2.1 Fabrication of GZO transparent ceramics
GZO nanopowders were prepared by the combustion method. Gd(NO3)3·6H2O (99.9%, Aladdin), Zr(NO3)4·5H2O (99.9%, Aladdin), and glycine (≥99%, Aladdin) were weighed at stoichiometric amounts and dissolved in deionized water. The appropriate amount of ammonia was slowly added to this solution until the pH value was adjusted to 4.0 and then heated in a water bath until it became a gel state. The gel was transferred to a muffle furnace and heated up to 300 °C for 30 min. The gel burned rapidly and formed a precursor powder when the temperature reached the ignition point of glycine. The precursor powder was calcined at 1200 °C for 2 h, and then ball-milled with deionized water for 24 h. The GZO powder was obtained after drying. The powder was dry pressed into disks and cold isostatically pressed at 200 MPa for 3 min. The disks were pre-sintered at 1400 °C for 3 h, then vacuum sintered at 1825 °C for 6 h, and the sintered samples were annealed at 1500 °C for 5 h finally. The preparation process was the same as described previously [26].
2.2 Leaching experiments
The parameters and conditions corresponding to each leaching experiment are listed in Table 2. Six GZO transparent ceramics were taken (numbered 0-5) for grinding and polishing with sample number 0 as a blank comparison. Then, samples numbered 1-5 were cut into regular rectangular blocks with dimensions of 7.7 × 7.8 × 0.9 mm, 7.7 × 7.8 × 0.9 mm, 7.7 × 8.1 × 1.0 mm, 7.7 × 8.7 × 0.9 mm and 7.8 × 7.8 × 0.9 mm, respectively. The samples were sonicated in deionized water repeatedly and dried to constant weight at 100 °C. The treated GZO transparent ceramics of No. 1-5 were suspended in the center of a PTFE vessel containing 80 ml liquid. The pH values of the liquids in the PTFE vessels of No. 1-5 were adjusted to 3.0, 5.0, 7.0, 9.0 and 11.0. The acidic conditions were configured with the nitric acid solution, neutral condition with deionized water, and alkaline conditions were adjusted with sodium hydroxide solution. Finally, all the above PTFE containers were transferred to a 90 °C oven for 1, 3, 7, 14, 21, 28, 35 and 42 days.
Table 2
Parameters of leaching experiment.
No.
|
pH of Solution
|
Sample’s size (W × L × H)
|
Volume of Solution
|
Experimental temperature
|
0
|
-
|
-
|
-
|
-
|
1
|
3
|
7.7 × 7.8 × 0.9 mm
|
80 ml
|
90℃
|
2
|
5
|
7.7 × 7.8 × 0.9 mm
|
80 ml
|
90℃
|
3
|
7
|
7.7 × 8.1 × 1.0 mm
|
80 ml
|
90℃
|
4
|
9
|
7.7 × 8.7 × 0.9 mm
|
80 ml
|
90℃
|
5
|
11
|
7.8 × 7.8 × 0.9 mm
|
80 ml
|
90℃
|
2.3 Characterizations
The leaching concentrations (Ci) of Gd and Zr were measured in the leachate under different pH conditions by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7700 ×). The normalized element rates LRi (g·m-2·d-1) are calculated according to the following formula:
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/B3ONrh86JSB1tfwP9lv1AdiYeumEJ9Gk0fPIletjaloiIFIyhJZiGrDDG3gVN2mEkVdT6qAlRYcbiB5Cjdm5FcHxdvlsNSmDs7e8hb8UWVAmAsOdpPGs+43xAMKNAVyxGFvHQtBX6Wvk8ERGRArGmhYiIiBSBIy1ERESkCAwtREREpAgMLURERKQIDC1ERESkCAwtREREpAgMLURERKQIDC1ERESkCAwtREREpAgMLURERKQIDC1ERESkCAwtREREpAgMLURERKQAwP8DwnXK2gEJATMAAAAASUVORK5CYII=)
where the five variables in the equation represent the concentration of the element i in the leached solutions after the leaching experiment (Ci, g/ L), the quality percentage of the element i in the specimen (fi, wt%), the surface area of the specimen (SA, m2), the volume of the leached solutions (Vi, L) and the duration time (t, d) [23,24,29]. Linear transmission spectra before and after leaching were measured in the 200-2500 nm wavelength range by solid ultraviolet absorption spectrophotometer (Solidspec-3700, Shimadzu, Japan). The changes in the crystal structure of GZO ceramics were analyzed by grazing incidence X-ray diffraction (GIXRD; Bruker; D8 Advance) with Cu Kα radiation (λ1=1.54056 Å, λ2=1.54439 Å and λave.=1.54248 Å). The Raman spectra were obtained using a Raman spectrometer (Raman; InVia, Renishaw, Britain) with an excitation wavelength of 785 nm. Surface morphology and root-mean-square (RMS) surface roughness of the samples were analyzed using atomic force microscopy (AFM; Seiko SPA-300HV, Japan) in tapping mode. The micro-morphology of the GZO ceramics was observed using field-emission scanning electron microscopy (FESEM; SU8020, Hitachi, Japan), and their elemental distribution was analyzed using an energy-dispersive X-ray spectrometer (EDX) attached to the FESEM device.