Activation Energy For Densification Research Articles

Solid state sintering behavior of zirconia-nickel composites

The densification kinetics of zirconia-nickel composites, containing up to 30 vol% of nickel with a medium particle size around 4 μm, was investigated. Green 3 mol% yttria stabilized zirconia (3YSZ)/Ni compacts with 56% of the theoretical density were submitted to dilatometry at constant heating rates of 5, 10 and 20 °C/min. The densification kinetics of the ceramic-metal composites (cermets) were evaluated by estimation of the apparent activation energy (Q), whereas scanning electron microscopy was used to analyze the microstructure. The addition of nickel caused an increase of activation energy of densification of the composites. The densification behavior was influenced by microstructural evolution of nickel phase and percolation phenomenon. The values of activation energy for densification increased as a function of Ni content. In the initial stage, the activation energy varied from (485 ± 45) J.mol−1, for pure zirconia, to (683 ± 28) J.mol−1, for composites with ZrO2(70%vol)/Ni(30%vol), 70Z-30Ni. In the intermediate, these values increased from (343 ± 13) to (480 ± 30) J.mol−1, for pure zirconia and 70Z-30Ni, respectively.

Low-temperature sintering and densification kinetics of sol-gel derived lithium-mica glass-ceramics through MgF2 addition

The aim of the current research is to investigate the low-temperature sintering of a nano-crystalline lithium-mica glass-ceramic. Sol-gel technique was used to prepare the glass-ceramics by application of LiMg3AlSi3(1+x)O10+6xF2 with various x values (0.25, 0.5, 0.75, and 1) and different amounts of MgF2 as the sintering aid. Use of appropriate amounts of additive and stoichiometric deviation (x) decreased the sintering temperature to 750 °C. Results also revealed that the highest relative density was achieved by applying the optimal amount of additive (4 wt%) with x = 1. The densification activation energy for the glass-ceramic containing 4 wt% MgF2 was calculated as 214.33 ± 17 kJ mol−1; it was lower than the sample containing 5 wt% additive; hence it was confirmed as optimal content. Consequently, considering densification and phase analysis, true optimum amounts of MgF2 were determined as 4, 4, 3 and 1 wt% for the glass-ceramics synthesized with x = 1, 0.75, 0.5 and 0.25, respectively.

Microstructure and mechanical behaviour of SrO doped Al2O3 ceramics

Microstructure and mechanical behaviour of the SrO doped Al2O3 ceramics have been studied with a dopant concentration levels of, 0 ppm (AS0), 1000 ppm (AS1), 3000 ppm (AS3), and 5000 ppm (AS5), respectively. High density (~99.1%) and finer grain size (~3.4 µm) have been obtained for the Al2O3 ceramics at an optimum dopant concentration of 5000 ppm SrO (AS5) attributed to the important effects of in-situ formed strontium hexaluminates (SrAl12O19). Controlled grain growth behaviour in SrO doped Al2O3 (AS5) may be ascribed to the zener pinning stresses and solute drag effect actuated by the grain boundary phase SrAl12O19 with platelet morphology. Improved densification behaviour in SrO doped Al2O3 (AS5), presumably because of the lower interfacial energy of newly formed Al2O3/SrAl12O19 interfaces and key benefits of secondary phase (SrAl12O19) inhibited grain growth mechanism. The activation energy for densification in SrO doped Al2O3 (AS5) is found to be 550 ± 30 kJ/mol, which is higher compared to undoped Al2O3 and the estimated activation energies have been phenomenonlogically correlated to the lower energy of Al2O3/SrAl12O19 interfaces. Improved mechanical properties in terms of fracture toughness (~3.7 MPa. m1/2), strength (~890 MPa) and hardness (~13.8 GPa) in optimum SrO doped Al2O3 (AS5) ceramics are triggered by a combination of toughening mechanisms primarily including SrAl12O19 platelet grain bridging, crack deflection, grain pullout, and crack tip healing.

