Uniaxial Hot Pressing Research Articles

Development and study of ZnO:In optical scintillation ceramic

Samples of the optical scintillation ceramic ZnO:In with an indium concentration from 0.075 to 1.6 mass% have been obtained by uniaxial hot pressing. Studies have established that indium has a significant and nontrivial influence on the structure, the character of the microstructure, the spectral transmittance, and the x-ray luminescence spectra and kinetics of ZnO:In. The optimum dopant concentration has been determined at which an optical scintillation ceramic is formed that has relatively intense edge luminescence with its maximum at 388 nm and a decay time of about 1.1 ns.

Thermal conductivity of β-FeSi2/Si endogenous composites formed by the eutectoid decomposition of α-Fe2Si5

Thermoelectric properties of semiconducting β-FeSi2 containing a homogeneous distribution of Si secondary phase have been studied. The synthesis was carried out using arc melting followed by the densification by uniaxial hot pressing. Endogenous β-FeSi2/Si composites were produced by the eutectoid decomposition of high-temperature α-Fe2Si5 phase. The aging heat treatments have been carried out at various temperatures below the equilibrium eutectoid temperature for various durations in order to tune the size of the eutectoid product. Thermal properties of the samples were studied in the temperature range of 100–350 °C. The microstructural investigations support the fact that the finest microstructure generated through the eutectoid decomposition of the α-Fe2Si5 metastable phase is responsible of the phonon scattering. The results suggest an opportunity to produce bulk iron silicide alloys with reduced thermal conductivity in order to enhance its thermoelectric performance.

Microstructure and properties of bulk copper matrix composites strengthened with various kinds of graphene nanoplatelets

Copper matrix composites strengthened with graphene were prepared by vacuum uniaxial hot pressing of ball milled mixtures of powders. Two grades of graphene platelets were used; one with thickness of 10–20nm and with lateral size of less than 14μm and another with much lower thickness of platelets of 2–4nm and in (002) plane crystallite size below 100nm. Hot pressing in vacuum allowed obtaining composites containing 1 and 2wt% of graphene. The addition of fine graphene led to about 50% higher hardness and about 30% lower electrical resistivity than composite with coarse graphene platelets. SEM studies of samples with fine graphene additions showed much more homogeneous microstructure than those containing coarse graphene. Based on TEM studies, copper particle size was estimated between 100 and 300nm, and smaller in the areas of copper particle boundaries, where plastic deformation of copper particles brought about mixing nanocrystalline copper and graphene phases. Graphene conglomerates were identified using electron diffraction and often consisted of elongated platelets of thickness up to 20nm in both graphene composites studied. Raman spectra confirmed nonhomogeneity of graphene in bulk composites and showed increase of defect density within graphene platelets, as was assessed from low intensity ratio I(2D)/I(G), in places of high graphene concentration in both kinds of samples. The weak signal in more homogeneously distributed graphene in samples with fine graphene additions confirmed similar structural features, however the lateral size of finer graphene remain unchanged in composites, contrary to that with coarser graphene additions.

High-Performance Three-Stage Cascade Thermoelectric Devices with 20% Efficiency

The use of advanced materials has resulted in a significant improvement in thermoelectric device conversion efficiency. Three-stage cascade devices were assembled, consisting of nano-bulk Bi2Te3-based materials on the cold side, PbTe and enhanced TAGS-85 [(AgSbTe2)15(GeTe)85] for the mid-stage, and half-Heusler alloys for the high-temperature top stage. In addition, an area aspect ratio optimization process was applied in order to account for asymmetric thermal transport down the individual n- and p-legs. The n- and p-type chalcogenide alloy materials were prepared by high-energy mechanical ball-milling and/or cryogenic ball-milling of elementary powders, with subsequent consolidation by high-pressure uniaxial hot-pressing. The low-temperature stage materials, nano-bulk Bi2Te3−x Sb x and Bi2Te3−x Se x , exhibit a unique mixture of nanoscale features that leads to an enhanced Seebeck coefficient and reduced lattice thermal conductivity, thereby achieving an average ZT of ~1.26 and ~1.7 in the 27°C to 100°C range for the n-type and p-type materials, respectively. Also, the addition of small amounts of selected rare earth elements has been shown to improve the ZT of TAGS-85 by 25%, compared with conventional or neat TAGS-85, resulting in a ZT = 1.5 at 400°C. The incorporation of these improved materials resulted in a peak device conversion efficiency of ~20% at a temperature difference of 750°C when corrected for radiation heat losses and thermal conduction losses through the lead wires. These high-efficiency results were shown to be reproducible across multiple cascade devices.

