Current Activated 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.

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.

Processing and Thermal Conductivity of Bulk Nanocrystalline Aluminum Nitride.

Producing bulk AlN with grain sizes in the nano regime and measuring its thermal conductivity is an important milestone in the development of materials for high energy optical applications. We present the synthesis and subsequent densification of nano-AlN powder to produce bulk nanocrystalline AlN. The nanopowder is synthesized by converting transition alumina (δ-Al2O3) with <40 nm grain size to AlN using a carbon free reduction/nitridation process. We consolidated the nano-AlN powder using current activated pressure assisted densification (CAPAD) and achieved a relative density of 98% at 1300 °C with average grain size, ~125 nm. By contrast, high quality commercially available AlN powder yields densities ~75% under the same CAPAD conditions. We used the 3-ω method to measure the thermal conductivity, κ of two nanocrystalline samples, 91% dense, = 110 nm and 99% dense, = 220 nm, respectively. The dense sample with 220 nm grains has a measured κ = 43 W/(m·K) at room temperature, which is relatively high for a nanocrystalline ceramic, but still low compared to single crystal and large grain sized polycrystalline AlN which can exceed 300 W/(m·K). The reduction in κ in both samples is understood as a combination of grain boundary scattering and porosity effects. We believe that these are finest reported in bulk dense AlN and is the first report of thermal conductivity for AlN with ≤220 nm grain size. The obtained κ values are higher than the vast majority of conventional optical materials, demonstrating the advantage of AlN for high-energy optical applications.

Kinetics of densification/phase transformation and transport properties of Mg-Sn cubic/trigonal composites

The Mg-Sn materials are interesting because of useful semiconducting properties and particularly because of their earth abundant, environmentally friendly constituent elements. The cubic Mg2Sn phase appears in the equilibrium phase diagram and has been extensively studied, while the transport properties of the trigonal phase—that does not appear in the phase diagram—have not been reported. We produced dense composites composed of cubic (stable) and trigonal (metastable) phases using a combination of high energy ball milling and current activated pressure assisted densification (CAPAD). We present a kinetic study of the densification and cubic to trigonal phase transformation. In addition, we present transport property measurements of cubic/trigonal composites. We determine densification rates and observe distinct densification regimes, attributed to the cubic-to-tetragonal phase transformation as well as porosity removal. Transport property measurements reveal that altering the amount of metastable phase in the composites changes both the electrical and thermal properties. The electrical conductivity significantly increases with increasing trigonal phase fraction. The thermal conductivities are similar for all compositions, which is likely due to the nanocrystalline character of the composites.

High (10−3 s−1) densification rates at

We measured the real-time densification rates in the current activated pressure assisted densification (CAPAD) of nanocrystalline MgO. The rates of samples densified from powders with grain sizes ~40 nm are similar to those measured previously for other nanocrystalline materials while those from 29 nm powder are higher. Exposing the powders to a humid environment increases the densification rates and allows densification at lower temperatures. The densification rate curves of samples with absorbed water display two peaks implying two controlling mechanisms, while the dry samples display one. We attribute lower temperature densification to Mg(OH)2-MgO conversion and the higher to diffusion-controlled densification.

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.

Effect of phase homogeneity and grain size on ferroelectric properties of 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BXT) lead-free ceramics

We used two different solid state synthesis routes in conjunction with Current Activated Pressure Assisted Densification (CAPAD) to produce lead-free 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (called BZT-BCT or BXT) ceramics. The materials are fully dense and have a wide range of grain sizes from nanocrystalline (90 nm) to more conventional micrometer size range (3 μm). We investigate the role of grain size and phase homogeneity on the dielectric and ferroelectric properties of BXT independent of porosity. One of the synthesis routes yields samples with comparably high switchable polarization, 2Pr at grain sizes ~7 times smaller (420 nm) than the former route.

