Reactive sintering of highly-doped YAG / Nd 3 + : YAG / YAG composite ceramics ✩

Multilayer YAG/Nd:YAG/YAG composite laser ceramics were obtained by the reactive sintering in vacuum. The effect of the neodymium ion concentration (1–4 at.%) on the formation of defects and optical quality of composite ceramics was studied. It was found that neodymium ions modify densification kinetics during solid-state reactive sintering of the highly-doped Nd:YAG ceramics by decreasing shrinkage rate in the temperature range 1320–1350 °C. Differences in phase transformation kinetics during reactive sintering lead to generation of pores at the interface of adjacent layers which decrease the optical homogeneity of fabricated YAG/Nd:YAG/YAG composite ceramics. The influence of layered structure on the laser performance of optical ceramics was investigated. It was shown that the ceramics with multilayer composite architecture have slope efficiency almost twice as the single-layer ceramics with the same composition (22% and 12.5%, respectively).


I. Introduction
Yttrium-aluminium garnet doped with neodymium ions Nd 3+ :Y 3 Al 5 O 12 (Nd 3+ :YAG) is widely used as an active medium of solid-state near-IR lasers due to its high mechanical and laser strength, transparency in a wide wavelength range, and high thermal conductivity.Optically isotropic cubic structure of YAG makes it possible to obtain transparent polycrystalline ceramics which have the key functional characteristics similar to that of corresponding single crystals [1,2].In contrast to the melt methods of single crystal growth the ceramic technology ensures formation of the homogeneous highly-doped Nd 3+ :YAG solid solutions.
The trend to decrease the size of the active medium in modern solid-state lasers could be achieved by short-ening the resonator length in order to obtain ultra-short pulses duration in the Q-switching operation.Simultaneously, this indicates a number of technical problems affecting the efficiency of laser generation and the laser beam quality.In particular, a small volume of the active medium at high concentration of laser ions and a use of the laser diode pumping with high radiation density lead to significant dispersion of the refractive index due to the inhomogeneous heating of the medium (the so-called thermal lensing).The use of a combined active medium consisting of a core doped with laser ions and surrounded by optically passive transparent layers makes it possible to inhibit negative effects of the thermal lensing significantly [3].Several methods were proposed to create composite active media based on the YAG/Nd 3+ :YAG ceramics by diffusion bonding of layers [4] or by tape casting [5].The ceramic technology combined with reactive sintering makes it possible to form a multilayer structure during moulding of the starting nanopowders.There are several reports on the ob-taining the composite ceramics by such method [6,7].However, optical quality of the produced samples is significantly lower in comparison with single-layer ceramics.The reason is the difference in the optimal sintering temperatures and kinetics of the solid-phase reactions during initial and intermediate sintering stages of layers having different chemical compositions.
The aim of the present work is to determine the influence of Nd 3+ ion concentrations on the kinetics of solidstate reaction during processing of multilayer transparent ceramics.In addition, the goals are to identify the features of multilayer YAG/Nd 3+ :YAG/YAG composite laser ceramics obtained by the reactive sintering, and to study functional characteristics of the fabricated ceramics.(where x = 0.03-0.12),were ground in a planetary ball mill in isopropyl alcohol using 5 mm alumina balls at 140 rpm rotation speed during 15 hours.The obtained suspension was dried and sieved through a 200 mesh screen.The neodymium concentration in the powder mixtures (in comparison to Y 3+ content) was varied in the range from 0 to 4 at.%.

