Freeze casting of porous Ni-YSZ cermets

Dual-phase porous Ni-YSZ cermets were fabricated via the freeze casting of a ceramic/camphene slurry. After removing the frozen camphene via sublimation at room temperature, the green samples were sintered for 3 h in air at various temperatures, ranging from 1100 to 1350 °C, and then reduced in an Ar-5% H₂ atmosphere at 700 °C for 3 h. The fabricated Ni-YSZ cermets showed 3-D pore channels formed by the replication of the entangled camphene dendrite network and small pores in the Ni-YSZ walls produced by partial sintering of the NiO-YSZ composite. Furthermore, the fabricated samples were found to possess reasonable electrical conductivities, thus rendering them suitable for use as the basic components of planar solid oxide fuel cells (SOFCs).     

Preparation and microwave dielectric properties of multilayer Li1.075Nb0.625Ti0.45O3 co-fired ceramics with aqueous tape casting

Multilayer Li_1.075Nb_0.625Ti_0.45O₃ (LNT) ceramics for low temperature co-fired ceramic (LTCC) application were prepared by aqueous tape casting. The rheological test indicated that the aqueous slurries exhibited a typical shear-thinning behavior without thixotropy suitable for tape casting process. The green tapes with 2 wt% V₂O₅ addition have satisfactory mechanical properties. Scanning electron microscopy studies revealed that the green tapes have a smooth defect-free surface and that the sintered LNT ceramics have a fine grain microstructure with a uniform grain size. Therefore, aqueous tape casting is suitable for the manufacture of high performance multilayer LNT ceramics. Silver inner-electrode was sintered with LNT tapes at 900 °C, and no reaction been observed between LNT ceramic and sliver layers. The addition of V₂O₅ does not induce much degradation on the microwave dielectric properties. In the case of 2 wt% V₂O₅ addition, the ceramics show good microwave dielectric properties of ε _r  = 65, Q × f = 6,350 GHz. It represents that LNT ceramics could be promising for multilayer LTCC application.     

Druckverlauf und Gasdruckschwingung im Schaufelkanal von Seitenkanalverdichtern

The pressure in the impeller, in the side channel and in the breaker of a side channel compressor with the polytropic compression and with the polytropic expansion of the breaker mass flow was investigated experimentally, and the occurring shock waves were detected during the expansion of the gas in the breaker. Thereby different pressure flows occur at the four measuring points in blade channel on the blade pressure and suction side of the related radius from r _i /r ₂=0.80 and r _i /r ₂=0.95, which give an indication of the blade channel flow. In addition to the periodic increase in pressure and the gas expansion in the breaker, the superimposed pressure oscillation in the side channel is caused by the blade rotation frequency. This superimposed pressure oscillation can be decomposed by Fourier transformation in the two pressure oscillation components. These phenomena are the reason for the more accurate investigation of the pressure flow in the blade channel during the polytropic compression and expansion of the breaker mass flow. The gradient of the effective values of the gas pressure oscillation in the blade channel leads not only to the operating characteristics of the side channel compressor ??p=f(?,n), but also to the breaker curve characteristic ??p _U =f(?,n).     

Modelling the removal of organic vehicle from ceramic or metal mouldings: The effect of gas permeation on the incidence of defects

