Influence of H2-N2 atmosphere composition and annealing duration on the selective surface oxidation of low-carbon steels

The selective surface oxidation of the Ti-stabilized interstitial free (Ti-IF) and dual phase (DP) steel during recrystallization annealing at 820 °C under the variety of exposure time and composition of protective H₂-N₂ atmosphere at low dew point (−40 °C) was examined. It was found that Mn, Al, Si and Cr oxides are formed in all cases, but higher Mn and Si concentration in DP steel leads to an increase in a density of coarse nonwettable MnSiO₃ particles. An increase in annealing time enhances surface coverage with oxides, while increased H₂ content in gas atmosphere results in less coverage with external oxides improving the wettability. The influence of the stronger reducing atmosphere is more exhibit on Ti-IF steel where after 120 s of annealing in 15vol.%H₂-85vol.%N₂ atmosphere the wetting angle θ stabilized to a value of the order of 6°. The presence of surface BN and the TiN particles also affects the full surface coverage by external oxides.     

Identification by photoelectrochemistry of oxide phases grown during the initial stages of thermal oxidation of AISI 441 ferritic stainless steel in air or in water vapour

Room temperature photoelectrochemistry was used to characterise oxide phases grown during the initial stages of oxidation of the ferritic stainless steel AISI441 at 650°C and 850°C in synthetic air or in water vapour. Grazing incidence X-ray diffraction and Raman spectroscopy were additionally used to discuss PEC results. Haematite Fe₂O₃ (∼2.0 eV), chromia Cr₂O₃ (3.0 and 3.5 eV) and their mutual solid solution (∼ 2.5 eV) were detected by their respective bandgap values determined from photocurrent vs. energy curves. The Cr/Fe ratio of the films increased with time/temperature and was higher in air-grown than in H₂O-grown oxides. Observation of photocurrent vs. potential curves indicated that chromia was N-type in all specimens, resulting from thermodynamic equilibrium with the metallic substrate and not with the gas phase.     

Enhanced magnetization and magnetoelectric coupling in hydrogen treated hexagonal YMnO3

The single phase hexagonal YMnO₃ has been synthesized via sol-gel route by adopting two different sintering conditions. In one case, sintering has been done at ∼700 °C in Ar/H₂ atmosphere and in other case it has been done at ∼1250 °C in air. Magnetic measurements of the samples, synthesized by sintering at relatively lower temperature in Ar/H₂ atmosphere, show the enhanced ferromagnetic behaviour at 10 K. M-H curve shows that the value of saturation magnetization (M_s) at 10 K is 8.04 emu/g for Ar/H₂ sintered sample while it is 2.93 emu/g for the air sintered sample. Moreover, a weak ferromagnetic signal at room temperature has been observed in YMnO₃ compound. Magnetization versus magnetic field (M-H) curves of hydrogen treated samples, measured at room temperature, show small kink in the linear variation near origin, possibly due to presence of weak ferromagnetic interactions in the samples at room temperature. However, the polarization-electric field (P-E) curve shows weak ferroelectric characteristics for the Ar/H₂ sintered samples. It is suggested that the enhanced ferromagnetism in Ar/H₂ sintered sample originates from the presence of oxygen vacancies in the Ar/H₂ sintered samples. Moreover, the magnetoelectric coupling coefficient at room temperature is improved to 106 mV/cm Oe for Ar/H₂ sintered sample as compared to 96 mV/cm Oe for air sintered sample at 40 kHz ac magnetic field frequency.     