Synthesis, processing and transport properties of cubic and trigonal solid solutions in the Mg-Si-Sn system

Mg-Si-Sn solid solutions have interesting and useful semiconducting properties particularly for thermoelectric applications. While cubic solid solutions have been prepared by a variety of methods, solid solutions with a trigonal crystal structure have not been reported. We employed a combination of high energy ball milling and current activated pressure assisted densification (CAPAD) to induce and complete phase transformation from cubic to trigonal phase, forming dense trigonal composites. This is the first successful preparation of the trigonal phase in Mg-Si-Sn system. In addition, we report the transport properties of both the cubic and trigonal composites measured in the 160–400 K range. The electrical conductivity of the trigonal composites was several times higher compared to the cubic counterparts while thermal conductivity is lower but in the range of previously reported Mg-Si-Sn materials. The Seebeck coefficient of the trigonal composites is lower compared to the cubic samples, leading to low ZT values. The ZT of the cubic samples, however is higher than previously reported for un-doped Mg-Si-Sn solid solutions which is promising for thermoelectric applications.

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%.

Synthesis and structural characterization of dense polycrystalline Mg9Sn5, a metastable Mg–Sn phase

We present a method for synthesizing bulk dense polycrystalline Mg9Sn5, a phase that is related to the familiar Mg2Sn phase but is metastable at ambient pressure and temperature. Our procedure requires significantly lower temperatures and pressures, which are in the 100MPa range compared to the previously reported methods that required pressures in the GPa range. We used a combination of high energy ball milling and current activated pressure assisted densification (CAPAD) to synthesize and retain the high pressure phase in a bulk (low porosity) form. We show the effect of CAPAD processing temperature and pressure on the density and phase composition of the material. We also present the first neutron diffraction study to confirm phase identity and refine the structure of Mg9Sn5.

Blue–Green Emission in Terbium‐Doped Alumina (Tb:Al2O3) Transparent Ceramics

AbstractAlumina (Al2O3) is one of the most versatile ceramics, utilized in an amazing range of structural and optical applications. In fact, chromium‐doped single crystal Al2O3 was the basis for the first laser. Today, most photoluminescent (PL) materials rely on rare earth (RE) rather than transition‐metal dopants because RE doping produces greater efficiencies and lower lasing thresholds. RE‐doped alumina could provide an extremely versatile PL ceramic, opening the door for a host of new applications and devices. However, producing a transparent RE:Al2O3 suitable for PL applications is a major challenge due to the very low equilibrium solubility of RE (∼10−3%) in Al2O3 in addition to alumina's optical anisotropy. A method is presented here to successfully incorporate Tb3+ ions up to a concentration of 0.5 at% into a dense alumina matrix, achieving a transparent light‐emitting ceramic. Sub‐micrometer alumina and nanometric RE oxide powders are simultaneously densified and reacted using current‐activated, pressure‐assisted densification (CAPAD), often called spark plasma sintering (SPS). These doped ceramics have a high transmission (∼75% at 800 nm) and display PL peaks centered at 485 nm and 543 nm, characteristic of Tb3+ emission. Additionally, the luminescent lifetimes are long and compare favorably with lifetimes of other laser ceramics. The high transparencies and PL properties of these ceramics have exciting prospects for high energy laser technology.

Transparent, luminescent terbium doped zirconia: development of optical-structural ceramics with integrated temperature measurement functionalities

We present a method for the preparation of transparent Tb doped zirconia (Tb:ZrO2) ceramics that luminesce in the visible. The visible luminescence is temperature dependent, yielding samples that have integrated temperatures sensing capabilities. Our approach is to simultaneously react and densify ZrO2 and Tb4O7 using current activated pressure assisted densification (CAPAD). The Tb dopant serves to both stabilize the tetragonal phase of zirconia and for emitting light. The Tb:ZrO2 ceramics have an excellent combination of structural and optical properties; the toughness is comparable to yttria stabilized zirconia and the transparency in the visible is high. Moreover, the luminescent lifetimes are long and amenable to luminescent thermometry. The ceramics have promise as thermal barrier materials and high-strength windows with “built-in” temperature measurement capabilities.

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 (&lt;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.