Identification of densification mechanisms of pressure-assisted sintering: application to hot pressing and spark plasma sintering of alumina

The identification of densification mechanism during hot uniaxial pressing is developed using an approach based on classical creep investigation. This approach is justified and generalised using continuum mechanics based sintering models. The benefit of this approach is to directly determine the densification parameters from the analysis of shrinkage rates of the porous material, rather than to transpose the creep mechanisms identified for dense material at given thermomechanical conditions to the densification progress. The suggested approach is applied to compare the densification mechanisms involved at the initial stage of sintering (i.e. for 60 % < relative density < 75 %) during hot pressing (HP) and spark plasma sintering (SPS) of a submicrometric alpha-alumina powder. From the stress exponent and activation energy values, it is shown that the main mechanism involves grain boundary sliding accommodated by dislocation motion and particle fracture in both cases. However, it appears that, in SPS, the high heating rate could reduce the existence of surface diffusion phenomena at the beginning of the consolidation process, as suggested by the higher activation energy compared to the one determined for HP.

Synthesis and study of nanocrystalline powders for an optical ceramic composed of magnesium aluminate spinel

Nanocrystalline powders of magnesium aluminate spinel (MgAl2O4) have been produced, and their morphological and structural properties have been studied. A comparative analysis of spinel samples synthesized by different liquid-phase methods has been carried out. Hot uniaxial pressing has been used to fabricate ceramic samples of spinel 15 mm in diameter and up to 2 mm thick.

Efficacy of a solution-based approach for making sodalite waste forms for an oxide reduction salt utilized in the reprocessing of used uranium oxide fuel

This paper describes the various approaches evaluated for making solution-derived sodalite with a LiCl–Li2O oxide reduction salt selected to dissolve used uranium oxide fuel so the uranium can be recovered and recycled. The approaches include modified sol–gel and solution-based synthesis processes. As-made products were mixed with 5 and 10mass% of a Na2O–B2O3–SiO2 glass binder and these, along with product without a binder, were heated using either a cold-press-and-sinter method or hot uniaxial pressing. The results demonstrate the limitation of sodalite yield due to the fast intermediate reactions between Na+ and Cl− to form halite in solution and Li2O and SiO2 to form lithium silicates (e.g., Li2SiO3 or Li2Si2O5) in the calcined and sintered pellets. The results show that pellets can be made with high sodalite fractions in the crystalline product (∼92mass%) and low porosities using a solution-based approach and this LiCl–Li2O salt but that the incorporation of Li into the sodalite is low.

Characterization of High Strength Stainless Steel/Al/Brass Composite Tri-layered Clad

Delamination is a major limitation of lamellar composite structures in general, wherein interface/interphase control is a major factor. Studying/investigating the mitigation of cracks as well as toughening mechanisms is paramount to establish the control of interface/interphase kinetics. Stainless steel 316, Al 6082, CU–Zn sandwich laminate composite-sheets, in a foil–foil processing technique, were fabricated by means of hot uniaxial pressing. A parametric study was introduced using temperature and pressure effects on the composite specimens. The composite-sheet interface and its bond strength were investigated using scanning electron microscopy and tensile test. Microstructural as well as mechanical characterization, were established via optical and scanning electron microscopy; energy dispersive X-ray spectroscopy and mechanical testing. Mechanical characterization of the interfacial shear strength is investigated using hardness testing. The interphase kinetic behavior is observed through localized micro plasticity, metal flow and delocalized Al oxide and Mg oxide. Results showed that low temperature heat treatment can improve the morphology of stainless steel 316, Al 6082, CU–Zn interface and increase its bond strength. Alloying element segregation with diffusion mechanisms induced the formation of localized metal flow along with the interface. Alloy segregation during solidification in a semisolid state processing induces directional solidification with fibrous entities and oxides on structural interphase. For the stainless steel 316, Al 6082, Cu–Zn composite, the reasonable heat treatment is approximately 823 K for 1 and 3 h.

