Synthesis and characterization of (Ni, Sb)-co-doped rutile ceramic pigment via mechanical activation-assisted solid-state reaction

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Synthesis and characterization of (Ni, Sb)-co-doped rutile ceramic pigment via mechanical activation-assisted solid-state reaction Shi Zhang, Zhidong Pan ∗ , Yanmin Wang ∗ School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 9 October 2017 Received in revised form 21 December 2017 Accepted 28 December 2017 Available online xxx Keywords: Mechanical activation Solid-state synthesis (Ni, Sb)-co-doped rutile Ceramic pigment

a b s t r a c t (Ni, Sb)-co-doped rutile yellow ceramic pigments were synthesized via a mechanical activation-assisted solid-state reaction in a stirred bead mill. The effects of mechanical grinding of the raw materials on the synthesis of (Ni, Sb)-co-doped rutile pigment were investigated. The results show that mechanical activation effectively decreased the crystallinity of the mixed precursors, thus enhancing their reactivity and the migration rate of ions. These effects lowered the temperature required for formation of the rutile phase. Compared with results from a mixture of separately ground precursors, the mixture of raw materials ground together was more effective for accelerating nickel/antimony ions doping into the TiO2 structure and improved the color performance of the pigment. The pigment from the mixed raw materials ground for 2.0 h and heat-treated at 1100 ◦ C could be used as an environmentally-friendly ceramic yellow pigment owing to its low-toxicity, bright yellow hue, good chemical/thermal durability, and high cost-effectiveness. © 2018 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Introduction Inorganic yellow pigments have a wide range of applications, such as porcelains, ceramic inks, plastics, cosmetics, and building materials (Bae, Tamura, & Imanaka, 2016; Grazenaite, Pinkas, Beganskiene, & Kareiva, 2016; Llusar et al., 2015; Moghtada, Shahrouzianfar, & Ashiri, 2017). In general, lead chromate (PbCrO4 ), antimony yellow (Pb3 (SbO4 )2 ), and cadmium yellow (CdS) are the most widely applied yellow pigments in industrial production. However, the applications of these yellow pigments are restricted because of the toxicity of the elements chromium (Cr), cadmium (Cd), antinomy (Sb), and lead (Pb), which are harmful to the human body and environment. Praseodymium-doped zirconium silicate (Pr-ZrSiO4 ) is another popular yellow ceramic pigment but contains a costly rare-earth element (i.e., praseodymium). Therefore, the development of environmentally friendly and cost-effective inorganic yellow pigments is required (He, Wang, Liu, Li, & Niu, 2017; Wang, Li, Byon, Mei, & Zhang, 2013). Elements such as nickel (Ni) and antimony (Sb) can diffuse into rutile lattices to form highly stable structures (Zou, 2013); hence, (Ni, Sb)-co-doped rutile pigments have been developed.

∗ Corresponding authors. E-mail addresses: [email protected] (Z. Pan), [email protected] (Y. Wang).

These materials feature a high refractive index, are cost-effective, show good chemical stability, and are environmentally friendly (Matteucci, Cruciani, Dondi, & Raimondo, 2006). Existing methods of synthesizing inorganic pigments include solid-state methods (Dondi, Cruciani, Guarini, Matteucci, & Raimondo, 2006; Gargori, Cerro, Galindo, & Monrós, 2010; Huguenin & Chopin, 1998; Sreeram, Aby, Nair, & Ramasami, 2008) and chemical methods, such as the Pechini method (Yuan, Fan, Ding, Wang, & Zhang, 2012), sol-gel method (Sadjadi, Zare, Khanahmadzadeh, & Enhessari, 2008; Taylor, Fleig, & Page, 2002), co-precipitation method, and the polymeric precursor method. Among these approaches, solidstate methods are simple and effective for synthesizing pigments. However, solid-state methods typically feature a high energy consumption owing to the requirements for high calcination temperatures. Mechanical activation in a grinding mill is a common pretreatment process of precursors prior to calcination reactions. This process can decrease the particle size, increase the specific surface area, introduce defects into particles, accelerate migration of defects in the bulk, and increase inter-particle contact (Avvakumov, Senna, & Kosova, 2001). This technique can thus lower the temperature required for the subsequent solid reactions (Ke et al., 2017). Mechanical activation has been used as a pretreatment to synthesize various ceramic pigments, such as Fe2 O3 –ZrSiO4 inclusion pigments (Cannio & Bondioli, 2012), Cr-doped sphene pigment

https://doi.org/10.1016/j.partic.2017.12.016 1674-2001/© 2018 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