Microwave dielectric properties and its compatibility with silver electrode of Li2MgTi3O8 ceramics

The effects of BaCu(B₂O₅) (BCB) additions on the sintering temperature and microwave dielectric properties of Li₂MgTi₃O₈ ceramic have been investigated. The pure Li₂MgTi₃O₈ ceramic shows a relative high sintering temperature (∼1000 °C) and good microwave dielectric properties as Q × f of 40,000 GHz, ?_r of 27.2, τ_f of 2.6 ppm/°C. It was found that the addition of a small amount of BCB can effectively lower the sintering temperature of Li₂MgTi₃O₈ ceramics from 1025 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 0.5 wt% BCB added Li₂MgTi₃O₈ ceramic sintered at 900 °C for 2 h exhibited good microwave dielectric properties of Q × f = 36,200 GHz (f = 7.31 GHz), ?_r = 26 and τ_f = −2 ppm/°C. Compatibility with Ag electrode indicates this material can be applied to low temperature-cofired ceramics (LTCC) devices.     

Effect of sintering temperature and time on densification, microstructure and properties of the PZT/ZnO nanowhisker piezoelectric composites

Lead zirconate titanate (PZT) based piezoelectric composites embedded with ZnO nanowhiskers (ZnO_w) were investigated to clarify the optimal sintering condition for densification, microstructure, and electrical properties. The samples are characterized by X-ray diffraction analysis and scanning electron microscopy. The results show that the increase of the sintering temperature and time is quite effective in improving the densification and piezoelectric properties of the PZT/ZnO_w composites. However, the relative density and piezoelectric properties deteriorate as the composites are sintered over the optimal sintering condition. Particularly, the PZT/ZnO_w composites sintered at 1150 °C for 2 h show excellent electrical properties of piezoelectric constant d₃₃ ∼ 471 pC/N, relative dielectric constant ? ∼ 3838, planar electromechanical coupling factor k_p ∼ 0.543, remnant polarization P_r ∼ 23.2 μC/cm² and coercive field E_c ∼ 9.2 kV/cm.     

Low-temperature sintering and dielectric properties of high-permittivity microwave (Ca, Nd)TiO3 ceramics

The effect of H₃BO₃-CuO-Li₂CO₃ combined additives on the sintering temperature, microstructure and microwave dielectric properties of (Ca_0.61Nd_0.26) (Ti_0.98Sn_0.02)O₃ (CNTS) ceramics was investigated. The H₃BO₃-CuO-Li₂CO₃ combined additives lowered the sintering temperature of CNTS ceramics effectively from 1300 to 950 °C. This may be due to the interim liquid-phase of Li₂O-CuO-B₂O₃, which were formed in the sintering process. (Li_0.5Nd_0.5)TiO₃ (LNT) demonstrated an effective compensation in τ_f value of the low-fired CNTS ceramics. The 0.4CNTS-0.6LNT ceramics with 5 wt% (H₃BO₃-CuO)-0.5 wt% Li₂CO₃ sintered at 900 °C for 2 h shows excellent dielectric properties: ?_r = 90.6, Q × f = 3400 GHz, and τ_f = 9 ppm/°C. Also, the LTCC material is compatible with Ag electrode.     

Development of high speed steel sintered using concentrated solar energy

The steel powders were sintered under N₂–H₂ atmosphere in a solar furnace and in a Fresnel lens, after compaction of the green parts, using much higher heating and/or cooling rates as compared to conventional tubular furnace. The effects of processing parameters and the use of concentrated solar energy on densification and mechanical properties were analyzed. Experimental results demonstrated the activating effect of concentrated solar energy on the sintering process showing that an optimum densification is achieved at 1150 °C on both solar installations in just 90 min for the solar furnace and in 30 min in the case of Fresnel lens installation compared with an optimum temperature of 1290 °C in ∼10 h of total cycle in the conventional tubular furnace. Better mechanical properties were obtained using concentrated solar energy with microhardness measurements ranging between 800 and 900 HV. Microstructural analyses by scanning and transmission electron microscopy reveal the presence of submicron sized vanadium nitrides and other nanometer sized particles in the samples that could be responsible for the high hardness values obtained.     

