Excess ZnO Research Articles

Effect of Porous Structure on the Magnetic Properties of NixMgyZn1−x−yFe2O4 Magnetic Materials

We deal with the preparation of NiMgZnFeIII-SO4 layered double hydroxides (LDHs) with the layered precursor method and introduce excessive ZnO into the NiMgZnFeIII-SO4 LDHs to produce NixMgyZn1−x−yFe2O4 ferrites that contain massive ZnO. Then the NixMgyZn1−x−yFe2O4 ferrites are treated with NaOH solution to remove ZnO to produce the porous NixMgyZn1−x−yFe2O4 magnetic material: when y = 0, porous NiZnFe2O4 ferrite magnetic materials are obtained; when y ≠ 0, porous NiMgZnFe2O4 ferrite magnetic materials are obtained. From analyses of these two ferrites, their pore-forming mechanism and comparison of their properties before and after they undergo the alkali treatment, we find that after being treated by the NaOH solution, NiZnFe2O4/NiMgZnFe2O4 have better uniform-structure pores, which will greatly expand their pore volume, widen their application scope and improve their magnetic properties.

Preparation and characterization of ZnO/TiO 2 for the photocatalytic reduction of Cr(VI) in aqueous solution

ZnO/TiO 2 photocatalysts coupled with various ZnO dosages (0.5–10 mol%) were prepared by wetness impregnation method and characterized in this study. X-ray photoelectron spectroscopy results indicated that zinc ions did not enter TiO 2 lattice and was more likely to bond with oxygen atoms to form ZnO coupled on the surface of TiO 2. The coupling of ZnO on the surface of TiO 2 retarded the crystal phase transformation from anatase to rutile phase for TiO 2 calcined at temperatures higher than 500 °C. The specific surface area of ZnO/TiO 2 calcined at 700 °C was notably higher than that of pure TiO 2 possibly because the aggregation and agglomeration of particles were inhibited by the indirect contact of TiO 2 particles during the calcination. Photocatalytic reduction of hexavalent chromium, Cr(VI), in aqueous solutions using ZnO/TiO 2 was evaluated under 365 nm UV light illumination. ZnO/TiO 2 containing 2.0 mol% ZnO exhibited the lowest photoluminescence emission intensity and the maximum photocatalytic reduction of Cr(VI) in aqueous solutions due to the enhancement of charge separation by interparticle electron transfer. However, excessive ZnO on the surface of TiO 2 may serve as recombination centers of electron–hole pairs to decrease the photocatalytic activity of ZnO/TiO 2. Moreover, the decreased photocatalytic reduction of Cr(VI) using ZnO/TiO 2 calcined at higher temperatures was assumed to be attributed to the decrease of specific surface areas.

Sintering Behavior and Conductivity Study of Yttrium‐Doped BaCeO3–BaZrO3 Solid Solutions Using ZnO Additives

The effect of ZnO on the crystal structure, sintering behavior, and electrical conductivity of yttrium‐doped BaCeO3–BaZrO3 was investigated by unfixing or fixing the yttrium content noted as BaCe0.5Zr0.3Y0.2−xZnxO2.9−0.5x and BaCe0.5Zr0.3Y0.2O2.9+yZnO, respectively. Studies on the two series revealed that BaO·ZnO eutectic, rather than ZnO, was responsible for the sintering densification. For BaCe0.5Zr0.3Y0.2−xZnxO2.9−0.5x, the evaporation of ZnO·BaO eutectic was observed after sintering at 1300°C for 10 h, and few impurities were detected by XRD with x<0.20. For BaCe0.5Zr0.3Y0.2O2.9+yZnO, the concomitant loss of BaO with ZnO caused A‐site deficiency and led to impurities of Y2O3 for y=0.08 and 0.14, and Y2BaZnO5 for y=0.20 during the sintering. For both series, ZnO enhanced the relative density, which was above 97% with x or y varying from 0.02 to 0.08. Energy‐dispersive X‐ray spectroscopy analysis revealed that ZnO hardly entered the perovskite phase. The conductivity study also suggested that ZnO did not serve as a dopant and that yttrium content was essential for sustaining a high ionic conduction. Excessive ZnO was especially detrimental to the grain boundary conduction and thus lowered the total electrical conduction. The optimized composition of BaCe0.5Zr0.3Y0.2O2.9+0.04ZnO has been obtained, with both a high relative density (∼98.5%) and a high electrical conductivity (1.35 × 10−2 S/cm at 600°C).