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(Pantic´ et al., 2013), and Nd2 Si2 O7 pigments (Ke et al., 2017). However, there have been few studies on the effects of mechanical activation on the synthesis of (Ni, Sb)-co-doped rutile yellow pigments. In this paper, a (Ni, Sb)-co-doped rutile yellow ceramic pigment was prepared by a mechanical activation-assisted solid-state method. The effects of milling time and calcination temperature on the performance of the pigment were investigated. The chemical and thermal resistances of the (Ni, Sb)-co-doped rutile ceramic pigments were examined. In addition, the color properties and durability of (Ni, Sb)-co-doped rutile pigment were also evaluated. Experimental Pigment synthesis Materials including titanium dioxide (TiO2 , 99.95%), antimony oxide (Sb2 O3 , 99.5%), and nickel nitrate (Ni(NO3 )2 ·6H2 O, 99.5%) (Tianjin Fuchen Chemical Reagent Co., Ltd., China) were used in this study. These precursors were mixed at a stoichiometric composition of Ti0.91 Ni0.03 Sb0.06 O2 . The solid content of the suspensions was 35 wt%. Suspensions with 1 wt% of grinding media (AD8030, Aoda Co., Ltd., China) as a grinding aid were mechanically activated in a model WS-0.1L ceramic stirrer bead mill (Sanxing Feirong Machine Ltd., China) at a stirring line speed of 6.89 m/s for 0.5, 1.0, 1.5, and 2.0 h. The grinding media were zirconium oxide ceramic beads with diameters of 0.1–0.3 mm and the loading of the grinding media was 80 vol%. Suspensions of the individual precursors, i.e., TiO2 and the mixture of Sb2 O3 and Ni(NO3 )2 ·6H2 O were also ground under the same conditions described above. After grinding, TiO2 and the mixture of Sb2 O3 and Ni(NO3 )2 ·6H2 O were mixed by mechanical stirring for 0.5 h at a stoichiometric composition of Ti0.91 Ni0.03 Sb0.06 O2 . The pretreated mixtures were then heated to 110 ◦ C and maintained at this temperature for 12.0 h to dry the precursors, which were then calcined at 800, 900, 1000, and 1100 ◦ C for 1.0 h at a heating rate of 10 ◦ C/min. The chemical and thermal resistances were also examined for the (Ni, Sb)-co-doped rutile pigments fabricated from the mixed raw materials ground for 2.0 h and calcined at 1100 ◦ C. For thermal resistance testing, (Ni, Sb)-codoped rutile powders were thermally treated in a muffle furnace at 1100 ◦ C for 1.0 h. For chemical resistance studies, the pigments were soaked in 5 wt% aqueous solutions of HCl, HNO3 , and NaOH for 1.0 h. Characterization The particle size and particle size distributions were measured with a model BT-9300S laser diffraction particle size analyzer (Bettersize Instruments Ltd., China). The thermal properties were analyzed by a model STA449C/3/MFC simultaneous thermogravimeter and differential scanning calorimeter (TG–DSC, Netzsch Co. Ltd., Germany), using ˛-Al2 O3 as a reference, at a heating rate of 10 ◦ C/min, in air. The phase composition was characterized by a model X’Pert Pro X-ray diffractometer (XRD, PANalytical Co. Ltd., The Netherlands), with Cu K˛ radiation ( = 1.15418 nm). The Raman spectra were obtained on an INVIA Raman analyzer (Renishaw PLC. Ltd., UK) with laser irradiation at 532 nm. The morphology and microstructure were inspected by a model EVO18 scanning electron microscope (SEM, Zeiss Co. Ltd., Germany). A model Lambda 950 UV–vis photospectrometer (UV–vis, Perkin Elmer Co., USA) was used to determine the optical absorbance spectra of powders, against a BaSO4 reference. The colorimetric values (L*/a*/b*) of the powders were determined with a colorimeter (X-RiteCo. Ltd., USA). Note that the parameter L* indicates the brightness (+L*) to darkness (−L*), the parameter a* presents the

