Nanostructure Evolution of ZnO in Ultra-fast Microwave Sintering

2011 ◽  
Vol 691 ◽  
pp. 65-71 ◽  
Author(s):  
Rodolfo F. K. Gunnewiek ◽  
Ruth Herta Goldsmith Aliaga Kiminami
Keyword(s):  
Grain Size ◽  
Zinc Oxide ◽  
Grain Growth ◽  
Microwave Sintering ◽  
High Heating Rate ◽  
Heating Rates ◽  
Grain Sizes ◽  
Average Grain Size ◽  
Conventional Sintering ◽  
Holding Temperature

Grain growth is inevitable in the sintering of pure nanopowder zinc oxide. Sintering depend on diffusion kinetics, thus this growth could be controlled by ultra-fast sintering techniques, as microwave sintering. The purpose of this work was to investigate the nanostructural evolution of zinc oxide nanopowder compacts (average grain size of 80 nm) subjected to ultra-rapid microwave sintering at a constant holding temperature of 900°C, applying different heating rates and temperature holding times. Fine dense microstructures were obtained, with controlled grain growth (grain size from 200 to 450nm at high heating rate) when compared to those obtained by conventional sintering (grain size around 1.13µm), which leads to excessively large average final grain sizes.

Microstructural Evolution of AZ31 Mg Wrought Alloy during Partial Remelting for Thixoextrusion

2006 ◽  
Vol 116-117 ◽  
pp. 251-254
Author(s):  
Young Ok Yoon ◽  
Shae K. Kim
Keyword(s):  
Grain Size ◽  
Microstructural Evolution ◽  
Liquid Fraction ◽  
Isothermal Holding ◽  
Holding Time ◽  
Interesting Point ◽  
Grain Sizes ◽  
Partial Remelting ◽  
Average Grain Size ◽  
Holding Temperature

An attempt has been made to investigate feasibility of thixoextrusion for AZ31 Mg wrought alloy through simple partial remelting. Microstructural evolution of AZ31 Mg wrought alloy for thixoextrusion was investigated as functions of isothermal holding temperature and time in the partially remelted semisolid state. The interesting point of this study was that the thixotropic structures of AZ31 Mg wrought alloy without additional pretreatment could be obtained through simple partial remelting. The uniform average grain size and liquid fraction according to the isothermal holding time were very important for the thixoextrusion. Because, it is must be considered on actual extrusion time. The liquid fraction was increased with increasing isothermal holding temperature and time. But, the liquid fraction was almost uniform after 10 min. The average grain size was decreased with increasing isothermal holding temperature. On the other hand, as the holding time increased, the average grain sizes were uniform at 615 and 622. This phenomenon is very useful for thixoextrusion in terms of process control such as actual extrusion time.

In Situ Observation of Grain Growth and Recrystallization of Steel at High Temperature

2010 ◽  
Vol 638-642 ◽  
pp. 1077-1082 ◽  
Author(s):  
Yasuhiro Yogo ◽  
Kouji Tanaka ◽  
Koukichi Nakanishi
Keyword(s):  
Grain Size ◽  
High Temperature ◽  
Grain Growth ◽  
In Situ Observation ◽  
Initial Structure ◽  
Grain Sizes ◽  
Dynamic Observation ◽  
Average Grain Size ◽  
Observation Method

An in-situ observation method for structures at high temperature is developed. The new observation device can reveal grain boundaries at high temperature and enables dynamic observation of these boundaries. Grain growth while maintaining microstructure at high temperature is observed by the new observation device with only one specimen for the entire observation, and grain sizes are quantified. The quantifying process reveals two advantages particular to the use of the new observation device: (1) the ability to quantify grain sizes of specified sizes and (2) the results of average grain size for many grains have significantly less errors because the initial structure is the same for the entire observation and the quantifying process. The new observation device has the function to deform a specimen while observing structures at high temperature, so that enables it to observe dynamic recrystallization of steel. The possibility to observe recrystallization is also shown.

Microwave Sintering of Zirconia Toughened Mullite (ZTM)

1994 ◽  
Vol 347 ◽  
Author(s):  
J. Cai ◽  
C. Y. Song ◽  
B. S. Li ◽  
X. X. Huang ◽  
J. K. Guo ◽  
...  
Keyword(s):  
Grain Size ◽  
Grain Growth ◽  
Microwave Sintering ◽  
Single Mode ◽  
Microwave Processing ◽  
Toughening Mechanism ◽  
Conventional Sintering

ABSTRACTMicrowave sintering of zirconia toughened mullite (ZTM) has been performed in a single mode applicator. In comparison with conventional sintering, microwave processing of ZTM leads to a higher density and finer grain size. Microstructure of microwave sintered ZTM was characterized by TEM and HRTEM techniques. The pinning of intergranular ZrO2 dispersoids retarded the grain growth of mullite matrix. The observation of a considerable number of trans-granular microcracks indicates that microcracking toughening is the main toughening mechanism for ZTM.

scholarly journals Micro structural control of stabilized Zirconia ceramics (8YSZ) through modified conventional sintering methodologies