Room temperature nanoindentation creep of hot-pressed B6O

Nanoindentation has become a widely used and versatile means of characterising the near-surface nanomechanical properties of a wide variety of materials. Yet, the nanomechanical properties of the polycrystalline boron suboxide (B6O) material prepared by uniaxial hot-pressing are sparsely known. We recently reported on the nanoindentation profiles, nanoindentation hardness, and elastic modulus determined by using the Oliver–Pharr method. To complement our earlier reports on the analysis of the load–displacement indentation response of hot-pressed B6O and for the first time, we present and discuss results giving an insight into the temporal evolution of the nanoindentation creep behaviour in B6O ceramics.

Fabrication and mechanical properties of Al-based metal matrix composites reinforced with Mg65Cu20Zn5Y10 metallic glass particles

In this work, Al-based composites reinforced with Mg-based metallic glass particles are synthesized by uniaxial hot pressing and their mechanical properties are analyzed. The composite powders were consolidated into highly dense bulk specimens at temperatures within the super-cooled liquid region of the metallic glass reinforcement. The uniform distribution of the glassy particles and the good reinforcement–matrix interface lead to a significant strength enhancement with respect to the unreinforced matrix along with good plastic deformation. The relationship between the mechanical properties and the structure is investigated and described by a modified shear lag model incorporating the matrix-strengthening mechanism. The calculations are in good agreement with the experimental results, revealing that the dislocation strengthening in the matrix due to the presence of the metallic glass particles plays a major role in determining the mechanical properties of the composites.

Mechanical properties of MWNT/Ni bulk composites: Influence of the microstructural refinement on the hardness

The grain growth stagnation produced by the addition of multiwalled carbon nanotubes to bulk nickel composites manufactured by hot uniaxial pressing is investigated. A hardness improvement up to 27% compared to pure nickel was achieved, mainly based on the microstructural refinement. However, this enhancement is only observable up to concentrations of 3wt%. Beyond this point, no significant improvement is observed, thus setting an empirical reinforcing limit. The main strengthening mechanism identified was the Hall–Petch effect, which hinders the dislocation movement and is strongly dependent on the final grain size of the composites. Finally, a model for the prediction of the final grain size is proposed based upon the Zener pinning equation for the particular system studied in this work. This model is intended to provide a basic tool for a further tailoring of the mechanical properties, adjusted for a certain application.

Microstructural study of an ODS stainless steel obtained by Hot Uni-axial Pressing

This study focuses on the consolidation of an ODS Fe–14Cr–1W–0.3Ti–0.3Y2O3 alloy obtained by powder metallurgy. The powder was sintered for 1h at 1100°C in a soft steel can and compacted by Hot Uni-axial Pressing (HUP) under 900MPa within less than 5s before air cooling. The HUPped material microstructure is prospected to determine relative density as well as microstructure and nano-precipitation. The HUP, which mimics the early stage of hot extrusion (HE), produces a fully dense metallurgical state. The HUPped microstructure is shown to be rather similar to the one obtained by HIPping for equivalent dwell time at high temperature. In both cases heterogeneous grain size microstructure was obtained. It is shown that the bigger grains come from static recrystallization occurring during the dwell time in furnace. Compared to HIP, the effect of the HUP is mainly to introduce intra-granular misorientations within these large recrystallized grains.