Phase Transformation Behavior of Bi<sub>2</sub>O<sub>3</sub>-ZnO-Nb<sub>2</sub>O<sub>5</sub> Ceramics Sintered at Low Temperature

To co-fire with commercial LTCC (Low Temperature Co-fired Ceramic) materials at 850oC ~ 880 oC, different contents of B2O3 were added to the Bi2O3-ZnO-Nb2O5 (BZN) ceramics. The dielectric properties of BZN ceramics sintered at low temperatures were studied. According to the test results, the cubic phase of BZN was transformed into orthorhombic in all the test materials. A BiNbO4 phase was formed in test materials with 2 ~ 5 wt% of B2O3 addition. The BiNbO4 phase was inhibited by extra ZnO addition. The phase transformation of cubic BZN was controlled during the synthesis process of cubic and orthorhombic ZnO-Nb2O5 phase with excess ZnO content. The Cubic and orthorhombic phases of BZN could coexist and be sintered densely at 850 oC/2hr.

Influence of Non‐Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: III. Effect of the Muffling Environment

The effect of different muffling environments on the structure and dielectric losses of Ba(Zn1/3Nb2/3)O3 (BZN) microwave ceramics was investigated. The microwave dielectric losses of stoichiometric BZN pellets heated in ZnO‐rich environments were severely degraded (e.g. Q×f∼15 000 in ZnO powder) compared with samples muffled in their own powder (Q×f∼80 000). Structure analyses and gravimetric measurements confirmed that the ceramics muffled in ZnO powder or vapor absorb excess ZnO to form non‐stoichiometric solid solutions with reduced cation order and Q. By using starting compositions in the (1−x)BZN−(x)BaNb4/5O3 binary (x=0.04), the stoichiometry can be tailored to ensure that after the uptake of ZnO, the ceramics remain well ordered and are located in a high Q region of the system. For example, ZnO‐vapor‐protected (0.96)BZN−(0.04)BaNb4/5O3 reached a very high Q×f (∼1 05 000) after sintering at 1400°C for 5 h.

Effects of excess ZnO addition on La-doped PZN–PZT ceramics prepared by hot pressing

Effects of excess ZnO addition on phase structure, dielectric and piezoelectric properties in La-doped Lead zinc niobate, Pb(Zn 1/3Nb 2/3)O 3 (PZN)–PZT systems have been investigated. Addition of excess ZnO helps to suppress the formation of the pyrochlore phase. Microstructure revealed the grains size to decrease with excess ZnO addition. Enhanced T max and d 33, and reduced ɛ max and Tan δ were observed. Adding proper excess ZnO in La-like doped PZN-based systems seems an effective approach to suppress the formation of the pyrochlore phase, enhance piezoelectric constant d 33 and reduce dielectric loss.

Excess ZnO Addition in Pure and La‐Doped PZN–PZT Ceramics

The effects of excess ZnO addition on phase structure, dielectric and piezoelectric properties in pure and La‐doped lead zinc nicobate (PZN)—lead zirconate titanate (PZT) systems have been investigated. Addition of excess ZnO helps suppress and prevents the formation of pyrochlore phase for both systems. However, the effects on electrical properties in the systems were very different, even opposite. For the PZN–PZT system, excess ions possibly exist as second‐phase inclusion particles. Based on Raman spectra and the changes in properties, in the La‐doped PZN–PZT system, excess ions are very likely to enter A‐site lead vacancies. Adding proper excess ZnO in La‐like‐doped PZN‐based systems is an effective approach to achieve a pure perovskite phase, enhanced piezoelectric constant d33, and reduced dielectric loss.

Excess ZnO addition in PZN–PLZT ceramics

Abstract0.3PZN–0.7PLZT (5/55/45) ceramics with and without nonstoichiometric excess ZnO addition were prepared by a conventional mixing method. The phase structure, microstructure, and dielectric and piezoelectric properties were investigated. Improvements of performances were observed. Excess ZnO could limit the formation of pyrochlore phase and enhance the electrical properties and the degree of the diffuse phase transition. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low Temperature Sintering and Piezoelectric Properties in Pb(ZrxTi1-x)O3–Pb(Zn1/3Nb2/3)O3–Pb(Ni1/3Nb2/3)O3 Ceramics

