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1.
Grain growth of ZnO during the liquid-phase sintering of binary ZnO–Bi2 O3 ceramics has been studied for Bi2 O3 contents from 3 to 12 wt% and sintering from 900° to 1400°C. The results are considered in combination with previously published studies of ZnO grain growth in the ZnO–Bi2 O3 system. For the Bi2 O3 contents of the present study, the rate of ZnO grain growth is found to decrease with increasing Bi2 O3 . Activation analysis, when combined with the results of similar analyses of the previous studies, reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-Bi2 O3 -content region of nearly constant activation energy values of about 150 kJ/mol, further Bi2 O3 additions cause an increase of the activation energy to about 270 kJ/mol. consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of Bi2 O3 changes from one of a phase-boundary reaction at low Bi2 O3 levels to one of diffusion through the liquid phase at about the 5 to 6 wt% Bi2 O3 level and above. 相似文献
2.
Grain growth in a high-purity ZnO and for the same ZnO with Bi2 O3 additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G n — G n 0 = K 0 t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn2+ lattice diffusion mechanism. Additions of Bi2 O3 to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi2 O3 content. The preexponential term K 0 was also independent of Bi2 O3 content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi2 O3 ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi2 O3 -rich liquid phase. 相似文献
3.
Miha Drofenik rej Znidarsic Darko Makovec 《Journal of the American Ceramic Society》1998,81(11):2841-2848
Additions of Bi2 O3 were used to promote grain growth and to increase magnetic permeability during sintering of MnZn ferrites. The results showed that small additions of Bi2 O3 of <0.05 wt% remarkably increase the permeability of MnZn ferrites. On the other hand, addition of 0.05 wt% Bi2 O3 induced the formation of a microstructure composed of giant grains with trapped pores embedded in a normal microstructure. The permeability of these samples showed a pronounced secondary maximum in permeability. At still higher Bi2 O3 concentrations, above 0.2 wt%, the grain growth was retarded and a normal microstructure appeared; however, the magnetic permeability was strongly reduced. 相似文献
4.
Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi2 O3 ceramic was investigated for A12 O3 additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI2 O3 reacted with the ZnO to form ZnAl2 O4 spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl2 O4 spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl2 O4 spinel particles, most probably controlled by the diffusion of O2- in the ZnAl2 O4 spinel structure. 相似文献
5.
Activated sintering in Bi2 O3 -doped ZnO has been studied with emphasis on the mechanistic role of intergranular amorphous films. The atomic-level microstructures and bismuth solute distributions in doped powders have been investigated using high-resolution electron microscopy and scanning transmission electron microscopy. Densification is observed to be significant below the bulk eutectic temperature in the presence of Bi2 O3 concentrations as low as 0.58 mol%. Transmission electron microscopy of as-calcined and sintered powders shows that significant neck growth and particle coarsening occur in the solid state. Intergranular amorphous films of ∼1 nm thickness, terminating in wetting menisci at sinter-necks, are observed to form concurrently with the onset of activated sintering. In a few instances, amorphous films are also observed at surfaces of the ZnO particles. These films appear to be the free-surface counterpart to equilibrium-thickness intergranular films. Activated sintering in this binary system is attributed to rapid mass transport through subeutectic, equilibrium-thickness intergranular films, with the amorphous phase also providing capillary pressure. 相似文献
6.
Keith G. Brooks Yolande Berta Vasantha R. W. Amarakoon 《Journal of the American Ceramic Society》1992,75(11):3065-3069
Preventing the incorporation of impurities in Li-Zn ferrite grains during sintering is essential for production of ceramics with reproducible magnetic and electrical properties. Li-Zn ferrites of composition Li0.3 Zn0.4 Mn0.05 Fe2.25 O4 were prepared with Bi2 O3 and borosilicate sintering additives. The distribution of impurity ions in the sintered ferrites was investigated using transmission electron microscopy (TEM) coupled with energy dispersive spectroscopy (EDS). Ceramics prepared with Bi2 O3 contained Si, Ca, and S impurities, located at grain boundaries and triple point regions. The low viscosity and good wetting properties of the Bi2 O3 and to a lesser extent the borosilicate liquid phase allowed impurities to be selectively removed from the growing ferrite phase during sintering, thus improving sample resistivities. 相似文献
7.
