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Studies on transparent laser ceramics continues to rapidly progress, this holds true for non-cubic ceramics as well. Cubic transparent ceramics have been demonstrated to be superior to their single crystal counterparts for laser applications. However, fabrication of anisotropic laser ceramics through ceramic processing is still a challenging problem in material science due to the birefringence inherent to these materials. Currently, there are two possible methods used to reduce the effects of birefringence in anisotropic laser ceramics: by achieving an orientated texture through the application of a high magnetic field, or by generating nanostructured grains through a fast sintering consolidation process. This research work presents an alternative method to process anisotropic Yb:S-FAP optical ceramics through a fast consolidation process. The methodology can be used as a versatile and practical way to develop nanostructured transparent ceramics with an anisotropic structure for laser and optical applications.
© 2014 Optical Society of America
1. Introduction
Rare-earth doped high transparency polycrystalline ceramics have recently emerged as promising candidates for next generation laser gain media in modern applied laser physics and optics [1–5]. Current research in the field of laser ceramics focuses on cubic polycrystalline materials (e.g., Y3Al5O12, Y2O3, Sc2O3, Lu2O3). These ceramics have the potential to dramatically reshape today’s solid-state laser marketplace by enabling the development of significantly less expensive lasers with an expanded range of capabilities [3]. The newly developed ceramic lasers could offer high output power and low loss, competitive with the current state-of-the-art commercial solid-state lasers [3]. Transparent laser ceramics can be manufactured to arbitrary geometries with variable dopant levels allowing the optical and physical characteristics of a ceramic laser to be tailored. In this way it becomes possible to design lasers with novel properties and functions that cannot be obtained with existing laser materials [1, 6].
Optical transparency in polycrystalline ceramics can be achieved when the scattering sources present in a material are minimized or eliminated. Sources of optical scattering may include grain boundaries, residual pores, point and line defects, dislocations of grains, or secondary phases. In addition to light scattering from the grain boundary, another challenge in the processing of transparent ceramics is to reduce the double refraction (birefringence) of the anisotropic structure. As shown in Fig. 1, anisotropic crystals have an inherent birefringence. However, it is possible to reduce or eliminate the birefringence either by aligning the grains so that their refractive indexes match along a single direction or by consolidating nano-scale particles into nanostructured polycrystalline ceramics. Exploration of the fundamental scientific and technical principles that govern non-cubic transparent ceramics is of critical importance to the advancement of lasers and optics [1, 7–12].For example, if polycrystalline hexagonal apatite transparent ceramics such as Yb:S-FAP, Nd:YVO4, Yb:KYW, can be successfully fabricated, laser gain media of larger sizes may be developed with optical properties currently unattainable through the Czochralski process [13, 14].
The anisotropic magnetic susceptibility of the grains allows for the use of a powerful magnetic field in the formation of ceramics with oriented or textured microstructures [15–21]. The asymmetric crystalline Yb:S-FAP particles express anisotropic magnetic susceptibilities according to the formula, Δχ = χ// – χ┴, where χ// and χ┴ represent the susceptibilities parallel and perpendicular to the magnetic principal axis. The driving force for the magnetic alignment of the Yb:S-FAP particles is relatively low in a typical magnetic field generated by permanent magnets. Powerful magnetic fields (>10 T) on the other hand are capable of causing a driving force strong enough to orient the Yb:S-FAP grains. The magnetization energy, U(θ), of the Yb:S-FAP particles depends on the angle between the crystal axis and the direction of magnetic field, and can be expressed by the formula (1) [16, 22]:
Here, µ0 is the magnetic permeability of vacuum, Δχ is the anisotropic susceptibility of the Yb:S-FAP particle, V is the volume of the Yb:S-FAP particle, θ is the angle between the magnetic field and the particle axis, B is the strength of magnetic field, and χ is the susceptibility of the aligned particle axis.Dr. Takunori Taira’s group has investigated the magnetic field-assisted alignment of grains to process transparent anisotropic ceramics, and has discovered a novel alignment technology of micro domain structure in laser ceramics assisted by rare-earth trivalent elements [23–25]. The new process shows superiority to the traditional electromagnetic process. The paper [23] summarizes that the control of crystal orientation of micrograins in polycrystalline ceramics has been investigated using various methods; with all of them developed from one principle, expressed in Eq. (1). The fifth term in this equation, M · dB, indicates that it is possible to align micrograins made of ferromagnetic media through the control of magnetic potential. Nd3+ and Yb3+ doped transparent fluorapatite ceramics have been fabricated based on this concept. After the alignment of crystal grains, the samples are pre-sintered for 2 hours at 1600°C in air and then sintered through hot isostatic pressure below 1600°C for 1 hour with the pressure set to 190 MPa in an atmosphere of Ar.
