Nondestructive Measurement of Fatigue Damage of Thermally Sprayed Al2O3/NiCr Using ESPI Method Under High Temperature

High-temperature fatigue (R = 0) damage and deformation behaviors of SUS304 steel thermally sprayed with an Al₂O₃/NiCr coating were investigated using a servopulse fatigue-testing machine, SEM, and an electronic speckle pattern interferometry (ESPI) method. The relation between crack/delamination and strain variation is discussed. Surface cracks occurred at the outer Al₂O₃ coating but stopped at the inner NiCr coating after one fatigue cycle when the tensile stress was 202 MPa at 873 K. They propagated into the NiCr coating but stopped at the substrate, and local delamination occurred along the NiCr/substrate interface after 1 × 10⁵ cycles test in condition (_max = 202 MPa, T = 873 K). Cracks and delamination largely decreased when _max = 115 MPa or T = 573 K. No influence of cycle frequencies (6.7 or 14 Hz) was detected. The strain value measured by ESPI method was confirmed to be almost the same as that obtained with strain gauges at 293 K. Strain values along cracks measured with the ESPI method were much larger than other areas as a result of crack opening under the tensile load, referred to as the strain concentration zone in this work. Positions of strain concentration zones on strain distribution figures by the ESPI method corresponded well to positions of cracks on sprayed coatings. Moreover, strain values largely decreased where local delamination occurred.     

Using ESPI system to measure high temperature fatigue deformation of ceramics thermally sprayed SUS304 steel

The strains in an Al₂O₃/NiCr coating, which was thermally sprayed on SUS304 steel, were analyzed using an electronic speckle pattern interferometry (ESPI) system during fatigue testing (R = 0, _max = 173 MPa) at high temperature of 873 K. The strain changes with the crack initiation in the coatings and the delamination at the coating/substrate interface are accordingly discussed.Surface cracks originated from the top coating of Al₂O₃ and stopped at the bond coating of NiCr after 2 cycles test at 873 K. Many surface cracks and delamination along the NiCr/substrate interface were confirmed after 1 × 10⁵ cycles test. The strain values of un-sprayed specimens obtained from the ESPI system agreed with those measured by the strain gauge when tensile stresses were applied at room temperature. The deformation by thermal expansion and stress application at high temperatures can also be easily measured using this method. The strain on sprayed specimens was almost the same with that on un-sprayed specimens at 873 K, indicating the deformation in the coatings are always associated with that of the substrate surfaces at high temperature. By comparing and analyzing the strain distribution on the coating surface, the presence of cracks in the coatings and delamination at the coating/substrate interface can be in-situ and nondestructively detected.     

Structure and electrical transport properties of pure and Li2O-doped CuO/MgO solid solution

The different electrical properties, σ, ?′, tan δ and E_σ of pure and Li₂O-doped CuO/MgO solid solution were investigated. The mole fraction of CuO (MF) was varied between 0.048 and 0.2. Pure and doped samples were subjected to heat treatments at 673 and 1073 K. The results revealed that the amount of CuO dissolved in MgO lattice increases progressively by increasing the MF as evidenced from the progressive decrease in the intensity of all diffraction lines of undissolved CuO phase. The dissolution process of copper ions in MgO lattice was accompanied by progressive increase in its lattice parameter. This process being conducted at 1073 K was accompanied by a significant progressive increase in the values of σ, ?′ and tan δ with subsequent decrease in the value of E_σ. The increase in the MF value of CuO from 0.048 to 0.2 led to a significant increase in the value of σ_DC, measured at room temperature, from 6.33 × 10⁻¹² to 9.9 × 10⁻¹⁰ Ω⁻¹ cm⁻¹ and E_σ decreases from 0.76 to 0.58 eV.Li₂O doping of the investigated system followed by calcination at 1073 K resulted in a measurable increase in values of σ, ?′ and tan δ with subsequent decrease in E_σ. These results were discussed in the light of the possible effective increase in the charge carriers concentration (Cu²⁺ions dissolved in MgO lattice) and also to an effective increase in mobility of these charge carriers by Li₂O doping.     

