Pressure-assisted Densification Research Articles

Acoustically transparent alumina-based cranial implants enhance ultrasound transmission through a combined mechano-acoustic resonant effect

Therapeutic ultrasound for brain stimulation has increased in the last years. This energy has shown promising results for treating Alzheimer’s disease, Parkinson’s disease and traumatic brain injury, among other conditions. However, the application of ultrasound in the brain should trespass a natural but highly attenuating and distorting barrier, the cranium. Implantable ceramic materials can be used to replace part of the cranium as an alternate method to enhance ultrasound transmission. In this work, it is presented the acoustic characterization of alumina ceramic disks that can be employed as cranial implants for acoustic windows-to-the-brain. Alumina samples were prepared using current-activated pressure-assisted densification and were acoustically characterized. Acoustic impedance and attenuation of the samples were determined for different porosities. Additionally, measured and modeled acoustic fields are presented and analyzed in terms of the total ultrasound transmitted through the ceramics. Results indicate a resonant behavior in the alumina disks when the thickness corresponds to a half-wavelength of ultrasound; this resonance permits a total of 95.4% of ultrasound transmission; for thicknesses out of the resonant zone, transmission is 53.0%. Alumina proves to be an excellent medium for ultrasound transmission that, in conjunction with its mechanical and optical properties, can be useful for cranium replacement in mixed opto-acoustic applications.

Material extrusion of high-density SiCp/7075Al composite via a high nitrogen-flowing assisted sintering

Material extrusion additive manufacturing of particle-reinforced metal matrix composite (MMC) attracts attention due to the features of uniformly sintered microstructure, easy-to-handle and low cost. However, 3D Al-MMC suffers from poor sinterability due to the stable alumina layer on alloy powder, and difficulty in removing of binders at low temperature. Here, we propose a high nitrogen-flow sintering approach for the thermal debinding and sintering of as-printed SiC particle-reinforced 7075Al (SiCp/7075Al) MMC, without using cumbersome high-vacuum system or pressure-assisted densification. Microcrystalline wax-based granular feedstock was developed based on contact angle testing experiment, which shows typical shear-thinning behavior and satisfactory printing performance. Roles of printing parameters and nitrogen flow rate in the microstructure, phases and mechanical properties of SiCp/7075Al MMC are investigated. Nitrogen flow rate of 2–2.5 L/min accelerates the breaking of oxide film, with the precipitation of intergranular MgAl2O4, Mg2Si phases, and intragranular Al2Cu phases. 3D SiCp/7075Al MMC with a high relative density of 97% and tensile strength of 308 MPa after heat treatment (HT) was achieved based on optimized printing parameters. The availability of high dense structure, combined with controlled microstructure, low-energy demand and low cost, provides a facile route for the application of 3D Al-MMC by material extrusion.

High-Throughput Multi-Principal Element Alloy Exploration Using a Novel Composition Gradient Sintering Technique

This work demonstrates the capabilities and advantages of a novel sintering technique to fabricate bulk composition gradient materials. Pressure distribution calculations were used to compare several tooling geometries for use with current-activated, pressure-assisted densification or spark plasma sintering to densify a gradient along the long dimension of the specimen. The selected rectangular tooling design retains a low aspect ratio to ensure a uniform pressure distribution during consolidation by using a side loading configuration to form the gradient along the longest dimension. Composition gradients of NixCu1−x, MoxNb1−x, and MoNbTaWHfx (x from 0 to 1) were fabricated with the tooling. The microstructure, composition, and crystal structure were characterized along the gradient in the as-sintered condition and after annealing to partially homogenize the layers. The successful fabrication of a composition gradient in a difficult-to-process material like the refractory multi-principal element alloy system MoNbTaWHfx shows the utility of this approach for high-throughput screening of large material composition spaces.

Nanocrystalline Yttria-Stabilized Zirconia Ceramics for Cranial Window Applications.