Effect of partial substitution of Cr for Ni on densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-Ni-based cermets

Four cermets of composition TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32 − x)Ni-xCr (x = 0, 3.2, 6.4 and 9.6 wt%) were prepared, to investigate the effect of the partial substitution of Cr for Ni on densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-Ni-based cermets. The partial substitution of Cr for Ni decreased full densification temperature, and the higher the content of Cr additive was, the lower full densification temperature was. The partial substitution of Cr for Ni had no significant effect of the formation of Mo₂C and Ti(C,N) and the dissolution of WC, and however, it had a significant effect on the dissolution of Mo₂C. Cr in Ni-based binder phase diffused into undissolved Mo₂C to form (Mo,Cr)₂C above 1000 °C at 6.4-9.6 wt% Cr additive, and a small amount of (Mo,Cr)₂C did not dissolve after sintering at 1410 °C for 1 h at 9.6 wt% Cr additive. In the final microstructure, Cr content in Ni-based binder phase increased with increasing the content of Cr additive, and however, regardless of the content of Cr additive, coarse Ti(C,N) grains generally consisted of black core, white inner rim and grey outer rim, and fine Ti(C,N) grains generally consisted of white core and grey rim. The partial substitution of Cr for Ni increased hardness and decreased transverse rupture strength (TRS). Ni-based binder phase became hard with increasing the content of Cr additive, therefore resulting in the increase of hardness and the decrease of TRS. TRS was fairly low at 9.6 wt% Cr additive, which was mainly attributed hardening of Ni-based binder phase and undissolved (Mo,Cr)₂C.     

Reactive wetting during hot-dip galvanizing of high manganese alloyed steel

The present study discusses reactive wetting during hot-dip galvanizing of high Mn alloyed steel (X-IP1000, 23 wt.% Mn) and is focused on investigating the influence of the metallic Mn concentration in the steel bulk composition on phase formation at the interface steel/coating. Samples were in-line bright annealed (1100 °C/ 60 s in N2-5%H2 at DP −50 °C) prior hot-dipping to avoid external MnO on the steel surface. This approach was applied to avoid influencing the wetting reaction by an aluminothermic MnO reduction, because this is considered to lead to an unwanted zeta-phase (FeZn13) formation in the coating by hot-dipping of Mn alloyed steels (< 5.0 wt.% Mn). The influence of hot-dipping parameters, which are contributing to the kinetics of the wetting reaction, was examined in terms of varying bath-Al content (0.17 and 0.22 wt.%), bath temperature (440-500 °C) and strip entry temperature (420-520 °C). The structure and chemical composition of both galvanized coating and interface steel/coating were characterized. While external MnO was verifiably avoided, brittle zeta-phase distinctively appeared at the interface steel coating together with the typical Fe₂Al₅ phase. This shows that the model of aluminothermic MnO reduction failed in the present case. This study suggests an alternative model explaining the appearance of zeta-phase with the removal of bath-Al by metallic Mn, which is dissolved out of the steel bulk into the Zn bath. The present investigation shows that alloying elements in the steel bulk may influence coating quality not only “indirectly” by external formation of nonwettable oxides, but also “directly” by influencing phase equilibria and kinetics of the wetting reaction. Understanding these phenomena will improve processing of (high) alloyed steel concepts as well as industrial Zn bath management.     

Smart magnetodielectric nano-materials for the very high frequency applications

Cobalt and copper doped Ni-Zn nano-ferrite with a composition of Ni_0.4Zn_0.4Co_0.2Cu_0.02Fe_1.98O₄ are prepared by a coprecipitation method. The structural, electromagnetic and magnetic properties are investigated by means of X-ray diffraction, impedance analyzer and vibrating sample magnetometer, respectively. Samples are calcinated at 600 °C and then subjected to different sintering temperatures. After sintering at 900 °C for 5 h, the average crystalline size is found to be 38 nm. The material shows almost constant permeability and permittivity, in the frequency range from 10 to 200 MHz, equal to ∼10.8 (loss tangent ∼ 0.04) and ∼6.5 (loss tangent ∼ 0.006), respectively. Relaxation phenomenon takes place beyond 200 MHz. The refractive index n is close to 8.3, and the reduced impedance Z/Z₀ is close to 1.3. The persistent and higher value of permeability than that of permittivity along with low losses enables this material useful for the very high frequency applications.     