2010 ◽  
Vol 42 (1) ◽  
pp. 91-97 ◽  
Author(s):  
K. Rajeswari ◽  
Rajasekhar Reddy ◽  
U.S. Hareesh ◽  
B.P. Saha ◽  
R. Johnson
Keyword(s):  
Grain Size ◽  
Grain Growth ◽  
Structural Control ◽  
Grain Size Analysis ◽  
Size Analysis ◽  
Stabilized Zirconia ◽  
Average Grain Size ◽  
Conventional Sintering ◽  
Slip Cast ◽  
Rate Controlled

Slip cast Y2O3 stabilized Zirconia (8YSZ) ceramics was subjected to Conventional Ramp and Hold (CRH), Rate Controlled (RCS) and Two Stage Sintering (2SS) methodologies. Sintered samples were characterized for their densities and grain size analysis by Scanning Electron Microscopy. The slip cast samples sintered by CRH and 2SS have achieved 98 - 99 % of theoretical densities while RCS samples have exhibited a low density of 97 %. The samples exhibited an average grain size of 2.64 ?m by 2SS sintering in comparison to 8.83 ?m in case of CRH and 3.45 ?m in case of RCS. Controlled pore growth associated with RCS, when compared to CRH methodology is mainly responsible for the relatively smaller grain size observed with RCS. A four fold decrease in grain size i.e. 2.64 ?m observed with the two step sintering can be attributed to the fact that the first heating step to high temperature of 1550?C for a shorter duration closes the porosity without significant grain growth. The second step at 1375?C for a longer period of time imparts densification with limited grain growth.

Production and Mechanical Properties of Nanocrystalline Intermetallics Based on TiAl3-X

2003 ◽  
Vol 791 ◽  
Author(s):  
H. A. Calderon ◽  
J. C. Aguilar-Virgen ◽  
F. Cruz-Gandarilla ◽  
M. Umemoto
Keyword(s):  
Mechanical Properties ◽  
Grain Size ◽  
Grain Growth ◽  
High Strength ◽  
Compression Tests ◽  
Grain Sizes ◽  
Intermetallic Materials ◽  
Plasma Sintering ◽  
Average Grain Size ◽  
Spark Plasma

ABSTRACTProduction of intermetallic materials in the system TiAl3-X (X = Cr, Mn, Fe) has been achieved by means of mechanical milling and sintering techniques. Spark plasma sintering is used since it reduces time at high temperature and inhibits grain growth. The produced materials have grain sizes in the nano and microscale depending on the material and processing variables. The TiAl3-X alloys are formed mostly by the cubic L12 phase. The average grain size ranges between 30 and 50 nm in the as sintered condition. Aging at elevated temperature has been used to promote grain growth. Compression tests have been performed to evaluate mechanical properties as a function of temperature and grain size. In all cases yield stresses higher that 700 MPa are obtained together with a ductility that depends upon temperature and grain size. No ductility is found for the smallest grains sizes tested (30 nm) at room temperature. Above 673 K, these materials show ductility and additionally they present a quasi superplastic behavior at temperatures higher that 973 K. On the other hand ductility can also be developed in the TiAl3-X alloys by inducing grain growth via annealing. Alloys with grains sizes around 500 nm show high ductility and a large density of microcraks after deformation suggesting that the yield strenght becomes lower than the stress to propagate the cracks. In such materials, a considerably high strength is retained up to 873 K.

Effect of High Heating Rates on Microstructure of Alumina and Aluminum Titanate Ceramics

2001 ◽  
Vol 7 (S2) ◽  
pp. 414-415
Author(s):  
Lia A. Stanciu ◽  
Joanna R. Groza
Keyword(s):  
Surface Diffusion ◽  
Grain Growth ◽  
Electrical Current ◽  
High Heating Rate ◽  
Heating Rates ◽  
Fast Heating ◽  
Current Application ◽  
High Heating ◽  
Conventional Sintering ◽  
Continuous Current

In conventional sintering, high heating rates are often used to enhance densification and reduce grain growth. The essence of this concept is to use fast heating rates to favor densification instead of grain growth, if the activation energy for densification is greater than that for grain growth. At low temperatures, surface diffusion is the most effective sintering mechanism but is associated with grain coarsening. Therefore, a high heating rate may take the system for densification at high temperatures before surface diffusion causes grain growth and decreases the sintering driving force. Field activated sintering technique (FAST) is a newly developed sintering method, which applies an external electrical field to enhance densification. The method applies pulsed and continuous current to powders subject to a modest pressure (< 100 MPa). Electrical current application enables fast heating rates. The sintering cycles are very short, typically less than 10 minutes for full densification.

