Investigation Of Regulating On Microstructure Of Rare Earth-doped Yttrium Aluminum Garnet Ceramics And Nano-powders And Their Optical Performance

Yttrium aluminum garnet?Y₃Al₅O₁2,YAG?is an important functional material.It has a very extensive application in civilian field,and occupies very important position in national defence science and technology field.The performances of Nd³⁺ doped YAG transparent ceramics,which can be applied as laser gain medium,are better than those of Nd:YAG single crystal.Nd:YAG transparent ceramics will be important alternative materials for high power laser.And the rare earth ion doped YAG phosphors can be used for light-emitting diode?LED?illuminant lighting.High light transmittance is a basic requirement for realizing the laser generation of Nd:YAG ceramics.The microstructures,especially grain size,grain boundary and porosity,are the determinants of ceramics transparency.In the preparation of laser ceramic,the properties of raw powder,such as purity,size and distribution,dispersity,morphology and crystallinity,can affect the microstructure of ceramics.Adding sintering aids into the nano-powder could improve sintering and regulate microstructure.Although a large number of literature have discussed the effects of sintering aids on the microstructure and performance of laser transparent ceramics,there are very small amount of investigation of synergistic effect of compound sintering aids.The particle size,morphology and crystallinity of rare earth doped YAG nano-phosphors influence directly its fluorescence properties.The phosphors prepared by solid state method,although with good crystallinity and high fluorescence quantum efficiency,are difficult in application due to their large particles.However,the phosphors prepared by wet chemical method with small particles possess poor crystallinity and low fluorescence efficiency.Existing technology for fluorescent nano-powder preparation could not meet the requirement of controllable morphology and particle size,high purity,good dispersity,high crystallinity phosphors.Therefore,in this thesis,rare earth doped YAG material is the main research object.By investigating and discussing the preparation technology of YAG materials,the regulating on microstructure of Nd:YAG transparent ceramics and controllable preparation of YAG:Ce nano-rod become reality,and the reaction mechanisms are uncovered.The main contents of this thesis are as following:1.The synergistic effect of tetraethoxysilane?TEOS?and magnesium citrate?MC?on microstructure modification of Nd:YAG transparent ceramics leading to high optical performance.?1?Neodymium doped yttrium aluminum garnet?Nd:YAG?transparent ceramics were fabricated by a reactive sintering method in vacuum using commercial Al₂O₃,Y2O3 and Nd2O3 powders as raw materials.TEOS and MC with various ratios were used as the sintering aids.The synergistic effects of the sintering aids on the microstructure of the samples and the performances of Nd:YAG transparent ceramics were discussed.Besides reducing the sintering temperature and period,the Si and Mg with an appropriate proportion could regulate the grain size and distribution,eliminate the micro-defects and promote the ceramic densification to improve the performance of Nd:YAG transparent ceramics.The additive with TEOS and MC in the ratio of 2:1 can realize the 2 at.%Nd:YAG transparent ceramic with relatively uniform grain size,pore-free microstructure,impurity-free and perfect optical performance.The optimized sample,whcih is 2 at.%Nd:YAG transparent ceramic with 7.5 wt.‰ TEOS and 3.75 wt.‰ MC,is with the average grain size of 5.5 ?m,and with the maximum transmittance of 83.7%at 1064 nm and 79.2%at 400 nm.The laser performance with such a Nd:YAG ceramic rod?3.0 mm × 3.0 mm × 6.0 mm?was demonstrated at 1064 nm with a slope efficiency of-25.2%under the resonant pumping at 808 nm.?2?By investigating the relationships between sintering aids,microstructure of the samples and performances of Nd:YAG transparent ceramics,a synergistic effect of TEOS and MC with an appropriate proportion was proposed in the fabrication of Nd:YAG transparent ceramics,which is mostly determined by the value equilibrium.The acting substance of synergistic effect of TEOS and MC sintering aids on microstructure of transparent ceramic is the substitution of Al3-in the Nd:YAG by Si⁴⁺ and Mg²⁺ ions.The doping of Si⁴⁺ and Mg²⁺ can regulate the grain growth rate and improve the