In Eq. (2), Gibbs free energy, entropy, temperature, curvature radius of the surface, boundary energy of the grain and chemical potential are represented by G, S, T, r, γg and μi. The molecular number of i-th grain, the stress tensor, and the strain tensor are given by Ni, eij, and σij. Finally, E, P, M, V, and B represent values for the strength of the electric field, the dielectric polarization, the density of magnetic flux, volume, and the magnetization.2. Non-cubic Yb:S-FAP transparent ceramics
Processing of non-cubic Yb:S-FAP ceramics from nanoparticles has been studied. Formation of these ceramics requires the synthesis of a dispersed powder with a narrow size distribution and a high level of chemical homogeneity. Yb:S-FAP is a uniaxial hexagonal crystal which is part of the apatite crystal family. It has the generic formula A5(MO4)3X, where A can be Ca, Sr, Ba; M can be P, V; X can be F, Cl. There are several methods capable of preparing the powder for sintering. Among these, a co-precipitation method was selected for the synthesis of the Yb:S-FAP powder due to its capacity for high chemical homogeneity, low processing temperatures, variable doping concentrations, and a finer control of the particle morphology. Figure 2 shows microstructure and an X-ray diffraction pattern of the synthesized powder.
The resulting powder is then loaded into a graphite mold, separated from the mold by graphite foil. The samples were sintered in a FCT System SPS furnace at a heating rate of 50 K/min to reach a sintering temperature of 1000-1050 °C, with a sintering pressure of 70-100 MPa. The sintering temperature was held for 8 min and monitored by a thermocouple wire which was inserted into the die through a very small hole drilled into the side. The application of pressure was expected to prevent the decomposition and evaporation of Yb:S-FAP particles at sintering temperatures. Ceramics successfully sintered by SPS have exhibited cleaner grain boundaries. The pressure applied in SPS not only has direct effects on the particle rearrangement to enhance the densification, but also has an influence on the chemical potential which promotes the mass transportation during the sintering process. Figure 3 shows a SEM image and XRD pattern of the sintered samples. The SEM image indicates that the average grain size is approximately 150 nm. The XRD results confirm that the sintered samples still exhibit a pure hexagonal phase structure.
The fast sintering process managed to consolidate the nano-scale powder to near maximum theoretical density with little grain growth due to the lower sintering temperatures and shorter holding times. In order to achieve nano-grained ceramics, the densification rate should be faster than the rate of grain growth during sintering, which is achievable using a fast sintering. Fast sintering is used to sinter powder compacts where the activation energy for densification is higher than that of the grain growth [26]. To fabricate anisotropic transparent Yb:S-FAP ceramics from a multi-domain green body, a sintering process must densify the ceramic green body, as well as concurrently removing and diminishing the scattering sources, including birefringence. It is imperative that the ceramic is fabricated to full density with pore-free clean grain boundaries [1, 2, 27]. Figure 4(a) shows an in-line optical transmittance spectrum in the wavelength range of 300-3300 nm, and an inset image of the transparent Yb:Sr5(PO4)3F ceramic with a thickness of 2 mm. Figure 4(b) shows an extinction spectrum to reveal an obvious peak associated with an Yb3+ energy transition. The absorption spectrum in Fig. 5 shows absorption cross-section of several Yb doped ceramics and single-crystals to identify the Yb site occupancies in the Yb:S-FAP transparent ceramics. The results demonstrate the Yb3+ ions mostly occupy the site I that is under a low crystal field.
Fig. 4 (a) in-line transmittance and an inset image of Yb:S-FAP ceramics, and (b) an extinction spectrum of Yb3+
3. Conclusion
Transparent laser ceramics with an anisotropic crystallographic structure can be processed through the magnetic-field-assisted alignment of grains or through the control of grain growth through fast sintering consolidation. For the magnetic field-assisted alignment of grains, stable, homogenous slurry with a high solids loading of well-dispersed particles is essential to allow for the effective alignment of particles under a high magnetic field. The alignment of particles operates on the principle that magnetic torque will rotate crystals to decrease their magnetization energy, stabilizing particles based on their anisotropic magnetic susceptibility. The alternative method is to sinter nanometer-sized particles in a fast sintering process to achieve nanostructured grains. Anisotropic Yb:S-FAP transparent ceramics have been synthesized by applying field-assisted sintering techniques, resulting in a high optical transmittance attributable to their small grain size. Microstructural analysis of ceramics produced via field assisted sintering shows an average grain of around 150 nm. With an understanding of the fundamentals of the process, it is expected that large-size polycrystalline anisotropic laser ceramics can be processed by fast sintering consolidation.
Acknowledgments
We gratefully acknowledge the US Air Force Office of Scientific Research (contract FA9550-14-1-0155) for funding and supporting this research. The author thanks Shi Chen, Thomas Olson, and Yan Yang for their assistance with experiment. We also thank Professor Romain Gaume at University of Central University for measuring the extinction spectrum and absorption spectrum.
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