A shrinking undegraded core and a porous outer layer result, if the organic vehicle used for shaping ceramic or metal powder mouldings recedes in the interparticle space of the moulded body during pyrolysis. In the present work, a numerical model has been used which simulates the undegraded shrinking core situation and quantifies degradation of the organic vehicle and the diffusion of the resulting products in solution in the organic phase during pyrolysis of a ceramic moulding. This model is extended to include gaseous mass transport in the porous outer layer for a moulding in the shape of an infinite cylinder. The effect of resistance to gaseous mass transport in the porous outer region on defects originating in inner regions was estimated. It is shown that the greatest obstruction to mass transport is diffusion of degradation products in solution in the organic phase. However, the permeability coefficient for gas transport in the outer region begins to affect the critical heating rate required for avoidance of defects only when it is less than 10^–15m².Nomenclature C Concentration,C=C (r, t), based on the total volume of ceramic suspension - d Effective molecular diameter of alphamethylstyrene - D Diffusion coefficient,D=D (C, T) - e Porosity of powder - E Activation energy for thermal degradation - h Remaining weight fraction of polymer - H_vap Enthalphy of vaporization - i Node number - I Pre-exponential constant in Equation 13 - j Time step - K _p Permeability coefficient - K ₀ Specific rate constant - m Mass of monomer displaced - M Mass of one alphamethylstyrene molecule - P Monomer vapour pressure - P _s Monomer vapour pressure at outer surface of the cylinder - P ₁ ⁰ Vapour pressure of monomer over its pure liquid - Q Rate of production of monomer, based on the total volume of ceramic suspension - r Radius of the cylinder - r _j Distance from central axis to the inner surface of the porous layer at time stepj - r ₀ Initial radius of the cylinder - R Universal gas constant - S ₀ Specific surface area of powder per unit solid volume - t Time - T Absolute temperature - T _c Temperature at maximum vapour pressure of monomer and atZ _c - V Volume of monomer - V _c Ceramic volume fraction - V _p Polymer volume fraction - w Mass of monomer stored in the porous annulus - Z Heating rate - Z _c Critical heating rate - ₁ Volume fraction of monomer in the polymer-monomer solution - Viscosity of the monomer vapour - _p Density of the polymer - Polymer-monomer interaction constant     

Dynamic behavior of selected ceramic powders

An experimental setup has been developed on the continuous recording of the stress profiles in ceramic powders subject to shock loading with manganin gauges. A series of plate impact experiments on highly porous ceramic powders such as Al₂O₃, SiC and B₄C were conducted at the laboratory's single stage powder gun facility. The relationship between shock wave velocity and particle velocity was measured to obtain the Hugoniot data. Plate impact onto powder sample experiments were conducted at loading stresses ranging from 1.6 to 4.2 GPa. The experimental results show that the shock wave speeds in various ceramic powders vary between 1 and 2 km/s. Linear Hugoniot relations between shock velocity (D) and particle velocity (u) are observed. The loading stress–specific volume form of Hugoniot relations (P–V) was constructed using the data from quasistatic compression test results, Hopkinson bar dynamic compression test results and powder gun plate impact experiment results. The P–V diagram shows that the crush strength of ceramic powders is comparable to the loading stress level. The B₄C and SiC powders with bigger particle size more easily reach the solid state Hugoniot than the powders with smaller particle size at the same loading condition. In the case of Al₂O₃, the material shows less sensitivity to particle size difference at the same level of loading rate as compared to B₄C and SiC.     

The effect of through-thickness cracks on the ballistic performance of ceramic armour systems

Full width through-thickness cracks were introduced into the ceramic tiles of ceramic faced composite armour panels. The ballistic limit velocity for projectiles striking directly on the crack was measured and compared with undamaged panels. The effect of the cracks was to lower the V₅₀ ballistic limit velocity to 744 m s⁻¹ compared to 764 m s⁻¹ for undamaged panels, a drop of only 3%. This means that the presence of cracks in a ceramic armour tile should not be sufficient reason to require replacement of the panel, a fact of some importance given the likehood of damage in the military environment. It is proposed that the small value of the reduction in performance is observed because the cracked ceramic is still effectively confined by the presence of a well bonded composite backing and a frontal spall shield. The presence of a large crack at the impact point has little effect as the ceramic in this area is anyway extensively comminuted ahead of the projectile upon impact. The backing and spall shield conserve the structural integrity of the panel and this acts to contain the radial stresses generated by the impact event. The performance of the armour system has also been assessed by measurement of the V₀ ballistic limit velocity determined from residual momentum of penetrating projectiles and armour fragments. The standard panels showed a V₀ of 743 m s⁻¹ compared to 699 ms⁻¹ for the pre-cracked panels.     

