Transparent Y3Al5O12 (YAG) ceramics haze in annealing and thermal-bonding because of second phase exsolution. To address this problem, SiO2 and fluorides co-doped transparent ceramics were prepared from YAG powders by slip casting and vacuum sintering method. 0~0.18 wt% of fluorides were doped into YAG ceramics as sintering aids along with 0~1.5 wt% of SiO2. Fluorides prompted sintering process and improved optical properties of sintered samples. SiO2 lightly doped (0.027 wt%) YAG transparent ceramics were obtained by co-doping with fluorides. Composition analysis showed that fluorides decreased Si content in sintered YAG ceramics and as a result, the formation of Si-riched second phase was successfully suppressed in annealing process.
© 2014 Optical Society of America
Yttrium aluminum garnet (YAG) is an attractive and intensively studied laser host material due to its excellent optical, thermal, mechanical and spectroscopic properties [1, 2]. YAG has a cubic crystal structure and is free of birefringence. Hence, highly transparent YAG polycrystalline can be produced by sintering to 99.99% density. Ikesue et al first achieved highly efficient laser gain in their transparent Nd:YAG ceramics by sintering Al2O3, Y2O3 and Nd2O3 powder mixture at 1750 °C in vacuum . This milestone work gave some important and well accepted recommendations including the use of SiO2 as sintering aid. Yagi et al developed another very promising method to obtain highly transparent YAG ceramics by sintering co-precipitated Nd:YAG nano-powders at 1700 °C [4, 5]. This method promises smaller grain size and less light scattering lose, hence gain increasing attention. Following these two routes, Nd:YAG , Er:YAG , Yb:YAG and Ho:YAG  transparent ceramics have been fabricated. These ceramics exhibit very inspiring properties. Ueda et al have shown that the laser damage for both rare earth dope and nondoped YAG ceramics are comparable to single crystal counterpart .
In the both methods, SiO2 plays a crucial role in sintering process of YAG ceramics. Calculation showed that SiO2 could largely decrease formation energies of cation vacancies and improve concentration of cation vacancies that leads to enhancement of lattice diffusion . Researchers also found that a liquid phase forms at 1390 °C and affects the intermediate stage densification followed by Si4+ substitution into YAG structure . The two above mechanisms both prompt densification of YAG ceramics during sintering process.
After sintering process, annealing is needed to eliminate defects in ceramics that are detrimental to emission properties of doped active ions . However, after annealing, Si-riched phase particles along grain boundaries and at triple points are always observed in SiO2 doped YAG ceramics, which decease transparency of the ceramics . In addition, thermal-bonding is a promising technique to obtain thick or composite ceramics. To achieve perfect bonding effect, relatively high temperature and long duration are preferable. However, Si-riched phase particles are prone to precipitate from YAG ceramics in this process at high temperature for a long duration . Therefore, doping level of SiO2 should better be limited. Efforts have been made to reduce the quantity of SiO2 [15–17]. Adam J. Stevenson et al reported B2O3-SiO2 doped Nd3+:YAG transparent ceramics . B2O3 increased densification rate in intermediate sintering process, and at the final sintering stage, B2O3 vaporized and was eliminated from YAG ceramics. By co-doping of B2O3, highly transparent YAG ceramics were obtained and SiO2 doping level was reduced to 0.112 wt%. However, the effect of the usage of B2O3 on the optical properties of annealed ceramics was not studied. In this article, we report YAG transparent ceramics sintered with the aids of AlF3, YF3 and SiO2. The effect of fluorides on YAG ceramics sintering and annealing was investigated.
To investigate how fluorides affect the microstructure and optical properties of YAG ceramics, different amounts of YF3 (99.99%, Alfa Aesar) and AlF3 (99.99%, Alfa Aesar) were introduced into YAG powders. The impurities in YAG powders are 3.8 ppm of Na, 6.8 ppm of K, 4.3 ppm of Fe, 25 ppm of Si, <1 ppm of Ca. Considering the YAG powder used is stoichiometric, the introduction of extra YF3 will cause Al3+ ions insufficient for the stoichiometric composition. To address this problem, AlF3 was added and the mole ratio of YF3 to AlF3 was set to be 3/5 in accordance with the stoichiometric composition of Y3Al5O12.
