latter. In addition, the former had higher microhardness than the latter. The results are rationalized using a shear-lag model, and it is suggested that grain edge lengths should be used as the reinforcement lengths when applying the model to tensile stress response descriptions.

The cleaning of nanoparticles from surface oxides or contaminants is critical during the initial stages of spark plasma sintering (SPS). However, so far only indirect evidence has suggested the occurrence of surface cleaning. In situ transmission electron microscopy was used to replicate the processing conditions during SPS to directly observe surface cleaning. Dielectric breakdown was discovered to lead to surface cleaning. Electrothermal depletion of surface oxides is obtained through defect formation, electron trapping and local field enhancement.

The development of vertical nanoelectronics is hindered by limited control over the growth of nanostructures in specific directions. A scanning tunneling microscopy tip was used as a guide to locally consolidate individual nanoparticles and grow metallic nanopillars with high aspect ratios. Consolidation from random agglomerates occurs through electromigration and diffusion in a temperature gradient. The results of this study provide a new avenue for the controlled growth of complex metallic nanostructures for future three-dimensional architectures.

To date, spark plasma sintering (SPS) has enjoyed much success in materials research and lesser, but growing, success in industrial applications. To some degree, the limitations in usage can be associated with the specific knowledge of the nature of the contributions and benefits that SPS provides over traditional sintering apparati. As sample dimension requirements are increased, the energy requirements of production become problematic and the scalability of the technique is an open question.

Functionally graded material (FGM) in terms of grain size gradation is fabricated from ZnO with a combination of modified Spark Plasma Sintering (SPS) graphite tooling, water sintering enhancements through transient liquid phase surface transport, and strategic SPS mechanical loading. The grain size gradation of the ZnO FGM spans from 180 nm grains to 1.2 micrometers in a fully dense material. This is the first semiconductor or ceramic to be graded microstructurally to this extent. Predictions of the microstructure with a Master Sintering Curve (MSC) approach were done with a series of isothermal experiments on two different FGM conditions revealing a slight offset due to a constrained mechanism. The mechanical properties were tested with Vickers micro hardness across the sample, showing a gradient in hardness from 2.6 GPa to 4.2 GPa. In addition, the thermoelectric properties of the FGM were measured and show a zT of 2 × 10−5 at 100 °C compared to uniform small- and large-grained samples of 1 × 10−6. This is an order of magnitude difference making a new path for improvements of bulk thermoelectric material.

The role of an applied electric field during the densification of ionic ceramics is still being debated. Here we describe how the polarization of a dielectric material contributes to the field strengths at particle surfaces and interfaces during the initial stage of sintering. Using numerical models, it is shown that significant increases in local field strengths can be expected during initial neck formation and continue, with decreasing contribution, through approximately half of the first stage of sintering. The field strengths achievable in common commercial and custom lab-scale electric field assisted sintering systems are found to be comparable to those at which electric fields have been shown to enhance the densification behavior of ionic ceramics.

Recent investigations regarding the role of applied fields on the grain growth and densification behavior of ionic ceramics are providing strong insights into the efficacy of Field Assisted Sintering Technique (FAST), aka Spark Plasma Sintering (SPS). Explanations of the observed behaviors, such as grain growth suppression and densification enhancement, are based upon the conjectured presence of a Joule heating driven temperature differential between grain interfaces and grain cores. These differentials were thought to be responsible for providing increased densification rates and lower densification temperatures through grain growth suppression and/or increased local kinetics at the forming necks. In this paper, we analyze the energetic, thermal, and practical details of this process in the context of the commonly accepted stages of sintering.

In order to establish the relative contributions of thermal and athermal mechanisms to densification in the absence of an extrinsic sintering pressure, nanometric powder compacts were sintered with and without applied fields using varied heating rates from 50 °C/min up to 800 °C/min. The relative contribution of the thermal and athermal mechanistic contributions to the densification behavior of two model dielectric ceramics, hydroxyapatite and zinc oxide, is evaluated in the context of the current leading theories of field-assisted sintering mechanisms. The effects of elevated heating rates in nanometric, dielectric ceramics are found to be minimal in the absence of a field. However, in the presence of an applied field there appears to be a synergistic effect with heating rate.

