Microstructure Control Of Y₂O₃Nanopowders Via Urea Precipitation Method And Its Application In The Preparation Of YAG Ceramics

Neodymium doped yttrium aluminum garnet (Nd:YAG) and yttria (Nd:Y2O3) are of important laser gain mediums. With the development of laser technology, high power laser and ultrafast laser have emerged as two main fields for the development of laser science. With its potential advantageous properties in growth technology, usable sample size, doping concentration of rare earth element and composite structure, transparent ceramics have became an important substitution for laser crystals since1960s.Among all preparation method, such as solid state reaction method, sol-gel method and combustion method, etc. for transparent ceramics, wet chemical methods, especially urea precipitation methods (UPM) in solution have attracted much attention since its easy accessible procedure. However, the controllable preparation of transparent ceramic from nanopowders remains a huge challenge. People still can not understand the exact precipitation mechanism including the specific physical and chemical process, nucleation and growth process, intrinsic symmetry and surface microstructure of the precursor during the precipitation process.In this thesis, effect of ammonium sulfate on the microstructure of Nd:Y2O3precursor prepared via UPM and its determination on sintering properties of Nd:Y2O3nanopowder were intensively investigated before we expand the method to the exploration of precipitation process of Y-Al dual cations system via UPM. The author proposed for the first time a partial wet chemical for the synthesis of YAG ceramics via UPM using nano Al2O3as the nucleus for the Y3+precipitant to form a Y-compound/Al2O3core-shell structure. In the end, effects of Y2O3constitution on structure and luminescence spectra of Nd:YAG ceramics were intensively investigated to provide data and basic analysis for the further exploration of Nd ion spectroscopy. Outline for the work of this thesis are listed as follows:Microstructure Control of Y2O3Nanopowders via UPMDifferent heating rates of the solution (1.4℃/min and0.7℃/min) were implemented during the preparation of Nd:Y2O3precursors via UPM to provide optimum reaction conditions for the preparation process. It is found that higher heating rate was important for the aggregation process of the precursor. Regular spherical, uniform Y2O3precursors were prepared with higher heating rate during the precipitation process.To further explore the mechanism of (NH4)2SO4regulation effect on the precursors and following calcination step, mixed solution of urea and Y3+ions with different dosage of (NH4)2SO4(0wt%,10wt%,20wt%,30wt%,40wt%) were studied at90℃. It is found that with the addition of (NH4)2SO4, the constitution, surface zeta potential, nucleation and growth pathway of the precursor were evidently modified. With different dosages of (NH4)2SO4, coral like, branch like and rod like precusors can be prepared while only spherical precursors can be obtained in (NH4)2SO4free solution (90℃). Transparent Nd:Y2O3ceramic can be prepared from the20wt%(NH4)2SO4added sample, indicating that the softly aggregated precursor from nanoparticles are optimum for nanopowders with high sinterability.Investigation on the mechanism of (NH4)2SO4regulation effect on microstructure of Y2O3nanopowders via UPM were carried out at25℃. Room temperature growth behaviors of precursors were investigated in a period up to180days. The precursors were regulated from amorphous aggregations to nanowires through nanoparticles. Experimental evidence of the regulation process was provided for the first time.In the ammonium sulfate saturated circumstance, flower like precursor assembled by the plate-shaped particles were obtained, indicating a stereo specific growth process happened during the precipitation process. During the calcination process at650℃, Y2O2SO4crystalline plate was prepared, offering stong evidence for the regulation effect of ammonium sulfate during the calcinations process.Sintering Ability Verification of Nd:Y2O3Nanopowders Prepared via UPM in Solid State Reaction Process and Mass and Charge Transfer in Y2O3/Al2O3/H2O Nano Suspension SystemUse material with nanoscale prepared via UPM and commercial AL2O3nano powder as the raw material for solid state reaction method to verify the sintering ability of Nd:Y2O3nanopowders prepared via UPM. Highly transparent Nd:YAG ceramics with transmittance over80%were prepared via the application of this method by sintering the1100℃calcined powder at1750℃for10h. The reaction process and diffusion of atoms were strengthened during the process. The easy accessible approach made it possible to preparation of Nd:YAG ceramics with high accuracy.During the preparation process, the rheological behaviors of the Y2O3/Al2O3/H2O nano suspension system were intensively studied. Negative charge on the surface of Al2O3particles was converted to positive charge with assembling of Y2O3on the surface of Al2O3nanoparticles in the colloidal system. A proposed mechanism that the paragenesis of hydrated of Y2O3with higher Ksp would attach and migrate onto the surface of another sediment Al2O3with smaller hydrate Ksp and opposite surface charge. This analogous Ostwald ripening process was first reported by the author. This different growth phenomenon is very important for the understanding of preparation process of YAG ceramics via solid state reaction method.Precipitation Mechanism of Y-Al Dual Cations System in UPM and Al2O3assisted preparation of Y-compound/Al2O3core-shell structureEffect of ammonium sulfate on the morphology and sinterability of YAG nanopowders prepared via UPM were studied. Different ratio of (NH4)SO4/YAG ranging from0wt%to30wt%were performed during the preparation process. It is found that the morphologies of the precursors were evidently regulated by ammonium sulfate. Regular particles of precursor can be obtained as the dosage of ammonium sulfate reached25%. Uniform nanopowders of YAG with smooth surface can also be prepared from that precursors.Intensive study was carried out on the effect of aging on the properties of nanopowders. A sharp decrease of Y/Al ratio in the precursors happened during a10h time after the precipitation process and then became steady. Based on the results aforementioned,10h was set as the aging time for lateral investigation. On the other hand, adjustment of Y/Al ratio in the original solution was performed to vary the Y/Al ratio in the final nanopowders. During the sintering step, the transparency of the ceramics increased as excessive Y reached2-3%and then decreased as more Y was added into the precipitation system.Synthesis of Y3Al5O12(YAG) powders respectively presents morphology control and chemical stoichiometry problems when employing the solid-state reaction or the wet-chemical route. This dissertation proposed for the first time the nano Al2O3assisted preparation of Y-compound/Al2O3core-shell structure. YAG nanopowder with scale around200nm retaining the morphology of Al2O3powder was designed and synthesized via the partial wet-chemical process with yttrium ions precipitating on the Al2O3particles. Effects of Y2O3Constitution on Structure and Luminescence Spectra of Nd:YAG CeramicsNd:YAG ceramics with different Y2O3constitution were synthesized by the solid-state reaction method. Microstructure, constitution, transmittance spectra and luminescence spectra of the ceramics are evidently affected by different Y2O3constitution. Different Y2O3constitution leads to morphology variation corresponding to different second phases in the ceramics. Micrograins of α-Al2O3remain in the YAG grain in Y-deficient samples, while the Y-rich compound recrystallizes in the grain boundary in Y-excessive samples. Existence of second phase leads to transmittance declining and stain in the YAG crystalline structure. The emission intensity of Nd ions in the non YAG stoichiometric samples was thereby declined. The replacement of Nd ions in Y-rich compound such as YAlO3, Y4Al2O9also causes the broadening of the luminescence spectra of the samples.