Preparation, Microstructure And Laser Properties Of Nd:YAG Transparent Ceramics

In1995, Japanese scientist A. Ikesue prepared high-quality Nd:YAG transparentceramic, and for the first time realized laser output and achieved a historic breakthroughfor the development of transparent ceramics. In1999, the Japanese Konoshima chemicalcompany used Nd:YAG nano-powder from co-precipitation method as raw materials,prepared high-quality transparent ceramic and realized KW level laser output. Transparentceramics rapidly become international research hot spots. In2006, large size and highquality of Nd:YAG transparent ceramics were confirmed as laser materials for high-powerlaser weapons by the U.S. military, since then Japan, the United States and some othercountries started embargo on transparent ceramics related products. Homemade Nd:YAGtransparent ceramics achieved1W CW laser output at the wavelength of1064nm in2006,although in subsequent years, the rare earth ion doped YAG transparent ceramics havebeen stuck in the sample size, laser performance （output power and optical conversionefficiency） and the composite structure preparation. There is a very big gap with foreignadvanced level.This thesis targeted on the shortness of Nd:YAG transparent ceramics, and conductedsystematic analysis on the influencing factors on material microstructure and opticalquality to optimize the preparation technology to solve the bottleneck problem of domestictransparent ceramics preparation. The objective of this study is to have independentintellectual property rights on preparation of large size, high-quality transparent ceramics,and to promote China’s development in the field of high-power solid-state lasers. Twomature preparation processes were leveraged from international advanced technology:（1）solid phase reaction method to fabricate Nd:YAG transparent ceramics （technology owners:Japan World Lab Company）;（2） co-precipitation synthesis of Nd:YAG nano powder andpreparation of transparent ceramics （technology owners: the Japanese KonoshimaCompany）. In this thesis, we carried out the work from the following aspects:1) Improve solid state reaction process by adding0.8wt.%CATB as a surfactant inthe commercial oxide raw materials, to improve the fluidity and uniformity of the slurry insolid-phase milling process and to improve the sintering activity of the powder; by adding0.8wt.%La₂O₃as sintering aids to lower the sintering temperature and refine Nd:YAG grain size to create more paths for pores’ elimination. Also, heating rate was controlled at1°C/min to lower the densification process and provide enough time for eliminating pores.With above improved parameters, the scattering centers were reduced maximally andhigh-quality Nd:YAG transparent ceramics were fabricated successfully. Sampletransmittance is82.5%at the visible wavelength of400nm and84.4%at the near infraredwavelength of1064nm, which are basically consistent with the theoretical transmittanceof crystalline materials. Judging from the surface and fraction SEM photos, the transparentceramics have a dense and clean microstructure, which guarantee the success ofpreparation of large-size ceramic technically.2) To avoid introducing new impurities, the large size biscuit molding process doesnot use any binder to improve the strength of the biscuit, but by controlling dry pressingpressure and biscuit thickness to ensure biscuit integrity. Dry pressure was controlled in80MPa¹20MPa, and the green body thickness was>7mm to form a nearly200mm lengthof the lath and a diameter of nearly200mm round flaky biscuit. Green body wasvacuum-sintered in1750°C for50h, then annealed in1450°C for20h in the air. Afterpolishing on both sides, Nd:YAG transparent ceramic disks （133×6mm²） were obtained.The optical transmittance of ceramic samples at1064nm and400nm were84.4%and82.8%, respectively. The scattering loss and absorption loss were0.004cm^-1and0.001cm-1, respectively. The optical transmittance and optical loss were both roughly equal withthose of crystals. The average grain size of the ceramic was about10μm, and the size ofthe grain boundary was less than1nm. Under this condition, the light scattering of thegrain boundary can be negligible. Nd:YAG transparent ceramic preparation process wasfine-tuned to ensure that pure YAG and Nd:YAG ceramics were able to realize thedensification of the microstructure under the same sintering process. No abnormal graingrowth appeared for different types of multiplestructure Nd:YAG transparent ceramics.The optical transmittances were84.2%at1064nm and above81.8%at400nm,respectively. The large-size and high-quality transparent ceramics were successfullyprepared, which was conducive to the realization of high-efficiency, and high power laseroutput of Nd: of YAG transparent ceramic.3) The research on the laser performance of large-size Nd:YAG transparent ceramicwas conducted. The Nd:YAG ceramic rod （6×100mm²） was side-pumped withLD-pumped system at the excitation wavelength of808nm and obtained425W laseroutput at1064nm and optical conversion efficiency of42.5%. These results were fullyconsistent with the same size and doping concentration of high-quality Nd:YAG crystal rodunder the same experimental conditions. Using master oscillator power amplifier system toimprove the efficiency of energy extraction, two Nd:YAG ceramic rod （6×100mm²）were side-pumped to achieve1.02KW laser output at the wavelength of1064nm, and optical conversion efficiency of29.7%. Nd:YAG ceramic slabs （93×30×3mm3） wereside-pumped with LD planar array at808nm. With the pump power of6.691KW,2.44KW laser output was obtained at1064nm, and optical conversion efficiency was36.5%.The results show that Nd:YAG transparent ceramic has the comparable performance withNd:YAG single crystal.Laser output at the wavelength of1.1μm from Nd: YAG transparent ceramics wereinvestigated. Nd: YAG ceramic rod （6×100mm²） was pumped with LD pump system at808nm, and248W laser output was obtained at1116nm, and the optical conversionefficiency was24.8%. Two Nd: YAG ceramic rod （6×100mm²） were side-pumpedand obtained509W laser output at the wavelength of1123nm with the optical conversionefficiency of25.8%. The main cause of the laser performances from1116nm and1123nm are worse than from1064nm is that the emission cross section of these twowavelengths are less than1/15of that of1064nm.1.1μm laser has shown great value inlighting, biomedical and other fields.4) Nd:YAG nano powder was synthesized using co-precipitation method with nitrateas raw materials. Controlling the pH value at8.2and the reaction temperature at25°C,Mg2+ions were added into the mixed nitrate solution. After calcining the precursor, Mg2+ions separated out on the Nd:YAG grain boundary in the formation of MgO whichinhibited grain growth and continuously played a grain refinement effect. Nd:YAG nanopowders with0.01wt.%MgO were well dispersed and had high sintering activity. Aftermilling mixed with0.5wt.%TEOS as sintering aids, the suppressed green body wassintered at1750℃for20h, then dense Nd:YAG transparent ceramic with the averagegrain size of⁸μm was obtained. No obvious pores and impurities were found on thesurface and the fracture of the samples. The optical transmittance of4mm thick samplewas more than82%at1064nm.In summary, the big-size and high quality Nd:YAG transparent ceramics fabricatedwith solid-state reaction method in this study are new chooses for laser materials in solidlaser, especially high power solid laser. High quality Nd:YAG transparent ceramicsfabricated with fine nano powders from co-precipitation method will help us get rid of thedependence on imported raw material powder in solid state reaction process, also good forthe development of completely independent intellectual property rights of Nd:YAGtransparent ceramics.