Iron/zinc doped 8 mol% yttria stabilized zirconia electrolytes for the green fuel cell technology: A comparative study of thermal analysis, crystalline structure, microstructure, mechanical and electrochemical properties

Abstract Pristine and Fe/Zn doped 8 mol% yttria stabilized zirconia (8YSZ) were prepared by ball milling technique for potential application in the green fuel cell technology. The series consisted of (8YSZ)0.97(Fe2O3)x(ZnO)3-x, where x indicates 0.5, 1.0, 1.5, 2.0 and 2.5 mol% Fe or Zn alternatively. A number of analytical techniques were used to characterize 8YSZ included X-ray diffraction, scanning electron microscopy, impedance spectroscopy, dilatometer, and hardness test. The analytical results indicated a decrease in the densification in the doped 8YSZ as the activation energy for densification decreased from 1087 kJ/mol in the pristine 8YSZ sample to 264 kJ/mol in the doped 8YSZ. The optimum doped 8YSZ was recognised with 2.0 mol% Fe and 1.0 mol% of Zn exhibit the highest percentage of the cubic phase approaching 62.4 wt%, while the shortest lattice parameter of 0.5132 nm and crystallite size equivalent to 26.34 nm. This composition also indicated pore-free morphology, the highest densification up to ∼94%, the greatest hardness value (above 740 MPa), while improve total conductivity about 7.398 × 10−6 S/cm at 300 °C. The results provide new insights on the effect of mixed dopants as ceramic composite as well as on the fuel cell technology.

Effective Activation Energy for the Solid-State Sintering of Silicon Carbide Ceramics

Effective activation energy for densification of SiC in the presence of C and B4C additives is determined by constant heating rate experiments through analysis of shrinkage in hot pressing. The activation energy (AE) for sintering increased linearly with the relative density (RD) in two different regimes. The effective AE increased from 407 ± 50 kJ/mole at 75 pct RD to 1132 ± 75 kJ/mole at 95 pct RD. Lattice diffusion is proposed as the predominant mechanism for SiC densification at higher density. This is also validated by the uniform distribution of sintering additive through electron probe microanalysis. The low AE in the regime of lower density could be attributed to the pressure-assisted particle rearrangement during hot pressing. The relative contribution of both the mechanisms between two density limits resulted in the change in AE for sintering. The mechanisms of defect generation resulting in densification are also discussed.

Kinetics and thermodynamics of densification and grain growth: Insights from lanthanum doped zirconia

The effect of dopants (or additives) on sintering is typically addressed from a mechanistic and diffusivity perspective by focusing on how dopants affect those parameters. However, a comprehensive description of sintering needs to address the role of dopants in the thermodynamics of the system, which affects local chemical potentials driving forces and is, therefore, ultimately linked to the mass transport mechanisms themselves by the thermodynamic extremal principle. In this work, Lanthanum doped Yttria-Stabilized-Zirconia (YSZ) sintering was studied from both kinetics and thermodynamic perspectives to demonstrate the need for those complementary analyses to allow proper processing control. La caused inhibition of both grain growth and densification, which was a result of a change of interfacial energies linked to La segregation as well as of the codependence of coarsening and densification on the grain boundary energy. While La caused a modest decrease in the activation energy for densification, surface and grain boundary energies decreased from 0.95 to 0.70 J/m2, respectively, for YSZ, to 0.80 and 0.41 J/m2 for 2 mol% La-YSZ, indicating an increase in sintering stress. The apparent contradiction between the thermodynamic data and the observed densification trend is attributed to the reduced grain growth that trapped the system in a metastable configuration.

Direct current-enhanced densification kinetics during spark plasma sintering of tungsten powder

Rapid densification associated with spark plasma sintering has been well documented; however theoretical understanding is still very limited. In this work we proposed a densification creep model to explicitly elucidate the effect of current density on densification kinetics, by taking electromigration into account. This new model was then validated experimentally. It is found that an increased current density decreases densification activation energy (energy barrier), thus accelerating powder densification. In essence, the rapid densification is caused by direct current that accelerates mass transport and increases dislocation mobility.

Comparison of apparent activation energies for densification of alumina powders by pulsed electric current sintering (spark plasma sintering) and conventional sintering—toward applications for transparent polycrystalline alumina

Abstract

FAST/SPS sintering of nanocrystalline zinc oxide—Part I: Enhanced densification and formation of hydrogen-related defects in presence of adsorbed water

This part is focused on the effect of surface bound water on the densification behavior and defect stoichiometry of zinc oxide. The second part [doi: 10.1016/j.jeurceramsoc.2015.12.008] concentrates on the effect of hydroxide complexions on the microstructural development, texture formation and anisotropic grain morphology. Nanocrystalline zinc oxide powder was humidified or dried followed by quick heating (100K/min) with field-assisted sintering technique/spark plasma sintering (FAST/SPS). Densification is strongly enhanced due to hydroxide-ion-diffusion mechanism, which shows species with lower valence and ionic radius in comparison to oxygen ions. The lowered activation energy for densification exhibits no impact of the sintering electric current on this enhanced densification behavior. The defect stoichiometry and structure of sintered zinc oxide was analyzed by several spectroscopic methods, indicating the formation of hydrogen-related defects for sintering in presence of bound water, while no hydrogen was detected for sintering of dried powder.