Fabrication and mechanical properties of monolithic MoSi2 by spark plasma sintering

Fine MoSi₂ powders containing a small amount of Mo₅Si₃ have been prepared by self-propagating high-temperature synthesis (SHS), followed by spark plasma sintering (SPS) for 10 min at 1200-1500°C and 30 MPa. Dense MoSi₂ materials, in which the grain size is ∼7.5 μm, have been fabricated at 1300°C. They exhibit excellent mechanical properties: Vicker’s hardness H_v (10.6 GPa), fracture toughness K_IC (4.5 MPa m^1/2), and bending strength σ_b (560 MPa). The strength of 325 MPa can be retained up to 1000°C.     

Synthesis, characterization and electrical properties of quaternary selenodiphosphates: AMP2SE6 with A = Cu, Ag and M = Bi, Sb

The new quaternary selenophosphate phases AMP₂Se₆ (A=Cu, Ag and M=Bi, Sb) were synthesized by ceramic methods at 1023 K. These phases were characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDX) and a.c. and d.c. electrical conductivity measurements. The phases all show values of electrical conductivity, σ, of about 10⁻⁴ Ω⁻¹ cm⁻¹ at 303 K and photoconductive effect. The conductivity is nearly five orders of magnitude larger than that of related phases.     

Lithium conducting glass ceramic with Nasicon structure

Lithium ion conducting glass and glass ceramic of the composition Li_1.4[Al_0.4Ge_1.6(PO₄)₃], have been synthesized. The monolithic glass pieces on thermal treatment resulted in single-phase glass ceramic with the Nasicon structure. Experiments with different electrodes proved that the lithium electrodes provide accurate values for the ionic conductivity using impedance spectroscopy. σ_ionic of the glass ceramic was found to be 3.8×10⁻⁵ S cm⁻¹ at 40°C with an activation energy (E_a) of 0.52 eV. The corresponding values for the glass are 2.7×10⁻⁹ S cm⁻¹ and 0.95 eV, respectively. The Arrhenius dependence of σ_ionic with temperature in glass and glass ceramic is interpreted with a hopping mechanism from which the microscopic characteristics of the lithium cation motion are deduced.     

Synthesis and Li ion conductivity of Li2O-Nb2O5-P2O5 glasses and glass-ceramics

Glasses with the compositions of xLi₂O-(70 − x)Nb₂O₅-30P₂O₅, x = 30-60, and their glass-ceramics are synthesized using a conventional melt-quenching method and heat treatments in an electric furnace, and Li⁺ ion conductivities of glasses and glass-ceramics are examined to clarify whether the glasses and glass-ceramics prepared have a potential as Li⁺ conductive electrolytes or not. The electrical conductivity (σ) of the glasses increases monotonously with increasing Li₂O content, and the glass of 60Li₂O-10Nb₂O₅-30P₂O₅ shows the value of σ = 2.35 × 10⁻⁶ S/cm at room temperature and the activation energy (E_a) of 0.48 eV for Li⁺ ion mobility in the temperature range of 25-200 °C. It is found that two kinds of the crystalline phases of Li₃PO₄ and NbPO₅ are formed in the crystallization of the glasses and the crystallization results in the decrease in Li⁺ ion conductivity in all samples, indicating that any high Li⁺ ion conducting crystalline phases have not been formed in the present glasses. 60Li₂O-10Nb₂O₅-30P₂O₅ glass shows a bulk nanocrystallization (Li₃PO₄ nanocrystals with a diameter of ∼70 nm) and the glass-ceramic obtained by a heat treatment at 544 °C for 3 h in air exhibits the values of σ = 1.23 × 10⁻⁷ S/cm at room temperature and E_a = 0.49 eV.     

The effect of strain on nonlinear temperature dependence of resistivity in SrMoO3 and SrMoO3−xNx films

Highly oriented SrMoO₃ thin films have been fabricated by pulsed laser deposition of SrMoO₄ in hydrogen. The films are found to grow along the (1 0 0) direction on LaAlO₃ (1 0 0) and SrTiO₃ (1 0 0) substrates. The method has been extended for the fabrication of oxynitride thin films, using ammonia as the reducing medium. The resistivity measurements show nonlinear temperature dependent (Tⁿ) behaviour in the temperature interval of 10-300 K. The conduction mechanism is largely affected by the strain due to the substrate lattice. A combination of T and T² dependence of resistivity on temperature is observed for films having lesser lattice mismatch with the substrate. The X-ray photoelectron spectroscopic studies confirm the formation of SrMoO₃ and SrMoO_3−xN_x films.     