color green (−a*) to red (+a*), and parameter b* presents color blue (−b*) to yellow (+b*) (CIE, 1932).

Results and discussion Fig. 1 shows the particle size distributions of the raw material mixtures ground at different times. The coarser particles in the mixtures were fractured into finer particles by the grinding process (Ohenoja, Illikainen, & Niinimäki, 2013; Zhang, 2004). The particle size of the raw materials decreased as the grinding time was increased. For the unground mixture of precursors, the median particle size was 1.25 ␮m. After grinding for 0.5 and 2.0 h, the median particle size of the mixture decreased to 0.64 and 0.50 ␮m, respectively. Mechanical milling is a process in which the particles are collided, pressed, trapped, and forged by moving beads. When shear, impact, and compressive stresses on particles are induced in high-energy milling, the particles are plastically deformed and fractured, resulting in a decrease of their size. Table 1 shows the CIE L*a*b* parameters (CIE, 1932) of raw material mixtures ground for different times. Clearly, the b* value increased while the −a* value decreased as the grinding time was increased, indicating that the yellow color of the raw material mixtures became more intense with extended milling time. This result implies that the crystal structure of TiO2 was disrupted and Ni ions were doped into the structure of TiO2 during mechanical milling, which enhanced the yellow color of the ground mixture. Fig. 2 shows X-ray diffraction patterns of the raw material mixtures ground for different times. Two phases were observed, i.e., anatase (PDF file number 21-1272) and Sb2 O3 (PDF file number 43-1071), in the precursors without grinding. After grinding, the Sb2 O3 phase was not observed. This result could be attributed to the crystal structure of Sb2 O3 being changed by high-energy milling. In addition, the half-width of the diffraction peaks of anatase increased and the diffraction intensity decreased for longer milling times. Clearly, the crystallinity of the anatase phase gradually decreased for longer grinding times. This result shows that the mechanical process disrupted the crystal structure. Fig. 3 shows X-ray diffraction patterns of mixtures of the raw materials ground at different times and heat-treated at different temperatures. For powders calcined at 800 ◦ C, anatase was the only phase in the mixture of precursors before grinding. However, rutile (PDF file number 21-1276) was the only phase observed in the mixture of precursors after grinding for 2.0 h (see Fig. 3(a)). For the powders heat-treated at 900 ◦ C, rutile became the only crystalline phase after a grinding time of 1.5 h (see Fig. 3(b)). The phase became rutile for all samples when the calcination temperature was increased to 1000 or 1100 ◦ C. In solid state reactions, a phase transformation from anatase to rutile occurs at 900 ◦ C, and a single pure rutile phase appears at 1000 ◦ C (Gargori et al., 2010). These results indicate that mechanical activation of precursors in the mill can increase the reaction rate and decrease the rutile formation temperature. Fig. 4 shows the particle size distributions of the mixtures of each raw material ground separately at different times. The particle sizes of the each of the ground raw materials decreased as the grinding time was increased. In addition, the median particle size of the mixed sample of the raw materials ground individually for 2.0 h was 0.49 ␮m. This size was similar to that of the mixture of raw material ground together for 2.0 h (i.e., 0.50 ␮m). Table 2 shows the CIE L*a*b* parameters of mixtures of each raw material ground separately for different times. Clearly, the b* and −a* values increased as the grinding time was prolonged, indicating that the mechanical grinding progress can promote the yellow and green colors of the mixtures of each raw material. However, for mixtures of the individually ground raw materials the value of b*,

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Fig. 1. Particle size distributions of mixture of raw materials ground together for different durations.