The YAG ceramics samples are divided into 3 series, A, B and C, as listed in Table 1. In A series, SiO2 is kept as a constant of 0.09 wt% while the fluoride content varies from 0 wt% to 1.5 wt%. In B series, the SiO2 content decrease from 0.18 wt% to 0 wt%, while fluorides were kept constant. C series has only one sample that doped with 0.027 wt% of SiO2 solely.
The transparent YAG ceramics were fabricated by slip casting and vacuum sintering method. YAG nano-powders, the sintering aids of fluoride powders (AlF3 and YF3) powders and tetraethyl orthosilicate (TEOS, 99.99%, Aladdin) were weighed according to Table 1, and ball milled together in triple-distilled water for 5 hours in polyurethane jars to obtain 37 vol.% slurry. To obtain stable slurry, 1.5 wt% of ammonium polymethacrylate was added to the above mixture before milling. The slurry was then casted into plaster molds and dried in air to form green bodies. After drying, the green bodies were calcined at 800 °C for 2h to remove water and organics, followed by sintering in vacuum below 1.0 × 10−2pa at 1720 °C for 10h.
The micro-morphology of the powders and the fresh fracture surfaces of YAG samples were observed by a scanning electron microscopy. The in-line transmittance of the polished samples was measured by an UV/VIS/NIR spectrometer (Lamda 750, PerkinElmer). Si content in the samples was detected by an Inductive Coupled Plasma Emission Spectrometer (ICP). F content in the samples was measured by a potentiometer (PHS-3C, INESA Scientific Instrument Co.,Ltd) equipped with an F- ion selective electrode. The potentiometer was calibrated by a NaF solution (5%) before measurement.
3. Results and discussion
Figure 1 shows the SEM images of A series samples that sintered at 1720 °C for 10 hours. It can be seen that the micro-morphology of the samples changed notably with the increase of fluorides content. The grain size grows from ~10 μm to ~20 μm as the fluorides content increased from 0 wt% to 1.5 wt%, which suggested fluorides can prompt the sintering process. A1, A2 and A3 have fine and well shaped grains and no obvious pores can be observed, while in A4, pores of different size scattered along grain boundaries and in grains. The pores may result from the rapid growth of grains or the vaporization of fluorides or both.
The in-line transmittance of A1, A2 and A3 sintered at 1720 °C is shown in Fig. 2(a). Since A4 is totally opaque, it transmittance was not measured. It is obvious that the addition of fluorides improved the optical quality of the sintered YAG ceramic samples. The transmittance of A1, A2 and A3 at 1064 nm is 70.6%, 74.1% and 82.1%, respectively. However, the use of fluorides must be limited, because excessive fluorides (A4, 1.5 wt% fluorides doped) caused lots of pores in the samples as revealed in Fig. 2 and led to totally non-transparent.
Annealing is necessary to eliminate defects that produced during sintering process. However, after being annealed at 1450 °C for 10 hours, A1 turned opaque. Its transmittance declined to below 10% as shown in Fig. 2(b). The transmittance of A2 and A3 also declined compared with un-annealed samples. Instead of turning opaque, A2 and A3 only slightly became hazy after annealing.
The micrographs of annealed A1~A3 are shown in Fig. 3. Second phase can be clearly seen along grain boundaries of all these three annealed samples, which decreased the in-line transmittance of the annealed samples. The second phase particles are revealed to be Si-riched phase by Energy Dispersive spectrum (EDS) measurement. With the increase of fluoride dosage, the second-phase particles become smaller and thinner, which means that the doped fluorides can suppress the formation of the second phase. As a result, more fluoride doped, higher transmittance the ceramics exhibit after annealing.
