DOI: 10.1021/cg9003005
Fabrication of Blue-Colored Zirconia Ceramics via Heterogeneous Nucleation Method
2009, Vol. 9 4373–4377
Wei Wang,† Zhipeng Xie,*,† Guanwei Liu,† and Weiyou Yang*,‡ †
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science & Engineering, Tsinghua University, Beijing 100084, P. R. China, and ‡Institute of Materials, Ningbo University of Technology, Ningbo, 315016, P.R. China Received March 15, 2009; Revised Manuscript Received August 11, 2009
ABSTRACT: We have demonstrated, for the first time, the fabrication of uniformly blue-colored CoAl2O4-ZrO2 ceramics via a heterogeneous nucleation method. Polyethylene glycol (PEG2000) was used as the dispersant for the ZrO2 powders followed by Al and Co hydrates introduced to the suspension for preparation of coated powders. Then NH3 was added into the suspension to tailor the pH values to favor the heterogeneous nucleation of CoAl2O4 spinels. It is found that heterogeneous nucleation can promote the formation of coloring phases, significantly decrease the volatilization, and improve the uniformity of coloring elements by reducing the mass transferring distance during sintering. It is believed that the method of heterogeneous nucleation could be a facile and cost saving route to facilitate solid-state reaction and guarantee the uniform distribution of new phases within the matrix for the fabrication of colored ceramics.
Introduction Fabrication of colored ZrO2 ceramics has attracted great interest due to their unique properties of beautiful colors, metallic lusters, and lack of hypersusceptibility. Along with their excellent mechanical properties and high wear resistance,1 colored ZrO2 ceramics could be excellent candidates to replace metallic decorative materials. Colored ZrO2 has widespread applications such as ornament components (watchbands, clock parts, etc.), exterior parts (cellular phones, mobile home electronics, etc.), optical components (ferrules, sleeves, etc.), structural parts (kitchen knife members, etc.), and so forth.2 Cobalt blue is a blue-hued inorganic pigment with spinel crystal structure.3,4 It has a double oxide and is well-known as Thenard’s blue color for its impressive optical effect. Conventional methods to append oxide spinels into ceramics usually involve a solid-state reaction of mechanically mixed parent metal oxides and zirconia, followed by sintering at high temperature (∼1500 °C). In such cases, serious volatilization of the costly colorants would happen. This would inevitably lead to the large consumption of coloring elements and serious environmental problems.5 In addition, when CoAl2O4 is introduced as a second phase via mechanical mixing, it cannot ensure the uniformity of the microstructure because of the agglomeration and long mass transferring distance during sintering. New techniques are highly desired to reduce the mass transferring distance and improve the uniformity of colorants within the ceramic matrix. Heterogeneous nucleation occurs much more often than homogeneous nucleation. It forms at preferential sites such as phase boundaries or impurities and requires less energy than homogeneous nucleation. At such preferential sites, the effective surface energy is lower, thus diminishing the free energy barrier and facilitating nucleation. For coated nanopowders, the large surface of the powder could be the preferred
heterogeneous nucleation sites. Meanwhile, the precipitation of the metal hydroxide via a heterogeneous nucleation method can be favored by tailoring the pH values of the solution in the proper region.6 In the current work, we reported the fabrication of uniformly colored ceramics via a heterogeneous nucleation method. Coated powders were prepared first, and then the pH values were tailored by introducing the NH3 into the solution, to favor the heterogeneous nucleation of CoAl2O4 colorants within the ZrO2 matrix. Experimental Method
*Corresponding author. Tel: þ86-10-62794603. Fax: þ86-10-62794603. E-mail:
[email protected] (Z.P.X.);
[email protected] (W.Y.Y.).
