Pyroelectric and electrocaloric effect of 〈1 1 1〉-oriented 0.9PMN-0.1PT single crystal

In this paper, the polarization vs. electric field hysteresis loops of 〈1 1 1〉-oriented 0.9PbMg_1/3Nb_2/3O₃-0.1PbTiO₃ (0.9PMN-0.1PT) single crystal at different temperatures (20-110 °C) were measured. The adiabatic temperature change ΔT of 〈1 1 1〉-oriented 0.9PMN-0.1PT single crystal due to the application or withdraw of electric field were calculated through the thermodynamic relation. The largest temperature change ΔT achieves ∼1 K with only a change of 40 kV/cm electric field, the mechanism of the electrocaloric effect (ECE) is discussed for 0.9PMN-0.1PT crystal. The pyroelectric coefficient of 0.9PMN-0.1PT under bias field was calculated according to the data of hysteresis loop. The result shows that 0.9PMN-0.1PT have large pyroelectric coefficient under bias field, the largest (∂P/∂T)_E value achieves −0.5 μC/cm² K.     

The maximum solubility of Y in α-Mg and composition ranges of Mg24Y5 − x and Mg2Y1 − x intermetallic phases in Mg-Y binary system

The solid solubility of Y in the α-Mg matrix and composition homogeneity ranges of Mg₂₄Y_5 − x and Mg₂Y_1 − x phases in the Mg-Y binary system are important parameters to understand solid-solution- and aging-strengthening effects of Mg-Y-based alloys. However, they are different among the various assessed Mg-Y phase diagrams which are based on limited experimental data, especially at temperatures below ∼850 K. Our experimental results by using both diffusion couple technique and alloy method are in good agreement each other, but much different from the presently accepted Mg-Y phase diagram. The results show that the maximum solubility of Y in the α-Mg matrix is 4.7 at.% Y, much larger than the current phase diagram. The determined composition homogeneity ranges of Mg₂₄Y_5 − x and Mg₂Y_1 − x phases are much wider than the present ones and shift remarkably to the Mg-rich corner of Mg-Y phase diagram.     

Influence of the oxygen content and the preparation method on the power factor of PrBaCo2O5+δ samples (0.54 ≤ δ ≤ 0.84)

In this work, we focus on the influence of the oxygen content and the preparation method on the power factor of different PrBaCo₂O_5+δ samples (0.54 ≤ δ ≤ 0.84). The samples have been initially synthesized by the Pechini method. Their oxygen content has been subsequently modified by annealing under argon/oxygen flow or by electrochemical oxidation/reduction. The oxygen stoichiometry has a high impact on the electrical conductivity and Seebeck coefficient of the resulting materials. Moreover, by adequately reducing their oxygen content while increasing their intergrain conductivity (by increasing the particle size and degree of sintering) the power factor of these samples can be drastically improved. The best result is shown by the PrBaCo₂O_5.54 sample, that was annealed at high temperature under argon flow, whose power factor is as high as 6.5 μW/K² cm⁻¹ at ∼135 K, more than two orders of magnitude higher than that shown by the initial PrBaCo₂O_5.76 reference sample.     

Thermoelectric properties of p-type skutterudites YbxFe3.5Ni0.5Sb12 (0.8 ? x ? 1)

p-Type skutterudites, with nominal compositions Yb_xFe_3.5Ni_0.5Sb₁₂ (0.8 ? x ? 1), have been synthesized by induction melting with subsequent annealing, and their thermoelectric properties evaluated from 3.5 to 745 K to assess their suitability for thermoelectric-based waste heat recovery applications. We report results for the synthesis and measurements of Seebeck coefficient (S), electrical resistivity (ρ), thermal conductivity (κ), Hall coefficient (R_H) and effective mass (m^*/m₀) of Yb_xFe_3.5Ni_0.5Sb₁₂ (0.8 ? x ? 1). Powder X-ray diffraction and electron probe microanalysis show that this system has a narrow filling fraction range of x ∼ 0.84-0.86 for Yb in the crystallographic voids. All samples show positive R_H for the entire temperature range studied, with carrier concentrations ranging from 9.6 × 10²⁰ to 2.8 × 10²¹ cm⁻³ at room temperature. Relatively high values of S result in high power factors up to 17 μW cm⁻¹ K⁻² at room temperature. However, large values of κ and a sharp reduction in the S at high temperature due to bipolar conduction prevent the attainment of high thermoelectric figure of merit.     

