To evaluate the features of red pigment R1 and yellow pigment R2 prepared by this new strategy, the red pigment T1 and yellow pigment T2 prepared by the traditional process are compared [19]. All these ceramic pigments are CdSxSe1−x encapsulated by ZrSiO4. X-ray diffraction (XRD) was performed to detect the crystal phases of the red pigments R1 and T1. Obviously, both of them are composed of the main ZrSiO4 phase, a small amount of CdSxSe1−x phase, and a few SiO2 phases (Fig. 1). Furthermore, the diffraction peak of CdSxSe1−x phase is weak due to the inclusion layer of ZrSiO4 crystal. The high purity of ZrSiO4 and CdSxSe1−x phases indicates the feasibility of this new preparation strategy.
As expected, the red pigment R1 and yellow pigment R2 used for ink-jet printing are brighter and purer than the red-brown pigment J1 and gold-brown pigment J2 used for commercial ink-jet printing (Fig. 2). Although the tones of the red pigment T1 and the yellow pigment T2 show better effects in brightness and purity, they can only be used in traditional technology due to their big particle size. In order to resist the erosion of high-temperature glaze, traditional inclusion pigments require a thicker ZrSiO4 coating layer. Therefore, using a higher calcination temperature makes the ZrSiO4 crystals have better crystallinity and larger grains, and the encapsulated CdSxSe1−x particles are fully converted into crystals, and finally show a brighter hue.
The crystal composition (Fe-Cr-Zn) and structure of the pigments J1 and J2 determine that their hues are not bright enough for ink-jet printing. The tiles that painted the inks I1 (R1), I2 (R2), I3 (J1), and I4 (J2) are shown in Fig. 3. The corresponding CIE Lab chromaticity values are displayed in Table 2. For the red pigment, the redness value (a*) of R1 is 30.66, while the value a* of J1 is only 18.37. Therefore, the pigment J1 cannot present more colors when designing the ink-jet pattern. Thus, the replacement of pigment J1 by pigment R1 can meet most of the needs for red color matching, and the color range is greatly expanded. Meantime, the tone of pigment R2 is very close to that of praseodymium doped zircon yellow pigment for ink-jet, and the color intensity of the ink with the same solid content prepared by pigment R2 is almost twice that of the ink prepared by praseodymium doped zircon yellow pigment. It is expected that praseodymium zircon doped yellow pigment for ink-jet will gradually be replaced by pigment R2.
Table 2
Chromaticity values of ink I1, I2, I3, and I4 after firing.
| Ink | L* | a* | b* |
| I1 (R1) | 62.68 | 30.66 | 22.54 |
| I2 (R2) | 84.97 | -0.17 | 48.05 |
| I3 (J1) | 60.15 | 18.37 | 28.65 |
| I4 (J2) | 47.81 | 14.31 | 15.61 |
Compared with the traditional inclusion pigments T1 and T2 (D50 = 6.5 µm, D97 = 16 µm), the as-prepared pigments R1 and R2 (D50 = 1.1 µm, D97 = 2.5 µm) have a small particle size and a narrow distribution, as shown in Fig. 4. In the process of ink grinding, as the crystal grains are broken and refined, the color decay of pigments R1 and R2 is much smaller than that of pigments T1 and T2, which also suggests that pigments R1 and R2 are more wear-resistant. When the pigments R1 and R2 are ground to D50 = 0.6 µm, the appearance of the ink still has a good color, and their color only fades by 30%. The coring intensity of the ink prepared by pigments R1 and R2 and its irreplaceable pure tone has great commercial value in ceramic ink at present. When the pigments T1 and T2 are ground to D50 = 1.2 µm, the appearance color fades by 60%, and the color fades by almost 100% after ink-jet printing and firing. This is because after the crystal grains are broken, the ZrSiO4 cannot completely encapsulate the CdSxSe1−x, and the exposed CdSxSe1−x will be decomposed into cadmium oxide, selenium oxide, and sulfur dioxide under high temperature. If soaking with strong acid at this time, the color will fade away completely. Therefore, under the particle size requirements of ceramic ink (solid particle size in the ink: D50 = 500 ± 200 nm, D100 ≤ 1.5 µm), the pigments T1 and T2 have not been successfully used in ink-jet printing, although they have incomparable bright colors.
