Effect of mechanical activation on solid-state synthesis process of neodymium disilicate ceramic pigment

Accepted Manuscript Effect of mechanical activation on solid-state synthesis process of neodymium disilicate ceramic pigment Shanjun Ke, Zhidong Pan, Yanmin Wang, Chengyun Ning, Shulong Zheng, Jianqi Huang PII:

S0143-7208(16)31408-5

DOI:

10.1016/j.dyepig.2017.06.005

Reference:

DYPI 6026

To appear in:

Dyes and Pigments

Received Date: 13 December 2016 Revised Date:

26 May 2017

Accepted Date: 3 June 2017

Please cite this article as: Ke S, Pan Z, Wang Y, Ning C, Zheng S, Huang J, Effect of mechanical activation on solid-state synthesis process of neodymium disilicate ceramic pigment, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.06.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Effect of mechanical activation on solid-state synthesis process of neodymium disilicate ceramic pigment Shanjun Ke a, Zhidong Pan a,b,*，Yanmin Wang a, Chengyun Ning a, Shulong Zheng b, Jianqi Huang b South China University of Technology, Guangzhou 510640, China b

Foshan Oceano Ceramics Co. Ltd., Foshan 528138, China

*Corresponding author. Tel.: +86 20 87114883; Fax: +86 20 87110273.

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a

E-mail address: [email protected] (Z.D. Pan), [email protected] (S.J. Ke).

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Abstract

The effect of mechanical activation of precursors ground in a high-energy density stirred bead

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mill on the synthesis of Nd2Si2O7 powders was investigated. The reaction of precursors after the mechanical treatment at different milling time was analyzed by laser particles size analysis, thermogravimetry and differential thermal analysis (TG-DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that the precursors without grinding and fired at 1200 oC cannot form the Nd2Si2O7 phase. Nd2Si2O7 can be formed at 986.9 oC when the

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precursors are ground for 3.0 h. To further clarify the effect of mechanical activation, two raw materials (i.e., SiO2 and Nd2O3) were pre-treated by different methods. The raw materials mixed and subsequently ground is conducive to decreasing the temperature of forming pure Nd2Si2O7

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phase. The results indicate that the mechanical activation favors the promotion of the crystal growth of Nd2Si2O7. In addition, the color properties of the synthesized Nd2Si2O7 powders were

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also evaluated by reflection differential colorimetry. It is indicated that Nd2Si2O7 powders that possess a high-temperature stability can be used as a ceramic pigment. Keywords: Mechanical activation; Solid-state synthesis; Neodymium disilicate; Ceramic pigment

1. Introduction

Neodymium disilicate (Nd2Si2O7) powder is considered as a potential functional pigment with “allochroic effect” under various illuminants, which could be a valuable ceramic pigment in the decoration [1]. The existing synthesis techniques for preparation of Nd2Si2O7 powder are solid-state reaction [2-4] and some chemical solution-based methods such as co-precipitation reaction [5] and sol-gel method [1,6,7]. Although the chemical solution methods provide a 1

ACCEPTED MANUSCRIPT high-quality Nd2Si2O7 powder, these methods have several inherent disadvantages associated with their rate of operation, production cost, and complexity. The solid state reaction is a conventional method for ceramic material processing. It is simple and low-cost to operate and involves cheap and easily available oxides as starting materials. However, this method usually requires the

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application of high temperatures and/or long processing time for calcination [8,9]. Therefore, it is necessary to identify and select an effective approach that is much simpler and energy-saving and can be easily scaled-up for large-scale preparation of fine powders like Nd2Si2O7 materials with some distinct advantages.

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Mechanical activation is a promising process for the preparation of precursor during the solid-state reaction, which is caused by high-energy ball milling [10]. In this process, as a result of

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high-energy milling, the particle size decreases and the inter-particle contact/activation surface area increases. In addition, the process of mechanical activation can diminish the temperature of the chemical reactions [11]. In fact, the high-energy milling has been applied to synthesis of various advanced materials [12-14]. In the past few years, some attempts have been made to synthesize various ceramic powders using mechanochemical activation starting from a powder

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mixture of carbonates and oxides, such as BaTiO3[15], LiFeO2[16], NaNbO3[17], PZT[18] and KNN[19]. However, only a little work on the mechanochemical activation on the synthesis of ceramic pigments has been reported [20].