Low-loss microwave dielectrics using rock salt oxide Li2MgTiO4

Rock-salt-structured Li₂MgTiO₄ ceramic was prepared by the conventional mixed oxide route and its microwave dielectric properties were investigated. The microstructures of the ceramics were characterized by SEM. The dielectric properties of the ceramics exhibited a significant dependence on the sintering condition and crystal structure. A new microwave dielectric material, Li₂MgTiO₄ sintered at 1360 °C has a dielectric constant (?_r) of ∼17.25, a Q × f of ∼97,300 GHz (where f = 9.86 GHz, is the resonant frequency) and a τ_f of ∼-27.2 ppm/°C. The microwave dielectric properties of the ceramic are reported for the first time.     

Thermal stability of superhard Ti-B-N coatings

Ternary transition-metal boron nitride Ti-B-N offers outstanding hardness and thermal stability, which are increasingly required for wear resistant applications, as the protective coatings are subjected to high temperature, causing thermal fatigue. Ti-B-N coatings with chemical compositions close to the quasibinary TiN-TiB₂ tie line and boron contents below ∼ 18 at.% contain a crystalline supersaturated NaCl structure phase, where B substitutes for N. Annealing above the deposition temperature causes precipitation of TiB₂, which influence dislocation mobility and hence the hardness of TiB_0.40N_0.83 remains at a very high level of ∼ 43 GPa with annealing temperature T_a up to 900 °C. Growth of Ti-B-N coatings with B contents above ∼ 18 at.% results in the formation of nm sized TiN and TiB₂ crystallites embedded in a high volume fraction of disordered boundary layer. The compaction of this disordered phase during annealing results in a hardness increase of TiB_0.80N_0.83 coatings from the as-deposited value of ∼ 37 GPa to ∼ 42 GPa at T_a = 800 °C. Excess B during growth of TiB_2.4 coatings causes the formation of bundles of ∼ 5 nm wide TiB₂ subcolumns encapsulated in a B-rich tissue phase. This nanocolumnar structure is thermally stable up to temperatures of ∼ 900 °C, and consequently the hardness remains at the very high level of ~ 48 GPa, as nucleation and growth of dislocations is inhibited by the nm sized columns. Furthermore, the high cohesive strength of the B-rich tissue phase prevents grain boundary sliding.     

Electrochemical performance of microwave synthesized Nd1.8Ce0.2CuO4±δ cathode for intermediate temperature solid oxide fuel cell applications

The microwave combustion of reagents followed by microwave sintering resulted in superfine crystalline Nd_1.8Ce_0.2CuO_4±δ solid solution. The Nd_1.8Ce_0.2CuO_4±δ crystal lattice volume increases with the reduction of crystallite size. Increase in sintering temperature increases crystallite size and sintered density. A good interfacial contact between Ce_0.9Gd_0.1O₂ electrolyte and Nd_1.8Ce_0.2CuO_4±δ cathode forms to yield symmetric cells for electrochemical studies. The electrochemical impedance data obtained at various temperatures simulated using electrical equivalent circuit. The low-frequency response well fitted to Gerischer element. The sintering temperature 900 °C during the cathode preparation is optimized on the basis of least area-specific-resistance (ASR) = 1.19 Ω cm⁻² at 700 °C.     

The effect of manganese and chromium on surface oxidation products formed during batch annealing of low carbon steel strip

Surface oxides formed at ferrite grain boundaries of low carbon steels annealed at 700 °C in 5% hydrogen 95% nitrogen atmosphere were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two different oxides (Fe,Mn)O and MnCr₂O₄ are observed at the grain boundaries and the former is five-fold coarser than the latter. It was found at the annealing temperature of 700 °C that the mean particle size of the (Fe,Mn)O depends on the manganese content, and the mean particle size and distribution of the MnCr₂O₄ dependent on chromium, but independent of manganese. It is unlikely the coarse (Fe,Mn)O precipitates pose any potential risks to the electrolytic tin coating quality as they will be removed by the pickling operation prior to tinning. The potential risks posed by the MnCr₂O₄ to the quality of the electrolytic tin coating of tinplate products can be minimized by restricting the chromium content of the steel.     