High-power ceramics of the composition 0.8Pb(Zr0.5Ti0.5)O3–0.2Pb[(Zn0.8Ni0.2)1/3Nb2/3]O3 (0.8PZT–0.2PZNN) + 0.5 wt% MnO2 have excellent piezoelectric and dielectric properties such as d33=320 pC/N, ε33T/εo=1227, kp=0.54, and Qm=1265. However, the sintering temperature of this composition is high, approximately 1200°C, restricting the use of cheap base metal electrodes in the fabrication of multilayer components. In this study, we provide a solution to this problem and report a low sintering temperature high-power composition in a PZT–PZNN system. CuO and Al2O3 were selected as sintering agents. It was found that the addition of CuO can reduce the sintering temperature to 920°C, but it can also significantly reduce the piezoelectric properties. The reduction of Qm magnitude was 81% when 0.5 wt% CuO was added. The addition of Al2O3 as a sintering agent was found to improve Qm magnitude but significant reductions in piezoelectric and dielectric properties were observed. The addition of excess ZnO and NiO was found to improve the density and piezoelectric properties, but it was also found to reduce Qm magnitude. Consequently, the optimization of the concentration of the sintering agents was carried out, and the optimized low sintering temperature high-power composition was found to be 0.875Pb(Zr0.5Ti0.5)O3–0.125Pb[(Zn0.8Ni0.2)1.5/3Nb2/3]O3 + 0.5 wt% MnO2 + 0.2 wt% CuO + 0.3 wt% Al2O3. The piezoelectric properties of the specimens of this composition sintered at 920°C were found to be kp=0.581, Qm=809, d33=345 pC/N, ε3T/εo=1345, and Tc=330°C.

Ordering and quality factor in 0.95BaZn1/3Ta2/3O3–0.05SrGa1/2Ta1/2O3 production resonators

Abstract Ordering of the B-site cations in UMTS (universal mobile telecommunications systems) standard resonator pucks composed of perovskite structured, 0.95BaZn 1/3 Ta 2/3 O 3 –0.05SrGa 1/2 Ta 1/2 O 3 (BZT–SGT) has been investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and powder neutron diffraction (PND). XRD patterns from samples sintered at 1550 °C/2 h but annealed and quenched at 50 °C intervals between 1400 and 1600 °C revealed that the order–disorder phase transition was at ∼1500 °C. In addition, a peak at ∼29.5° 2θ attributed to a Ba 8 ZnTa 6 O 24 phase was present due to ZnO loss. Electron diffraction patterns revealed that samples heat treated ⩾1500 °C (including as sintered samples, 1525 °C/2h,) exhibited short-range 1:2 ordering along all directions giving rise to an average short-range face centred cubic structure. Samples annealed and quenched from below 1500 °C showed 1:2 order. To avoid excessive ZnO loss, an annealing temperature was chosen at 1275 °C (for 24 and 168 h). Neutron diffraction data were best refined using two ordered BZT phases with slightly different lattice parameters. TEM revealed a microstructure in each case consisting of 1:2 small ordered domains in the centre of all grains but with every second grain exhibiting a concentric shell composed of an ordered single domain, containing elongated translational (APBs) but not orientational domains. The formation of the concentric ordered shell was attributed to grain boundary migration during grain growth. As-sintered samples gave unloaded quality factors ( Q )=54,000 at 2 GHz which rose to 78,000 at 2 GHz after annealing for 24 h. No further improvement in Q was observed for longer annealing times.

Growth of High‐Quality Pb((Zn1/3Nb2/3)0.91Ti0.09)O3 Single Crystals by Excess ZnO Addition

A high‐quality single crystal of Pb((Zn1/3Nb2/3)0.91Ti0.09)O3 (PZNT 91/9), 40 mm in diameter and 20 mm in length, was successfully grown using the solution Bridgman method with a slight excess amount of ZnO. High‐quality wafers were sliced from the light‐brown single crystal. No PbO inclusions or opaque areas were observed in the transparent wafers. An array probe for echocardiography was constructed with the single‐crystal wafer, and its superior performance was demonstrated.

Negative ion resputtering in Ta2Zn3O8 thin films

Thin films have been deposited by radio frequency magnetron sputtering from a stoichiometric Ta2Zn3O8 ceramic target (3ZnO+1Ta2O5). Negative ion resputtering effects have been observed in the stoichiometric Ta2Zn3O8 target and have been attributed to O− ion formation from primarily Ta–O bonds. Zinc deficient thin films were deposited as a result of the preferential resputtering of Zn versus Ta. The negative ion resputtering effects are exacerbated at higher powers and lower pressure. This observation is correlated to the oxygen ion transport through the dark space and the plasma, which ultimately controls the energy distribution of the oxygen particles that arrive at the substrate. Another ceramic target with excess ZnO (6ZnO+1Ta2O5) was also sputtered under similar conditions, however predominantly ZnO films were deposited. Finally, a mosaic ZnO–Ta target was sputtered, which resulted in stoichiometric and luminescent thin films.