Table 1 L*, a*, b* parameters of the mixture of raw materials ground at different times. Grinding time (h)

L*

a*

b*

0 0.5 1.0 1.5 2.0

94.72 90.14 89.17 87.41 89.97

−3.98 −3.46 −2.51 −1.90 −1.65

2.08 12.29 16.55 18.00 18.95

which indicates yellow, was far lower than that of the raw material mixtures ground together for the same time (see Tables 2 and 1). Hence, the yellow color of the precursor mixtures ground together was more intense than that of the mixture of individually ground materials. This result can be attributed to an increase of defects in the particles and inter-particle contacts and defect migration in the bulk, which were enhanced in the mixture ground together. Also, Fig. 5 shows X-ray diffraction patterns of the materials preground by different approaches and subsequently heat-treated at different temperatures. For the raw material mixtures pre-ground for 2.0 h (see Fig. 5(a)), rutile was the sole phase in the pigment when the heat-treating temperature was >800 ◦ C. However, for mixtures of each raw material ground separately for 2.0 h, the Sb2 O3 phase persisted in the precursors, and rutile became the sole phase only when the heat-treatment temperature was >900 ◦ C (see Fig. 5(b)). These results indicate that as mechanical stress is exerted on the particles in the mixture of the raw material, the crystalline

Fig. 2. X-ray diffraction patterns of mixture of raw materials ground together for different durations.

lattices are subjected to distortion, and doping of nickel (Ni2+ ) and antimony (Sb5+ ) ions into TiO2 can be accelerated, thus lowering the formation temperature of rutile in the subsequent heat treatment. Fig. 6 shows Raman spectra of the raw material mixtures ground for different times. For the unground raw material mixture, intense bands appeared at 142.4, 195.6, 394.7, 515.5, and 636.9 cm−1 . All these bands were assigned to anatase, which can be attributed to the five Raman active modes of anatase phase with symmetries of Eg (143 cm−1 ), Eg (196 cm−1 ), B1g (395 cm−1 ), A1g (516 cm−1 ), and Eg (639 cm−1 ), respectively (Sekiya, Ohta, Kamei, Hanakawa, & Kurita, 2001). A band at 255.5 cm−1 was assigned to Sb2 O3 (Mestl, Ruiz, Delmon, & Knozinger, 1994). After grinding, the bands attributed to anatase and Sb2 O3 became broader and some even disappeared. This observation indicates that the mechanical force of the highenergy stirrer bead milling on the particles led to disruption of anatase/Sb2 O3 crystallographic bonds and accelerated the doping of Ni and Sb ions into TiO2 . The mechanical grinding process decreased the size and crystallinity of the solids, induced chemical interactions among the solids (Avvakumov et al., 2001), and induced mechano–chemical reactions (Cannio & Bondioli, 2012). In addition, some bands at 169.5, 337.0, 316.5, and 357.5 cm−1 , which were attributed to the B3g (169 cm−1 ), B3g (327 cm−1 ), B1g (318 cm−1 ), and B2g (366 cm−1 ) modes of brookite (Iliev, Hadjiev, & Litvinchuk, 2013), appeared in the ground precursor mixtures. After grinding for 2.0 h, two intense bands appeared in the precursor mixtures at 604 and 426 cm−1 , which were similar to the A1 g (612 cm−1 ) and Eg (447 cm−1 ) rutile modes (Ocana, GarciaRamos, & Serna, 1992). These results indicate that mechanical activation favors the formation of new phases, i.e., brookite and rutile. Mechanical grinding processes in a stirrer bead mill can form various crystal defects (i.e., ruptured chemical bonds, stacking faults, dislocations, and new grain boundaries) (Ke et al., 2017). These crystal defects are beneficial for inducing the formation of new phases. Note that the relatively small amount of brookite/rutile seeds, localized on the outer layer of the particles could be detected by Raman spectroscopy owing to the enhancement of the Raman signals from the shells. Fig. 7 shows TG–DSC curves of the mixtures of raw materials ground together at different times. Three stages of weight loss were present in the TG curves. For the unground mixture of raw materials (see Fig. 7(a)), the first stage of weight loss (i.e., 1.33%) was attributed to the removal of water of crystallization (such as that in Ni(NO3 )2 ·6H2 O → Ni(NO3 )2 ·3H2 O + 3H2 O; Ni(NO3 )2 ·3H2 O → Ni(NO3 )2 ·2H2 O + H2 O) and burning out of organic compounds (such as the polymer dispersant). The second stage of weight loss (i.e., 2.14%)