II. Experimental procedure
Moulding of the ceramic composite with layer-bylayer configuration (YAG/Nd 3+ :YAG/YAG) was carried out as follows.The powder mixtures with composition corresponding to the undoped YAG as outer layers and Nd 3+ :YAG as the inner layer were charged in metal mould and pressed uniaxially at 30 MPa.Then the composite green body was isostatically pressed at 250 MPa.The single-layer compacts with different compositions were prepared under the same conditions for the comparison.The pressed green bodies were annealed in air at 800 °C during 4 hours in order to remove organic impurities.Reactive sintering was carried out in tungsten crucibles in a vacuum furnace with tungsten heaters at 1750 °C for 10 hours.In order to retain oxygen stoichiometry and to relax microstructural stresses, ceramic samples were annealed in air at 1300 °C for 15 hours.Finally, the ceramics was ground and polished.The typical 2 at.%Nd 3+ :YAG and YAG/2 at.%Nd 3+ :YAG/YAG samples obtained after sintering and polishing are shown in Fig. 1.
The densification kinetics of powder compacts was studied by the dilatometric analysis on NETZSCH-402 ED dilatometer.Measurements were carried out in vacuum in the temperature range of 20-1470 °C at a heating rate of 5 °C/min.Microstructure of ceramics was studied using transmission optical microscope Zeiss axioscop 40 with 100×-magnification.The concentration of neodymium ions in composite ceramics was measured using the electron-probe microanalysis by scanning electron microscope JSM-6390LV equipped with energy-dispersive spectrometer AZtecEnergy Xmax N 50.In-line optical transmission spectra of the Nd 3+ :YAG ceramics were measured on Perkin-Elmer Lambda-35 spectrophotometer in the range of 250-1100 nm.The phase composition was investigated by X-ray diffraction (XRD) with Siemens D-500 diffractometer (Siemens AG, Germany) with CuKα radiation.Phases were identified using the PDF-1 JCPDS X-ray database of the EVA survey system.Laser generation in the uncoated ceramic samples was obtained under the semiconductor laser diode pumping with the wavelength λ = 808 nm.The resonator with the length of L = 9 mm consisted of a plane high-reflectivity mirror (R 1064 > 0.999) and a translucent out-coupling mirror with a transmittance T 1064 = 0.06.The samples were fixed in a copper holder.

Characterization
The prepared samples were characterized by XRD and a typical pattern of the YAG doped with 2 at.%Nd 3+ (2 at.%Nd 3+ :YAG) ceramics was shown in Fig. 2  Transmission spectra of the multilayer ceramics with a thickness of 3 mm are shown in Fig. 3b.A significant decrease in the optical quality is observed with the increase of Nd 3+ concentration.The in-line optical trans-mission at the wavelength λ = 1064 nm for YAG/2 at.%Nd 3+ :YAG/YAG ceramics was 80.5%, while it decreases to 74.5% for YAG/4 at.%Nd 3+ :YAG/YAG sample.The transmittance of highly-doped samples with composite architecture decreases significantly in the short-wavelength spectral range due to the light scattering on the nanosized and submicron residual pores [8,9].
Figure 4 shows a typical microstructure of YAG/Nd 3+ :YAG/YAG ceramics observed by a transmission optical microscopy and by SEM.The observed dark defects in the optical images of ceramics are residual pores that appear also on SEM images.Left (Fig. 4a,c) and right (Fig. 4b,d) pairs of images correspond to the samples with 2 at.% and 4 at.% of Nd in doped layer, respectively.The highly-doped composite ceramics contains a significant amount of residual pores which cause transmission decrease.According to the optical microscopy data, the increase of the optical losses in highly-doped composite ceramics is caused by the rising concentration of residual pores.An increase in the neodymium concentration up to 4 at.%does not affect the pore concentration and optical quality of the single-layer samples (Fig. 3a), while the typical porosity of Nd 3+ :YAG ceramics obtained by the reactive sintering method is ∼ 10 −3 -10 −4 vol.% [10].Thus, defect formation in the ceramics with "sandwich" structure was caused by pore formation on the interfaces of doped and undoped layers during sintering.