A commercially available nanosized Y-TZP powder (ZrO2-3 mol %Y2O3, YSZ-DM-3.0, Farmeiya advanced materials Co. Ltd., China) with a purity of 99.4% was used as the starting powders. The average size and the specific surface area of the powders are ∼124 nm and 17.9 m2/g, respectively. In a typical process, Y-TZP particles, mixed with dispersants of polyethylene glycol (PEG2000), were first dispersed in a bath under ultrasound for 60 min to limit the particle agglomeration. The precursor solution was prepared by dissolution of metal nitrates in aqueous solution. 0.1 mol/L Co(NO3)2 and 0.2 mol/L Al(NO3)3 (Sinopharm Chemicals, analytically pure) were slowly added into the powder suspension under magnetic stirring at room temperature. At this stage, the pH value of the obtained solution was measured to be ∼3.0. Then 0.01 mol/L aqueous NH3 was dropped into the suspension under strong magnetic stirring to tailor the pH value of the suspension in the range of 9.1-9.5. The resulting suspension was continuously stirred for 2 h and then filtered. The obtained powder was washed using the deionized water more than three times and dried in a conventional oven at 60 °C for 24 h. The as-prepared powder was then pressed into pellets and sintered at 1000-1400 °C for 2 h in air followed by furnace cooling. In comparison, blue-colored ZrO2 ceramics were fabricated by using the traditional method of mechanical mixture with other similar experimental parameters, in which Al2O3, Co2O3, and Y-TZP were used as the raw materials. The average particle sizes of Y-TZP powders were measured by dynamic light scattering (3000HS Zetasizer, Malvern, England). Differential scanning calorimetry (DSC) and thermogravimetric (TG) analyses (STA 409 PC/PG, NETZSCH, Germany) were performed in air to investigate the weight loss and mass balance of the colored ZrO2 during the sintering process. The X-ray diffraction (XRD) patterns were recorded on a RINT-2500 spectrometer
r 2009 American Chemical Society
Published on Web 08/26/2009
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Figure 1. (a) Dispersion of Y-TZP powders after 24 h stabilization in deionized water with (i) no dispersant; (ii) isopropyl alcohol; (iii) PEG2000. (b) Size distributions of the powders with no dispersant and PEG2000 as the dispersants. (Rigaku, Japan) with CuKR radiation. The microstructure observation was carried out by using a scanning electron microscope (SEM, LEO1530, Germany) operated at 15 kV. A transmission electron microscope (JEM-2010F, JEOL, Japan), equipped with an energy dispersive X-ray analyzer (EDX), was used for the powder morphology and composition analysis. The UV-vis spectra were measured by a spectrophotometer (U-3310, Hitachi, Japan) at room temperature.
Results and Discussion To fabricate the colored ceramics via heterogeneous nucleation, it is vitally important to prepare a stable suspension with a good dispersion of powders.7 It is generally accepted that the surface and interface phenomena of the powders play an important role in the stability of suspensions.8 There are two basic strategies to modify the surface chemistry of the powders. One is the repulsive forces derived from a charged electric double layer (electrostatic stabilization), or from noncharged and charged polymers adsorbed on the surface (steric and electrosteric stabilization, respectively).9 The higher the molecular weight of the polyelectrolytes, the more stability of the suspensions driven by the steric effect instead by electrostatic repulsive forces can be obtained.10 The other is the chemical interaction forces between the anionic dispersants and powders induced by the change of the pH value.11 In the current work, the surface chemical properties of zirconia suspension are tailored by the introduced isopropyl alcohol and PEG2000 (Figure 1a, ii and iii). It clearly suggests that ZrO2 powder suspensions with the most stable dispersion can be obtained by using PEG2000 (Figure 1a, iii), compared to that without dispersants (Figure 1a, i) and with isopropyl alcohol (Figure 1a, ii) as the dispersants (pictures are recorded after 24 h stabilization). The agglomeration of the powders can also be greatly limited when using PEG2000 as the dispersants (Figure 1b). Thereby, we choose PEG2000 as the dispersant in the following work for the fabrication of colored ZrO2 ceramics. Figure 2 shows the microstructures of the obtained ZrO2 powders dispersed by PEG2000 followed by hydrates deposition. The composite powder is of typical core-shell structure with a relatively smoothed out layer of ∼10 nm thickness (Figure 2a,b). The composite powders seem to be agglomerated due to the hydrogen effects. However, the surface of the layer becomes uneven with a thickness ranging from 10 to 40 nm after hydrates of both Al and Co deposited (Figure 2c). It could be attributed to a charge effect among the cations of Al3þ and Co2þ, which leads to the formation of an unstable suspension system. Figure 2d is a typical HRTEM image
Figure 2. (a, b) Typical TEM images of coated powders deposited only by Al hydrates at different magnifications. (c, d) Representative TEM images of coated powders deposited with both Al and Co hydrates. Panels (e) and (f) are respective EDS spectra recorded from the core and shell of the coated powders.