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Fig. 3. X-ray diffraction patterns of mixture of raw materials ground together for different durations and heat-treated at different temperatures, (a) 800 ◦ C, (b) 900 ◦ C, (c) 1000 ◦ C, and (d) 1100 ◦ C.

Fig. 4. Particle size distributions of the mixture of raw material individually ground for different durations.

Table 2 L*, a*, b* parameters of the mixture of each raw material ground separately at different times. Grinding time (h)

L*

a*

b*

0 0.5 1.0 1.5 2.0

94.72 92.52 91.75 91.73 91.33

−3.98 −4.53 −4.78 −4.86 −4.95

2.08 7.42 9.46 10.40 10.90

was attributed to the decomposition of intermediates (Ni(NO3 )2 ·2H2 O → 1/3Ni(OH)2 ·2Ni(NO3 )2 ·2H2 O + 2/3H2 O + 2/3HNO3 ; 1/3Ni(OH)2 ·2Ni(NO3 )2 ·2H2 O → NiO + 4/3NO2 + 1/3O2 + H2 O) and the removal of constitutional water. Brockner, Ehrhardt, and Gjikaj (2007) also found that the decomposition temperature of Ni(NO3 )2 ·2H2 O likes between 140–190 ◦ C, and the intermediate can decomposed to NiO at 300 ◦ C. There is a small weight loss in the third stage relating to the formation of some new phases. The TG curves for each sample are similar, while the DSC curves

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Fig. 5. X-ray diffraction patterns of mixtures of materials after being ground and heat-treated at different temperatures in different sequences: (a) TiO2 , Sb2 O3 , and Ni(NO3 )2 ·6H2 O were first mixed and then ground for 2.0 h and (b) TiO2 , Sb2 O3 , and Ni(NO3 )2 ·6H2 O were first separately ground for 2.0 h and then mixed. Table 3 Crystallographic parameters of pigments from mixture of raw materials ground together at different times and heat-treated at 1100 ◦ C.

Fig. 6. Raman spectra of mixture of raw materials ground for different durations.

show differences. In the unground mixture of raw materials, endothermic and exothermic peaks appeared at 273.4, 514.5, and 1010.4 ◦ C, which correspond to decomposition of the intermediate Ni(NO3 )(OH)1.5 O0.25 ·H2 O, formation of the anatase phase, and formation of the rutile phase, respectively. However, in the ground raw material mixtures, the endothermic peak at 273.4 ◦ C disappeared, indicating that mechanical activation promotes the decomposition of nickel nitrate at a lower temperature. Furthermore, the temperature of anatase formation gradually decreased as the grinding time was prolonged. For a grinding time of 2.0 h, the temperature decreased to 187.5 ◦ C. Similarly, the formation temperature of rutile decreased in the ground raw material mixture and at a grinding time of 2.0 h, the temperature for the complete transformation of anatase to rutile was 723.4 ◦ C. These results indicate that mechanical activation of the raw material mixture in the mill can effectively lower the formation temperature of rutile. The mechanical grinding process can effectively decrease the crystallinity and destroy the crystal structure, thus enhancing the reactivity of the mixed precursors. Table 3 shows the crystallographic parameters of the pigments formed from the raw material mixtures ground at different times and heat-treated at 1100 ◦ C. Clearly, the crystallographic parameters (a, c) of these pigments were larger than those of the rutile

