Preparation and Properties of Co-Doped Magnesium Lanthanum Hexaluminat Blue Ceramics

: The types of blue ceramics are monotonous, mainly alumina and zirconia ceramics, and their colors are not pure, with some green tones ( − a* value is not close to 0). In this paper, aluminate blue ceramics (LaMgAl 11 − x Co x O 19 ) doped with Co were prepared by the high temperature solid-phase reaction method, which enriched the blue ceramics system. The e ﬀ ect of Co content on the color of ceramics was studied, and the optimal doping amount of Co was found. X = 1.0 is the bluest color of the material ( − b* = 35.36), and there is almost no noise e ﬀ ect ( − a* = − 2.71). By studying the e ﬀ ect of temperature on the system, it is found that the color e ﬀ ect is best when the temperature reaches 1450 ◦ C. When the temperature exceeds 1450 ◦ C, it can only promote the synthesis of LaMgAl 11 O 19 phase, and has no e ﬀ ect on the color of ceramics samples. Based on the material’s pure and bright colors, and good color stability at room temperature, it has great potential in the decoration industry, such as the preparation of jewelry or building decoration materials.


Introduction
In recent years, colored ceramics have been widely used in the decoration industry, such as mobile phone back plates, bracelets, and cases [1]. The blue hue is bright and popular with the people, and there are many researches on blue ceramics. Toriumi et al. [2] studied the mixing enthalpy of CoAl 2 O 4 solid solution blue material. Walsh et al. [3] studied the optical effects and optical catalytic properties of CoAl 2 O 4 spinel. Zhi et al. [4] used alumina and cobalt oxide to synthesize spinel-type CoAl 2 O 4 blue ceramics using a high-temperature solid-phase method, and found that calcining m(CoO): m(Al 2 O 3 ) = 0.540 at a sintering temperature of 1200 • C can provide a better cobalt blue powder material. Lv HD, Bao JX et al. [5] Prepared blue-violet ceramics with Co as the coloring ion and ZrO 2 as the toughening matrix. We find that the blue ceramics studied in recent decades are mainly alumina ceramics and ZrO 2 ceramics. However, the color is not bright enough, as the chromaticity value of this system almost meets −b* < 35 (the larger the −b* value, the bluer the color), −a* > −10 (the larger the −a value, the stronger the green tone) [1][2][3][4][5][6]. This means that the development of blue ceramics in other systems has great research value. Mg-based lanthanum hexaaluminate (LaMgAl 11 O 19 ) is a Hibonite structure with good thermal stability, optical properties, and catalytic properties [7]. There are many studies on the preparation methods of Mg-based lanthanum hexaaluminate materials. Zhu et al. [8] prepared LaMgAl 11 O 19 powder with regular morphology and more uniform particle size by sol-gel method and solid-phase sintering method. He et al. [9] used solid-phase reaction method to synthesize magnesium-based lanthanum hexaaluminate (LaMgAl 11 O 19 ) for thermal barrier coatings with uniform particle size and good fluidity. The current research reports on LaMgAl 11 O 19 are mainly focused on thermal barrier coatings and phosphor matrix materials [10,11], but few studies have been done with ceramic pigments or colored ceramics. Melo et al. [12] synthesized La 1−x Ca x CoO 3 (x = 0-0.4) pigment by polymer precursor method, and Jun Li et al. [13] studied the coloring potential of pyrite structure by replacing Al 3+ in tetrahedron with Ni 2+ , but the chromaticity value −b* is only around 18.5.
As we all know, transition elements have better coloring ability and are mostly used in the preparation of ceramic pigments [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. In the present work, we have studied the LaMgAl 11−x Co x O 19 blue ceramics (x = 0.5, 1.0, 1.5, 2.0, 2.5) and explored the optimal doping amount of the system. In addition, the influence of sintering temperature on the synthesis of phase and the optical properties of the sample was studied, and the color stability of the sample at room temperature was tested. The results show that this kind of material is suitable for the preparation of barely leaking decorative materials, such as watches, bracelets, necklaces, and other architectural decorations.