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In this paper, a mechanochemical technique was used to synthesize single crystalline phase Nd2Si2O7 powder with the starting material of Nd2O3 and SiO2. The effect of the mechanical

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activation of the precursor on the thermal reaction process and formation of Nd2Si2O7 was investigated. In addition, the color properties of the synthesized Nd2Si2O7 powder were also evaluated.

2. Experimental 2.1. Materials

Neodymium oxide (Nd2O3, 99.5%, Ganzhou Ruihua Rare Earth Co., Ltd., China) and silicon oxide (SiO2, AR, Sinopharm Chemical Reagent Co., Ltd., China) were used as starting materials. The particle size distributions of Nd2O3 and SiO2 were analyzed by a model BT-9300S laser diffraction particle size analyzer (Dangdong Bettersize Instruments Ltd., China). In Fig. 1, the 2

ACCEPTED MANUSCRIPT median particle sizes (d50) of Nd2O3 and SiO2 are 11.4 µm and 23.3 µm, respectively. A grinding polymer aid named WF-211 (LR, Minchem Cheminal, Co. Ltd., France) was used. Deionized water (H2O, AR, Guangzhou Qianhui Bose Instrument Co. Ltd., China) was used throughout the experiments.

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2.2. Powder synthesis Nd2O3 and SiO2 were mixed to obtain the stoichiometric composition of neodymium disilicate at 5 wt.% of WF-211 as a grinding aid. Aqueous suspensions containing Nd2O3 and SiO2 particles at a solid fraction of 40 wt.% were ground in a model WS-0.3 ceramic stirred beads mill (Sanxing

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Feirong Machine Ltd., China) at 1995 rpm for 0.5, 1.0, 2.0 and 3.0 h, respectively. Zirconium oxide beads with the diameters of 0.3-0.5 mm were used as grinding media at a loading of 80

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vol.%. Also, the raw materials of SiO2 and Nd2O3 were ground severally under the same other conditions at different milling time (i.e., 0.5, 1.0, 2.0 and 3.0 h). The ground SiO2 and Nd2O3, according to a stoichiometric composition of Nd2Si2O7, were mixed uniformly by a planetary ball mill for 30 min. After grinding, the mixture solution was heated to 80 oC and kept at this temperature for 24.0 h. The dried mixtures were ground in an agate mortar and calcined at 900,

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1000, 1100 and 1200 oC for 5.0 h, respectively. 2.3. Powder characterization

The particle size of the powders was determined by a model BT-9300S laser diffraction particle

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size analyzer (Bettersize Instruments Ltd., China). The thermal analysis of the prepared precursors was carried out by a simultaneous thermogravimeter and differential thermal analyzer (TG-DTA,

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Netzsch Instruments Ltd., Germany), at a heating rate of 10 oC/min, in air, using α-Al2O3 as a reference. The crystalline phases of the powder were examined on a model PW-1710 X-ray diffractometer (XRD, Philips Co. Ltd., The Netherlands), using Cu Kα radiation. The morphology of the powder samples was inspected by a model EVO-18 scanning electron microscope (SEM, Carl Zeiss AG, Germany). The color of the powder was determined via the measurement of the value of L*a*b* using a model Color Premier 8200 reflection differential colorimeter (X-Rite Co., USA).

3. Results and discussion Fig. 2 shows the particle size distributions of the raw material mixtures ground at different time. 3

ACCEPTED MANUSCRIPT Clearly, the particle size of raw materials decreases with the increase of grinding time. For the sample without grinding, the median particle size is 14.13 µm. At the grinding time of 0.5 h, the median particle size decreases to 0.67 µm. The median particle size is 0.42 µm after further milling for 3.0 h. In fact, high-energy grinding is just a process of energy transfer [21]. During

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grinding process, collision and shearing of the zirconium oxide beads play an important role of energy transferring from beads to particles of raw materials. When mechanical energy from beads exerts the particles of raw materials, the particles are ruptured and their particle sizes decrease accordingly.