Figure 4 shows the transmittance of the sample B1, B2 and B3 sintered at 1720 °C before and after annealing. It can be seen from Fig. 4 that when fluoride content kept a constant value of 0.57 wt%, the decrease of SiO2 content from 0.18 wt% to 0.027 wt% remarkably hinder the optical deterioration of the annealed YAG ceramics. This observation can be expected because the deterioration was caused by Si-riched second phase. The less Si was doped, the less second phase particles were formed. When the SiO2 doping level decreased to 0.027 wt%,no obvious second phase particles were observed in the annealed sample as can been seen from Fig. 3(d). However, SiO2 is essential to obtain transparent YAG ceramics. When SiO2 was not used at all, transparent ceramics could not be obtained only with 0.57 wt% of fluoride sintering aids. It should be noted that fluorides are also very necessary. Compared to B3 (0.027 wt% of SiO2 and 0.57 wt% fluorides co-doped), the sample of C0 doped with 0.027 wt% of SiO2 solely was not transparent even translucent without fluorides, which proved fluorides effective sintering aids.
Table 2 showed Si, F, Y, and Al content in A3 and B1 samples. Ikesue found that detuning Y/Al away from stoichiometry can suppress second phase formation . However, from Table 2, it can be calculated that the Y/Al ratios of A3 and B1 after sintering were both kept 0.6. Therefore, the suppression of second phase in our experiment was not caused by detuning of Y/Al ratio. According to Schober and Thilo’s study , AlF3 and SiO2 react and form fluor-topaz between 750 °C to 950 °C. The fluor-topaz decomposes to Al2O3 and SiF4 above 1100 °C. In our experiment, fluor-topaz might form during the calcinations of the slip-casted tablets at 800 °C and the subsequent sintering process, and then decompose during the following sintering process in vacuum. As a result, the content of Si in the sintered YAG ceramics might decrease as a result of SiF4 formation and its vaporization from the bulk. The contents of Si in annealed A3 and B1 were 0.024 wt% and 0.042 wt%, respectively as can be seen in Table 2. These values are equal to 44.9% and 51.8% of Si loss when compared with the original Si content as shown in the parenthesis in Table 1. Though the Si content did decrease when fluorides were added, the loss of Si was not proportional to the amount of doped F. The respective original content of F was 0.012 wt% and 0.016 wt% for A3 and B1. It can be calculated that 94.5% and 93.2% of F were lost for A3 and B1, respectively. These results suggested that only part of doped fluorides reacted with Si, formed SiF4 and subsequently released from the bulk, and that the rest part of them directly vaporized during sintering process. It can be explained by that most doped fluorides and SiO2 were separated from each other by YAG powders and only part of them had a chance to react before they vaporized when sintered.
Transparent YAG ceramics were sintered using YAG nano-powders. Transparent YAG ceramics doped with very low content of SiO2 were obtained with the help of fluorides. Excessive fluorides deteriorated optical properties of the obtained ceramics. Appropriate amount of fluorides improved optical properties of the sintered ceramics and suppressed the formation of Si-riched second phase during annealing process in two ways. Firstly, fluorides prompted sintering process and the necessary amount of doped SiO2 could be reduced. Secondly, fluorides decreased Si concentration in sintering process. This result provides an effective method to deal with the second phase separation in SiO2 doped YAG ceramics in annealing.
However, the mechanism of sintering enhancement by the use of fluorides is not clear yet. The evaporation of the doped fluorides in vacuum at high temperature and their reaction with SiO2 during sintering are lack of study. Researches on these subjects are underway in our laboratory.
The authors acknowledge the financial support by the National Nature Science Founds of China (Grant No. 61378069 and 51102257), the National Youth Natural Science Foundation of China (Grant No. 61108062, 51102253 and 51302284), Shanghai City Star Program (Grant No. 14QB1400900 and 14QB1402100).
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