Apparent Activation Energy For Sintering Research Articles

Densification process of a 95% alumina ceramic during the intermediate stage of sintering in air, and O2

Sintering in pure O2 can enhance the densification of oxide ceramics during the final stage of sintering, but little attention has been paid on its effect on the densification process during the intermediate stage of sintering. This paper measures the shrinkage processes of a 95% alumina ceramic during the intermediate stage of sintering in air and O2 at different heating rates, and analyzes them by the master sintering curve (MSC) theory. It is found that the densification of the alumina ceramic sintered in air is obviously rapider than that in O2 at the heating rate of 15 K/min, while it is slower than that in O2 at the heating rate of 2.5 K/min. MSC analysis reveals that the apparent activation energy of sintering in O2 is lower than that in air, and this might be attributed to it that O2 restrains the diffusion of aluminum vacancies together with oxygen vacancies stoichiometrically, but favors the diffusion of aluminum vacancies and the transformation between O2 and aluminum vacancies on the surface.

Sintering of Ce3+-doped yttria nanoparticles prepared by precipitation method

Cerium doped yttrium oxide nanoparticles with various Ce3+ concentrations between 0.001 and 0.010 at% have been synthesised by precipitation method using ammonium hydroxide as a precipitation agent. The synthesised powders are characterised by a mean particle size of ca. 55 nm. Highly dense specimens, with a relative density> 98.8%, were obtained by sintering the green compacts shaped by pressure filtration, at 1550 °C for 3 h in air. The sintering behaviour of Ce3+ doped Y2O3 was studied by constructing Master Sintering Curves (MSC); the results showed that the apparent activation energy of sintering for Ce3+ doped Y2O3 increases with the increase of cerium concentration. The segregation of larger Ce3+ cations in the grain boundaries is likely to be responsible for the increase in the sintering activation energy.

Analysis of sintering behavior of alumina green bodies molded under a strong magnetic field based on master sintering curve theory

Understanding and controlling the anisotropic sintering shrinkage behavior of the green body in the strong magnetic field molding technique bring out the superior functions of ceramics. This study aimed to elucidate the sintering behavior of green bodies molded under a strong magnetic field using the master sintering curve (MSC) theory. Green bodies were prepared to obtain MSCs in the directions perpendicular and parallel to the applied magnetic field. A unique MSC of the alumina green bodies molded with and without a magnetic field was obtained using sintering shrinkage ratio estimated by various heating rates. The apparent activation energy of sintering in the direction perpendicular and parallel to the applied magnetic field was 663 kJ/mol, independent of the measured direction, which is higher than that without an applied magnetic field (562 kJ/mol). The (0001) planes of the sintered body obtained from the green body molded in the magnetic field were oriented perpendicular to the magnetic field, whereas the samples without a magnetic field were randomly oriented. Consequently, we found that the grain boundaries with high consistency in the sample applied to a magnetic field should increase the apparent activation energy because of its grain orientation.

Water‐assisted cold isostatic pressing to enhance sinterability of alumina ceramics

AbstractWater‐Assisted cold isostatic Pressing (WAP) enabled pressure‐less sintering of fine grained (0.6 μm) and nearly full dense (99.3%) weakly translucent alumina ceramics. As a term of comparison, Dry Pressed (DP) samples, prepared under identical processing conditions, maintained an opaque appearance, and their relative density did not exceed 97.7%. The enhanced compaction of WAP allowed to lower the sintering temperature down to 1350°C, which is approximately 100–150°C lower than that of DP powder. WAP samples resulted in an apparent activation energy for sintering (640 kJ/mol) matching literature values, confirming that the enhanced sinterability was attributed to an improved green density rather than a change in sintering mechanisms.

The Master Sinter Curve and Its Application to Binder Jetting Additive Manufacturing

AbstractThe master sinter curve (MSC) is an empirical model used to predict the density of a part after being sintered. The model is typically applied to components that undergo isotropic shrinkage. Parts manufactured using binder jetting additive manufacturing (BJAM) are known to have nonuniform powder systems and high levels of anisotropy. This work explores the application of the master sinter curve to components made by BJAM. Cylindrical samples were manufactured with the long axis parallel (vertical), perpendicular (horizontal), and 45 deg to the printing direction. A bimodal blend of titanium powder (0–45 µm and 106–150 µm) was used to make samples with consistent green densities (ranging from 47.2% to 52.3%) between the different orientations. Samples were then sintered at heating rates of 1, 3, and 5 °C/min to a maximum of 1400 °C. After sintering, the samples showed significant variation between the different orientations, with vertical samples on average 7.6 ± 2.98% and 4.7 ± 1.20% denser than the horizontal and the 45 deg samples, respectively. The calculated apparent activation energies for sintering were within the same range for all orientations, 200–260 kJ/mol for vertical and 45 deg, and 140–260 kJ/mol for horizontal samples. Validation sinter runs showed that the density prediction errors of the master sinter curves were between 0.9% and 4.3%. This work shows that the master sinter curve can be applied to predict the sintered density of components manufactured by binder jetting additive manufacturing.