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Fig. 3 shows the XRD patterns of the raw material mixtures. Clearly, the pattern of the sample without grinding shows the presence of Nd2O3, SiO2 and Nd(OH)3. The phase of Nd(OH)3 is due

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to the reaction result of Nd2O3 and water. Hamano et al.[22] investigated the adsorption of water on Nd2O3 at room temperature. The hydration process is that Nd2O3 first changes to NdOOH and then to Nd(OH)3. The phase of Nd2O3 is not detected when the grinding time is 0.5 h. It means that the grinding process is conducive to the formation of Nd(OH)3. In addition, the diffraction intensities of SiO2 and Nd(OH)3 decrease, while the half-width of the diffraction peaks

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of SiO2 and Nd(OH)3 increases with the increase of grinding time. That is, the crystallinities of SiO2 and Nd(OH)3 reduce. It can be explained that the high-energy milling process affects the internal structures of SiO2 and Nd(OH)3.

Fig. 4 shows the XRD patterns of raw material mixtures ground at different time and

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heat-treated temperatures. For the sample heat-treated at 900 oC, there are only two phases (i.e.,

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Nd2O3 and SiO2), and no other phase is detected in the XRD pattern when the raw material mixtures are not ground. After grinding, the phase is Nd4Si3O12 for all the samples (see Fig. 4(a)). For the samples heat-treated at 1000 oC, the result is similar to the former case. Furthermore, there exists a little Nd2Si2O7 crystalline phase when the grinding time is 3.0 h (see Fig. 4(b)). The sample without grinding forms Nd4Si3O12 crystalline phase when the sintering temperature increases to 1100 oC. In addition, Nd2Si2O7 crystalline phase with a small amount of Nd4Si3O12 appears when the grinding time is 0.5 h. Nd2Si2O7 becomes a sole crystalline phase and Nd4Si3O12 disappears completely when the grinding time is 2.0 h (see Fig. 4(c)). When the sintering temperature increases to 1200 oC, there is no Nd2Si2O7 crystalline phase detected in the XRD pattern of the sample without grinding. For the ground samples, Nd2Si2O7 becomes a sole 4

ACCEPTED MANUSCRIPT crystalline phase, whose peaks are more intense (see Fig. 4(d)). These results indicate that high-energy milling process can improve the reactivity of raw materials and decrease the reaction temperature of Nd2Si2O7. Fig. 5 shows the particle size distributions of the raw materials of SiO2 and Nd2O3 ground at

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different time. It is seen that the particle size of SiO2 powder decreases with the increase of grinding time. When the grinding time is 3 h, the maximum particle size of the Nd2Si2O7 powder is only 0.95 µm (see Figs. 5(a) and 5(b)). In Figs. 5(c) and 5(d), Nd2O3 powder with a d50 of 0.49 µm is obtained after milling for 3 h, which is analogous to the previous result for the mixture of

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SiO2 and Nd2O3 ground for 3 h (see Fig. 2). Fig. 6 shows the XRD patterns of the heat-treated samples ground by different approaches. For the samples of SiO2 and Nd2O3 mixed before

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grinding for 3 h (see Fig. 6(a)), the main phase is Nd4Si3O12 after heat-treatment at 900 oC. When the sintering temperature increases to 1000 oC, a small number of Nd2Si2O7 crystalline phase appear. Furthermore, Nd2Si2O7 becomes a sole crystalline phase and Nd4Si3O12 disappears completely when the sintering temperature is 1100 oC. However, for the samples of SiO2 and Nd2O3 ground for 3 h before mixing (see Fig. 6(b)), there still exists Nd4Si3O12 crystalline phase

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detected in the XRD pattern when the sintering temperature increases to 1200 oC. These results also indicate that the mechanical activation is conducive to accelerate the formation process of the pure phase of Nd2Si2O7.

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The boundary of two or multi-components of solids may be activated by following reasons: When the particles of the sample are fractured between two beads colliding or shearing, they

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undergo deforming plastically, and are repeatedly fractured and rewelded. The forces of colliding or shearing on the particles lead to the breakage of the crystallographic bonds and the appearance of new surfaces. The new surfaces created enable the particles to weld together easily and this leads to an increase in the rate of dissolution of solid material. The chemical and physico-chemical reactions may also take place when the surface energy of the fractured particles increases. Moreover, for solid-phase reactions, grain boundary diffusion plays an important role in the whole sintering process[23]. The process of high-energy milling can form a variety of crystal defects such as increased number of grain boundaries, dislocations, stacking faults, and deformed and ruptured chemical bonds. This defect structure enhances the diffusivity of molten elements in the sintering process. Consequently, high-energy milling results in micro-homogenization of starting 5