Sample Preparation
The initial raw materials are La 2 O 3 (Hangzhou Wanjing Co., Ltd., Hangzhou, China, purity ≥ 99.99%), Al(OH) 3 , MgO (Weifang Guanxu Chemical Co., Ltd., Shangdong, China, purity ≥ 99.99%) and Co 2 O 3 (Henan Zhengkai Chemical Co., Ltd, Henan, China). In order to remove the moisture contained in the raw materials, the purchased raw materials were calcined at 400 • C for 6 h. Pre-calcined raw materials (La 2 O 3 , Al(OH) 3 , MgO and Co 2 O 3 ) are accurately weighed according to the stoichiometric ratio. The raw materials weighed according to the stoichiometric ratio are ball-milled in an ethanol environment for 24 h, and then the mixed slurry that has been ball-milled for 24 h is transferred to a drying box at 75 • C for 12 h to remove the absolute ethanol in the slurry. The powder obtained after drying was ground with an agate mortar and pressed into a rough embryo with a Ø25 mold under a pressure of 20 MPa. The prepared crude embryo sample was sintered in air at 1450 • C for 3 h without pressure.

Microstructural Characterization
The phase structure of the cobalt-doped lanthanum hexaaluminate ceramic was determined, and the crystal lattice range of 20-80 • was detected by X-ray diffraction (XRD, BRUKER D8 ADVANCE, Germany) at a scanning speed of 2 • /min. We use the Jade software to analyze the measured XRD data, compare the diffraction peaks with a PDF card, and then use different symbols to mark the diffraction peaks that can match the PDF card. The microstructure indicated by the sample was characterized by a field emission scanning electron microscope (FE-SEM, Zeiss, Sigma 500), and the sample was sonicated in absolute ethanol for 50 min to remove impurities on the surface of the sample. The bulk density of the Mg-based lanthanum hexaaluminate ceramic sample in absolute ethanol was determined by Archimedes' principle, and the relative density was calculated. The theoretical density here is 4.268 g/cm 3 [15].

Optical Performance Characterization
The color of the blue lanthanum hexaaluminate ceramics were measured by the ultraviolet-visible spectrophotometer (UV-3900, Shanghai, China) from 200 nm to 800 nm, and the spectrophotometer (CM-3600A, Konica Minolta Japan Co., Shenzhen, China.) characterizes the color change of the sample with increasing cobalt content. In the face of incident light, white samples will have total reflection in the range of ultraviolet and visible light (200-800 nm), black samples will be completely absorbed, and colored samples will have diffuse reflection in a specific range of ultraviolet and visible light. For example, the peak of the UV diffuse reflection of the cobalt blue colorant sample appears at 430-500 nm [16]. Make a tangent line at the reflection peak positions of all samples using the intercept line method, and then use Formula (1) to calculate the band gap value of each sample, where E g is the band gap value of the sample, and λ is the intersection of the tangent and the abscissa.
CIE1976Lab uniform color space can be calculated using the following function formula.
In this functional formula, XYZ is the tristimulus value of the object color, and X 0 Y 0 Z 0 is the tristimulus value of the light source color. The colorimetric value (L*, a*, b*) of the samples were measured with a spectrophotometer, where L* represents the sample saturation, the larger the value of L*, the higher the saturation, the brighter the color observed by the naked eye, a* Indicates that the samples appear red or green (a* > 0 is red, a* < 0 is green), b* indicates that the sample is yellow or blue (b* > 0 is yellow, b* < 0 is blue). peaks were also found. The reason for this phenomenon is that according to most raw materials, LaMgAl 11 O 19 will be obtained by reaction. A small amount of Al(OH) 3 will change to Al 2 O 3 as the temperature rises, and react with La 2 O 3 to obtain LaAl 11 O 18 . The excess Al 2 O 3 and Co 2 O 3 will react to obtain CoAl 2 O 4 . The specific reaction process is shown in the following reaction formulas:

Phase Analysis
With the increase of Co 2 O 3 content, Al 3+ in LaMgAl 11 O 19 lattice is replaced by Co 3+ , and the remaining amount of Co 2 O 3 increases, which leads to the enhancement of the diffraction peak of With the increase of Co2O3 content, Al 3+ in LaMgAl11O19 lattice is replaced by Co 3+ , and the remaining amount of Co2O3 increases, which leads to the enhancement of the diffraction peak of CoAl2O4.   Figure 2 shows a FE-SEM image of a Co-doped magnesium-based lanthanum hexaaluminate ceramics sintered at 1450 °C. The polyhedral CoAl2O4 phase and lamellar LaMgAl11O19 phase can be found in the figure. At the same time, it was found that with the increase of the Co content, the phase content of CoAl2O4 gradually increased, and the compactness of the samples gradually decreased [17,18], which was consistent with the bulk density change values recorded in Table 1. When x = 2.0, obvious pores appear on the ceramic surface, which may be caused by the soft and porous structure of CoAl2O4 [19].   At the same time, it was found that with the increase of the Co content, the phase content of CoAl 2 O 4 gradually increased, and the compactness of the samples gradually decreased [17,18], which was consistent with the bulk density change values recorded in Table 1. When x = 2.0, obvious pores appear on the ceramic surface, which may be caused by the soft and porous structure of CoAl 2 O 4 [19].