ceramic densification.However,the radius and charge of doping ion are different.With an appropriate ratio of Si⁴⁺ and Mg²⁺,the doping can counteract the lattice micro-strain induced by the Nd³⁺ doping and compensate the vacancies and charges caused by ions substitution.The synergistic effect can be applied for fabrication other kinds of transparent ceramic.2.Investigation on controllable preparation of YAG nano-powder and the reaction mechanism of core-shell structure precursor.?I?The granular YAG nano-powder with uniform size,uniform composition,good crystallinity,good dispersity and excellent sintering activity was prepared by calcining the YAG precursor with Al₂O₃/Y-compound core-shell nanostructures,which was synthesized via a partial wet chemical route using Al₂O₃ nanoparticles as template and Al source,Y?NO₃?₃ solution as Y source and urea as precipitant.The Al₂O₃/Y-compound core-shell structure of YAG precursor and the evolution process of the reaction were characterized and confirmed by XRD,SEM,TEM and FTIR tests.?2?The mechanism of the core-shell structured yttrium aluminum garnet?YAG?precursor formation is intensively studied.By simulating the reaction conditions with urea and NH₄HCO₃ as precipitants,a two stages formation mechanism with different dynamic processes is proposed to describe the core-shell structure formation process.First.the surface of Al₂O₃ particles adsorb carbonate and hydroxyl groups and form some tiny Y-compound nuclei on the surface of Al₂O₃ nanoparticles before the burst nucleation occurring.Second,burst nucleation process occurs at urea decomposition temperature,and plenty of tiny Y-compound nuclei are formed simultaneously all over the suspension.By self-assembling of the Y-compound nanoparticles on the surface of the precoated Al₂O₃ nanoparticles,the Al₂O₃/Y-compound core-shell nanostructures are formed.During the most of the whole process,the electrostatic attraction between the opposite surface charges of Al₂O₃ and Y-compound nanoparticles is deemed to one apparent driven force.However,the high surface energy of Y-compound nanoparticles originated from the burst nucleation process plays a key role to complete the core-shell structure formation.Surface modification with negative charged PAA-will block both the precoating process and assembling process,leading to the failure in the core-shell structure formation.By making the core-shell structure formation clear,the partial wet chemical route with a burst nucleation process can be applied on the morphology controllable synthesis of many other binary oxides.3.Investigation on YAG:Ce nanorod phosphors prepared via a partial wet chemical route with good fluorescence properties and biolabeling applications.?1?One-dimensional YAG:Ce nanorod phosphors were prepared via a two-step procedure involving a hydrothermal method and a partial wet chemical route.First,Al₂O₃ nanorods were prepared by calcining one-dimensional NH₄AI?OH?₂CO₃,which was controllable synthesized by hydrothermal method using Al³⁺ solution and urea as precipitant.Second,YAG:Ce nanorods were prepared via a partial wet chemical route followed by a calcination process by using Al₂O₃ nanorods as both templates and the reactant and urea as precipitant.These novel well-crystallized YAG:Ce phosphors with a 200-300 nm diameter and a 2-3 ?m length have a high specific surface area while being virtually devoid of surface defects due to their high crystallinity.The evolution process of the reaction were characterized and uncovered by XRD,SEM and TEM tests.?2?The YAG:Ce nanorod phosphors possess good luminescent properties compared with granular YAG:Ce phosphors.The photoluminescence QY of YAG:6 at.%Ce³⁺ and YAG:2 at.%Ce³⁺ nanorod phosphors are 30.28%and 40.12%,respectively,which are higher than those of granular ones.The reason is that although the specific surface area of nanorod phosphor is smaller,the crystallization of nanorod phosphor is better and the surface defects of nanorod phosphor is less than that of granular ones.Due to the unique morphology,YAG:Ce nanorod phosphors possess two luminescent decay times,a short lifetime and a long lifetime.These phosphors were found to be able to label bone marrow mesenchymal stem cells efficiently and exhibit a relatively good biocompatibility.The good luminescent properties and biocompatibility of these YAG:Ce nanorod phosphors indicate that they are promising candidates for a more systematic biolabeling usage.