ACCEPTED MANUSCRIPT components, which is beneficial to inducing the formation of new phases. Fig. 7 shows the TG and DTA curves of raw material mixtures ground at different time. For the sample without grinding (see Fig. 7(a)), there are three stages of weight loss on the TG curve. The first stage of weight loss (i.e., 8.84%) is due to the dehydroxylation (Nd(OH)3 → NdOOH + H2O)

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and burning out of organic compounds (like WF-211 as a grinding aid). Nagao et al. [24] also reported that Nd(OH)3 can be changed to NdOOH at 200 oC-400 oC. The second stage of weight loss (i.e., 2.3%) corresponds to the dehydroxylation (2NdOOH → Nd2O3 + H2O). Zhu et al. [25] found that the decomposition temperature of NdOOH was between 500 and 800 oC. The third

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stage shows a very little weight loss. In this stage, some new phases can be formed. For the sample with grinding, the TG curves are similar to those of the sample without grinding. However,

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the DTA curves have significant differences in the high temperature range, especially at 800 oC -1200 oC. The exothermic peaks at a high temperature indicate the crystallization of new phase. For the sample without grinding, the exothermic peak at 1033.3 oC is corresponding to the formation of Nd4Si3O12. The result is similar to that of XRD, as shown in Fig. 4(c). After grinding, the formation temperature of Nd4Si3O12 decreases to 883.8 oC. Meanwhile, the formation

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temperature of Nd2Si2O7 gradually reduces with the increase of the grinding time. The temperature for Nd2Si2O7 formation is 986.9 oC when the grinding time reaches 3.0 h. The results by thermal analysis also indicate that the mechanical activation can effectively reduce the formation

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temperature of Nd2Si2O7.

Fig. 8 shows the SEM micrographs of raw material mixtures ground at different time and

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heat-treated at 1200 oC. In Figs. 8(a) and 8(b), there are different particles with three morphologies. The spherical particles are the residual SiO2. Some aggregates with the massive fine particles are the unreacted Nd2O3. The vermiform particles are Nd4Si3O12. When the grinding time is 0.5 h, Nd2Si2O7 is a sole crystalline phase, but the grain of Nd2Si2O7 is not obvious and the shape is irregular, as shown in Figs. 8(c) and 8(d). The outline of Nd2Si2O7 grains becomes more distinct when the grinding time increases, indicating that the mechanical activation process favors the promotion of the crystal growth of Nd2Si2O7. The value of a* is positive and the value of b* is negative, implying that Nd2Si2O7 powders have red and blue hues, thus resulting in appearing purple in color. The a* and -b* values increase with the increase of the grinding time from 0.5 to 3.0 h, indicating that the color of the Nd2Si2O7 6

ACCEPTED MANUSCRIPT powders become deeper. When the sample is ground for 3.0 h and fired at 1200 oC, the value of a* is 11.08, and the value of b* is -13.11. The color properties of an object basically depend on the absorbed and scattered light at each wavelength in the region of 400-700 nm [26]. Fig. 9 shows the reflectance spectra of the raw

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material mixtures ground at different time and fired at 1200 oC in the visible wavelength range. The absorption spectra present typical Nd3+ absorption bands at 431, 476, 526, 582, 624 and 680 nm. All the absorption bands are assigned to the 4I9/2 → 2P1/2, 4I9/2 → 2G9/2, 4I9/2 → 4G7/2, 4I9/2 → 4

G5/2 + 2G7/2, 4I9/2 → 2H11/2 and 4I9/2 → 4F9/2 transition of Nd3+ ion, respectively [27]. Especially,

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there exist two intense absorption bands in a wavelength range of 500-600 nm (i.e., green and yellow). The reflectance spectra also exhibit a high reflectivity in low wavelength regions (i.e.,

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violet and blue). As a result, Nd2Si2O7 powder possesses a light-violet color.

Pigments applied in the molten glaze at a high temperature must possess a superior dyeing force. In this study, the sample ground for 3.0 h and heat-treated at 1200 oC was subjected to wet-milling in a planetary ball mill with zirconia balls for 20 min. The ground powders were added into transparent frit glaze and coated on the ceramic tile. The coated ceramic tile was sintered at 1200 C for 10 min. Fig. 10 shows the color of the glaze containing Nd2Si2O7 powders under different

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illuminants. The glaze presents violet red color under an incandescent lamp, while presents a blue color under a fluorescent lamp. The mechanism of color change under different illuminations was

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4. Conclusions

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discussed in detail in a previous work [1].