Grinding time (h)

Average crystal size (nm)

0 0.5 1.0 1.5 2.0

62.7 69.2 71.6 74.0 75.2

Lattice parameters

a (Å)

c (Å)

4.5967 4.5993 4.5998 4.6012 4.6024

2.9617 2.9629 2.9634 2.9639 2.9642

Cell volume (Å3 )

62.580 62.676 62.700 62.749 62.788

phase (a = 4.5933 Å, c = 2.9592 Å) (Amraoui, Doghmane, Chettibi, & Laefer, 2017). Furthermore, the average grain size, crystallographic parameters (a, c), and cell volume of the pigments increased as the grinding time increased. These findings support the assumption that nickel ions (Ni2+ ) are well dissolved into the tetragonal rutile lattice of TiO2 , and that doping of nickel ions (Ni2+ ) increases for longer grinding times. This behavior is expected according to the difference between the ionic radii of titanium ions Ti4+ (r = 0.61 Å) and nickel ions Ni2+ (r = 0.69 Å). The cell volume changes almost linearly as nickel ions (Ni2+ ) are substituted into the lattice of the TiO2 structure (Ahmed, 2017). We also examined the material by SEM imaging, as shown in Fig. 8. The grain size increased as the grinding time was prolonged. Mechanical activation effectively decreased the crystallinity of the mixed precursors and enhanced the migration rate of ions, thus increasing the grain size. For the pigments formed from the unground precursors, the morphology of the particles was spherical or ellipsoidal (see Fig. 8(a) and (b)). However, for the pigments from the ground precursors, the particles gradually changed to a short columnar morphology, which is typically associated with a well-developed rutile phase. Clearly, mechanical activation of the precursor mixture in the mill before the heat treatment effectively promoted the formation and grain growth of the rutile phase. To characterize the color performance of the pigments from precursor mixtures ground at different times and heat-treated at 1100 ◦ C, the UV–vis absorption spectra, chromatic diagram, and color parameters are shown in Figs. 9 and 10, and Table 4, respectively. A broad band in the blue region (i.e., 400–500 nm) featured in the spectra of the pigments arising from the spin-allowed transition 3 (3 A2g → 3 T1g (3 p)) of Ni2+ (3d8 ion) in an octahedral environment. A broad band at 600–800 nm was considered to arise from spin-allowed transitions 2 (3 A2g → 3 T1g (3 F)) in a Ni2+ (3d8 ion)

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Fig. 7. TG–DSC curves of mixture of raw materials ground together for different durations: (a) 0, (b) 0.5, (c) 1.0, (d) 1.5, and (e) 2.0 h.

octahedral environment. A band at 250–400 nm was attributed to the O2− –Ti4+ (i.e., 250–300 nm) and O2− –Ni2+ (i.e., 300–400 nm) charge transfer transitions (Llusar et al., 2015). He et al. (2017) reported that intense absorptions in the blue (3 ) and green regions of NiTiO3 ceramic pigments (i.e., 490–560 nm) are responsible for its yellow color. In this work, the broad and intense absorption in the blue (3 ) and green regions (i.e., 380–560 nm) of the pigments heat-treated at 1100 ◦ C are responsible for the yellow color. The ligands are further away from the central ion when Ni2+ ions (r = 0.69 Å) substitute the central ions (i.e., Ti4+ , r = 0.61 Å) in the lattice of TiO2 structure (see Table 3). Thus the transition energy from the higher energy level to a lower energy level is decreased. Consequently, the absorption bands of the pigments shift to longer wavelengths (i.e., blue/green region), giving rise to the yellow color (Pantic´ et al., 2013). In addition, the absorption bands in the blue/green region (i.e., 380–560 nm) of the pigments became more intense as the grinding time was prolonged. The color should become more intense as the absorption intensity increases.