Transparent yttria-stabilized zirconia (YSZ) ceramics are promising for cranial window applications because of their good mechanical and optical properties as well as biocompatibility. YSZ discs with different yttria concentrations were either processed via current-activated pressure-assisted densification (CAPAD) using commercial nanoparticles or densified via spark plasma sintering (SPS) using pyrolysis-synthesized nanoparticles in-house. This study provided critical results to screen composition, processing, microstructure, and cytocompatibility of transparent YSZ discs for cranial window applications. CAPAD-processed YSZ discs with 6 or 8 mol % yttria (6YSZ and 8YSZ) and SPS-densified YSZ discs with 4 mol % yttria (4YSZ_P) showed 200-350 nm polycrystalline grains containing 20-30 nm crystallite domains. SPS-densified YSZ discs with 8 mol % yttria (8YSZ_P) showed larger polycrystalline grains of 819 ± 155 nm with 29 ± 5 nm crystallite domains. CAPAD-processed YSZ discs with 3 mol % yttria (3YSZ) showed 39 ± 9 nm grains. Bone-marrow-derived stem cells (BMSCs) on the polished YSZ discs showed statistically higher spreading areas than those on the unpolished YSZ discs of the same compositions. Generally, polished 8YSZ, 4YSZ_P, and 8YSZ_P discs and unpolished 8YSZ_R, 4YSZ_PR, and 8YSZ_PR discs had lower average cell adhesion densities than other YSZ discs under direct contact conditions. Under indirect contact conditions, all the YSZ disc groups showed similar average cell adhesion densities to the Cell-only control. The groups of polished 4YSZ_P and 8YSZ_P discs, unpolished 4YSZ_PR and 8YSZ_PR discs, and particle control of 8YSZ_Pnp showed higher Y3+ ion concentrations than other groups. No mineral deposition was detected on the polished YSZ discs after cell culture. Considering multiple factors such as cytocompatibility, cell adhesion density, Y3+ ion release, mineral deposition, and optical transparency collectively, 8YSZ may be the best candidate for the cranial window applications. Further studies are needed to evaluate the long-term transparency and biocompatibility of YSZ discs.

Tough metal-ceramic composites with multifunctional nacre-like architecture

The brick-and-mortar architecture of biological nacre has inspired the development of synthetic composites with enhanced fracture toughness and multiple functionalities. While the use of metals as the “mortar” phase is an attractive option to maximize fracture toughness of bulk composites, non-mechanical functionalities potentially enabled by the presence of a metal in the structure remain relatively limited and unexplored. Using iron as the mortar phase, we develop and investigate nacre-like composites with high fracture toughness and stiffness combined with unique magnetic, electrical and thermal functionalities. Such metal-ceramic composites are prepared through the sol–gel deposition of iron-based coatings on alumina platelets and the magnetically-driven assembly of the pre-coated platelets into nacre-like architectures, followed by pressure-assisted densification at 1450 °C. With the help of state-of-the-art characterization techniques, we show that this processing route leads to lightweight inorganic structures that display outstanding fracture resistance, show noticeable magnetization and are amenable to fast induction heating. Materials with this set of properties might find use in transport, aerospace and robotic applications that require weight minimization combined with magnetic, electrical or thermal functionalities.

Effect of Thermomechanical Treatment Conditions on the Structure and Properties of MgB2

The effect of pressure-assisted sintering and preliminary densification by hydrostatic pressure of commercial powder on the microstructure and superconducting properties of MgB2 ceramics is studied in this work. Various thermomechanical actions are shown to result in the formation of different structural defects, which can be considered probable centers of magnetic flux pinning.

A theoretical analysis of cold sintering

ABSTRACTCold sintering is an innovative low-temperature processing technique which allows consolidation of several ceramics. Despite recent research activities on the cold sintering of functional and structural ceramics, an analytical study accounting for consolidation and grain growth phenomena is still missing in the literature. In this work, we provide a theoretical analysis of the mechanisms active during cold sintering. The analysis considers two cold sintering approaches, characterised by the application either of isostatic or uniaxial pressure. Physical phenomena and microstructural features are discussed in view of the applied cold sintering approach. The developed pressure-assisted densification models indicate that the processes governing densification during uniaxial cold sintering are more complex than those of conventional liquid phase sintering. A key role is played by the water/material interaction which promotes several effects such as formation of surface defects and secondary phases, dynamic recrystallization and other phenomena still partially unknown.

Synthesis, properties and thermal decomposition of the Ta4AlC3 MAX phase

The present work describes a synthesis route for bulk Ta4AlC3 MAX phase ceramics with high phase purity. Pressure-assisted densification was achieved by both hot pressing and spark plasma sintering of Ta2H, Al and C powder mixtures in the 1200–1650 °C range. The phases present and microstructures were characterized as a function of the sintering temperature by X-ray diffraction and scanning electron microscopy. High-purity α-Ta4AlC3 was obtained by hot pressing at 1500 °C for 30 min at 30 MPa. The β-Ta4AlC3 allotrope was observed in the samples produced by SPS. The Young’s modulus, Vickers hardness, flexural strength and single-edge V-notch beam fracture toughness of the high-purity bulk sample were determined. The thermal decomposition of Ta4AlC3 into TaCx and Al vapour in high (˜10−5 mbar) vacuum at 1200 °C and 1250 °C was also investigated, as a possible processing route to produce porous TaCx components.