Nd2Si2O7 powders were formed via mechanical activation-assisted solid-state synthesis. The raw materials including Nd2O3 and SiO2 were pretreated at different grinding time using a high-energy density stirred beads mill. The precursors without grinding and fired at 1200 oC could not form Nd2Si2O7 phase. The temperature for Nd2Si2O7 formation was 986.9 oC after the precursors were ground for 3.0 h. The results of thermal analysis implied that the mechanical activation increased the reactivity of the precursors and reduced the characteristic temperature of Nd2Si2O7 formation. In addition, the raw materials (i.e., SiO2 and Nd2O3) pre-ground by different methods were heat-treated at different temperatures. For the mixed samples of SiO2 and Nd2O3 ground for 3 h, Nd2Si2O7 became a sole crystalline phase and Nd4Si3O12 disappeared completely 7

ACCEPTED MANUSCRIPT when the sintering temperature was 1100 oC. For the respective samples of SiO2 and Nd2O3 ground for 3 h and subsequently mixed, however, there still existed Nd4Si3O12 crystalline phase when the sintering temperature increased to 1200 oC. These results also indicated that the mechanical activation could be conducive to accelerating the formation process of the pure phase

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of Nd2Si2O7. The SEM result showed that the outline of Nd2Si2O7 grains became more distinct when the grinding time increased, indicating that the mechanical activation favored the promotion of the crystal growth of Nd2Si2O7. Moreover, Nd2Si2O7 powders possessed a high-temperature stability and could be used as a ceramic pigment with “allochroic effect” under various

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illuminants.

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Acknowledgement

This work was supported by the China Postdoctoral Science Foundation (No. 2016M602465), the Fundamental Research Funds for the Central Universities (No. 2017MS013), the Major Scientific and Technological Projects of Guangdong Province (No. 2015B090927002) and the Industry, Education and Research Integration Project of Guangdong Province (No.

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2014B090903003).

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Table 1 L*, a* and b* parameters of samples ground at different time and fired at 1200 oC. Grinding time

L*

a*

b*

0h 0.5 h 1h 2h 3h

86.82 80.05 80.42 79.40 75.68

4.10 9.39 9.49 9.72 11.08

-8.35 -11.48 -11.85 -12.03 -13.11

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Figure captions

Fig. 1 Particle size distributions of raw materials.

Fig. 3 XRD patterns of raw material mixtures ground at different time.

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Fig. 2 Particle size distributions of raw material mixtures ground at different time.

Fig. 4 XRD patterns of samples heat-treated at different temperatures, (a) 900 oC, (b) 1000 oC, (c) 1100 oC and (d) 1200 oC.

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Fig. 5 Particle size distributions of raw materials ground at different time, (a)(b) SiO2 and (c)(d) Nd2O3

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Fig. 6 XRD patterns of the heat-treated samples preground by different approaches, (a) SiO2 and Nd2O3 were mixed before grinding for 3 h, and (b) SiO2 and Nd2O3 were ground severally for 3 h before mixing.

Fig. 7 TG-DTA curves of raw material mixtures ground at different time, (a) 0 h, (b) 0.5 h, (c) 1.0 h, (d) 2.0 h, (e) 3.0 h and (f) DTA curves of all the samples from 800 to 1200 oC.

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Fig. 8 SEM images of raw material mixtures ground at different time and fired at 1200 oC, (a)(b) 0 h, (c)(d) 0.5 h, (e)(f) 1.0 h, (g)(h) 2.0 h and (i)(j) 3.0 h. Fig. 9 Reflectance spectra of raw material mixtures ground at different time and fired at 1200 oC.

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Fig. 10 Images of the samples under various illuminations.

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Nd2Si2O7 was synthesized by a mechanical activation-assisted solid-state synthesis method. Mechanical activation reduced the characteristic temperature of Nd2Si2O7 formation.

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Mechanical activation favored the promotion of the crystal growth of Nd2Si2O7.

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Color properties of the synthesized Nd2Si2O7 powder were evaluated.

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