Obtaining highly dense YSZ nanoceramics by pressureless, unassisted sintering

For technological applications, zirconia is commonly blended with other oxides to stabilise the tetragonal and/or cubic phases at low temperature, being yttria the most frequently added dopant. It is generally desirable to obtain highly dense ceramics while maintaining grain sizes in the nanoscale ( < 100 nm). Small grains contribute to stabilise the tetragonal phase and to improve the toughness and flexural strength. Moreover, a higher ionic conductivity for cubic zirconia electrolytes is achieved with smaller grain size and lower thickness of the intergranular regions. The sintering onset temperatures required for nanometric particles are significantly reduced when compared to conventional micrometric powders. However, densification is generally accompanied by an undesirable grain coarsening. A Ramp and Hold Sintering (RHS) is the simplest densification schedule, consisting of heating up to the peak temperature followed by a holding time at that temperature. Another approach, called Two-Step Sintering (TSS) is based on the principle that the activation energy for grain growth is lower than the activation energy of densification. The key elements in this method are heating up to a high temperature to achieve a density >75% Theoretical Density (TD) to render the pores unstable, and then cooling down rapidly to a lower temperature to finish sintering and hinder grain growth. Alternatively, considerable efforts have been made in increasing the heating rate and/or reducing the hold time at the peak temperature during sintering cycles in processes that are generally referred to as ‘Fast Firing’ (FF) or ‘rapid sintering’. This review summarises the attempts in the literature for obtaining dense monolithic nanocrystalline Yttria-Stabilized Zirconia (YSZ) ceramics by pressureless sintering schedules carried out in conventional furnaces. RHS, FF and TSS schedules are reported only from YSZ as starting powders, i.e. without the aid of any additional dopant or grain growth inhibitor. For the sake of comparison, the discussion is focused mainly on YSZ nanoceramics with final densities >99% TD and average grain size < 100 nm. Powder and shaping effects on microstructure and properties of bulk nanoceramics are discussed. A comparison among sintering approaches is then made taking into account the microstructural development of the nanostructures and some key properties of the products.

Construction of master sintering curves to understand the effect of Na addition on ZnO-based varistors

This work describes the effect of a small sodium addition on the densification of Bi2O3-based ZnO varistors. The characterisation was performed using both the master sintering curve tool and the analyses of structural and microstructural characteristics of the materials. It was found that sodium-free varistor material exhibited very low activation energy for densification (149.2kJ/mol). This was explained by the liquid phase promoting diffusion through the enhanced reaction between ZnO and Bi2O3. For sodium-doped material, the calculated activation energy was much higher (292.5kJ/mol) and was very close to the one obtained for pure ZnO, which is often related to the grain boundary solid-state diffusion. It was shown that a small sodium addition prevented Bi2O3 from satisfactorily reacting with ZnO. Finally, a mechanism was proposed to explain how a slight sodium amount altered the reactivity between Bi2O3 and the ZnO matrix, resulting in some major changes in the final microstructures.

Activated liquid phase sintering of W–Cu and Mo–Cu

A model for densification based on the master sintering curve concept is applied to solid-state sintering of W and Mo and to liquid-phase sintering of W–10Cu and Mo–18Cu. The effects of small additions of Ni, Co, Fe, and Pd to W–10Cu and Mo–18Cu are investigated. Separate grain growth models for solid-state and liquid-phase sintering are also applied to these systems. The model predictions are compared to experimental results to determine the effects of the activators on densification and grain growth mechanisms. Ni and Pd additions only slightly reduce the activation energy for densification, while Co and Fe have a much larger effect. The effects of Co and Fe additions on grain growth rate constants are not adequately described by consideration of their effects on the solubility of W and Mo in the liquid phase. The experimental results with Co and Fe additions are better described by a solid-state grain growth model. The effects of Co and Fe additions on densification and grain growth are consistent with enhanced diffusion through a segregated solid phase during liquid-phase sintering.