Near room temperature magnetic entropy changes in as-cast Gd100−xMnx (x = 0, 5, 10, 15, and 20 at.%) alloys

A series of Gd_100−xMn_x (x = 0, 5, 10, 15, and 20 at.%) alloys were prepared by arc-melting. The Curie temperature (T_C) associated with the ferromagnetic-paramagnetic transitions, derived from M-T curves, show decrease in T_C for as-cast alloys (∼279 K) as compared to as-cast Gd (∼292 K). No appreciable decrease in the |ΔS_M|_max values ∼4.6 J/kg K (0-2 T) and ∼8.6 J/kg K (0-5 T) were observed upon alloying Gd with Mn up to x ≤ 15 at.%. Refrigerant capacity (q) showed negligible variation ∼195 J/kg (0-2 T) and ∼450 J/kg (0-5 T) with increasing Mn (up to x ≤ 15 at.%) content. Similar values of |ΔS_M|_max and q coupled with ∼13 K decrease in T_C for as-cast Gd_100−xMn_x (0 ≤ x ≤ 15) alloys as compared to Gd, suggests expansion of working temperature region of Gd upon alloying with Mn up to 15 at.%. Low cost, adjustable T_C, favorable magnetocaloric properties make Gd_100−xMn_x alloys potential candidates as second-order transition based magnetic refrigerants for near room temperature air-conditioning and magnetic refrigeration.     

Tailoring powder processing and thermal treatment to optimize the densification of W–CuO powder mixtures

W–CuO powder mixtures were prepared by attritor mixing/milling of commercial powders. The reduction steps during heating under He/H₂ gas flow were identified by thermogravimetric analysis (TGA). W–Cu powder mixtures with three different O-content were prepared by adjusting the reduction conditions in a furnace. The effect of O-content on the sintering of powder compacts was then especially investigated. Two sintering steps were identified by dilatometry during liquid phase sintering under He/H₂ atmosphere. The first step was associated to rearrangement after copper melting. The second step was related to the presence of W-oxides at the particle surface: shrinkage was enhanced and the second step was shifted to lower temperatures by using initial powders with low oxygen content, by decreasing the heating rate or by introducing a holding time at 1050 °C. This behaviour was related to a gradual reduction of W-oxides from the edge to the bulk of the samples. Microstructural observations were performed at different stages to confirm the analysis. Powder processing and thermal cycle were optimized to obtain materials with 96–97% relative density.     

The oxidation behavior of Fe-20Cr alloy foils in a synthetic exhaust-gas atmosphere

Oxidation tests of rare-earth-modified and Ti-modified Fe–20Cr alloy foils, which are under consideration for catalytic converter supports, were performed in a synthetic exhaust-gas atmosphere (N₂+H₂O+CO₂) between 900°C and 650°C. Between 900°C and 750°C, the rare earths had no effect on oxide growth rates while Ti increased growth rates. Oxide growth rates for the rareearth alloys at 800°C and 750°C are much lower than those found in the literature for oxidation of Fe–Cr alloys or pure Cr in O₂-rich atmospheres. The slow growth rates for the rare-earth alloys agree with literature data for oxidation of stainless steels containing >20% Cr in wet atmospheres and are caused by growth of an oxide scale only one grain thick. At temperatures 700°C, Fe–20Cr alloys grow massive Fe oxides; however, this can be suppressed by adding rare earths or Ti. To ensure good oxide adherence, free sulfur must be eliminated in the alloy by tying it up with a reactive-element addition. Both Ti and the rare earths can be used to tie up S, but the rare earths are more effective. For converter applications, the optimum alloy composition may contain rare earths for good oxide adherence and a small amount of Ti to suppress growth of Fe-rich oxides.     