Thermoelectric properties of the tetradymite-type Bi2Te2S-Sb2Te2S solid solution

The thermoelectric properties of the tetradymite-type Bi_2−xSb_xTe₂S solid solution (0 ≤ x ≤ 2) are reported for the temperature range 5-300 K. The properties of non-stoichiometric, Cl and Sn doped n- and p-type variants are reported as well. The Seebeck coefficients for these materials range from −170 to +270 μV K⁻¹ while the resistivities range from those of semimetals, 2 mΩ cm, to semiconductors, >1000 mΩ cm. Thermal conductivities were low for most compositions, typically 1.5 W m⁻¹ K⁻¹. Nominally undoped Bi₂Te₂S shows the highest thermoelectric efficiency amongst the tested materials with a ZT = 0.26 at 300 K that decreased to 0.04 at 100 K. The crystal structure of Sb₂Te₂S, a novel tetradymite-type material, is also reported.     

Mechanical and thermal properties of LaMgAl11O19

Lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉) powders were synthesized successfully at 1300 °C for 4 h by solid-state reaction, and LaMgAl₁₁O₁₉ ceramic was prepared at 1700 °C for 6 h by pressureless sintering. Phase composition, microstructure, mechanical and thermophysical properties of LaMgAl₁₁O₁₉ ceramic were investigated. Results show that the flexural strength and fracture toughness of LaMgAl₁₁O₁₉ ceramic are 353.3 ± 12.5 MPa and 4.60 ± 0.46 MPa m^1/2. Young's Modulus and Poisson ratio is 295 GPa and 0.23, respectively. The linear thermal expansion coefficient of LaMgAl₁₁O₁₉ ceramic from 473 K to 1473 K is 9.17 × 10⁻⁶/K, and thermal conductivity at 1273 K is 2.55 W/m K.     

Evaluation of elastic/mechanical properties of some glasses and nanocrystallized glass by cube resonance and nanoindentation methods

Elastic and mechanical properties of 10La₂O₃·30Bi₂O₃·60B₂O₃ (LaBiB) glass, 15K₂O·15Nb₂O₅·68TeO₂·2MoO₃ (KNbTeMo) glass and a transparent KNbTeMo nanocrystallized (particle size: ∼40 nm) glass were examined using cube resonance and nanoindentation methods. The values of Poisson’s ratio, Young’s modulus (E), Debye temperature (θ_D), fractal bond connectivity, Martens hardness, indentation hardness, indentation Young’s modulus, elastic recovery, Vickers hardness, fracture toughness (K_c) and brittleness for the samples were evaluated, and the relation with the structure and nanocrystallization were clarified. LaBiB glass containing high oxygen-coordinated La³⁺ ions and two-dimensional BO₃ structural units shows excellent properties of E=90.6 GPa, θ_D=404 K and K_c=0.72 MPa m^1/2 and a high resistance against deformation during Vickers indentation. KNbTeMo glass with the three-dimensional network structure and consisting of weak Te-O bonds has small values of E=51.4 GPa and K_c=0.29 MPa m^1/2. It was demonstrated that the elastic and mechanical properties of KNbTeMo precursor glass are largely improved by nanocrystallization, e.g., E=69.7 GPa and K_c=0.32 MPa m^1/2. The nanocrystallization also induces a high resistance against deformation during Vickers indentation.     

Sintering of stacked pellets for controlling the temperature coefficient of dielectrics

Bearing in mind the excellent dielectric properties at high frequency of the oxides Ba₅Nb₄O₁₅, BaNb₂O₆, ZnNb₂O₆ and Zn₃Nb₂O₈ (?_r ∼ 20-45, tan(δ) < 10 × 10⁻⁴, ρ_i > 10¹⁰ Ω cm), “glass composites” of these materials were investigated in order to control their permittivity temperature coefficients for various applications. In a first method, samples constituted of mixed powders with temperature coefficient of opposite sign have been conventionally sintered. The obtained properties are not the expected ones because of the high reactivity between the mixed phases. In a second method, samples constituted of two stacked pellets with temperature coefficient of opposite sign have been co-sintered by the sinter-forging process, i.e. while maintaining a pressure, the two stacked pellets were co-sintered, leading to a good mechanical behaviour. More importantly we show that this technique avoids the delamination at the interface and that the so obtained dielectric properties are globally in good agreement with the mixing rule. Clearly, it is demonstrated that the temperature coefficient of these dielectrics can be tuned using the sinter-forging process, opening a new route for the optimization of multilayer capacitors.     