Influence of chemical composition and holding time on austenite (γ) → ferrite (α) transformation in ductile iron occurring within the intercritical interval

This work focuses on the influence exerted by the chemical composition on the characteristics of the γ → α reaction occurring within the intercritical interval of the Fe-C-Si diagram, in order to produce a new “dual phase ADI” variant.Three ductile iron melts with different chemical compositions were used and several thermal cycles were performed. The results show a strong dependence between the alloy composition and the characteristics of the γ → α reaction, affecting the amount as well as the morphology of the precipitated allotriomorphic ferrite. Low alloy content promotes ferritic nucleation around graphite nodules and fast growth. Moreover, when the alloy content increases, the ferrite nucleates preferentially at the grain boundaries of the recrystallized austenite, and grows very slowly forming a continuous net. This novel microstructure is expected to enhance the mechanical properties of ductile cast iron.     

Thermoelectric properties of n-type Ca1−xDyxMn1−yNbyO3−δ compounds (x = 0, 0.02, 0.1 and y = 0, 0.02) prepared by spray-drying method

We report the high temperature thermoelectric properties of Ca_1−xDy_xMn_1−yNb_yO_3−δ (x = 0, 0.02, 0.1 and y = 0, 0.02) synthesized by spray-drying method. A maximum power factor (PF) value of 2.65 μW K⁻² cm⁻¹ is obtained at 1100 K for CaMn_0.98Nb_0.02O_3−δ. This represents an improvement of about 75% with respect to undoped CaMnO_3−δ sample at the same temperature. We also provide a complete structural characterization of the samples.     

Spinel formation in thermal barrier systems with a Pt-enriched γ-Ni + γ′-Ni3Al bond coat

The oxidation of electron beam physical vapour deposited thermal barrier coatings with a Pt-enriched γ-Ni + γ′-Ni₃Al bond coat was investigated. Due to the growth of the thermally grown oxide (TGO), γ-Ni formed underneath the TGO as a result of Al depletion. Phase characterisation by X-ray diffraction, as well as microstructural observations, indicated that a NiAl₂O₄ spinel phase formed at the TGO/bond coat interface after prolonged oxidation. It is proposed that the formation of spinel occurs when local cracks present at the interface and the underlying bond coat is Al-depleted. The cracks provide a direct path for oxygen and nickel oxide forms at the bond coat surface. With further oxidation, the spinel forms at the interface through solid state reaction between the TGO and nickel oxide.     

Effects of α- and β-Si3N4 as precursors on combustion synthesis of (α + β)-SiAlON composites

Preparation of (α + β)-SiAlON composites from the powder compacts of Si, Si₃N₄, SiO₂, Al, and AlN was investigated by self-propagating high-temperature synthesis (SHS) in nitrogen of 2.17 MPa. Test samples adopted not only pure α- and β-Si₃N₄, but also a mixture of (α + β)-Si₃N₄. The combustion temperature and flame-front propagation velocity decreased with increasing ratio of Si/Si₃N₄, but they increased with proportion of α/β-Si₃N₄. For the sample containing pure α-Si₃N₄, the synthesis reaction yielded only α-SiAlON with various morphologies including equiaxed crystals, elongated grains, and fine whiskers. As a mixture of (α + β)-Si₃N₄ was employed, the resulting products were (α + β)-SiAlON composites, within which the content of β-SiAlON increased with increasing β-Si₃N₄. For the sample adopting 100% β-Si₃N₄, comparable amounts of α- and β-SiAlON were produced. Additionally, the morphology of (α + β)-SiAlON composites was dominated by elongated grains with a high aspect ratio.     