Mattias Elfwing Ragnar Österlund Eva Olsson 《Journal of the American Ceramic Society》2000,83(9):2311-2314
Detailed analysis of the microstructure of grain boundaries, especially triple-grain and multiple-grain junctions, in ZnO varistor materials has been performed using transmission electron microscopy. Different polymorphs of Bi2 O3 are shown to exhibit different wetting properties on ZnO interfaces. Recent investigations suggest that the equilibrium configuration consists of crystalline Bi2 O3 in the triple-grain and multiple-grain junctions and an amorphous bismuth-rich film in the ZnO/ZnO grain boundaries. The present investigation supports this suggestion for δ-Bi2 O3 and also adds to the microstructural image and wetting properties of α-Bi2 O3 . 相似文献
8.
Pomin Su Anil V. Virkar Camden R. Hubbard O. Burl Cavin Wallace D. Porter 《Journal of the American Ceramic Society》1993,76(10):2513-2520
Gd2 O3 -doped Bi2 O3 polycrystalline ceramics containing between 2 and 7 mol% Gd2 O3 were fabricated by pressureless sintering powder compacts. The as-sintered samples were tetragonal at room temperature. Hightemperature X-ray diffraction (XRD) traces showed that the samples were cubic at elevated temperatures and transformed into the tetragonal polymorph during cooling. On the basis of conductivity measurements as a function of temperature and differential scanning calorimetry (DSC), the cubic → tetragonal as well as tetragonal → cubic → teansition temperatures were determined as a function of Gd2 O3 concentration. The cubic → tetragonal transformation appears to be a displacive transformation. It was observed that additions of ZrO2 as a dopant, which is known to suppress cation interdiffusion in rare-earth oxide–Bi2 O3 systems, did not suppress the transition, consistent with it being a displacive transition. Annealing of samples at temperatures 660°C for several hundred hours led to decomposition into a mixture of monoclinic and rhombohedral phases. This shows that the tetragonal polymorph is a metastable phase. 相似文献
9.
J. WONG 《Journal of the American Ceramic Society》1974,57(8):357-359
The intergranular phase obtained by sintering a binary mixture of ZnO + 0.5 mol% Bi2 O3 was isolated by using a dilute solution of HCIO4 , which etches ZnO preferentially. The combined results of selected-area electron diffraction and microscopy, microprobe analysis, and X-ray diffraction strongly indicate that the intergranular material is a polycrystalline phase of tetragonal β-Bi2 O3 ( P 421 c ), rather than the amorphous ZnO-Bi2 O3 phase reported earlier. It appears that the nonohmic behavior in this prototype metal-oxide varistor must be an interfacial property associated with the semiconducting ZnO grains separated by thin layers of high-resistivity Bi2 O3 . 相似文献
10.
Michel W. Barsoum Alexander Elkind Fadel A. Selim 《Journal of the American Ceramic Society》1996,79(4):962-966
Diffusion of molten Bi2 O3 into the grain boundaries of sintered, alumina-doped (0.23 and 0.7 mol%) ZnO pellets resulted in varistors with breakdown voltages in the 3–5 V range and nonlinearity coefficients of 10–24. The varistors were fabricated by spreading a thin layer of Bi2 O3 powder on the surface of ZnO pellets and heating the combination to various temperatures (860–1155°C) and different times. The highest nonlinearity coefficients (20–24) and lowest breakdown voltages (3–5 V) were recorded in samples annealed at 860°C for 35 min. Longer annealing times and/or higher temperatures resulted in progressively higher breakdown voltages. Eventually the devices became insulating, which was attributed to the formation of an insulating Bi2 O3 layer between the grains. Separate wetting experiments have shown that the penetration of Bi2 O3 into ZnO grain boundaries was a strong function of alumina doping —the penetration rate was decreased by a factor of 5–7 as the ZnO was doped with as little as 0.2 mol% alumina. It is this slowing down of the penetration of the ZnO grain boundaries that is believed to be critical in the development of the low breakdown voltages observed. 相似文献
11.