Waveguide-like structures written in transparent polycrystalline ceramics with an ultra-low fluence femtosecond laser

We present a laser processing study of optically transparent ytrria stabilized zirconia (YSZ) ceramics (ZrO2-8 mol. % Y2O3) using unamplified femtosecond (fs) laser pulses of a few nJ and high repetition rate (70 MHz). The ceramics were fabricated using current activated pressure assisted densification (CAPAD) and have fine grain size and minimal porosity, producing a transparent material. Irradiation using fs laser pulses caused permanent changes in the optical properties of the irradiated zone. These laser written structures were found to confine He-Ne laser light (632 nm) in effect functioning as waveguide like structures and were written into the YSZ ceramics using a remarkably low per-pulse energy (5nJ). The number of passes with the laser i.e total incident pulses per unit area was found to significantly affect the waveguide writing. We believe that waveguides are regions were the concentration of oxygen vacancies and/or their associated free electrons have been altered by laser irradiation. We are not aware of previous reports of low fluence fs laser pulses being used to influence vacancy related defects to produce waveguides in ceramics. This new mechanism opens the door for writing strictures in optical ceramics with lower power than previously thought feasible.

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.

Simultaneous synthesis and densification of transparent, photoluminescent polycrystalline YAG by current activated pressure assisted densification (CAPAD)

Abstract We report a method for the synthesis and processing of transparent bulk polycrystalline yttrium aluminum garnet (YAG) and photoluminescent Ce-doped YAG ceramics via solid-state reactive-current activated pressure assisted densification (CAPAD). The process uses commercially available γ-Al 2 O 3 , Y 2 O 3 , and CeO 2 nanopowders. The nanopowders were reacted and densified simultaneously at temperatures between 850 °C and 1550 °C and at a maximum pressure of 105 MPa. The solid-state reaction to phase pure YAG occurs in under 4 min at processing temperatures 1100 °C which is significantly faster (on the order of tens of hours) and occurs at much lower temperatures (∼600 °C) compared to conventional reaction sintering. We found that the reaction significantly improves densification – the shrinkage rate of reaction-produced YAG was three times higher than that of YAG using pre-reacted powder. The Ce additions were found to retard the reaction driven shrinkage kinetics by a factor ∼3, but are still faster (by a factor ∼1.6) than those associated with direct densification (no synthesis). Densities >99% were achieved in both pure YAG and Ce doped YAG (Ce:YAG). Results of optical measurements show good transparency in the visible and photoluminescence (PL) in the Ce:YAG. The PL peak is broad and appears white when excited using blue light confirming that the ceramics can be used in solid state lighting to produce white light.

Thermal Conductivity of Nanocrystalline Silicon: Importance of Grain Size and Frequency-Dependent Mean Free Paths

The thermal conductivity reduction due to grain boundary scattering is widely interpreted using a scattering length assumed equal to the grain size and independent of the phonon frequency (gray). To assess these assumptions and decouple the contributions of porosity and grain size, five samples of undoped nanocrystalline silicon have been measured with average grain sizes ranging from 550 to 64 nm and porosities from 17% to less than 1%, at temperatures from 310 to 16 K. The samples were prepared using current activated, pressure assisted densification (CAPAD). At low temperature the thermal conductivities of all samples show a T(2) dependence which cannot be explained by any traditional gray model. The measurements are explained over the entire temperature range by a new frequency-dependent model in which the mean free path for grain boundary scattering is inversely proportional to the phonon frequency, which is shown to be consistent with asymptotic analysis of atomistic simulations from the literature. In all cases the recommended boundary scattering length is smaller than the average grain size. These results should prove useful for the integration of nanocrystalline materials in devices such as advanced thermoelectrics.

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.

The current activated pressure assisted densification technique for producing nanocrystalline materials

The harnessing of length-scale-dependent material properties has been an area of intense research in the past two decades. Retention of nanocrystalline features in fully dense bulk materials has been particularly elusive. Recently the current activated pressure assisted densification (CAPAD) technique has emerged as one of the most successful methods for the production of functional and structural nanocrystalline materials with large sizes. In this article we review some ongoing efforts in using the CAPAD technique for producing single component and nanocomposite systems from nanocrystalline powders. The properties of these nanocrystalline materials include improved light transmittance and magnetic properties caused by interfacial coupling ofanti-ferromagnetic/ferrimagnetic phases.