Nanoscale integration of oxides and metals in bulk 3D composites: leveraging SrFe12O19/Co interfaces for magnetic exchange coupling

The integration of different material classes (e.g, oxides and metals) with nanoscale dimensions in large 3D materials remains a fundamental challenge in nanocomposite fabrication. The incentive is that some of the most interesting properties occur at nanoscale interfaces, while the challenge arises from the difficulty in densifying the materials without deleterious reaction at the interface. Here, we introduce a method based on the synthesis of core–shell powders followed by efficient, relatively low-temperature densification with current-activated pressure-assisted densification. The composition of the bulk nanocomposites can be controlled by varying the core–shell weight ratio, leading to controllable thicknesses of the hard/soft magnetic phases. We demonstrate intimate mixtures of nanoscale strontium ferrite (hard magnetic phase) and Co–Fe (soft magnetic phase) with minimal reaction. The high volume content of high-quality oxide/metal interfaces leads to magnetic exchange coupling in the composites.

The grain growth in alumina compacts sintered under high pressure

Abstract In this study, alumina ceramics were produced with high density and nanograin size without additives from nano-size γ-Al2O3. The effects of sintering pressure (3 GPa and 7 GPa) and temperature (600 °C and 1 200 °C) on the grain size of sintered samples were studied. It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase. Pressure-assisted densification decreases grain size, so that nano-scale grains can be achieved in the sintered structure. The effect of various sintering conditions on grain growth in alumina was investigated during high-pressure sintering for better understanding of pressure and temperature effects on microstructural properties.

On the Stability of c-BN-Reinforcing Particles in Ceramic Matrix Materials.

Cubic boron nitride (c-BN) composites produced at high pressures and temperatures are widely used as cutting tool materials. The advent of new, effective pressure-assisted densification methods, such as spark plasma sintering (SPS), has stimulated attempts to produce these composites at low pressures. Under low-pressure conditions, however, transformation of c-BN to the soft hexagonal BN (h-BN) phase can occur, with a strong deterioration in hardness and wear. In the present work, the influence of secondary phases (B2O3, Si3N4, and oxide glasses) on the transformation of c-BN was studied in the temperature range between 1100 °C and 1575 °C. The different heat treated c-BN particles and c-BN composites were analyzed by SEM, X-ray diffraction, and Raman spectroscopy. The transformation mechanism was found to be kinetically controlled solution–diffusion–precipitation. Given a sufficiently low liquid phase viscosity, the transformation could be observed at temperatures as low as 1200 °C for the c-BN–glass composites. In contrast, no transformation was found at temperatures up to 1575 °C when no liquid oxide phase is present in the composite. The results were compared with previous studies concerning the c-BN stability and the c-BN phase diagram.

Fabrication of Miniature Components from ZrO2 Powder by Combining Electrical-field Activated Sintering Technique and Micro-forming

There is an increased demand for miniature/micro products (such as MEMS) and nanotechnology-based products (such as nano-materials). Micro-manufacturing is a link between Macro-and Nano Manufacturing and an effective means for transferring nanotechnology-product designs into volume production. The micro forming has the potential for low-cost, high volume manufacturing applications. In order to meet the high demands on miniaturised products, a rapid production technique and the system, high flexibility, cost-effectiveness and processing a wide range of materials are needed. Recently, a series of studies have been undertaken to investigate forming miniature/micro-components by using a combination of micro-forming and Electrical-field activated sintering (Micro-FAST). The process uses low voltage and high current density, pressure-assisted densification and synthesis technique, which renders several significant merits. The work to be reported in this paper will be focused on the forming of miniature components from Zirconia (ZrO2) powder, without using binders. Several processing parameters have been investigated, such as pressure, heating rate, heating temperature and holding time, which helped to obtain high-quality parts. Using graphite dies and punches, sample parts (solid cylinders of Ø4.00mm × 4.00 mm) were formed. These were subjected to detailed examinations and analysis, such as analysis of the relative density, hardness at the necks formed among the particles and in the particle bodies, as well as the microstructures. The results showed that directly forming the parts from loose powder is feasible, and by properly designing and control the processing parameters, high-quality parts could be achieved, among which heating temperature and holding time are extremely important. At the same time, due to low conductivity of the powder material, carefully designing the tooling is essentials for ensuring properly heating, pressurisation and cooling.

Effects of applied pressure on the densification rates in current-activated pressure-assisted densification (CAPAD) of nanocrystalline materials

We discuss the role of applied pressure on the densification rates of nanocrystalline materials (aluminum, silicon and zirconia) using current-activated pressure-assisted densification. Varying pressure in the 35–106 MPa range does not significantly affect the temperature at which the maximum densification rate (ρ̇Max) occurs, suggesting that the densification mechanism(s) do not change. However, the pressure dependence of ρ̇Max is remarkable for all three materials, and can be as high as 120% in the case of Si when increasing the pressure by 30%.