Thermoelectric performance of nanostructured p-type Zr0.5Hf0.5Co0.4Rh0.6Sb1−xSnx half-Heusler alloys

Several compositions of the p-type half-Heusler alloys Zr₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb1–xSnx (0≤x≤0.4) were synthesized by mechanically alloying high purity elemental powders using hardened steel jars and balls on a high energy shaker mill. Powder X-ray diffraction (XRD) investigations of several aliquots taken after regularly spaced milling time suggested that single phase products with half-Heusler (HH) structure can be obtained after 10 h. However, XRD and transmission electron microscopy (TEM) studies of several specimens obtained from compacted polycrystalline powders of Z₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb1–xSnx alloys using a uniaxial hot press (HP) revealed the presence of CoSb inclusions with various sizes embedded inside the HH matrix. Hall effect, electrical conductivity, and thermopower data collected between 300 K and 775 K on several compositions suggested that electronic transport in the synthesized Zr₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb1–xSnx/CoSb composites strongly depends on the average size and/or mole fraction of the embedded CoSb inclusions rather than the fraction (x) of Sn substituting for Sb. Among the samples investigated, the nanocomposite with x=0.2, which contains nanometer-scale CoSb inclusions, showed the largest power factor (800 μW/K² m at 775 K) and the lowest lattice thermal conductivity (~2.2 W/m K at 775 K) leading to a six-fold enhancement in the figure of merit when compared to the Zr₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb₀.₉₉Sn₀.₀₁ bulkmore » matrix. - Graphical abstract: CoSb nanoinclusions embedded into a p-type Zr₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb1–xSnx half-Heusler matrix simultaneously boost the thermopower and carrier mobility leading to a drastic enhancement of the power factor of the resulting bulk nanostructured materials. Highlights: • The phase composition of half-Heusler (HH) alloy is very sensitive to the synthesis method. • Mechanical alloying (MA) of elements results in bulk HH matrix with CoSb inclusions. • CoSb nanoinclusions simultaneously boost the thermopower and carrier mobility with the HH matrix. • The MA process reduces lattice thermal conductivity due to high density of grain boundaries.« less

Hydroxyapatite/Al2O3/diopside ceramic composites and their behaviour in simulated body fluid

In the present work, alumina and diopside were introduced in hydroxyapatite matrix, and hydroxyapatite/Al2O3/diopside ceramic composites with good mechanical properties were fabricated by uniaxial hot pressing. The behaviours of hydroxyapatite/Al2O3/diopside ceramic composites in simulated body fluid were studied by SEM, Fourier transform infrared spectroscopy and electron probe microanalyser. Scanning electron microscopy images showed an obvious mineral layer formed on the soaked composite surface, which indicated that the introduction of Al2O3 and diopside in hydroxyapatite matrix could not only improve the strength and toughness of the composites but also maintain its ability to precipitate an apatite layer.

Protective Coatings for Niobium Alloys: Manufacture, Characterization and Oxidation Behaviour of (TiXCr)7Si6 with X = Fe, Co and Ni

Intermetallic single phases (TiXCr)7Si6 (with X = Fe, Co or Ni) were manufactured by uniaxial hot pressing. Their full characterization (SEM + XRD) highlighted that the nature of the metal constituent X influenced the chromium content in the silicide compositions. Both isothermal and cyclic oxidation studies at a temperature as high as 1300 °C evidenced very low degradation rate for all compounds and lifetime at 1100 °C exceeded 4700 cycles. All phases developed a duplex protective layer of chromia and silica. It was observed that the chromia:silica ratio depended on the nature of X, and, particularly for the Ni-containing specimen, also on the temperature. Co- and Ni-containing specimens developed pure chromia during oxidation whereas Fe ones develop (CrxFe1−x)2O3, reducing drastically the CrO3 volatilization during exposure at temperature higher than 1000 °C.