The consolidation of crystalline powders to obtain dense microstructures is typically achieved through a combination of volume and grain boundary diffusion. In situ transmission electron microscopy was utilized to study neck formation between adjacent nickel particles during the early stages of sintering. It was found that the presence of carbon during consolidation of Ni lowers the reduction temperature of nickel oxides on the particle surface and therefore has the potential to accelerate consolidation. In the absence of carbon, the surface oxides remain present during the early stage of sintering and neck formation between particles is limited by self-diffusion of nickel through the oxide layer. This study provides direct experimental evidence that corroborates related earlier hypotheses of self-cleaning on the surface of the nanoparticles that precedes neck formation and growth.

Ceramic nanocomposites of alumina and carbon nanotubes (CNTs) are experimentally studied for use as microwave absorbers in particle accelerators. The weight percentage of multi-walled CNTs in SPS sintered nanocomposite samples is varied from 0.5 to 10% and the complex permittivity is measured. The RF absorption is strong and relatively flat in the frequency band 1–40 GHz for a CNT weight percentage in the range 1–2.5%, which is just above the percolation threshold. The permittivity is observed to increase dramatically with increasing CNT weight percentage above the percolation threshold as observed elsewhere, and in accordance with theoretical treatments. The electromagnetic properties of the nanocomposites are little changed in going from 294 K to 77 K. The DC conductivity of the alumina-CNT nanocomposite is also sufficient to drain static charge in particle accelerator beamline environments, even at cryogenic temperatures. Fabrication of the nanocomposites using an industrial RF sintering process compatible with large sizes shows that the microwave absorption properties of small samples are similar to those of the SPS sintered samples.

The fracture toughness of a series of fully dense polycrystalline alumina samples prepared by electrical field assisted sintering with grain sizes varying from ∼290 nm to ∼3.3 μm was measured using the surface crack in flexure method. Fracture toughness values for different grain sizes were statistically analyzed based on the Weibull distribution. It was found that the fracture toughness in fine grained alumina is independent of grain size. The characteristic fracture toughness calculated from the Weibull distribution is 3.37 MPa m1/2.

The deformation behavior of spark plasma sintered Ni powders with unimodal and bimodal grain size distributions, made up of nanocrystalline (NC) and ultra-fine grained (UFG) was studied at homologous temperatures from 0.36 to 0.40. TEM and SEM analysis have determined that minimal grain growth occurs during testing, while the bimodal specimens show a propensity for twin formation and growth. Purely nanocrystalline and microcrystalline sample activation volumes and strain-rate sensitivities were found to be consistent with previous work on electrodeposited Ni of similar grain sizes. The microstructures, activation volumes/energies, and strain rate sensitivities are found to be discontinuously changing as a function of increased volume fractions of UFG grains. Current models of nc deformation do not fully explain the observed behavior. A new rate controlling process is suggested in which both competitive and cooperative mechanisms function to produce the observed, novel behavior.

The consolidation of nanometric powders of WC by the spark plasma sintering (SPS) method was investigated over the temperature range 1425–1800 °C under a uniaxial pressure of 126 MPa. Nominally stoichiometric WC powders with a grain size in the range 40–70 nm could be consolidated to near theoretical densities (99.1%) with a grain size of 305 nm when heated at a high rate to 1750 °C with no hold time. The sintered material, however, contained W2C and the effect of addition of carbon on the presence of this phase was investigated with these powders and with powders having a grain size of 12 nm. The effect of carbon on abnormal grain growth (AGG) was investigated as a function of temperature and carbon addition. The effect of heating rate up to the sintering temperature and the hold time at temperature was also investigated.

Poly(vinyl alcohol) was modified with pendant acrylate groups to create a multifunctional macromer that is crosslinkable via photopolymerization and degradable through hydrolytically cleavable ester groups within the crosslinks. The chemistry and functionality of the macromer, as well as the polymerization conditions, were varied to produce hydrogel networks with a wide range of physical properties and degradation behavior. The resulting networks were characterized by measuring the volumetric swelling ratio and mass loss profiles with degradation time. Experimental results show that the volumetric swelling ratio increases exponentially during degradation of these gels, and this increase can be predicted through a pseudo-first order reaction depending only on the kinetic constant for hydrolysis and the number of degradable links in the crosslinker. Coupled to the hydrolytic degradation, the network structure also influences the mass loss or erosion profile and, ultimately, the overall time for degradation.