Synthesis, characterization and electrical conductivity studies on A3Bi2P3O12 (A = Na, K) materials

Crystalline Na₃Bi₂P₃O₁₂, K₃Bi₂P₃O₁₂ and glassy K₃Bi₂P₃O₁₂ compounds were prepared by solid-state reaction method. The prepared samples are characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry. The crystalline materials are found to be orthorhombic. The electrical conductivity measurements on the crystalline and glassy samples show that at ∼373 K, the σ_DC for crystalline K₃Bi₂P₃O₁₂ (0.81 × 10⁻⁸ S/cm) is about two orders of magnitude higher than the corresponding glassy phase (1.25 × 10⁻¹⁰ S/cm). The scaling results show that the conductivity relaxation mechanism is independent of temperature.     

Insertion of trivalent cations in the layered MPS3 (Mn, Cd) materials

The Al_0.21Mn_0.78PS₃, Al_0.15Cd_0.83PS₃, In_0.20Cd_0.70PS₃ and Ga_0.28Cd_0.58PS₃ compounds have been synthesized by the cation-exchange process used for other cationic species of the same family. These compounds were characterized by X-ray diffraction , Fourier transform infrared spectroscopy, ICP plasma analyses, photoconductivity and electrochemical impedance spectroscopy. The compounds synthesized show electrical conductivities (σ) of the order of 10⁻⁹ to 10⁻¹⁰ S cm⁻¹ at 298 K and photoconductive effect. The physical properties of the new Al³⁺ materials reveal the same behavior as our previous report on In³⁺ and Ga³⁺ compounds.     

Influence of SnO2 addition on the thermoelectric properties of Zn1 − xSnxO (0.01 ≤ x ≤ 0.05)

The grain size and the density of the Zn_1 − xSn_xO (0 ≤ x ≤ 0.05) samples decreased with increasing SnO₂ content. The addition of a small amount of SnO₂ (x ≤ 0.01) to ZnO led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient, resulting in a significant increase in the power factor. The thermoelectric power factor was maximized to a value of 1.25 × 10⁻³ Wm⁻¹ K⁻² at 1073 K for the Zn_0.99Sn_0.01O sample.     

Crystal structure and properties of the new complex vanadium oxide K2SrV3O9

The new complex vanadium oxide K₂SrV₃O₉ has been synthesized and investigated by means of X-ray powder diffraction (XPD), electron microscopy and magnetic susceptibility measurements. The oxide has an orthorhombic unit cell with lattice parameters a = 10.1922(2) Å, b = 5.4171(1) Å, c = 16.1425(3) Å, space group Pnma and Z = 4. The crystal structure of K₂SrV₃O₉ has been refined by Rietveld method using X-ray powder diffraction data. The structure contains infinite chains built by V⁴⁺O₅ square pyramids linked to each other via VO₄ tetrahedra. The chains form layers and potassium and strontium cations orderly occupy structural interstices between these layers. Electron diffraction as well as high resolution electron microscopy confirmed the structure solution. Magnetic susceptibility measurements revealed an antiferromagnetic interaction with J of the order of 100 K inside the chains and no long-range magnetic order above 2 K. The origin of the magnetic exchange is likely a result of super-exchange interaction through the two VO₄ tetrahedra linking the polyhedra with the magnetic V⁴⁺ cations.     