Densification and grain growth behavior study of trivalent MO1.5 (M = Gd, Bi) doped ceria systems

Densification and microstructure for Ce0.9Gd0.1O1.95−δ and trivalent co-doped CeO2 (Ce0.9Gd0.06Bi0.04O1.95−δ) were investigated. The sinterability of co-doped nano-crystalline CeO2 was heightened compared to Gd-doped CeO2 according to the dilatometer measurements. The activation energy for densification was 745 and 419kJmol−1 for Gd-doped CeO2 and co-doped CeO2, respectively. The improved sintering behavior was attributed to the lower activation energy of co-doped CeO2. Investigations on the grain growth in the ceramics showed the suppressing effects of co-doped CeO2. The grain growth kinetics study indicated that the grain growth exponent, m, was 3 for Gd-doped CeO2, and 4 for co-doped CeO2, respectively.

Sintering and grain growth kinetics in La0.85Sr0.15MnO3–Ce0.9Gd0.1O1.95 (LSM–CGO) porous composite

Abstract The sintering kinetics in La 0.85 Sr 0.15 MnO 3 –Ce 0.9 Gd 0.1 O 1.95 (LSM–CGO) porous composite was studied by applying a two-stage master sintering curve (MSC) approach and comparing with LSM and CGO single-phase materials. In the two-stage MSC, sintering mechanisms occurring at different stages were separated with respect of density, giving a typical apparent activation energy values for each sintering stage of the LSM–CGO system. Compared with the single-phase materials, retardant effect of the different phases on mass diffusion leads to much higher apparent activation energy for densification of the composite. Similarly, constrain effect was also observed in grain growth in the composite. Particularly, in the investigated temperature range (1100–1250 °C), the determined grain boundary mobility of CGO in the LSM–CGO composite (10 −18 –10 −16 m 3 N −1 s −1 ) is comparable with the single-phase CGO, while the grain boundary mobility of LSM in the composite (10 −17 –10 −16 m 3 N −1 s −1 ) is around 1 order of magnitude smaller than the single-phase LSM.

Sintering kinetics of YAG ceramics

Solid state reactive (SSR) sintering kinetics was observed for YAG ceramics. There were two densification stages in sintering process due to its reaction. After the first stage, samples began to expand, then, the second densification stage began. At a heating rate of 10 °C/min, the sample warped down and warped back to straight. The apparent activation energy of the first densification process was about 522 kJ/mol for the initial shrinkage of Al2O3 and Y2O3 mixed powder green-body, which increased in the following process due to the solid state reaction. In the second densification stage, synthesis reaction of YAG still worked. Green-bodies processed with higher heating rate got more shrinkage at the same temperature than lower heating rate green bodies. And its kinetic field diagram was abnormal, compared with that of other reported ceramics, such as Al2O3. It was found that the reaction of YAG provided positive effect to the sintering driving force. The apparent activation energy for densification of SSR YAG sintered in ArH5 atmosphere was 855 kJ/mol at temperature holding sintering. And the apparent activation energy for grain growth was 1053 kJ/mol.

Microwave versus conventional sintering: Estimate of the apparent activation energy for densification of α-alumina and zinc oxide

Abstract A comparative study between the conventional and 2.45 GHz microwave multimode sintering behavior of insulator (α-Al2O3) and semi-conductive ceramic (ZnO) was systematically investigated. The apparent activation energy of nonisothermal sintering was determined by way of the Arrhenius plot of densification data at constant heating rates (CHR) and the concepts of Master Sintering Curves (MSCs), respectively. During microwave densification process, the apparent activation energy was about 90 kJ/mol less than the value for conventional sintering of Al2O3 applying these two estimation methods. However, an opposite result was obtained in the case of ZnO, although its densification process had been also accelerated by microwave as well as Al2O3. The significant differences in activation energy give a good proof of the difference in diffusion mechanism induced by the electromagnetic field underlying microwave sintering.