Manufacturing of fine-grained titanium by cryogenic milling and spark plasma sintering

Commercially pure (CP) gas-atomized titanium powder was processed by wet cryogenic milling in liquid argon and compacted by spark plasma sintering. The time-dependent sintering evolution at different temperatures was evaluated by using the master sintering curve (MSC) approach with the aim of achieving a material with maximum relative density and minimum grain size. Carrier-gas hot extraction (CGHE) confirmed a purity consistent with ASTM standard of Grade 4 CP Ti. Grain size and texture were determined by EBSD. An apparent activation energy of sintering of 115 kJ/mol was found based on the MSC approach. It is significantly lower than the activation energy of self-diffusion in Ti. This is attributed to an enhanced diffusion rate due to high concentration of defects in the powder after milling. The relative density was correlated with the resulting grain size and a general trade-off relationship between achieving high relative density and maintaining small grain size was found. The distribution of oxygen after milling and subsequent sintering at low temperatures is heterogeneous as determined by complementary XRD, CGHE and microhardness measurements. The distribution of oxygen becomes homogeneous with increasing sintering temperature. The microhardness of the material was shown to depend on residual porosity, the content and distribution of oxygen and also on texture. Processing parameters of milling and sintering were optimized to achieve fully dense, fine grained material with a low contamination by nitrogen and oxygen.

A differentiating approach to the master sintering curve analysis

The master sintering curve (MSC) has been widely applied in analyzing and predicting the densification process during sintering. The usual approach to the MSC analysis supposes that the apparent activation energy of sintering Q keeps constant during the whole sintering process, but some studies reported that the density influenced Q. This paper proposed a differentiating approach to the MSC analysis, in which Q was taken as a function of the density or the linear shrinkage of the compact. The MSC of a SiO2-CaO co-doped alumina ceramic during sintering was built by the differentiating approach and the usual approach. It was found that the differentiating approach was more accurate in predicting the densification process than the usual approach, and it was less time-consuming in the numerical computation than the usual approach. The evolution of Q with the density calculated by the differentiating approach can be used to analyze the effect of the density on the densification rate during sintering.

Analysis of the sintering process in gadolinia-doped ceria by thermodilatometry and correlation with microstructure evolution

The non-isothermal sintering process of cerium dioxide containing gadolinium sesquioxide powders within a wide range of specific surface area was investigated by dilatometry. Linear shrinkage data of powder compacts were recorded under several constant rates of heating. Dilatometry data were analyzed by two methodologies enabling to preview the relative density for any temperature/time profile, and determination of the apparent activation energy for sintering. Correlation of dilatometry results with microstructure evolution was also carried out. Remarkable differences in sintering powders with different specific surface areas were found. The apparent activation energy for sintering increases with decreasing specific surface area and, in most cases, it does not change significantly in the approximately 70–85% range of relative density.

Effect of Heating Rate on the Pressureless Sintering Densification of a Nickel-Based Superalloy

Pressureless sintering of Inconel 718 has important technological applications for the densification of metal injection molding or additive manufacturing of parts with powder/binder systems. The effect of heating rates ranging from 15 to 200 K/minute on the sintering behavior of fine (−325 mesh) Inconel 718 powders was studied using the master sintering curve (MSC) concept. A pressureless pulsed electric current sintering setup was used to heat samples. The temperature at the onset of sintering increased as the heating rate increased. The formation of a supersolidus liquid fraction was shifted toward higher temperatures for increased heating rates. The apparent activation energy of sintering was obtained by least squares fitting of the sintering data to the MSC and was in good agreement with the lattice diffusion activation energy of the alloying elements present in Inconel 718. The MSC followed different kinetics for low heating rates (≤50 K/minute) and high heating rates (≥75 K/minute), and these differences were related to liquation kinetics.

Master sintering curves for UO2 and UO2–SiC composite processed by spark plasma sintering

Master sintering curve (MSC) theory has been applied successfully to UO2 and UO2–SiC composite ceramics processed using spark plasma sintering (SPS). By applying the constant heating rate method, where the powder compact was heated at a constant rate to a specified maximum temperature and then cooled naturally, the apparent activation energies for sintering have been determined to be 140kJ/mol for UO2 and 420kJ/mol for UO2–SiC composite. The ability of the derived MSCs to control and predict final density in the sintered compact was demonstrated by additional experimental runs using the isothermal heating method, in which the powder is held for a specified time at the maximum sintering temperature. It is shown that the master sintering curve is an effective tool to characterize the densification and sinterability behavior of a given nuclear powder compact during spark plasma sintering. The reason for significantly lower activation energy in SPS processed UO2 pellets compared to conventional sintering has been rationalized on the basis of field activation in SPS process.