One‐Step Synthesis of Monodispersed Mesoporous Carbon Nanospheres for High‐Performance Flexible Quasi‐Solid‐State Micro‐Supercapacitors

Cost‐effective synthesis of carbon nanospheres with a desirable mesoporous network for diversified energy storage applications remains a challenge. Herein, a direct templating strategy is developed to fabricate monodispersed N‐doped mesoporous carbon nanospheres (NMCSs) with an average particle size of 100 nm, a pore diameter of 4 nm, and a specific area of 1093 m² g^?1. Hexadecyl trimethyl ammonium bromide and tetraethyl orthosilicate not only play key roles in the evolution of mesopores but also guide the assembly of phenolic resins to generate carbon nanospheres. Benefiting from the high surface area and optimum mesopore structure, NMCSs deliver a large specific capacitance up to 433 F g^?1 in 1 m H₂SO₄. The NMCS electrodes–based symmetric sandwich supercapacitor has an output voltage of 1.4 V in polyvinyl alcohol/H₂SO₄ gel electrolyte and delivers an energy density of 10.9 Wh kg^?1 at a power density of 14014.5 W kg^?1. Notably, NMCSs can be directly applied through the mask‐assisted casting technique by a doctor blade to fabricate micro‐supercapacitors. The micro‐supercapacitors exhibit excellent mechanical flexibility, long‐term stability, and reliable power output.     

Properties of tape-cast Y-substituted strontium titanate for planar anode substrates in SOFC applications

This work evaluates the use of fine-grained yttrium-substituted strontium titanate powders for the preparation of planar anode supported solid oxide fuel cells. Starting from a submicron-sized powder of Y-substituted strontium titanate Sr_0.895Y_0.07TiO₃ (SYT), which was synthesised via spray pyrolysis followed by a grinding process, suspensions of high solid concentration were prepared by steric stabilisation. From these suspensions, tape casting slurries of up to 25 vol% were produced and further processed to ceramic green tapes using the doctor blade technique. The rheological behaviour of the slurries was investigated in dependence on the content of solids and organic additives. Furthermore, the binder burnout and sintering behaviour of the green sheets were characterised. After firing, crack-free substrates of high planarity were obtained. The achieved properties of the sintered tapes such as density, porosity, warping, mechanical strength, and electrical conductivity were determined in dependence on sintering temperature.     

Processing of Al2O3/SiC ceramic cake preforms and their liquid Al metal infiltration

In order to prepare ceramic preforms, chemical processes were used rather than using mixing of ceramic powders to obtain porous Al₂O₃/SiC ceramic foams. A slurry was prepared by mixing aluminium sulphate and ammonium sulphate in the water, and silicon carbide powder was added into the slurry so that a uniform mixture of Al₂O₃/SiC cake could be produced. The resulting product was (NH₄)₂SO₄·Al₂(SO₄)₃·24H₂O plus silicon carbide particles (SiC_p) after dissolving chemicals in the water. This product was heated up in a ceramic crucible in the furnace. With the effect of heat it foamed and Al₂O₃/SiC cake was obtained. Resulting Al₂O₃ grains were arranged in a 3D honeycomb structure and the SiC particles were surrounded by the alumina grains. Consequently, homogeneous powder mixing and porosity distribution were obtained within the cake. The morphology of the powder connections was networking with flake like particles. These alumina particles resulted in large amounts of porosity which was desired for ceramic preforms to allow liquid metal flow during infiltration. The resulting high porous ceramic cake (preform) was placed in a sealed die and liquid aluminium was infiltrated by Ar pressure. The infiltration was achieved successfully and microstructures of the composites were examined.     

Sintering characteristic,crystal structure and microwave dielectric properties of a novel thermally stable ultra-low-firing Na2BiMg2V3O12 ceramic

A novel ultra low-firing microwave dielectric ceramic with the composition of Na₂BiMg₂V₃O₁₂ was fabricated by a solid state reaction method. The phase structure, sintering behavior and microwave dielectric properties of ceramics were investigated. The ceramic can be well densified at 660 °C for 4 h. X-ray diffraction data show that Na₂BiMg₂V₃O₁₂ has a cubic garnet structure with lattice parameters of a = 12.4929 Å, V = 1,949.80 Å³, Z = 8 and ρ = 4.42 g/cm³. The microwave dielectric properties are strongly related to the density and morphology of the samples. The ceramic sintered 660 °C exhibits good microwave dielectric properties with ε_r = 23.2, Q × f = 3,700 GHz and τ _f  = 8.2 ppm/°C. These results indicate that Na₂BiMg₂V₃O₁₂ ceramic is a candidate for low temperature co-fired ceramics devices, such as chip multi-layer LC filter, microstrip bandpass filter, multilayer antenna.     