Proton conduction and chemical stability of (La0.5Sr0.5)(Mg0.5+yNb0.5−y)O3−δ

Electrical conduction properties of complex perovskite-type oxides in the (La_0.5Sr_0.5)(Mg_0.5+yNb_0.5−y)O_3−δ (y = 0.02-0.06) series at intermediate-high temperatures were investigated; introduction of protons by hydration of oxide-ion vacancies was expected by increasing the Mg/Nb ratio from unity. The conductivity depended on y and a maximum conductivity was obtained at y = 0.04: σ = 4.9 × 10⁻⁶ S cm⁻¹ at 400 °C in wet H₂ atmospheres. From electromotive force measurements of hydrogen and water vapor concentration cells, electrical conduction in wet H₂ atmospheres can be attributed to ionic conduction, and proton conduction is dominant below 700 °C. Unlike other perovskite-type proton conductors, (La_0.5Sr_0.5)(Mg_0.54Nb_0.46)O_3−δ was stable in CO₂ atmospheres even in the low-intermediate temperature region due to dilution of reactive strontium by lanthanum.     

Thermoelectric properties of Zn4Sb3 prepared by hot pressing

Polycrystalline specimens of the thermoelectric material Zn₄Sb₃ were prepared by the hot-pressing method at various temperatures and pressures and their thermoelectric properties were evaluated in a temperature range from 298 K to 673 K. A single phase of Zn₄Sb₃ was obtained in the samples prepared at 673 K with a pressure above 150 MPa, whereas ZnSb was placed in the Zn₄Sb₃ matrix for the samples prepared at 100 MPa. The electrical transport properties of the single phase compound showed p-type conduction and metallic transport behavior based on the temperature dependence. The sample produced at 673 K under a pressure of 200 MPa exhibited the highest ZT value of 1.36 at 673 K. This study suggests that the dense and single-phase Zn₄Sb₃ compound is a route to achieve a high thermoelectric performance.     

Microstructure and mechanical properties of titanium carbonitride whisker reinforced β-sialon matrix composites

The microstructure, hardness, fracture toughness and thermal shock resistance were investigated for 15 vol.% TiC_0.3N_0.7 whisker reinforced β-sialon (Si_6−zAl_zO₂N_8−z with z=0.6) composites with additions of three different volume fractions 2, 5 and 20 vol.%, of an yttrium-containing glass oxynitride phase. The composites were prepared by hot pressing at 1750°C for 90 min under a uniaxial pressure of 30 MPa in nitrogen atmosphere. The TiC_0.3N_0.7 whiskers were found to survive without deteriorating in morphology or reacting with the β-sialon matrix and/or the glass phase. The TiC_0.3N_0.7 whiskers had no obvious influence on the matrix microstructure, but their presence improved both the hardness and the fracture toughness of the composites. The highest hardness was obtained for the whisker composite with 2 vol.% glass phase (H_v=18.6 GPa). The fracture toughness and thermal shock resistance improved with increasing glass content. The whisker reinforced composite containing 20 vol.% glass showed the highest fracture toughness (K_1C=6.8 MPa m^1/2). No unstable crack extension occurred during the thermal shock test of the obtained composites in the temperature interval 90-700°C, but above 700°C severe oxidation of the whiskers precludes further evaluation of thermal shock properties by the indentation-quench method applied.     

Hygroscopicity and bulk thermal expansion in Y2W3O12

Negative thermal expansion material, Y₂W₃O₁₂ has been synthesized by the solid-state method and bulk thermal expansion of the material has been investigated from 300 to 1100 K. The material reversibly forms a trihydrate composition whose X-ray diffraction pattern can be indexed to an orthorhombic unit cell with a = 10.098(1) Å, b = 13.315(3) Å, c = 9.691(4) Å. The cell volume of the hydrated pattern is 7% smaller than the unhydrated cell volume. According to the dilatometric studies, the material shows a 3-6% increase in the linear strain at about 400 K, which can be attributed to the removal of water. Sintering the material at 1473 K leads to large grain size of >100 μm, which results in a large hysteresis in the bulk thermal expansion behavior. Hot pressing at 1273 K under a uniaxial pressure of 25 MPa results in a fine-grained (2-5 μm) ceramic. Glazing the ceramic prevents moisture pick up and a linear thermal expansion over the entire temperature range 1100-300 K and an average linear thermal expansion co-efficient of −9.65 × 10⁻⁶/K is observed. The effect of water on the thermal expansion behavior of this system is discussed.