Comparative investigation of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti2 − хCrхAlC targets

A comparative investigation of the structure and properties of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti_2 − хCr_хAlC targets (where x = 0, 0.5, 1.5, and 2) in an Ar atmosphere or in a gaseous mixture of Ar + N₂ is presented. The coatings were characterized in terms of their structure, elemental and phase composition, hardness, elastic modulus, elastic recovery, thermal stability, friction coefficient, wear rate, corrosion, and high-temperature oxidation resistance. The structure of the coatings was studied by means of X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, glow discharge optical emission spectroscopy, electron energy loss spectroscopy, and Raman spectroscopy. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum or in air at temperatures 600-1200 °C. The results obtained show that the TiAlCN coatings possess high hardness of 32-35 GPa, low friction coefficient against WC-Co well below 0.25, high thermal stability up to 1200 °C, and superior performance in dry milling tests against high Cr steel. Meanwhile, the coatings with high Cr content demonstrated improved oxidation resistance up to 1000 °C and superior electrochemical behavior, but their mechanical and tribological properties were deteriorated.     

A new TiB2 + CrSi2 composite – Densification,characterization and oxidation studies

A new composite of TiB₂ with CrSi₂ has been prepared with excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM with EDAX. Mechanical and physical properties were evaluated. Extensive oxidation studies were also carried out. A near theoretical density (99.9% TD) was obtained with a small addition of 2.5 wt.% CrSi₂ by hot pressing at 1700 °C under a pressure of 28 MPa for 1 h. The microstructure of the composite revealed three distinct phases, (a) dark grey matrix of TiB₂, (b) black phase – rich in Si and (c) white phase – Cr laden TiB₂. Hardness and fracture toughness were measured as 29 ± 2 GPa and 5.97 ± 0.61 MPa m^1/2, respectively. Crack branching, deflection and bridging mechanisms were responsible for the higher fracture toughness. With increase in CrSi₂ content, density, hardness and fracture toughness values of the composite decreased. Thermo gravimetric studies revealed the start of oxidation of the composite at 600 °C in O₂ atmosphere. Isothermal oxidation of these composites showed better oxidation resistance by formation of a protective oxide layer. TiO₂, Cr₂O₃ and SiO₂ phases were identified on the oxidized surface. Effects of CrSi₂ content, temperature and duration of oxidation on the oxide layer formation are reported. Activation energy of the composite was calculated as ∼110 kJ/mol using Arrhenius equation. Diffusion controlled mechanism of oxidation was observed in all the composites.     

Pre-treatment and oxidation behavior of sol–gel Co coating on 430 steel in 750 °C air with thermal cycling

Research and development efforts continue to improve oxidation resistance and electrical conductivity of solid oxide fuel cell (SOFC) interconnects. This research evaluates the performance of a ferritic stainless steel (AISI 430 — a candidate SOFC interconnect material) with and without a Co coating (via sol–gel dip coating technique) with various pre-treatments, during cyclic oxidation exposures up to 750 °C in laboratory air. A pre-treatment exposure of Co coated samples to a 750 °C reducing atmosphere (prior to oxidation exposures) led to the formation of an effective Co-based spinel oxide coating that lowers oxidation rates by more than 40 times, and substantially lowers area specific resistance (ASR) in comparison to uncoated specimens. The development of effective sol–gel Co coatings with reducing pre-treatments, and their evolution within simulated SOFC interconnect environments is presented and discussed.     