Redox intercalation of alkylammonium ions into VOAO4.nH2O (A=P,As)

The layered, mixed-oxide hydrates, V^VOAsO₄.3H₂O and V^VOPO₄.2H₂O react with alkylammonium iodides to form (RNH₃)_xV^IV_xV^V_1?xOAO₄.H₂O, where R=PhCH₂, or C_nH_2n+1 (n=3 to 8), and x= .4 to .6The intercalate materials exhibit a layered structure derived from the starting solids. After reaction the latter undergo a lamellar swelling which is in agreement with a “pillar” arrangement of alkylchains.     

Electric isolation of porous silicon by electro generated polyethyleneimine film,comparison to thermal oxide

Electrochemical oxidation of diethylenetriamine (DETA) charged with 0.1 M LiSO₃CF₃, leads to a polymeric coating (polyethyleneimine film: PEI) on the porous silicon (PS) surface. The obtained passivated electrode is characterized through IRTF spectrometry and through C(V) curves on PS/PEI structures showing good insulating properties of the PEI film. The passivated electrode presents a linear pH sensitivity close to 62 mV per pH unit. Such sensitivity is higher compared to flat PEI passivated silicon electrode. The results were fitted with the generalized site-binding theory and the following parameters were found: the surface site density N_s was found to be equal to 3.6 × 10¹⁶ cm^− 2 and pK_H (K_H acido-basic constant of the amino group) is − 5.54.     

Plastic rotational factor and J-COD relationship of three point bend specimen

COD calculation formulas in DD(19) and BS 5762 have been compared theoretically. It has been shown that differences between the results of these calculation formulas generally are about 10%.An experimental method for determining the plastic rotational factor r_p of three point bend specimen has been described. This method uses two clip gauges and records q-V and q-V plots simultaneously, and r_p can be determined from the slope of q-V curve. Experimental results of three steels show that under smaller amount of crack growth, the r_p remains a constant about 0.45 and is not dependent on a/W noticeably.Optical microscopic observations on half-thickness sections of specimens show that under small amount of crack growth observed fatigue precrack tip opening displacement and crack tip opening displacement calculated by using formula with r_p = 0.45 are in agreement.P-V and P-q plots were simultaneously obtained on the same specimen. J integral values and COD values were obtained from both P-V and P-q plots. Two series of J integral values and two series of COD values of three steels are in agreement within 5%. Experimental data of J_p and δ_p of three steels were used for checking J-COD relationship within 8%.     

Synthesis,microstructure, and mechanical properties of boron suboxide materials doped with chromium boride

Improvement on the densification and fracture toughness of ceramic materials based on boron suboxide (B₆O) has been of great importance. The mechanical properties of B₆O with and without chromium boride addition, hot pressed at a temperature range of 1850–1900°C and pressures of 50–80?MPa for 20 minutes, were investigated. The relative density, phase relationship, microstructures and mechanical properties of the processed ceramics were examined. More than 96% of the theoretical density was obtained for both ceramic systems. A good combination of mechanical properties was obtained with the B₆O-CrB₂ material (H_V?=?32.1?GPa, K_IC?=?4.5?MPa?·?m^0.5) compared to the pure B₆O material (H_V?=?30.5?GPa, K_IC?=?brittle). The addition of 1.7?wt.% CrB₂ resulted in a pronounced improvement in both the hardness and fracture toughness values. Crack bridging and deflection are some of the toughening mechanisms liable for the enhanced fracture toughness of the sintered ceramic materials.     