Superhydrophobic surfaces (i.e., surfaces extremely repellent to water) allow water droplets to bead up and easily roll off from the surface. While a few methods have been developed to fabricate metallic superhydrophobic surfaces, these methods typically involve expensive equipment, environmental hazards, or multi-step processes. In this work, we developed a universal, scalable, solvent-free, one-step methodology based on thermal sensitization to create appropriate surface texture and fabricate metallic superhydrophobic surfaces. To demonstrate the feasibility of our methodology and elucidate the underlying mechanism, we fabricated superhydrophobic surfaces using ferritic (430) and austenitic (316) stainless steels (representative alloys) with roll off angles as low as 4° and 7°, respectively. We envision that our approach will enable the fabrication of superhydrophobic metal alloys for a wide range of civilian and military applications.

Studying particle-agglomerate systems compared to two-particle systems elucidates different stages of sintering by monitoring both pores and particles. We report on in situ sintering of 3% yttria-stablized zirconia particle agglomerates in the transmission electron microscope (TEM). Real-time TEM observations indicate neck formation and growth, particle coalescence and pore closure. A MATLAB-based image processing tool was developed to calculate the projected area of the agglomerate with and without internal pores during in situ sintering. We demonstrate the first densification curves generated from sequentially acquired TEM images. The in situ sintering onset temperature was then determined to be at 960 °C. Densification curves illustrated that the agglomerate projected area which excludes the internal observed pores also shrinks during in situ sintering. To overcome the common projection problem for TEM analyses, agglomerate mass-thickness maps were obtained from low energy-loss analysis combined with STEM imaging. The decrease in the projected area was directly related to the increase in mass-thickness of the agglomerate, likely caused by hidden pores existing in the direction of the beam. Access to shrinkage curves through in situ TEM analysis provides a new avenue to investigate fundamental mechanisms of sintering through directly correlating microstructural changes during consolidation with mesoscale densification behavior.

A functionally graded boron carbide (B 4 C) with precipitous property and microstructural gradients has been synthesized using pulsed electric current sintering (PECS) in tubular shape forms. During PECS the amorphous powders react and partially consolidate forming a density gradient radially across the tube walls. Modeling results support the presence of a large radial temperature gradient sufficient to produce B 4 C in smoothly varying densities from inside to outside diameters. This material and geometry is both novel and of particular use in applications requiring a cylinder of differing surface structures that transition smoothly into a metallic substructure or assembly.

Strain hardening capability is critical for metallic materials to achieve high ductility during plastic deformation. A majority of nanocrystalline metals, however, have inherently low work hardening capability with few exceptions. Interpretations on work hardening mechanisms in nanocrystalline metals are still controversial due to the lack of in situ experimental evidence. Here we report, by using an in situ transmission electron microscope nanoindentation tool, the direct observation of dynamic work hardening event in nanocrystalline nickel. During strain hardening stage, abundant Lomer-Cottrell (L-C) locks formed both within nanograins and against twin boundaries. Two major mechanisms were identified during interactions between L-C locks and twin boundaries. Quantitative nanoindentation experiments recorded show an increase of yield strength from 1.64 to 2.29 GPa during multiple loading-unloading cycles. This study provides both the evidence to explain the roots of work hardening at small length scales and the insight for future design of ductile nanocrystalline metals.

Field assisted sintering studies have produced a wealth of data about the densification behaviors of many powder systems. However, the sheer volume of work has not met with sufficient mechanistic descriptions of the processes in metal or ceramic systems. This fact has, and is, limiting the acceptance and widespread use of this promising technique in larger than laboratory scale manufacturing. We describe here the nature of the influences of electric fields and/or currents, changes in heating rate, and the effects of applied pressures upon ceramic and metal systems in context of the commonly accepted stages of sintering. As many of the specific mechanisms discussed have not been directly characterized within field assisted sintering studies we focus on the established theoretical underpinnings to better understand their influence and to enable definitive future experimentation on this area of research.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

This study reports the in situ transmission electron microscopy (TEM) observation of pressure-less field-assisted sintering of agglomerated nanometric nickel particles. Scanning tunneling microscopy inside the TEM was used to apply an electrical current directly to the powder particles. Electrical testing during the experiment reveals that consolidation occurs in the absence of an external heat source. Neck formation between adjacent particles and attendant increase in local Joule heating causes rapid densification. The results represent a first stepping stone towards achieving a fundamental mechanistic understanding of the atomic-scale processes that enable field-enhanced sintering of conductive nanogranular materials.