XRD pattern showing the purity, crystallinity and two-phase nature of heat-treated MgO-Y2O3 composite powders synthesized by spraying. With these materials, the grain plays an important role in the transparency and mechanical durability of the windows.
Synthesis of Magnesium Oxide
Much research has been done on the heat treatment temperature for the decomposition of magnesium hydroxide to magnesium oxide. The heat treatment temperature plays an important role in the processing behavior of the powder and ultimately in its sinterability.
Synthesis of Yttrium Oxide
Wen et al.58 studied the effect of mixing process on the particle size and morphology of yttrium oxide. Sohn et al.57 found that the concentration of yttrium ions during synthesis influences the final particle size and morphology of the precipitated yttrium.
Synthesis and Importance of the Magnesia-Yttria System
The advantages of using a nanocomposite are evident through the improvement of the mechanical properties and the maintenance of the optical properties of a consolidated sample. This has been seen in the case of the MgO/Y2O3 system, where the mechanical properties are greater than those of any individual component and the optical properties are preserved.
Reverse Micelle Synthesis of Oxide Ceramics in the
Many have studied and confirmed the fact that the ratio of water to surfactant can change the size and shape of the particles. Gardner et al.94 performed molecular dynamic simulations to study the size and shape of reverse micelles as affected by the water-to-surfactant ratio of the AOT-isooctane-water reverse micelle system.
Spark Plasma Sintering of Optical Ceramics
SPS of Optical Ceramic Oxides
A similar study by Jiang and Mukherjee70 determined that the infrared transmission results were excellent, which they attributed to the small grain size and homogeneity of the microstructure in the same system. With recent capabilities to achieve pressures greater than 150 MPa in SPS, the effect of high pressure was explored.
Solution Synthesis
- Chemical Precipitation
- Spray Precipitation
- Collection and Washing
- Milling
- Reverse Micelle Synthesis
- Heat Treatment
An excess of five times the required amount of ammonium hydroxide is added to deionized water to make a 50% diluted solution. Ethanol is added to the vials, which are then shaken and sonicated to deagglomerate and remove any remaining ions. This mixture is added to a polyethylene plastic bottle with 3 mm zirconium milling media added ¼ the height of the bottle.
The supernatant was then decanted and water was added to the vial, which was then shaken and ultrasonicated.
Spark Plasma Sintering
Conventional Low Pressure SPS
The drying was then placed in a controlled set-point oven (F1848, Lindberg, Riverside, MI) at low temperature (less than 100°C) and rapidly heated (approximately 60°C/min) to the temperature of specified.
High Pressure SPS
The reverse micelle powders were sintered at 1000°C and 500 MPa, according to the same parameters as the previous high pressure experiments.
Characterization
Powder
A thermogravimetric instrument (SDT 2960 Simultaneous DSC-TGA, TA Instruments, New Castle, DE) was used to obtain calorimetric measurements. A surface area and porosity analyzer (TriStar, Micromeretics, Norcross, GA) with 11-point multipoint analysis was used for surface area measurements. Digital image analysis software (ImageJ, National Institutes of Health, Bethesda, MD) was used to measure grain size.
A water immersion density measurement technique (ASTM C20) was used to determine the density of the sintered samples.130 The samples were boiled in water for 2 hours and allowed to cool and soak overnight.
Chemical Precipitation Synthesis
Magnesium Oxide
The suspension medium plays an important role in the dispersion of the powders and ultimately the measurement of the particle size. Shown in Figure 16, the synthesized powders are large and agglomerated, which is also evident in the DLS data. The morphology of the particle appears to consist of multiple crystallites agglomerated together with rounded edges.
Using BET surface analysis techniques, the surface area of the precipitated, calcined MgO powder was determined to be 72.8 m2/g.
Yttrium Oxide
Therefore, it seemed logical that a composite of these two materials could be produced by a co-precipitation method. TGA plot showing the weight loss versus temperature, indicating the decomposition temperature of yttrium hydroxide to yttrium oxide. XRD pattern showing the purity, crystallinity and mineralogy of heat-treated Y2O3 synthesized by chemical precipitation method.
Magnesium Oxide-Yttrium Oxide Composite
Using BET surface analysis techniques, the surface area of the precipitated heat-treated MgO-Y2O3 composite powders was determined to be 55.03 m2/g. Chemical precipitation and co-precipitation synthesis methods were found to be successful for the preparation of MgO, Y2O3 and MgO-Y2O3 composite materials. XRD pattern showing the purity, crystallinity and biphasic nature of heat-treated MgO-Y2O3 composite powders.
Peaks indicated by circles are representative peaks for the Y2O3 PDF with intensities of eight or more.
Spray Precipitation Synthesis
In-situ XRD patterns of as-synthesized MgO-Y2O3 composite powders by spray precipitation technique. TEM micrographs of as-synthesized and heat-treated MgO-Y2O3 composite powders synthesized by spray precipitation, showing the morphology. DLS after milling, before and after heat treatment of MgO-Y2O3 composite powders synthesized by spray precipitation.
SEM of sintered sample showing improved homogeneity after milling of the MgO-Y2O3 composite powders synthesized by spray precipitation.
Reverse Micelle Synthesis
Stability Studies
Another factor affecting the destabilization concentration of the reverse micelle system is the ratio of water to surfactant. Using yttrium nitrate as the ionic solution, three different ratios of water to surfactant and their destabilization concentrations were investigated. To remain within the stable range during the synthesis of the composite powders, a total concentration of 0.3 M was chosen for the metal nitrate solution.