Densification of fine-grained alumina ceramics doped by magnesia, yttria and zirconia evaluated by two different sintering models

Abstract The influence of various dopants (500 ppm MgO and Y2O3; 250 ppm ZrO2) on sintering of fine-grained alumina ceramics was evaluated by high-temperature dilatometry. The apparent activation energy of sintering was estimated with the help of Master Sintering Curve and a model proposed by Wang and Raj. The densification kinetics was controlled by at least two mechanisms operating at low (higher activation energy) and high (lower activation energy) densities. Good agreement between the activation energies calculated with both models was observed for low as well as for high densities. The lowest value of activation energy exhibited undoped alumina; the addition of MgO resulted in slight increase of the activation energy. Y2O3 and ZrO2 significantly inhibited the densification, which was reflected in the higher sintering activation energies. The low activation energies in the final sintering step indicates the importance of proper choice of sintering temperature, namely in the two-step sintering process.

Sintering and grain growth kinetics in undoped and silica doped zinc oxide ceramics

Dilatometric sintering and grain growth kinetics of undoped and silica doped (2, 4, 6wt% respectively) zinc oxide samples were systematically investigated. Silica (SiO2) as a dopant retards the densification and changes the sintering behavior of zinc oxide (ZnO) ceramics by increasing the apparent activation energy of sintering and changing densification mechanism at the initial stage. At the same time, SiO2 addition restricts the grain growth of ZnO compacts by influencing the mechanism and activation energy of the process. It is observed that the increase in temperature of sintering was more crucial for grain growth than the isothermal holding time. These effects were pronounced up to 4wt% SiO2 doping, and beyond this level, there was a tendency of saturation (marginal change) regarding the shrinkage behavior as well as grain growth of the compacts.

Sintering kinetic study of the nano-crystalline 3-mol% yttria-samaria codoped tetragonal zirconia polycrystal ceramics

Dilatometric shrinkage data was utilized to study the sintering kinetics of the in-house synthesized nano-crystalline 3-mol% yttria-samaria codoped tetragonal zirconia polycrystal (TZP) ceramics. The objective was to determine activation energy (Q) of sintering and the sintering mechanism (n) relevant to the initial stage of sintering. The product of activation energy and sintering parameter, i.e., “nQ” was calculated from the shrinkage data acquired from the constant rate of heating experiment. The apparent activation energy of sintering (Q) was calculated using modified-“Dorn” method. Modified Johnson’s equation was used to determine value of “n” using the activation energy obtained from the Dorn method. Stepwise isothermal dilatometry technique was utilized as an independent method to determine the “n” value. The activation energy of sintering was in the range of 400–525 kJ mol−1 and found to be dependent on the dopant concentration. The value of “n” was found to be ~0.33 for both 3 mol% yttria-doped (3Y-TZP) and yttria-samaria codoped (3(Y,S)-TZP) TZP, whereas for 3 mol% samaria-doped tetragonal zirconia (3S-TZP), the value of “n” was ~0.40. From the obtained “n” values, it may be concluded that grain boundary diffusion (GBD) was the dominant sintering mechanism in 3Y-TZP and 3(Y,S)-TZP, whereas an intermediate of GBD and volume diffusion influences the initial sintering stage in the 3S-TZP.

Effects of Y2O3 on SiC/MoSi2 composite by mechanical-assistant combustion synthesis

MoSi2 based materials are considered as a potential high temperature structural parts. In this work, a 0.5wt% Y2O3–20vol% SiC/MoSi2 composite was successfully prepared by pressureless sintering from mechanical-assistant combustion synthesized powders. Adding a small amount of Y2O3 to the SiC/MoSi2 composite decreased the apparent activation energy of sintering by 10.4%, resulting in a denser composite with finer grains. The relative density, flexural strength, Vickers hardness and fracture toughness of 0.5wt% Y2O3–20vol% SiC/MoSi2 increased by 5.3%, 27.7%, 27.2% and 35.8% as compared to 20vol% SiC/MoSi2, respectively. The oxidation mass gain of Y2O3–20vol% SiC/MoSi2 at 1200°C was higher than that of 20vol% SiC/MoSi2 for 16.9%, while it still exhibited very good oxidation resistance at this temperature.

Sintering behavior of an ultrafine alumina powder shaped by pressure filtration and dry pressing

Master sintering curve (MSC) was used as a practical approach to analyse the sintering behavior of an ultrafine alumina powder. Non-isothermal sintering was carried out at three heating rates. The concept of the MSC has been used to investigate the effect of different shaping methods, i.e. dry pressing (DP) and pressure filtration (PF) on the apparent activation energy for sintering. Although no change in the dominant densification mechanism is expected, the interplay between different diffusion mechanisms taking place during the heating ramp may be affected by the shaping method. Values of 605 ± 15 and 700 ± 20 kJ/mol were determined for PF and DP samples, respectively. The lower activation energy for PF samples may be explained by the higher green density and homogeneity, smaller pore size in comparison with the DP samples. Furthermore, the mechanical properties (hardness and fracture toughness) of the samples are presented.