The effects of temperature and composition on the thermal conductivities of [(ZrO2)1−x(CeO2)x]0.92(Y2O3)0.08 (0 ? x ? 1) solid solutions

The thermal conductivities of [(ZrO₂)_1−x(CeO₂)_x]_0.92(Y₂O₃)_0.08 (0 ? x ? 1) solid solutions are studied in this paper. The incorporation of ZrO₂ and CeO₂ in the solid solution decreases the thermal conductivity compared with their end members (YSZ and YDC). The thermal conductivities of the solid solutions show clearly different temperature dependences in the ZrO₂-rich (0 ? x ? 0.5) region and in the CeO₂-rich region (0.5 ? x ? 1). The composition and the temperature dependence of the thermal conductivities are discussed based on established phonon scattering theories. We have concluded that the composition dependence of the thermal conductivity of this system is mainly controlled by the mass difference between Zr⁴⁺ and Ce⁴⁺, while the thermal conductivity-temperature relationship is dominated by the randomness of the defect distribution.     

Stress-driven migration of symmetrical 〈1 0 0〉 tilt grain boundaries in Al bicrystals

Stress-induced migration of planar grain boundaries in aluminum bicrystals was measured for both low- and high-angle symmetrical 〈1 0 0〉 tilt grain boundaries across the entire misorientation range (0–90°). Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains. Boundaries with misorientations smaller than 31° and larger than 36° moved in opposite directions under the same applied external stress. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with theoretical predictions. The coupled boundary motion was measured in the temperature range between 280 and 400 °C, and the corresponding activation parameters were determined. The results revealed that for mechanically induced grain-boundary motion there is a misorientation dependence of migration activation parameters. The obtained results are discussed with respect of the mechanism of grain-boundary motion.     

Atomistic structure of the Cu(1 1 1)/α-Al2O3(0 0 0 1) interface in terms of interatomic potentials fitted to ab initio results

We propose an atomistic model to describe the copper/sapphire interface by means of simple interatomic potentials involving only a few fitting parameters. Successful results are achieved when the copper atoms in the monatomic layer closest to the interface have properties different from the bulk. This layer is to accommodate the ionic/covalent bonding in the ceramics to the metallic bonding in copper. For an oxygen terminated interface, we fit the parameters of the potentials to the results of a rigid tensile test (explained in the text) simulated from first principles. The results of atomic relaxation near the interface are shown to be consistent with ab initio and experimental results available in the literature. Calculations reveal highly interesting relaxation dynamics near the interface. In the early stage of relaxation, a periodic network of partial misfit dislocations is formed, which later transforms into an irregular network due to the instability of the layer of copper atoms atop the oxygen atoms. This explains the interface incoherency observed in high-resolution electron microscopy. Calculations based on the FK model reproduce this effect.     

Decarburization of TiC in Ni activated sintered W–xTiC (x = 0, 5, 10, 15 wt%) composites and the effects of heat treatment on the microstructural and physical properties

Blended elemental W–xTiC (x = 0, 5, 10, 15 wt%) powders were mechanically alloyed (MA’d) for 30 h in a SPEX Mixer/Mill at room temperature. About 1 wt% Ni was added to each MA’d batch as sintering aid which were further milled for 1 h. MA’d powders were sintered at 1400 °C for 2 h under Ar, H₂ gas flowing conditions and annealed at 1600 °C for 6 h under Ar atmosphere. Microstructural characterizations of as-sintered and annealed samples were conducted using XRD and SEM. XRD patterns of the as-sintered and annealed samples revealed the presence of the matrix W and Ni phases, whereas (Ti_x,W_1−x) solid solution phase came into existence after annealing. In addition to XRD patterns, hot combustion and infrared detection measurements revealed the decarburization of TiC. Relative density values varied between 85.2% and 96.4% after sintering. The density values of sintered samples decreased with increasing TiC content. After annealing, a maximum relative density value of 99.8% was achieved. Vickers microhardness values varied between 5.11 GPa and 10.79 GPa for as-sintered samples and a maximum microhardness value of 8.1 GPa was measured after annealing. Wear resistance of the as-sintered samples increased with increasing TiC content.