Effects of Diffusion of Co/Ni Cations on Sol-Gel Derived ZrO 2 Polymorphic Transformations
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respectively for 4 h under vigorous stirring. Filtrating and drying (100°C, 2 h) then gave the metal ion loaded gel powders. The resulting powders are denoted as CZ and CNZ (C = cobalt, N = nickel, and Z = zirconia). The above metal-loaded gels were calcined at 300, 400, 500, 600, 700, 800 and 900'C respectively for 5 h in static air in a furnace (Carbolite). The resultant gel samples are identified as CZn and CNZn in the text, where the n stands for the calcination temperature in hundreds (QC)for a designated sample. The crystallographic structure and polymorphic transformation of the gels upon calcination treatments were examined with the Fourier-transform infrared (FTIR) spectroscopic method, and the population of each thermostable or metastable phase at room temperature was quantified with the X-ray diffraction technique (XRD, Philips PW1710). Phase volume populations of the as prepared and metal stabilized tetragonal phases (t' and Ct)in the metalloaded gel series CZn and CNZn upon calcination temperature were calculated from the ratio of the XRD peak intensities: (t'+ t")% = (l11),.,/[1.6(1ll)m + (I I ]),,,] [6]. The differential thermal analyses (DTA, Shimazu DTA-50) were carried out for the parent ZrO 2 gel as well as for all the metal loaded ZrO 2 samples. RESULTS AND DISCUSSION In all sample gels, IR absorptions at around 735, 657, 575, and 420 cm' can be clearly identified [7,8], indicating the gels are predominantly in monoclinic phase (m). This observation is also confirmed with the XRD results by examining characteristic diffraction peaks [6]. It is important to realize that within each set of the metal-loaded samples, the amount of the doped metal(s) is a constant. Under normal circumstances, at low sintering temperatures, the metal doped ZrO 2 phase is expected to be small and solely in surface or near surface regions, while at high-temperatures, the doped regions will be expanded but the
C:
Fig. I Representative DTA spectra of Co-loaded ZrO 2 (250 -> 14000C -> 250) for CZ7-CZ9, and Co-Ni-loaded ZrO 2 (25' 0 -> 1400 C -- 250) for CNZ7-CNZ9.
CZ 0
E 0) 0 C:
CNZ9
945.4 cQ
946.21
1
Lii
951.9
996.8
I1 200
600
1000
1400
200
600
Temperatur (°C)
656
1000
1400
dopant concentration should be lowered. Using the X-ray photoelectron spectroscopy (XPS), the found reduction in molar ratio of M/Zr (M = Ni or Co) can be as large as 4 folds on the ZrO 2 matrix surfaces, and the final thermodynamically stable M/Zr ratio is around 5 mol% for both the Ni/ZrO2 and Co/ZrO 2 systems [4,5]. As all gels are predominantly in monoclinic form, endothermic bands located at 962, 964, and 9970 C of CZ7-CZ9 in Fig. 1 can be assigned to the m -- t (t = high-temperature tetragonal) transformation, although it may also include a small degree of t'---ý m --* t [9]. In view of the presence of metals in the bulk matrices, endothermic bands observed in hightemperature range of most DTA spectra can be assigned accordingly to metal-modified high temperature tetragonal - cubic (c) transformation (T.,, ranging from 1225
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