This molarity allows ammonium hydroxide to be added to the precipitation system and still remain within the stable concentration range.
Precipitation in Reverse Micelles and Characterization
In-situ high temperature (ranging from room temperature to 1000˚C) XRD patterns of as-synthesized MgO-Y2O3 composite powders synthesized by reverse micelle technique. XRD pattern of calcined MgO-Y2O3 composite powders synthesized by reverse micelle technique, calcined at 1000 °C for one hour. DLS of synthesized and heat-treated (600˚C and 1000˚C) MgO-Y2O3 composite powders synthesized by reverse micelle technique.
TEM micrographs of (a) as-synthesized and (b) 600°C and (c) 1000°C calcined MgO-Y2O3 composite powders synthesized by the reverse micelle technique.
Spark Plasma Sintering
Low Pressure Sintering
High Pressure Sintering
SEM of the fracture surfaces of MgO-Y2O3 composite powders synthesized by the reverse micellar technique, sintered using the high-pressure SPS technique at 500 MPa (a) fine-grained microstructure (b) mixed microstructure. The final grain sizes for the spray deposition powders were measured, averaged, and presented in Figure 51. In all cases, the average grain size is around 60 nm, which is the initial average particle size of the powders.
The other region contains large plate-like grains, with an average thickness of 75 nm and an average width of 700 nm.
Reverse Micelle Stability
To vary the water to surfactant ratio, the water concentrations remained the same and the amount of surfactant was varied. Bohidar et al.83 studying the same aqueous system isooctane-AOT- used DLS characterization as a method to determine the micelle size in relation to water to surfactant ratio between 1 and 50. The first trend in Figure 37 shows a decrease in size of the micelle with an increase in the water-surfactant ratio.
As the ratio of water to surfactant increases, less surfactant is present in the system and the micelle solution cannot stabilize at the desired micelle size due to the lack of surfactant.
Collection and Washing
Importance of Solvents
However, they did not provide insight into stability or study the effect of metal ions in the water domains. Depending on the ion concentration, the reverse micelle solution is stabilized to a certain size due to the charge effects of the ions on the surfactant. When the synthesis reaction is carried out in the reverse micelle, the stability of the solution is crucial to produce unimodal and uniform materials.
After the addition of the precipitating solution, the reverse micelle size decreases slightly, as seen in Figure 38.
Inhomogeneity
In the case of water, the surface remains with an oxygen bridge, causing aggregation, while ethanol on the surface leads to a hydrogen-bonded bond, which is less likely to result in bonding and lead to aggregation. Since the use of ethanol will help limit the aggregation of the powders, the powders are washed once in ethanol to remove all water from the surface of the powders after synthesis in water. The powders are ground by hand using a pestle and mortar after drying and calcination, however this method is not efficient enough to homogenize the powder.
It is used as a suspending medium in the grinding procedure to further remove water from the surface of the particles.
Reverse Micelle Washing
Calcination Studies
Thermogravimetric Analysis
The total weight loss of the reverse micelle synthesized powders (Figure 40) is approx. 10% greater than the chemical precipitation methods. This additional weight loss is attributed to the removal of residual organic substances or hydrocarbons from the surface of the particles. The powders have a larger surface, so more organic substances will be absorbed on the surfaces.
X-ray Diffraction
Importance of Calcination Temperature
Crystallization of the powders also leads to cracking of the particles which further reduces the size. Change in average particle size before and after characterization for (1.) MgO by chemical precipitation (2.) Y2O3 by chemical precipitation (3.) MgO-Y2O3 by chemical precipitation (4.) MgO-Y2O3 by spray precipitation (5.) MgO- Y2O3 by spray precipitation, milled (6.) MgO-Y2O3 by reverse micelle synthesis. After calcination of the composite, the yttrium component had a smaller crystallite size compared to magnesia.
Comparison of particle size with crystallite size for (1.) MgO by chemical precipitation (2.) Y2O3 by chemical precipitation (3.) MgO-Y2O3 by chemical precipitation (4.) MgO-Y2O3 by spray precipitation (5.) MgO- Y2O3 by reverse micelle synthesis (Note: . The size of the Y2O3 crystallites was not measured).
Comparison of Powders by Different Synthesis Methods
However, this method is difficult because washing and collection leads to agglomeration of the powders, resulting in a particle size that is similar to that of spray precipitation powders. Of the three methods investigated for composite material synthesis, the one that resulted in the smallest particle size, obtained by DLS, was the powders synthesized by spray precipitation. The particle size of the spray precipitation powders is smaller, but the crystallite sizes of the reverse micelle powders are smaller, Figure 56 and Figure 55.
The severity of agglomeration, meaning hard or soft, is important as it affects the sintering of the powders.
Spark Plasma Sintering
Low Pressure SPS
The fourth part indicates the completion of the hash, and the remaining hold ensures that the hash is complete. The power is turned off, the force is slowly released and the die cools down naturally. In this segment, displacement begins just below the sintering temperature and the rate of displacement is fastest at the final sintering temperature (beginning of the second segment).
The power is turned off, the pressure is slowly released and the die cools down naturally.
Comparison of Results
In the melt of the composite material, the density increases only slightly with increasing pressure. The stability of the reverse micelle system is extremely important for ceramic composite synthesis. By using high pressures during sintering, the final sintered grain size can be related to that of the initial powder particles and crystallite size.
Changes can be made to the mold that can help optimize the sintering cycle.