Effect of Nd 3+ concentration on densification
The influence of neodymium ions concentration on densification peculiarities of Nd 3+ :YAG ceramics was studied by dilatometric analysis method performed for The shrinkage of green bodies at various temperatures is mainly determined by these reactions (1-3).The smaller specific volume of YAP phase relative to that of starting oxides (approximately by 13%) leads to a sharp increase in density of green body during the reaction (2) (T = 1100-1250 °C in Fig. 5a).In the 1250-1350 °C temperature range, densification of the green body is determined by the competition of sintering of powder mixture and YAG formation reaction (3).The sintering process is aimed at densification of a porous green body by removal of pores out of the volume and approaching of particle centres.The reaction (3) is accompanied by an increase in the specific volume by 11%.As a result, a significant reduction in the shrinkage rate of ceramics is observed at this stage.
The neodymium ion concentration affects significantly the densification rate of the prepared ceramics.Fig. 5b shows that the compaction rate decreases with increasing neodymium concentration, and for the compositions with C Nd > 2 at.% it becomes negative resulting in volume expansion.The observed effect can be caused by formation of NdAlO 3 (NdAP) intermediate phase in the step (2) which partially replaces YAP phase [11].This changes the garnet formation kinetics via reaction (3).Heating of green body above the temperature of 1400 °C causes transformation of intermediate phases and promotes neodymium ions to enter the garnet structure as substitutional ones.Moreover, neodymium ions have an impact on the sintering kinetics by forming compounds with sintering aid (silica) [12], which determines largely the shrinkage rate.
Thus, during sintering of the composite ceramics in the temperature range of 1320-1350 °C the undoped YAG layers retain a positive sintering dynamics, while the central doped layer undergoes swelling.The appearing mechanical stresses can be relaxed by generating pores at the interface of adjacent layers, which are responsible for the increased optical losses of ceramics with composite architecture.

Nd 3+ ions diffusion during sintering
Sintering of YAG/Nd 3+ :YAG ceramics is accompanied by the diffusion of activator ions towards the undoped layer.The neodymium ions concentration distribution in YAG/4 at.%Nd 3+ :YAG/YAG ceramics is shown in Fig. 6.In order to describe the diffusion process, the first Fick's law was applied for the onedimensional diffusion through a flat boundary.According to this law, the concentration C Nd in the undoped layer at the point x after some time t can be approxi-mately described by the equation: where C 0 is the impurity concentration at the interface, erf(x, t) is the error function, x 0 is the coordinate of the interface (we used coordinates for x = 0), and D e f f is the effective diffusion coefficient.The effective diffusion coefficient was determined by approximating the experimental data by Eq. ( 4) (Fig. 6, solid line).It was found that D e f f ≈ 1.5 • 10 −15 m 2 /s which corresponds to an actual diffusion rate of approximately 50 µm/h.

Laser performance
The effect of structure of ceramic active media on the laser characteristics was studied using the multilayer sample doped with 2 at.% of Nd 3+ .The singlelayer 2 at.%Nd 3+ :YAG ceramics was used as reference.Figure 7 shows CW laser output at the wavelength of 1064 nm as a function of the absorbed pump power for the single-layer 2 at.%YAG:Nd 3+ and composite YAG/2 at.%Nd 3+ :YAG/YAG ceramics.Solid lines correspond to the linear fits of the experimental data.The slope efficiencies of the composite ceramics and the reference sample were 22% and 12.6%, respectively.Thus, the multilayer structure allows one to almost double the slope efficiency of ceramic laser.It should be noted that a threshold of laser emission for the single-layer ceram-ics is significantly lower than that for composite ceramics (0.75 and 1.1 W, respectively).This difference arises due to the lower optical quality of the latter (Fig. 7).The laser characteristics of ceramics could be substantially improved by optimizing the measurement conditions, applying anti-reflective coatings on the samples, and using an active cooling of samples.It has been found that the shrinkage rate of Nd 3+ :YAG ceramics decreases with an increase of concentration of neodymium ions in the starting mixture, and it becomes negative at the Nd 3+ content higher than 2 at.%.This extreme on the sintering trajectory of ceramics was connected with competition of the densification processes and the garnet phase formation, since the latter results in specific volume decrease.The change in the shrinkage rate could be explained by the partial substitution of YAlO 3 by NdAlO 3 that influence the kinetics of the solid-state reaction 3 YAlO The structure of the ceramic gain media has significant effect on their laser performance.It has been determined that the presence of cladding layers on the neodymium-doped active layer decreases the thermal lensing effect and doubles the slope efficiency of ceramic laser.

Figure 7 .
Figure 7. Laser output as a function of the pumping power of the single-layer 2 at.%Nd 3+ :YAG and composite YAG/2 at.%Nd 3+ :YAG/YAG ceramics

3 +
Al 2 O 3 − −− → Y 3 Al 5 O 12 .The local swelling effects observed at the interface of adjacent layers during reactive sintering of highlydoped YAG/Nd 3+ :YAG/YAG composite ceramics lead to the generation of structural defects and decrease optical quality of synthesized samples.