The diffuse Reflectance and the Chromatic Properties
The blue Mg-based lanthanum aluminate ceramics prepared by experiments were pure blue and uniform in color. Figure 3 shows the UV-vis diffuse reflection spectrum of Co-doped LaMgAl11O19 ceramics. With the increase of Co content, the reflectance gradually decreases, the reflectance decreases from about 50% to 25%, and the sample color brightness decreases. The figure shows that the sample has strong diffuse reflection at about 431 nm, and the absorption peak appears at 494 nm. The samples reflect blue light and absorbs light other than blue light. All samples found very small absorption peaks at the 475 nm position. This may be caused by the CoAl2O4 spinel phase absorbing visible light around 430 nm and overlapping with the diffraction peaks of LaMgAl11-xCoxO19 [20]. All samples have a strong reflection in the visible range after the 725 nm position, which increases the brightness of the sample. The intercept method was used to calculate the forbidden band width of

The Diffuse Reflectance and the Chromatic Properties
The blue Mg-based lanthanum aluminate ceramics prepared by experiments were pure blue and uniform in color. Figure 3 shows the UV-vis diffuse reflection spectrum of Co-doped LaMgAl 11 O 19 ceramics. With the increase of Co content, the reflectance gradually decreases, the reflectance decreases from about 50% to 25%, and the sample color brightness decreases. The figure shows that the sample has strong diffuse reflection at about 431 nm, and the absorption peak appears at 494 nm. The samples reflect blue light and absorbs light other than blue light. All samples found very small absorption peaks at the 475 nm position. This may be caused by the CoAl 2 O 4 spinel phase absorbing visible light around 430 nm and overlapping with the diffraction peaks of LaMgAl 11−x Co x O 19 [20]. All samples have a strong reflection in the visible range after the 725 nm position, which increases the brightness of the sample. The intercept method was used to calculate the forbidden band width of each sample, and each sample was calculated and averaged multiple times. After collating the data, the relationship between Co content and forbidden band width (E g ) is obtained, as shown in Figure 4. In the figure, we find that the band width of the sample increases with the increase of the Co content, indicating that the color of the sample becomes darker when the band width increases within a certain range.
Ceramics 2020, 3 FOR PEER REVIEW 6 each sample, and each sample was calculated and averaged multiple times. After collating the data, the relationship between Co content and forbidden band width (Eg) is obtained, as shown in Figure  4. In the figure, we find that the band width of the sample increases with the increase of the Co content, indicating that the color of the sample becomes darker when the band width increases within a certain range.    Ceramics 2020, 3 FOR PEER REVIEW 6 each sample, and each sample was calculated and averaged multiple times. After collating the data, the relationship between Co content and forbidden band width (Eg) is obtained, as shown in Figure  4. In the figure, we find that the band width of the sample increases with the increase of the Co content, indicating that the color of the sample becomes darker when the band width increases within a certain range.    The color coordinate values (L*, a* and b*) and chromaticity change curves of the synthesized lanthanum aluminate ceramics are shown in Table 2 and Figure 5. It can be seen from Figure 5 that L* shows a downward trend with the increase of the content of Co, and the color of the sample becomes darker. The a* values of all samples are distributed around 0, and the −b* values representing blue are greater than 20, and they increase first and then decrease. When x = 1.0, the maximum value of 35.36 is reached, and the sample has the strongest blue hue. Jun Li et al. [13] Prepared the LaAl 11 NiO 19 material with a blue tone of −b* = 23. It can be seen that the magnesium-based lanthanum hexaaluminate material we prepared is relatively blue and bright. The physical picture of the sample is shown in Figure 6. In summary, when the doping amount is x = 1.0, the sample band gap value E g = 4.26. At this time, the bluest ceramic is obtained. are greater than 20, and they increase first and then decrease. When x = 1.0, the maximum value of 35.36 is reached, and the sample has the strongest blue hue. Jun Li et al. [13] Prepared the LaAl11NiO19 material with a blue tone of −b * = 23. It can be seen that the magnesium-based lanthanum hexaaluminate material we prepared is relatively blue and bright. The physical picture of the sample is shown in Figure 6. In summary, when the doping amount is x = 1.0, the sample band gap value Eg = 4.26. At this time, the bluest ceramic is obtained.    are greater than 20, and they increase first and then decrease. When x = 1.0, the maximum value of 35.36 is reached, and the sample has the strongest blue hue. Jun Li et al. [13] Prepared the LaAl11NiO19 material with a blue tone of −b * = 23. It can be seen that the magnesium-based lanthanum hexaaluminate material we prepared is relatively blue and bright. The physical picture of the sample is shown in Figure 6. In summary, when the doping amount is x = 1.0, the sample band gap value Eg = 4.26. At this time, the bluest ceramic is obtained.     Figure 7 shows the effect of temperature on the optimal doping amount of LaMgAl 11−x Co x O 19 . Unsintered samples with optimal doping amount x = 1.0 were selected and calcined at 1350 • C, 1450 • C, and 1550 • C, respectively. Their X-ray diffraction (Figure 7a) and optical properties (Figure 7b) were examined. Figure 7a shows that LaMgAl 11 O 19 is the main phase in these three samples calcined at different temperatures. The diffraction peak intensity of CoAl 2 O 4 gradually decreases with increasing temperature, which may be caused by the volatile Co element at high temperature [21]. It is found from Figure 7b that the temperature has a certain effect on the brightness of the sample. The sintered sample at 1450 • C is significantly bluer than the sample prepared at 1350 • C. The blue tone does not increase significantly at 1550 • C. Considering the energy consumption, we found that it is more reasonable to calcine the sample at 1450 • C. Figure 8 shows the change of sample color when the best-doped sample (x = 1.0) calcined at 1450 • C was placed in air at normal temperature for different times. It was found that the sample that was just out of the oven was left in the air for 0.8 years, and the color of the sample did not change, which indicates that this sample has good color stability at room temperature.