An efficient catalyst for the production of isobutanol and methanol from syngas. XI. K‐ and Pd‐promoted Zn/Cr/Mn spinel (excess ZnO)

A 2.25 wt% K‐ and 6.0 wt% Pd‐promoted Zn/Cr/Mn spinel (excess ZnO) higher alcohol synthesis (HAS) catalyst has been tested for the production of isobutanol and methanol at 440°C and 1500 psig. An isobutanol production rate of 179 g kg−1 h−1, a methanol‐to‐isobutanol mole ratio of 1.3 and a total alcohol rate of 304 g kg−1 h− are attained. This ratio is slightly higher than the desired value of 1.0 for MTBE production. These results are superior to others presented in the literature thus far. Surface characterization data indicate that the near‐surface region of the catalyst consists primarily of ZnO and K. Pretreating the catalyst in a reductive environment similar to that given to the catalyst before reaction tests, causes an enrichment of the K promoter at the surface. The product stream composition does not significantly change during five days of testing.

Reaction and Surface Characterization Study of Higher Alcohol Synthesis Catalysts: IX. Pd- and Alkali-Promoted Zn/Cr-Based Spinels Containing Excess ZnO

A Zn/Cr spinel support material was prepared which contains excess ZnO and then was promoted with 5.9 wt% Pd and varying amounts ranging from 0 to 7 wt% of either K or Cs. Each of these catalysts was tested at four different reactor operating conditions (T of 400 or 440°C and P of 1000 or 1500 psig) for higher alcohol synthesis (HAS) using a syngas feedstream (1 : 1 CO : H2) after reductive pretreatment. High isobutanol production rates in conjunction with low methanol-to-isobutanol mole ratios (≤1.0) and low hydrocarbon byproduct rates are desired. For the K-promoted catalysts the highest isobutanol production rates are obtained at the higher pressure and temperature settings of 1500 psig and 440°C, and methanol-to-isobutanol mole ratios below the ideal value of 1.0, which is required for downstream methyl tertiary-butyl ether (MTBE) synthesis, are obtained. The Cs-promoted catalysts generally yield higher isobutanol production rates than the K-promoted catalysts. The highest isobutanol production rate of 170 g/kg-h is obtained using the 3 wt% Cs-promoted catalyst at 1000 psig and 440°C. The lower pressure is economically advantageous with regard to process costs. Most importantly, this isobutanol production rate is quite high compared to others presented in the literature and demonstrates that Pd does enhance the synthesis of isobutanol. The catalytic activity remained stable over a 5-day test period for each catalyst. X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) were used to characterize these catalysts. The results obtained from these techniques indicate that pretreating the catalysts in 1×10−7Torr of H2at 300°C for 4 h causes an enrichment of the near-surface alkali-promotor concentration. A layered structure forms which consists of this enriched alkali layer and an enriched Pd and ZnO layer which reside above the Zn/Cr spinel support material. The reaction process decreases the thickness of the alkali and ZnO layers and seems to trap the Pd in a thin layer above the support material but below the alkali promotors which reside at the surface. The surface areas of these catalysts are about 80 m2/g, and they are stable with aging. The primary role of the Zn/Cr spinel is to provide a high surface-area support for the promoted ZnO.

98/02002 Higher-alcohol synthesis reaction study. V. Effect of excess ZnO on catalyst performance : Minahan, D. M. et al. Appl. Catal. A, 1998, 166, (2), 375–385

98/02002 Higher-alcohol synthesis reaction study. V. Effect of excess ZnO on catalyst performance : Minahan, D. M. et al. Appl. Catal. A, 1998, 166, (2), 375–385

Higher-alcohol synthesis reaction study V. Effect of excess ZnO on catalyst performance

A nonpromoted, K-promoted, and Cs-promoted 1 : 1 Zn/Cr spinel as well as K- and Cs-promoted Zn/Cr spinel catalysts containing excess ZnO were tested for higher alcohol synthesis (HAS) using a syngas feedstream (1 : 1 CO : H 2). Two reactor operating pressures, 1000 and 1500 psig, and two reactor bed temperatures, 400 and 440°C, were used in order to determine the influence of these operating parameters, and the effects of promotor concentrations and excess ZnO on the product stream composition. Of the catalysts tested, the 3 wt% Cs/Zn/Cr catalyst with excess ZnO yields the highest isobutanol production rate of 171 g/kg h at 440°C and 1500 psig. Although lower methanol-to-isobutanol mole ratios were attained, this catalyst yields a ratio of 1.4 which is nearly ideal for downstream synthesis of methyl tertiary-butyl ether (MTBE). Superior performance is achieved using Cs as the promotor on the Zn/Cr spinels in comparison to the K promotor. Lower hydrocarbon production rates and corresponding increased selectivities to total alcohols, however, are obtained using K as the promotor of the 1 : 1 Zn/Cr spinel catalysts while Cs is more effective in reducing the hydrocarbon production in the presence of excess ZnO. The addition of excess ZnO to the Cs-promoted catalyst enhances not only the isobutanol rate but also results in increased selectivity, lower hydrocarbon production rates and lower methanol-to-isobutanol mole ratios at the higher Cs loadings. Higher isobutanol production rates are also obtained over the K-promoted Zn/Cr spinels containing excess ZnO in comparison to the K-supported 1 : 1 Zn/Cr spinels if 3 wt% or greater K addition is employed, but higher hydrocarbon production rates result in lower selectivities to the desired alcohol products.