Enhanced performance of sandwich structure Pb0.8La0.1Ca0.1Ti0.975O3 thin film for pyroelectric applications

Using the advantages of low-temperature crystallization and high orientation in Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ (PLCT) film, a dense PLCT/porous PLCT/dense PLCT sandwich structure was obtained in the present study. It is found that dense PLCT layer can both sustain the porous density in the core layer and also lead to better preferential orientation of the sandwich structure. In the sandwich structure, low dielectric constant (ε_r = 43) and leakage current density (J < 9 × 10^− 5A/cm²) are simultaneously achieved. Because of high orientation in sandwich structure, the pyrocoefficient (p > 185 µC/m² K) is still keeping a relatively large value. The resulting high figure of merit (F_V′ = 4.5 µC/m² K, F_d′ = 228 µC/m² K) make the sandwich structure films good candidate for pyroelectric thin-film devices.     

Structural and piezoelectric properties of barium-modified lead-free (K0.455Li0.045Na0.5)(Nb0.9Ta0.1)O3 ceramics

Ferroelectric (K_0.455Li_0.045Na_0.5)(Nb_0.9Ta_0.1)O₃ + x mol% BaCO₃ ceramic compositions with Ba²⁺ as an A-site dopant in the range of x = 0–1.2 mol% were synthesized by conventional ceramic processing route. Effect of Ba²⁺ content on the microstructure, ferroelectric, dielectric, and piezoelectric properties of the ceramics was investigated. The results of X-ray diffraction reveal that Ba²⁺ diffuse into the (K_0.455Li_0.045Na_0.5)(Nb_0.9Ta_0.1)O₃ lattices to form a solid solution with a perovskite structure having typical orthorhombic symmetry. As Ba²⁺ content increases, cell volume and tetragonality increase in the crystal structure of the ceramics. Increasing doping level of Ba²⁺ inhibits grain growth in the ceramics and reduces both the Curie temperature (T _c) and tetragonal–orthorhombic phase transition temperature (T _o-t). The bulk density, remnant polarization P _r, room-temperature dielectric constant (ε′_RT), planar electromechanical coupling factor k _p , and piezoelectric charge coefficient d ₃₃ are found to increase as Ba²⁺ concentration increases from 0 to 0.8 mol% and then decrease as Ba²⁺ content increases further from 0.8 to 1.2 mol%. High piezoelectric properties of d ₃₃ = 187 pC/N and k _p  = 48 % are found in 0.8 mol% Ba²⁺ composition. Optimum amount of Ba²⁺ dopant takes the polymorphic phase boundary region consisting of orthorhombic and tetragonal crystal structures of the ceramic system near the room temperature and enhances its piezoelectric properties.     

Preparation of Aluminum Nitride Ceramics by Aqueous Tape Casting

High compacted aluminum nitride (AlN) substrates were prepared by aqueous tape casting. Aluminium dihydrogen phosphate was used to modify AlN particle surface from hydrolysis during casting process. The absolute value of zeta potential for the AlN particle treated by Al(H₂PO₄)₂ was 60mv at pH = 9. The slurry suitable for tape casting was prepared which possessed shear-thinning behavior and appropriate viscosity. The results showed that the green tapes own excellent properties including high bulk density, uniform pore distribution and low porosity. The samples were fired at 1700°C (lower than melting point of Pt) with pressureless sintering process which can be used to fabricate complex device.     

Fourier series based reliability analysis of aeroengine turbine blade under linear fuzzy safety state

A Fourier series method is established to compute fuzzy reliability of aeroengine turbine blade. Based on Fourier series of Normal probability density function (pdf), reliability analysis is carried out under linear fuzzy safety states defined by forms I–III. For aeroengine turbine blade root flange subjected to Normal random variables using form I, fuzzy reliability (P_r) evaluated by Fourier series integration is 0.999787. Moreover in blade edge using form II, Fourier series P_r is 0.999684 by balancing between the fluctuation level and the series number. Furthermore for blade connection under Normal distributed stress using form III, Fourier series P_r is 0.918523. Caused by undetermined factors, error ratios of numerical results range from 1.31 × 10⁻⁵ to 2.38 × 10⁻⁴. As the exactness, convergence and computational capability of Fourier series method are validated, it can be used in fuzzy reliability of aeroengine turbine blade.     