Microwave sintering of ZnO at ultra high heating rates

2001 ◽  
Vol 16 (10) ◽  
pp. 2850-2858 ◽  
Author(s):  
Geng-fu Xu ◽  
Isabel K. Lloyd ◽  
Yuval Carmel ◽  
Tayo Olorunyolemi ◽  
Otto C. Wilson
Keyword(s):  
Grain Size ◽  
Heating Rate ◽  
Microwave Heating ◽  
Grain Growth ◽  
Rapid Heating ◽  
Microwave Sintering ◽  
Complete Suppression ◽  
Heating Rates ◽  
High Heating ◽  
Size Uniformity

In this paper, a unique processing approach for producing a tailored, externally controlled microstructure in zinc oxide using very high heating rates (to 4900 °C/min) in a microwave environment is discussed. Detailed data on the densification, grain growth, and grain size uniformity as a function of heating rate are presented. With increasing heating rate, the grain size decreased while grain size uniformity increased. At extremely high heating rates, high density can be achieved with almost complete suppression of grain growth. Ultrarapid microwave heating of ZnO also enhanced densification rates by up to 4 orders of magnitude compared to slow microwave heating. The results indicate that the densification mechanisms are different for slow and rapid heating rates. Since the mechanical, thermal, dielectric, and optical properties of ceramics depend on microstructure, ultrarapid heating may lead to advanced ceramics with tailored microstructure and enhanced properties.

scholarly journals Grain Growth Control of ZnO-V2O5-Cr2O3 Based Varistors by PrMnO3 Addition

2021 ◽  
Author(s):  
Chankun Cai ◽  
Yu Shi ◽  
Manyi Xie ◽  
Ke Xue ◽  
Maofeng Xu ◽  
...  
Keyword(s):  
Activation Energy ◽  
Grain Size ◽  
Apparent Activation Energy ◽  
Size Distribution ◽  
Grain Growth ◽  
Growth Control ◽  
Growth Behaviour ◽  
Grain Sizes ◽  
Pinning Effect ◽  
Average Grain Size

Abstract In this work, the grain growth behaviour of ZnO+V2O5(1 mol%)+Cr2O3(0.35 mol%)-based ceramics with 0.25–0.75 mol% additions of PrMnO3 was systematically investigated during sintering from 850°C to 925°C with the aim to control the ZnO grain size for their application as varistors. It was found that with the increased addition of PrMnO3, not only did the average grain size decrease, but the grain size distribution also narrowed and eventually changed from a bimodal to unimodal distribution after a 0.75 mol% PrMnO3 addition. The grain growth control was achieved by a pinning effect of the secondary ZnCr2O4 and PrVO4 phases at the ZnO grain boundaries. The apparent activation energy of the ZnO grain growth in these ceramics was found to increase with increased additions of PrVO4; hence, the observed reduction in the ZnO grain sizes.

Preparation of Industrial Watchcase of 5Y-Tetragonal ZrO2 by Microwave Sintering

2014 ◽  
Vol 602-603 ◽  
pp. 323-326
Author(s):  
Xiao Xuan Pian ◽  
Bing Bing Fan ◽  
Xin Zhang ◽  
Hao Chen ◽  
Chen Yang Wang ◽  
...  
Keyword(s):  
Grain Size ◽  
Phase Change ◽  
Vickers Hardness ◽  
Sintering Temperature ◽  
Microwave Sintering ◽  
Resonant Mode ◽  
Homogeneous Microstructure ◽  
Average Grain Size ◽  
Conventional Sintering ◽  
Lower Temperature

The industrial watchcases of 5Y-Tetragonal ZrO2were prepared by microwave sintering. Samples were sintered in a microwave chamber with TE666 resonant mode at 2.45GHz. The sintering temperature was from 1250°C to 1400°C. XRD and SEM techniques were used to characterize the samples. It is found that microwave sintering improved the phase change from t-ZrO2to m-ZrO2. Dense and homogeneous microstructure was obtained within the microwave-sintered samples and the average grain size was about 500nm. Compared to conventional sintering, microwave-sintered samples show higher density and hardness. And microwave-sintered samples were performed 130°C lower temperature. Optimized sample with the density of 6.1 g/cm3and Vickers Hardness of 16.2MPa is microwave-sintered at 1350°C for 30min.

Densification of Nanocrystalline Ceramics by Combustion Reaction and Quick Pressing

2014 ◽  
Vol 616 ◽  
pp. 204-211
Author(s):  
Jiang Hao Liu ◽  
Zheng Yi Fu
Keyword(s):  
Grain Size ◽  
Relative Density ◽  
Heating Rate ◽  
Grain Growth ◽  
Applied Pressure ◽  
Combustion Reaction ◽  
High Heating Rate ◽  
Nanocrystalline Ceramics ◽  
Average Grain Size ◽  
High Heating

The technique of combustion reaction and quick pressing was adopted to prepare dense nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 100 nm was investigated, under the applied pressure of up to 170 MPa, and the temperature of 1740–2080 K with ultra-high heating rate of above 1700 K/min. As a result, pure magnesia ceramics with a relative density of 98.8% and an average grain size of 120 nm was obtained at 1740 K and 170 MPa, while the ones with decreased relative density and increased grain size were produced under the increasing temperature and the identical pressure conditions. The results indicated that grain growth of the nanocrystalline magnesia was effectively restrained by the combined effect of the ultra-high heating rate and the high pressure. Moreover, under the particular sintering conditions, there existed an appropriate temperature range for the preparation of dense nanocrystalline magnesia, and the excessive temperature would not only exaggerate grain growth but also impede densification.

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