Al-doped ZnO nanostructured powders by emulsion detonation synthesis – Improving materials for high quality sputtering targets manufacturing

Abstract Emulsion detonation synthesis method was used to produce undoped and Al-doped ZnO nanostructured powders (0.5–2.0 wt.% Al2O3). The synthesized powders present a controlled composition and a morphology which is independent on the doping level. The XRD results indicate wurtzite as the single phase for undoped ZnO and the presence of gahnite as secondary phase for Al-doped ZnO powders. The sintering behavior of each powder was studied based on their linear shrinkage and shrinkage rate curves, showing the high sinterability of the powders. Activation energies for densification in the earlier stage were calculated for all compositions and possible sintering mechanisms are suggested depending on the doping level. The high chemical homogeneity and sinterability and the lower electrical resistivity of the bulk Al-doped sintered samples demonstrates the feasibility of emulsion detonation synthesis for the production of high quality Al-doped ZnO powders to be used in ceramic sputtering targets manufacture.

Influence of La Doping on the Apparent Activation Energy of α-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; during the Densification Process at Constant Heating-Rate

The apparent activation energy for densification is a characteristic quantity that elucidates the fundamental diffusion mechanisms during the sintering process. Based on the Arrhenius theory, the activation energy for densification of the pure and La doped α-Al2O3at constant heating-rates sintering has been estimated. Sintering of α-Al2O3and La doped powder has been executed by the way of a push rod type dilatometer. It was shown that the shrinkage and the relative density of the samples decreased with increasing heating rates. At the same sintering condition, the shrinkage of the pure α-Al2O3was higher than that of the La doped. According to the calculation, it was found that the apparent activation energy had a single value and did not depend on the relative density for both samples. The apparent activation energy of the pure α-Al2O3was 374.3kJ/mol, which was lower than that of La doped (966.5kJ/mol). So the densification process for α-Al2O3was retarded due to La doping, which was consistent with experimental results of the shrinkage.

Effect of Heating Rate on Densification and Grain Growth During Spark Plasma Sintering of 93W-5.6Ni-1.4Fe Heavy Alloys

Blended 93W-5.6Ni-1.4Fe powders were sintered via the spark plasma sintering (SPS) technique using heating rates from 10 K min−1 to 380 K min−1 (10 °C min−1 to 380 °C min−1). The kinetics of densification and grain growth were analyzed to identify heating rate effects during the SPS of 93W-5.6Ni-1.4Fe powders. The activation energies for densification were calculated and compared with the experimental values for diffusion and other mass transport phenomena. The results show that for the slowly heated specimens [heating rate <100 K min−1 (100 °C min−1)], densification occurs mainly through dissolution–precipitation of W through the matrix phase and W grain boundary diffusion. The concurrent grain growth is dominated by surface diffusion at a low sintering temperature and by solution–reprecipitation and Ni-enhanced W grain boundary diffusion at a higher temperature. For the specimens sintered with heating rates higher than 100 K min−1 (100 °C min−1), the apparent activation energy value for the mechanism controlling densification is a strong function of the relative density, and fast densification controlled by multiple diffusion mechanisms and intensive viscous flow dominates over the grain growth. High SPS heating rate is favorable to obtain high density and fine-grained tungsten heavy alloys.

Sintering and electrochemical performance of Y2O3-doped barium zirconate with Bi2O3 as sintering aids

Decreasing Y2O3-doped barium zirconate (BZY) sintering temperature is crucial for its implementation in solid oxide fuel cells (SOFC). Bismuth oxide as sintering aid for BZY is evaluated. It is found that 3 mol% Bi2O3 doped BZY (BZY-3) improves its shrinkage from 10.4% to 19.02% at about 1480 °C, with the maximum shrinkage rate increasing from −0.20 mm min−1 to −0.34 mm min−1. The activation energy for densification is 5.40 eV for BZY-3. The SEM images confirms that it is porous for BZY without Bi2O3 (BZY-0), but that less small pores could be seen for BZY-3 and 5 mol% Bi2O3 doped BZY (BZY-5) sintered at 1400 °C for 24 h. Bi2O3 addition decreases the grain boundary conductivity for BZY-3 and BZY-5 obviously, but is less significant for BZY-2. It is 1.13 × 10−3 S cm−1 for BZY-2 in wet hydrogen atmosphere at 600 °C sintered at 1400 °C for 24 h, nearly equal to that of BZY-0. The open circuit voltage is 0.95 V, with the ionic transport number of 0.83 for BZY-2 under SOFC operating conditions.