Zinc oxide (ZnO) nanoparticles coated with 1–5 wt% Bi2 O3 were prepared by precipitating a Bi(NO3 )3 solution onto a ZnO precursor. Transmission electron microscopy showed that a homogeneous Bi2 O3 layer coated the surface of the ZnO nanoparticles and that the ZnO particle size was ∼30–50 nm. Scanning electron microscopy showed that ZnO grains sintered at 1150°C were homogeneous in size and surrounded by a uniform Bi2 O3 layer. When the ZnO grains were surrounded fully by Bi2 O3 liquid phases, further increases in the ZnO grain size were not affected by the Bi2 O3 content. This predesigned ZnO nanoparticle structure was shown to promote homogeneous ZnO grains with perfect crystal growth. 相似文献
12.
Nina Daneu Aleksander Renik Slavko Bernik Drago Kolar 《Journal of the American Ceramic Society》2000,83(12):3165-3171
The effects of adding small quantities of SnO2 to the basic ZnO–Bi2 O3 varistor composition were studied in terms of phase reactions, microstructural development, and the formation of inversion boundaries. Scanning and transmission electron microscopy studies showed that the inversion boundaries, triggered by the addition of SnO2 , cause anisotropic grain growth in the early stages of sintering. ZnO grains that include inversion boundaries grow exaggeratedly, at the expense of normal grains, until they dominate the microstructure. Higher additions of SnO2 lead to an increase in number of grains with inversion boundaries and to a more fine-grained microstructure. The increasing amount of secondary phases is also related to a higher level of SnO2 addition; however, the influence of these phases on ZnO grain growth is subordinate to the role of inversion boundaries. 相似文献
13.
Jin-Seong Kim Ngoc-Huan Nguyen Myung-Eun Song Jong-Bong Lim Dong-Soo Paik Sahn Nahm Jong-Hoo Paik Byung-Hyun Choi Soon-Jae Yu 《International Journal of Applied Ceramic Technology》2009,6(5):581-586
Bi2 O3 was added to a nominal composition of Zn1.8 SiO3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi2 O3 content was <8.0 mol%, a porous microstructure with Bi4 (SiO4 )3 and SiO2 second phases was developed in the specimen sintered at 885°C. However, when the Bi2 O3 content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛr =7.6, and τf =−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi2 O3 sintered at 885°C for 2 h. 相似文献
14.
Lin Cong Xu Zheng Peng Hu Sun Dan-feng 《Journal of the American Ceramic Society》2007,90(9):2791-2794
Vaporization of Bi2 O3 in microwave-sintered ZnO varistors is discussed in this study. The Bi2 O3 vaporization of ZnO varistors sintered by a conventional electric furnace is also studied for comparison. The results show that the Bi2 O3 vaporization in microwave-sintered ZnO varistors is more homogenous from the surface to the inside of the sample, which results from the special thermal gradient inside the microwave-sintered samples, and we also find out that the Bi2 O3 vaporization directly affects the electrical properties of ZnO varistors. Microwave-sintered samples exhibit more excellent electrical properties than the conventional ones because the homogenous Bi2 O3 vaporization leads to more uniform microstructures. 相似文献
15.
Hongguo Zhang Longtu Li Ji Zhou Zhenxing Yue Zhilun Gui 《Journal of the American Ceramic Society》2001,84(12):2889-2894
The effect of the addition of Bi2 O3 on the densification, low-temperature sintering, and electromagnetic properties of Z-type planar hexaferrite was investigated. The results show that Bi2 O3 additives can improve the densification and promote low-temperature sintering of Z-type hexaferrite prepared by a solid-state reaction method. The presence of Bi2 O3 in the grain boundaries and the generation of Fe2+ degrade the initial permeability of the samples but make the quality factor and cut-off frequency increase. Various possible mechanisms involved in generating these effects were also discussed. 相似文献
16.