Dense La0.9Sr0.1Ga0.8Mg0.2O2.85 electrolyte for IT-SOFC's: Sintering study and electrochemical characterization

This paper presents the sintering behaviour of a La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 coral-like microstructure powder. This is prepared by a successive freeze-drying and self-ignition process followed by calcination at 1200 °C during 1 h. This synthesis method gives great uniformity of the powder and allows shaping into compacts without requiring a grinding step. The grain size distribution (between 0.5 and 4 μm) favours a good sintering behaviour: open porosity disappear at 1400 °C and relative densities over 99% can be achieved after 6 h at 1450 °C. The same powder can also be sintered into a thin disc of ∼100 μm thickness. The characterization of the dense material by impedance spectroscopy shows that the activation energies below and above 600 °C are 1.0 eV and 0.7 eV, respectively. The conductivity at 800 °C is ∼0.11 S cm⁻¹. Special attention is devoted to the temperature range between 200 °C and 400 °C, where the intragrain and intergrain contributions can be distinguished. The analysis of the parameters describing the intragrain constant phase element in the equivalent circuit suggests that, above 325 °C, the system evolves from a distribution of relaxation time to only one relaxation time. The analysis of the data by the complexes permittivity show that ionic oxide conduction mechanism would occur in two steps. In the first, an oxygen vacancy would be released and, in the second, the migration of the ionic oxide would take place in the material.     

SmBaCoCuO5+x as cathode material based on GDC electrolyte for intermediate-temperature solid oxide fuel cells

The performance of SmBaCoCuO_5+x (SBCCO) cathode has been investigated for their potential utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The powder X-ray diffraction (XRD), thermal expansion and electrochemical performance on Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte are evaluated. XRD results show that there is no chemical reaction between SBCCO cathode and GDC electrolyte when the temperature is below 950 °C. The thermal expansion coefficient (TEC) value of SBCCO is 15.53 × 10⁻⁶ K⁻¹, which is ∼23% lower than the TEC of the SmBaCo₂O_5+x (SBCO) sample. The electrochemical impedance spectra reveals that SBCCO symmetrical half-cells by sintering at 950 °C has the best electrochemical performance and the area specific resistance (ASR) of SBCCO cathode is as low as 0.086 Ω cm² at 800 °C. An electrolyte-supported fuel cell generates good performance with the maximum power density of 517 mW cm⁻² at 800 °C in H₂. Preliminary results indicate that SBCCO is promising as a cathode for IT-SOFCs.     

Effect of addition of Ba(W0.5Cu0.5)O3 in Pb(Mg1/3Nb2/3)O3-Pb(Zn1/3Nb2/3)O3-Pb(Zr0.52Ti0.48)O3 ceramics on the sintering temperature, electrical properties and phase transition

In this work, we report on the Pb(Mg_1/3Nb_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃-Pb(Zr_0.52Ti_0.48)O₃ (PMN-PZN-PZT) ceramics with Ba(W_0.5Cu_0.5)O₃ as the sintering aid that was manufactured in order to develop the low-temperature sintering materials for piezoelectric device applications. The phase transition, microstructure, dielectric, piezoelectric properties, and the temperature stability of the ceramics were investigated. The results showed that the addition of Ba(W_0.5Cu_0.5)O₃ significantly improved the sintering temperature of PMN-PZN-PZT ceramics and could lower the sintering temperature from 1005 to 920 °C. Besides, the obtained Ba(W_0.5Cu_0.5)O₃-doped ceramics sintered at 920 °C have optimized electrical properties, which are listed as follows: (K_p = 0.63, Q_m = 1415 and d₃₃ = 351 pC/N), and high depolarization temperature above 320 °C. These results indicated that this material was a promising candidate for high-power multilayer piezoelectric device applications.