A trapped field of >3 T in bulk MgB2 fabricated by uniaxial hot pressing

A trapped field of over 3 T has been measured at 17.5 K in a magnetized stack of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness 5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which is a readily scalable, industrial technique, to 91% of their maximum theoretical density. The macroscopic critical current density derived from the trapped field data using the Biot–Savart law is consistent with the measured local critical current density. From this we conclude that critical current density, and therefore trapped field performance, is limited by the flux pinning available in MgB2, rather than by lack of connectivity. This suggests strongly that both increasing sample size and enhancing pinning through doping will allow further increases in trapped field performance of bulk MgB2.

Dielectric and Pyroelectric Properties of Ferroelectric SBN–PVDF Composites Prepared by Hot‐Uniaxial Press

Composites of polyvinyledene fluoride (PVDF) and strontium barium niobate (SrxBa1−xNb2O6 for x = 0.53/SBN)/ФSBN − (1 − Ф)PVDF with Ф = 0.1 to 0.3 have been prepared by hot uniaxial press. Structural, morphological, dielectric and pyrolectric properties of the composites have been studied. Self‐sustaining thick films with a 0–3 connectivity have been synthesized. The dielectric constant (εr) was found to increase with the increase in ceramic volume fraction (Ф) in the composite. The composite with Ф = 0.3 is found to have highest values of pyroelectric coefficient (pi) ~13.475 μC/m2K, FOMI ~0.481 μC/m2K and FOMII ~20.13 μC/m2K with good mechanical flexibility.

Alternative route for the preparation of CoSb3 and Mg2Si derivatives

An alternative manufacturing route has been developed for cobalt triantimonide and magnesium disilicide derivatives. Elemental powders were mixed in stoichiometric proportions, cold pressed into cylindrical preforms and heated in oxygen-free environment to initiate the exothermic reaction. According to DTA/TG measurements and observations under high-temperature microscope, the onset of reaction occurred at a temperature not exceeding the melting point of the more volatile component, i.e. antimony in the case of CoSb3 and magnesium in the case of Mg2Si. The reaction products were additionally heat treated to secure homogenization. Dense sinters were obtained by hot uniaxial pressing of the obtained powders in moderate temperature-and-pressure conditions. Several advantages were identified in the proposed technology: absence of liquid phases, relatively short time of the synthesis, possibility of in-situ or ex-situ doping and grain size control.

Microstructure and thermal conductivity of surfactant-free NiO nanostructures

High purity, nanometer sized surfactant-free nickel oxide (NiO) particles were produced in gram scale using a solution combustion method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), gas pycnometry and gas adsorption analysis (BET). The average particle size of the as-synthesized NiO increases significantly with the preheating temperature of the furnace, while the specific surface area decreases. A BET specific surface area of ∼100m2/g was obtained for NiO nanoparticles with size as small as 3nm synthesized at 300°C. The thermal conductivity (κ) of pressed pellets of the synthesized NiO nanoparticles obtained using spark plasma sintering (SPS) and uniaxial hot pressing is drastically decreased (∼60%) compared to that of NiO single crystal. This strong reduction in κ with particle size suggests the suitability of the synthesized surfactant-free NiO nanoparticles for use as nanoinclusions when designing high performance materials for waste heat recovery.

Scintillation optical ceramics based on Gd_2O_2S doped with Pr, Tb, or Eu

This paper discusses the structural, optical, and spectrokinetic characteristics of ceramics based on gadolinium oxysulfide Gd2O2S:RE (RE = Pr, Tb, Eu), obtained by the method of uniaxial hot pressing. It is shown that the composition and conditions of hot pressing affect the parameters of the Gd2O2S lattice and the formation of the predominant grain orientation in the (001) plane of the pinacoid. All the ceramics possess transparency that is high for an anisotropic material (about 40% at a thickness of 1.65 mm) in the intrinsic emission region. The overall x-ray luminescence intensities (relative to CsI:Tl) for the Gd2O2S:Pr,Ce, Gd2O2S:Tb(Ce), and Gd2O2S:Eu ceramics are 50, 160 (120), and 200%, respectively, while the damping time constants are 3.2, 600–700, and 450 µs, respectively.