Hongliang Du Daijun Liu Fusheng Tang Dongmei Zhu Wancheng Zhou Shaobo Qu 《Journal of the American Ceramic Society》2007,90(9):2824-2829
Lead-free piezoelectric (K0.5 Na0.5 )NbO3 – x wt% Bi2 O3 ceramics have been synthesized by an ordinary sintering technique. The addition of Bi2 O3 increases the melting point of the system and improves the sintering temperature of (K0.5 Na0.5 )NbO3 ceramics. All samples show a pure perovskite phase with a typical orthorhombic symmetry when the Bi2 O3 content <0.7 wt%. The phase transition temperature of orthorhombic–tetragonal ( T O − T ) and tetragonal–cubic ( T C ) slightly decreased when a small amount of Bi2 O3 was added. The remnant polarization P r increased and the coercive field E c decreased with increasing addition of Bi2 O3 . The piezoelectric properties of (K0.5 Na0.5 )NbO3 ceramics increased when a small amount of Bi2 O3 was added. The optimum piezoelectric properties are d 33 =140 pC/N, k p =0.46, Q m =167, and T C =410°C for (K0.5 Na0.5 )NbO3 –0.5 wt% Bi2 O3 ceramics. 相似文献
17.
Jinho Kim Toshio Kimura Takashi Yamaguchi 《Journal of the American Ceramic Society》1989,72(8):1541-1544
Densification and microstructure de velopment in Bi2 O3 -doped ZnO have been studied with a special emphasis on the effect of the Bi2 O3 content. A small amount of Bi2 O3 in ZnO (0.1 mol%) retarded densification, but the addition of Bi2 O3 to more than 0.5 mol% promoted densification by the formation of a liquid phase above the eutectic temperature (∼740°C). The liquid phase increased grain-boundary mobility, which was responsible for the formation of intragrain pores and the decrease in the sintered density. The increase in the Bi2 O3 content increased the probability of the formation of skeleton structure, which reduced the grain growth rate and the sintered density. 相似文献
18.
Extended defects in ZnO ceramics containing, 6 wt% Bi4 Ti3 O12 were studied by analytical electron microscopy. Apart from basal plane condensation stacking faults, which are also present in as-received ZnO, extended defects related to the presence of Bi4 Ti3 O12 were observed. In samples sintered at 900°C they lie in the basal or in the prismatic planes and they quite often form closed loops, whereas they form serpentine-shaped boundaries in samples sintered at 1200°C. Evidence is given that they are inversion boundaries. Their TEM image characteristics, as well as the unambiguous presence of Ti at the boundaries, suggest that they are formed due to the presence of 2-D coherent precipitates of Ti-rich (possibly Zn2 TiO4 -type spinel) phase. 相似文献
19.
The microstructure of strontium titanate internal boundary layer capacitors at various stages in their processing was studied by transmission electron microscopy of rapidly quenched and normally cooled samples. Compositions containing excess TiO2 , Al2 O3 , and SiO2 have a completely wetting liquid phase at the sintering temperature; during cooling Tin O2 n −1 , Magneli phases precipitate at multiple grain junctions. Diffused metal oxides and flux (Bi2 O3 , PbO, CuO, and B2 O3 ) rapidly penetrate as a liquid phase along boundaries in postsintering heat treatment. This liquid phase disappears during slow cooling. 相似文献
20.
The influence of Nd2 O3 doping on the reaction process and sintering behavior of BaCeO3 is investigated. Formation of BaCeO3 is initiated at 800°C and completed at 1000°C. When Nd2 O3 is added to the starting materials, the formation of BaCe1–x Ndx O3–δ is delayed and the temperature for complete reaction is increased to 1100°C. Only a BaCe1-x Ndx O3–δ solid solution with an orthorhombic crystal structure is present in the specimens for x ≤ 0.1. A secondary phase rich in Ce and Nd is formed within grains and at grain boundaries, when the Nd2 O3 content is greater than the solubility limit (x ≥ 0.2). Pure BaCeO3 is difficult to sinter, even at 1500°C, and only a porous microstructure could be obtained. However, doping BaCeO3 with Nd2 O3 markedly enhances its sinterability. The enhancement of the sinterability of Nd2 O3 -doped specimens at x ≤ 0.1 is attributed to the increase in the concentration of oxygen ion vacancies, which increases the diffusion rate. At x ≥ 0.2, the grain size is abnormally coarsened, which is caused by the formation of a liquid phase. While this liquid phase accelerates sintering, its beneficial effect on densification is counteracted by the segregation of the secondary grain-boundary phase which inhibits sintering. 相似文献