Influence of Other Factors on the Optimal Doping Amount
Ceramics 2020, 3 FOR PEER REVIEW 8 Figure 7 shows the effect of temperature on the optimal doping amount of LaMgAl11-xCoxO19. Unsintered samples with optimal doping amount x = 1.0 were selected and calcined at 1350 °C, 1450 °C, and 1550 °C, respectively. Their X-ray diffraction (Figure 7a) and optical properties (Figure 7b) were examined. Figure 7a shows that LaMgAl11O19 is the main phase in these three samples calcined at different temperatures. The diffraction peak intensity of CoAl2O4 gradually decreases with increasing temperature, which may be caused by the volatile Co element at high temperature [21]. It is found from Figure 7b that the temperature has a certain effect on the brightness of the sample. The sintered sample at 1450 °C is significantly bluer than the sample prepared at 1350 °C. The blue tone does not increase significantly at 1550 °C. Considering the energy consumption, we found that it is more reasonable to calcine the sample at 1450 °C. Figure 8 shows the change of sample color when the best-doped sample (x = 1.0) calcined at 1450 °C was placed in air at normal temperature for different times. It was found that the sample that was just out of the oven was left in the air for 0.8 years, and the color of the sample did not change, which indicates that this sample has good color stability at room temperature.

Conclusions
In this paper, cobalt-doped Mg-based lanthanum hexaaluminate ceramics were prepared by a solid-phase high-temperature reaction method. The effects of doping amount and temperature on optical properties were studied, and the stability of the samples at room temperature was analyzed. The specific conclusions are as follows: (1) The doping content of Co is the key factor for the color development of the sample, and x = 1.0 is the optimal doping amount. At this time, the blue tone of the sample −b * reached the maximum value (−b * = 35.36). (2) The forbidden band width also plays a certain role in the color development of the sample.
When Eg = 4.26 ev in this system, the sample color is the bluest. (3) Increasing the temperature can promote the synthesis of the phase, thereby increasing the color depth of the sample. In this system, the color of the sample reaches the deepest at 1450 °C. Continuing to increase the temperature will only promote the synthesis of the phase with little effect on the color of the sample. (4) This material can maintain color stability when exposed to air, and can be used to prepare various jewelry and decorative items.