Stabilization of perovskite phase and dielectric properties of Pb(Zn, Mg)1/3Nb2/3O3-PbTiO3 ceramics prepared by excess constituent oxides

The perovskite phase in PZN-PMN-PT (PbZn1/3Nb2/3O3-PbMg1/3Nb2/3O3-PbTiO3) pseudoternary ceramics was stabilized by the addition of excess constituent divalent oxides (PbO, MgO, and ZnO). 5 mol% excess MgO or 7.5 mol% excess PbO was sufficient to eliminate the remnant cubic pyrochlore phase after sintering at 1100 °C for 1 h. The enhanced diffuse phase transition (DPT) and the decrease in the electrical resistivity were observed in the presence of excess ZnO or MgO. These were interpreted in terms of the additional formation of negatively charged, short-range ordered 1: 1 domains with a concomitant generation of charge carriers (holes). The behavior of excess MgO or ZnO at concentrations above 5 mol% was studied by examining complex impedance patterns.

Roles of spinel and maghemite phases in the oxidative dehydrogenation of butene over iron complex oxides

Abstract Commercial catalysts for oxidative dehydrogenation of butene experience slight but noticeable deactivation, particularly, near the exit of adiabatic reactors. Thus the understanding of roles played by each individual phase in these catalysts will be beneficial for further improvement of commercial catalysts. Based on results shown in part I of this article, the derivative infrared spectroscopic method is used to examine the existence of small amounts of γ-Fe2O3 in oxidative dehydrogenation catalysts. The promotion effect of excess γ-Fe2O3 or MgO and the deleterious effect of excess ZnO in nonstoichiometric ferrite are demonstrated and explained in view of epitaxial growth of the two oxides and vacancy development along phase boundaries with ferrite spinels. A mechanism is suggested for the oxidative dehydrogenation of butene.

Temperature dependence of the cation distribution in zinc ferrite (ZnFe2O4) from powder XRD structural refinements

Zinc ferrite (ZnFe 2 O 4 ) has the cubic spinel structure with an (approximately) normal cation distribution. It was synthesised by direct reaction of the oxides at 800 ° C, and also by using a sodium tungstate flux with excess ZnO. Lattice parameters (a o ) of samples from both syntheses were emasured at room temperature, on samples annealed in air between 500 and 1200 ° C and then quenched into water, a o decreases from 8.4419(2) for samples annealed at 500 ° C, to 8.4400(2) for 950 ° C, and there is no significant difference between samples from either synthesis method. Annealing at higher temperatures causes no further decrease in a o in the quenched samples. The change of a o with time during isothermal annealing at 500 ° C established that a steady value was reached in about 24 hours. The cation distributions in quenched samples of the flux-grown material were determined by powder X-ray diffraction, using the Rietveld method of structural refinement. The degree of inversion in ZnFe 2 O 4 increases from 0.02(2) at 500 ° C to 0.19(2) at 950 ° C. Samples quenched from higher temperatures show no further increase in x, in agreement with the trend in a 0 . This is probably due to the rate of re-ordering being too fast to quench above ∼950 ° C. Fe Mossbauer spectra were collected for a few samples at 80K and 298K. All spectra show only a single doublet with hyperfine parameters characteristic of octahedral Fe 3+ . The tetrahedral Fe 3+ deduced from the XRD refinements cannot be resolved

Structure stabilization and dielectric properties of pzn ferroelectric ceramics

Abstract Structure stabilization and dielectric properties of Pb (Zn1/3 Nb2/3)O3 -PbTiO3 -BaTio3 (PZN-PT-BT) ferroelectric ceramics have been carefully studied. Around morphotropic phase boundary (MPB) of PZN-PT system, heterovalant substitution of K+ for Pb2+ stabilizes the pesrovskite Structure, while substitution of La3+ for Pb2+ goes toward the other end. Nonstoichiometry addition of excess ZnO or BaO can also suppress the formation of pyrochlore phase.