Application of Work-of-sintering concepts in powder metals

One of the current challenges facing the particulate materials industry is developing simple, accurate models to predict sintered properties. Work-of-sintering concepts, where time-temperature integrals are used in such models, offer a solution to this problem. In this study, the master sintering curve concept is applied to several powder metal systems: 17 to 4PH stainless steel, 316L stainless steel, nickel, niobium, tungsten heavy alloys with two different compositions, and molybdenum. A detailed explanation of the construction of these curves is given, including methods used to calculate the apparent activation energy for sintering and to curve-fit experimental data to a sigmoid function describing the master sintering curve. Discussion of the results shows that the master sintering curve can be applied to powder metal systems, even those that use liquid phase during sintering.

Densification behavior of tungsten heavy alloy based on master sintering curve concept

The master sintering curve (MSC) theory is modified by substituting the densification ratio (φ) for the densification parameter (ψ) to identify regions where shrinkage occurs by a similar combination of sintering mechanisms. The modified MSC theory is used to analyze the results of dilatometry experiments conducted with W-Ni-Fe heavy alloys, in which a phase change occurs during sintering. Apparent activation energies for sintering in three regions (solid state, transition, and liquid phase) are calculated. These activation energies are compared with experimental values for diffusion and other mass-transport phenomena to identify the dominant mechanisms in each region. A series of master sinter curves for varying W contents are developed into a master sintering surface that includes tungsten content and integral work.

Control of Shrinkage during Sintering of Alumina Ceramics Based on Master Sintering Curve Theory

The Master Sintering Curve (MSC) is quite useful for analyzing the shrinkage behavior of ceramics. It is possible to compare shrinkage behavior using MSCs that are obtained from different firing profiles. In this study, shrinkage behavior during sintering of green bodies of several kinds of Al2O3 based ceramics were evaluated, using an electric furnace equipped with a dilatometer to be controlled based on the MSC theory. Although all of the samples shrank monotonically, shrinkage behavior depended on the additive and heating rate. The MSC theory was applied to analyze shrinkage behavior. As a result, a different MSC could be obtained in Al2O3 with and without the addition of MgO. In the pure Al2O3, a single MSC could be obtained from shrinkage curves by firing at a heating rate of 7.5-20oC/min, though the shrinkage curve at a heating rate of 3-5oC/min did not correspond with the MSC. In contrast, shrinkage curves at heating rate of 5-20oC/min were converged in the case of the MgO doped Al2O3 to obtain a unique MSC independent of firing profile. Apparent activation energy for sintering was estimated as 555 kJ/mol in the pure Al2O3 and 880 kJ/mol in the MgO doped Al2O3. The firing profile to obtain a requested sintering shrinkage curve was predicted from the resultant MSC. A comparison between the predicted and the experimental shrinkage curves, showed good consistency, thus confirming that it is possible to control shrinkage behavior using the MSC.

Sintering of ultrafine zirconium diboride powder prepared by modified SHS technique

Pressureless sintering of ultrafine zirconium diboride ZrB2 powder produced by a self-propagating high temperature synthesis (SHS) route was investigated at 1973–2223 K with continuous flow of pure argon gas (20 mL min−1). Ultrafine ZrB2 powder produced by the addition of an inert SHS diluent showed excellent sinterability, and a maximum densification of 97·5% was observed at 2223 K. In contrast, the identically sintered diboride powder produced without the addition of SHS diluent showed only 81·2% densification. The apparent activation energy for sintering was found to vary with particle size from around 37 to 43 kJ. Boron diffusion through ZrB2 is likely the rate controlling factor for the sintering reaction.

Synthesis and sintering characteristics of Y?Ba?Cu-oxide superconductors

Y–Ba–Cu-Oxide (YBCO) powder were prepared by three different techniques and the isothermal sintering characteristics of the synthesized powder were systematically investigated using a thermo-mechanical analyzer (TMA). Expectedly the apparent densities of the sintered samples increased with the decrease of particle sizes of the synthesized powder due to high specific surface energy. The nano-sized powder generated by a chemical pyrophoric reaction exhibited excellent sinterability than the powder synthesized by either the conventional ceramic process (CCP) or the co-precipitation process (CP). The variation of apparent activation energies for sintering (Q) and sintering kinetic parameter (n) with the change of particle sizes of the green compacts indicated differing nature of mass transport phenomena operating in the system during sintering. The apparent activation energies for sintering were estimated from the respective Arrhenius plots.