corresponding to the marked area of A in Figure 2c, displaying the detailed core-shell structure of the obtained composite powders. The core is typically crystalline with a regular lattice and the shell is amorphous (Figure 2d). It suggests that the mesoporosities have not been induced into the materials since no pores can be observed within the coating layer of PEG2000 (Figure 2b,d). Figure 2e,f presents the typical EDX spectra recorded from the shell and core of the composite powder, respectively, suggesting a relatively large amount of Al and Co encapsulated in the shell. Both the TEM images and EDX indicate that the ZrO2 particles are completely encapsulated by an amorphous layer with Al and Co hydrates. It is worth noting that this is the key for the fabrication of colored ceramics via heterogeneous nucleation. Although heterogeneous and homogeneous nucleation compete with
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Figure 3. (a) XRD pattern of the as-prepared coated powders. (b) XRD pattern of the samples by heterogeneous nucleation and sintering at 1000 °C. (c) XRD pattern of the sample by heterogeneous nucleation and sintering at 1400 °C. (d) XRD pattern of the sample by mechanical mixture and sintering at 1400 °C.
each other for crystal growth, the former is dominant in coated powders because crystal growth is favored by thermodynamics and more likely to happen, once the pH values of the system remain in a proper regime.6 In current work, we introduce PEG2000 with Al and Co hydrates to the suspension for the preparation of coated powders, and add NH3 into the suspension to tailor the pH value (9.1-9.5) to limit the homogeneous nucleation, and in turn, to favor the heterogeneous nucleation for the fabrication of colored ceramics. Figure 3 shows the phase evaluation of the composite powders. Figure 3a is the XRD pattern of the powders dispersed by PEG2000 and deposited by Al and Co hydrates. No other phases can be observed besides tetragonal and monoclinic ZrO2, which are the same as that of the raw materials. It suggests that the hydrates of Al and Co are amorphous, and would be transformed to the colored phase of CoAl2O4 when the sintering temperature is up to 1000 °C (Figure 3b). When sintered at 1400 °C, peaks of CoAl2O4 are rather clear, indicating that CoAl2O4 spinel crystals have developed well. For the formation of CoAl2O4, hydrates of Al and Co would first decompose to Al2O3 and CoO, leading to the formation of a quaternary Zr-Al-Co-O liquid phase when the temperature is up to 1400 °C.12 Because of the very low concentration of CoO in the liquid phase, CoO would be precipitated to react with Al2O3, resulting in the heterogeneous nucleation and growth of CoAl2O4 on the surface of ZrO2 powders.3 The transformation of monoclinic ZrO2 to tetragonal phase is due to the more stable tetragonal phase at 1400 °C (Figure 3c). Figure 3d provides the XRD pattern of the sample prepared via the method of mechanical mixture, disclosing the existence of a small amount of Al2O3, which might be due to both waste of CoO and nonhomogeneous distribution of Al within the matrix. These results imply that fabrication of colored ceramics via heterogeneous nucleation
can limit the volatilization of toxic CoO greatly, suggesting a relatively green and cost saving way to fabricate colored ceramics. Figure 4a displays the as-prepared blue-colored CoAl2O4-ZrO2 ceramic specimen via the heterogeneous nucleation followed by sintering in a conventional furnace at 1400 °C for 2 h. The weight loss after sintering with respect to the volatilization of the colorants is ∼2 wt % according to TG and DSC analysis. The ceramics have been thoroughly bluecolored (Figure 4a, fracture surface). Figure 4b-e are the respective backscattered electron images (BEI) of the CoAl2O4-ZrO2 ceramics fabricated via methods of heterogeneous nucleation (Figure 4b,c) and mechanical mixing (Figure 4d,e) under different magnifications, confirming the formation of CoAl2O4 spinels within the matrix of both samples. CoAl2O4 spinels exhibit obvious stratum structure with accurate grain edges (Figure 4b). In the sample fabricated via the heterogeneous nucleation, the spinels exhibit a uniform distribution within the ZrO2 matrix with an average size of ∼300 nm (Figure 4c), which should be critically important for the ceramic to be uniformly colored. However, in the samples prepared by mechanical mixture, the spinels show a very nonuniform distribution (Figure 4d,e). Many regions of pure ZrO2 grains without any spinels, sized ∼10-30 μm, could be clearly identified. It can be attributed to following two points: one is the agglomeration of ZrO2 powders might form during the mechanical mixing process; the other could be caused by the nature of nonhomogenous distribution of Al and Co cations in the ZrO2 powders via mechanical mixing. In the formation of CoAl2O4 in the ZrO2 matrix, CoO transfers toward Al2O3 mainly through two ways:3 one is by vaporization and condensation of CoO because of its high vapor pressure at elevated temperatures; the other is by solidstate diffusion through intergranular liquid channels in the
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Figure 4. (a) As-prepared blue-colored ZrO2 specimen via heterogeneous nucleation; (b, c) backscattered electron images of the bluecolored ZrO2 ceramics under different magnifications fabricated via heterogeneous nucleation; (d, e) backscattered electron images of the blue-colored ZrO2 ceramics under different magnifications fabricated via mechanical mixture.