Based on the scanning electron microscope (SEM), the grain size of pigment R1 is uniform, basically between 200 ~ 1000 nm, which is close to the particle size required for the ink (Fig. 5a,b). This result is consistent with the size distribution result above. Only about 15% of the grains need to be further refined, so the grinding time required for grinding the ink is relatively shorter. The microstructure of pigment R2 is similar to that of pigment R1 due to the similar preparation process. The appropriate size of the grinding ball should be selected to avoid the particles that have reached the particle size requirements from being destroyed again, which can protect the hue of the pigment to a great extent. Compared with the pigments T1 and T2 prepared by the traditional process (Fig. 5c,d), the grain size of pigments R1 and R2 is significantly reduced and the grains have no obvious acid etch holes. The developed synthesis strategy optimizes the ratio of ZrSiO4 and CdSxSe1−x, and reduces the firing temperature and holding time, which reduces the growth speed of the coating layer crystals and the chromogenic crystals, so as to achieve the purpose of controlling the grain size of the pigment. Although the color strength and temperature resistance of the pigments prepared by this process are slightly weakened in the glazing system, which is no advantage in the traditional application field, yet it has better resistance and finer grain, which can meet the requirements of ink-jet pigment.
The microstructure of the prepared pigment R1 was observed by transmission electron microscope (TEM), as shown in Fig. 6a,b. The ZrSiO4 layer is very complete, and the coated CdSxSe1−x chromogenic grains are spherical and uniformly dispersed, with a particle size of 20 ~ 50 nm. The CdSxSe1−x grains are not adhered to and penetrated each other. Therefore, even if one particle is exposed during ball milling, it will not affect the other chromogenic particles. This is close to the structure under the assumed conditions in the Lei’s inclusion calculation model [20]. Under the same particle size, the effective inclusion rate of such structure will be much higher than that of traditional grains. However, some of the chromogenic particles of the traditional grains have been etched by acid to form the holes, and the size of the chromogenic particles is uneven, ranging from 20 to 300 nm, as observed in Fig. 6c,d. Thus, after crushing, the surface area of the broken particles is larger, the ZrSiO4 coating layer is relatively thinner, and the large CdSxSe1−x particles are more likely to be exposed. Once it is not successfully encapsulated by ZrSiO4, the entire particle will be eroded under high temperature or strong acid, and become invalid chromogenic particles. In addition, the particles prepared by the traditional process exist obvious agglomeration and penetration of the CdSxSe1−x grains. Once the particles are exposed, other particles will be "linked". Therefore, when the average particle size of the traditional coating pigment is about 1.2 µm, there is no effective coating of chromogenic particles.
In liquid-phase synthesis, monoethanolamine and Hyperdispersant are added to alleviate the agglomeration phenomenon in the sol-precipitation system. Compared with the dispersion plan reported by Liu et al. [17], this synthesis strategy reduces the amount of ethanolamine and increases the amount of high-efficiency hyperdispersant solsperse 27000. This is because when ethanolamine is excessive, the carbon element is not completely discharged during the firing process, and it is easy to be encapsulated in the crystal growth of ZrSiO4, leading to the darkening of the pigment and also affecting the inclusion efficiency. Ethanolamine has the function of complexing and anchoring metal ions (cadmium and zirconium), the latter mainly plays a steric resistance effect. With the cooperation of these two functions, it plays a key role in controlling the agglomeration of cadmium selenide and zirconium hydroxide, and controlling the uneven crystal growth, as suggested in Fig. 5 and Fig. 6. Furthermore, silicon can form gaseous silicon fluoride during the firing process, which has the ability of migration and homogenization. So long as zircon and cadmium selenide are well dispersed, the crystals grow uniformly, and the final CdSxSe1−x@ZrSiO4 grains can grow according to the dispersion state of the precursor, and the inclusion rate and particle size are guaranteed.
In addition, raw materials are selected in this synthesis strategy, and the reactivity among various raw materials is well matched. If the activity of the precursor particles is too high, the chromogenic particles will grow up rapidly during firing, and the ZrSiO4 cannot quickly encapsulate it and cannot reach the ideal inclusion state. Simlarly, when the ZrSiO4 crystal grows too fast, the CdSxSe1−x precursor crystal grows insufficient, and even if it is successfully encapsulated, the final pigment color will be dark. Therefore, the activity of the precursor is related to factors such as manufacturing process of the raw material, the content of impurities, and the types of impurities.