On the Structure of the Superfluid State and Quasiparticle Spectrum in a Bose Liquid with a Suppressed Bose–Einstein Condensate

A self-consistent model of the superfluid (SF) state of a Bose liquid with strong interaction between bosons and a weak single-particle Bose–Einstein condensate (BEC) is considered. The ratio of the BEC density n ₀ to the total particle density n of the Bose liquid is used as a small parameter of the model, n ₀/n?1, unlike in the Bogolyubov theory of a quasi-ideal Bose gas, in which the small parameter is the ratio of the number of supracondensate excitations to the number of particles in an intensive BEC, (n?n ₀)/n ₀?1. A closed system of nonlinear integral equations for the normal ~Σ₁₁(p, ω) and anomalous ~Σ₁₂(p, ω) self-energy parts is obtained with account for terms of first order in the BEC density. A renormalized perturbation theory is used, which is built on combined hydrodynamic (at p→0) and field (at p≠0) variables with analytic functions ~Σ _ij (p, ε) at pε0 and ε→0 and a nonzero SF order parameter ~Σ₁₂(0, 0)≠0, proportional to the density ρ _s of the SF component. Various pair interaction potentials U(r) with inflection points in the radial dependence and with an oscillating sign-changing momentum dependence of the Fourier component V(p) are considered. Collective many-body effects of renormalization (“screening”) of the initial interaction, which are described by the bosonic polarization operator Π(p, ω), lead to a suppression of the repulsion [V(p)<0] and an enhancement of the effective attraction [V(p)<0] in the respective domains of nonzero momentum transfer, due to the negative sign of the real part of Π(p, ω) on the “mass shell” ω=E(p). In the framework of the “soft spheres” model with the single fitting parameter—the value of the repulsion potential at r=0—the quasiparticle spectrum E(p) is calculated, which is in good accordance with the experimental spectrum E _exp(p) of elementary excitations in superfluid ⁴He. It is shown that the roton minimum in the quasiparticle spectrum is directly associated with the first negative minimum of the Fourier component of the renormalized (“screened”) potential of pair interaction between bosons.     

Microstructure,ferroelectric, piezoelectric and ferromagnetic properties of BiFeO3–BaTiO3–Bi(Zn0.5Ti0.5)O3 lead-free multiferroic ceramics

Multiferroic ceramics of (0.70?x)BiFeO₃–0.30BaTiO₃–xBi(Zn_0.5Ti_0.5)O₃ + 1 mol% MnO₂ with perovskite structure were prepared by a conventional ceramic technique and the effects of Bi(Zn_0.5Ti_0.5)O₃ doping and sintering temperature on the microstructure, multiferroic and piezoelectric properties of the ceramics were studied. All the ceramics possess a pure perovskite structure and no second phases can be detected. After the addition of a small amount of Bi(Zn_0.5Ti_0.5)O₃ (x ≤ 0.05), the ferroelectric and piezoelectric properties of the ceramics are improved and the grain growth is promoted. However, excess Bi(Zn_0.5Ti_0.5)O₃ (x ≥ 0.10) retards the grain growth, degrades the ferroelectricity and piezoelectricity, and induces two dielectric anomalies at high temperature. The ceramics can be well sintered at the very wide range of low sintering temperatures (880–980 °C) and exhibit good densification (relative density: 96.2–98.4 %) and strong electric insulation. The increase in the sintering temperature promotes the grain growth and improves the ferroelectricity of the ceramics. The ceramic with x = 0.05 sintered at 880–980 °C possesses improved ferroelectric and piezoelectric properties with remanent polarizations P _r of 21.9–28.1 μm/cm², piezoelectric constants d ₃₃ of 125–139 pC/N and planar electromechanical coupling factors k _p of 30.1–32.4 %, and high Curie temperatures T _C of 523–565 °C. A weak ferromagnetism with remanent magnetizations M _r of 0.0411–0.0422 emu/g and coercive fields H _c of 1.70–1.99 kOe were observed in the ceramics with x = 0–0.025.