ZrO2 matrix. For solid-state reaction through mechanical mixing, due to the nature of nonhomogeneous distribution of coloring elements, the mass transferring distance is mainly determined by original particle sizes of both parent compounds and matrix. Generally, the particle size would be several hundred or even one thousand nanometers (Figure 1b). Such a long diffusion distance would result in more CoO transferred through vaporization and condensation. Thus, a part of the CoO vapor would be lost in the sintering process, leading to the excess and individual formation of Al2O3 (Figure 3d). In terms of current work, the circumstance is much better for the nucleation and growth of CoAl2O4 in the ZrO2 matrix. As demonstrated in Figure 2, the parent compounds are uniformly deposited on the surfaces of ZrO2 powders in the amorphous forms of hydrates. Thus, the mass transferring distance would be shorter than the thickness of the whole deposited layers, which are sized at just tens of nanometers. It can facilitate solidstate diffusion greatly and reduce the volatilization of CoO during the sintering process. This is partly supported by the fact that the smaller blue area has been formed after sintering on the alumina substrate used to support specimens by heterogeneous nucleation than that by mechanical mixture. Figure 5a shows the ring-shaped part fabricated by using the heterogeneous nucleation method. The as-prepared powder was first dry pressed in a ring-shaped die, and then sintered in a conventional furnace at 1400 °C for 2 h in air, followed by furnace cooling to ambient temperature. It clearly suggests the uniform and vivid blue-colored appearance of ZrO2
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Figure 5. (a) As-fabricated ring-shaped and blue-colored ZrO2 parts via heterogeneous nucleation. (b) UV-visible spectrum of blue-colored ZrO2 ceramics.
ceramics has been achieved. The colorimetric properties of the as-prepared part were investigated under a spectrophotometer at room temperature. On the basis of the reflectance spectra (Figure 5b), CIE Lab chromatic coordinates of L*, a*, and b* are calculated to be ∼45.56, ∼10.34, and ∼ -31.41 (shown as the inset table in Figure 5b), respectively, confirming the blue-colored ZrO2 ceramics have been successfully prepared. The energy level for Co2þ (3d7 configuration) in both octahedral and tetrahedral ligand fields presents three spin-allowed transitions. The UV-vis spectra of CoAl2O4-ZrO2 composites shows three triple bands, which are ascribed to [4A2(F) f 4T1(P)] transition at about υ1 = 638 nm, υ2 = 585 nm, and υ3 = 546 nm.13 These triple bands can be attributed to a Jahn-Teller distortion of the tetrahedral structure.14 The transition of Co2þ ions in an octahedral ligand field at υ4 = 480 nm (blue region) can be assigned to [4T1(F) f 4T1(P)] transition, which is due to the transitions between octahedral and tetrahedral sites. Conclusions In summary, blue-colored CoAl2O4-ZrO2 ceramics have been successfully fabricated through a heterogeneous nucleation method. The as-prepared ZrO2 ceramic parts exhibit a uniform and vivid blue-colored appearance. Coated ZrO2 powders have been prepared by using PEG2000 as the dispersants, followed by Al and Co hydrates introduced to the suspension. Heterogeneous nucleation of CoAl2O4 within the matrix has been achieved by adding NH3 into the
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suspension to tailor the pH value. It is found that heterogeneous nucleation can promote the formation of CoAl2O4, significantly decrease the volatilization, and improve the uniformity of coloring phases within ceramics by reducing the mass transferring distance during sintering. It is believed that heterogeneous nucleation could be a facile and cost saving route to facilitate solid-state reaction and guarantee the uniform distribution of new phases within the matrix for the fabrication of colored ceramics.
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Acknowledgment. The work was financially supported by the National Science Foundation of China (NSFC, Grant No. 50572049) and the National High Technology Research and Development Program of China (863 Program, Grant No. 2007AA03Z522).
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