Ultra-high heating rate densification of nanocrystalline magnesia at high pressure and investigation on densification mechanisms

Abstract The pressure-assisted densification method based on combustion reaction heating was applied to prepare dense nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 50 nm was investigated, under the pressure range of 0–170 MPa, and the temperature range of 1620–1880 K with ultra-high heating rate (above 1600 K/min). The pressure was found to have an effect on enhancing densification while suppressing grain growth, and the higher sintering temperature lead to the larger grain size and lower density of the compact. Pure magnesia nanocrystalline ceramics with a relative density of 99.1% was obtained at 1620 K and 170 MPa, and the concurrent grain growth was almost completely restrained. Furthermore, the investigation on the pressure-dependent densification mechanisms including plastic flow, diffusion and power-law creep was also carried out. The result indicated the rate-controlling mechanism was the plastic flow accommodated by grain-boundary diffusion creep.

Transmitting, emitting and controlling light: Processing of transparent ceramics using current-activated pressure-assisted densification

We review recent advances in using current-activated pressure-assisted densification (CAPAD) for fabricating polycrystalline ceramics that are transparent to visible and infrared light. In addition, we discuss important one-dimensional (vacancies), two-dimensional (grain boundaries) and three-dimensional (pores) defects that can affect optical properties. We pay special attention to procedures whereby the benefits of CAPAD can be used to ensure the precise control of microstructure that facilitates transparency and other functionalities.

Impact of Rapid Thermal Annealing on Thermoelectric Properties of Bulk Nanostructured Zinc Oxide

ABSTRACTIn search for non-toxic thermoelectric materials that are stable in air at elevated temperatures, zinc oxide has been shown to be one of only few efficient n-type oxidic materials. Our bottom-up approach starts with very small (<10 nm) Al-doped ZnO nanoparticles prepared from organometallic precursors by chemical vapor synthesis using nominal doping concentrations of 2 at% and 8 at%. In order to obtain bulk nanostructured solids, the powders were compacted in a current-activated pressure-assisted densification process. Rapid thermal annealing was studied systematically as a means of further dopant activation. The thermoelectric properties are evaluated with regard to charge carrier concentration and mobility. A Jonker-type analysis reveals the potential of our approach to achieve high power factors. In the present study, power factors larger than 4×10-4 Wm-1K-2 were measured at temperatures higher than 600 °C.

The effect of Peltier heat during current activated densification

It is shown that current-activated pressure-assisted densification (CAPAD) is sensitive to the Peltier effect. Under CAPAD, the Peltier effect leads to a significant redistribution of heat within the sample during the densification. The densification of highly p-doped silicon nanoparticles during CAPAD and the properties of the obtained samples are investigated experimentally and by computer simulation. Both, simulation and experiments, indicate clearly a higher temperature on the cathode side and a decreasing temperature from the center to the outer shell. Furthermore, computer simulations provide additional insights into the temperature profile which explain the anisotropic properties of the measured sample.

Graphene nanoplatelet/silicon nitride composites with high electrical conductivity

Silicon nitride (Si3N4) processed with up to 25vol.% of graphene nanoplatelets (GNPs) gives conductive composites with the highest electrical conductivity (40Scm−1) reported for these ceramics with added conductive particles. During compaction and pressure-assisted densification of the composites in the spark plasma sintering (SPS), a preferred orientation of GNPs occurs. Consequently, the electrical conductivity measured along the direction perpendicular to the SPS pressing axis is more than one order of magnitude higher than the one measured along the parallel direction.Percolation in the composites is observed for 7–9 vol.% of GNPs, depending on the measuring direction, perpendicular or parallel to the pressing axis. Different conduction mechanisms are apparent for the two orthogonal orientations. Charge transport along the direction defined by the graphene ab-plane (perpendicular direction) may be explained by a two dimensional variable range hopping mechanism, whereas conduction in the parallel direction shows a more complex behavior, with a metallic-type transition (dσ/dT<0) for high GNP contents. A thin amorphous layer was identified at the Si3N4/GNPs interface that may affect the conduction for the parallel configuration.

Low temperature, transient liquid phase sintering of B2O3-SiO2-doped Nd:YAG transparent ceramics

Abstract

Current-Activated, Pressure-Assisted Densification of Materials

This review of current-activated, pressure-assisted densification (CAPAD) focuses on both fundamental and practical issues. We provide some useful background for researchers interested in the process and critically assess the state of the technique.