Clearly, the Ni-doping into rutile increased for longer grinding times because the mechanical stress on the particles promoted doping of nickel ions into TiO2 (see Tables 1–3). A corresponding chromatic diagram of the pigments formed from the raw material mixtures ground at different times and those heat-treated at 1100 ◦ C is shown in Fig. 10 (CIE, 1932). Clearly, the yellow color of the pigments became more intense as the grinding time was prolonged, which is consistent with the results of the UV–vis absorption spectra (see Fig. 9). (Ni, Sb)-co-doped rutile pigment presented with a yellow color while the rutile titanium dioxide presented with a white color. These results can be attributed to the substitution of nickel ions (Ni2+ ) into the lattice of the TiO2 structure, thus decreasing the valence states of the central ions, and increasing the distance from the ligands to the central ion. Therefore, the crystal field splitting energy  decreased according to the crystal field theory (CFT) (Stephanos & Addison, 2017). The absorption band shifted to longer wavelengths (i.e., blue/green region) on the basis of the equation  = hc/, where h is the Planck

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Fig. 8. SEM images of pigments from mixture of raw materials ground together for different durations and heat-treated at 1100 ◦ C: (a and b) 0, (c and d) 0.5, (e and f) 1.0, (g and h) 1.5, and (i and j) 2.0 h.

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Table 5 Color parameters and color difference (E*ab ) for (Ni, Sb)-co-doped rutile pigment before and after the thermal and chemical resistance tests.

Fig. 9. UV–vis absorbance spectra of pigments from mixtures of raw materials ground for different durations and heat-treated at 1100 ◦ C.

Fig. 10. Chromatic diagram of pigments formed from mixture of raw materials ground together for different durations and heat-treated at 1100 ◦ C. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.) Table 4 L*, a*, and b* parameters of pigments from mixture of raw materials ground together at different times and heat-treated at 1100 ◦ C and those of other yellow ceramic pigments. Pigments

Grinding time (h)

(Ni, Sb)-co-doped rutile

0 0.5 1.0 1.5 2.0

Pr-ZrSiO4 (De la Luz et al., 2013) Ni-geikielite (Llusar et al., 2015) Cr doped rutile (Zou, 2013) (Cr, Sb)-co-doped rutile (Calatayud et al., 2016) (Ni, Sb)-co-doped rutile (Dondi et al., 2006)

Color parameters

L*

a*

b*

89.15 87.10 86.93 86.23 85.85 84

−6.2 −4.26 −4.03 −3.46 −2.54 3

47.08 60.05 60.56 61.56 62.83 63

83.6

4.6

44.6

75.46 75

15.24 15

39.46 39

79

−3

50

Samples

L*

a*

b*

E*ab

1100 ◦ C heat-treatment 5 wt% HCl 5 wt% HNO3 5 wt% NaOH

84.11 84.49 84.24 85.10

−2.03 −2.45 −2.13 −2.40

63.30 62.90 62.37 62.23

1.87 1.36 1.72 0.97

constant, c is the speed of light, and  is wavelength (Zhang, 1997), thus giving rise to a yellow color. Table 4 shows the color parameters of the pigments formed from the mixture of raw materials ground for different times and heat-treated at 1100 ◦ C. The L* and −a* values clearly decreased while the b* value increased as the grinding time was prolonged, indicating that the yellow color of the (Ni, Sb)-co-doped rutile pigments became more intense for longer grinding time (see Figs. 10 and 11). Mechanical activation can effectively decrease the crystallinity of the mixed precursors, to accelerate the doping of nickel ions (Ni2+ ) into TiO2 , and promote the grain growth of the rutile phase, thus the pigment presented with a more intense yellow color. In addition, the pigment from the precursor mixtures ground for 2.0 h and heat-treated at 1100 ◦ C showed good color properties (i.e., L*/a*/b* = 85.85/−2.54/62.83). However, the pigment formed from the unground precursor mixture heat-treated at 1100 ◦ C showed poor color performance with a higher green hue and much lower yellow hue (i.e., L*/a*/b* = 89.15/−6.2/47.08). Table 4 also shows the color parameters of other yellow ceramic pigments. The color parameters of the (Ni, Sb)-co-doped rutile pigment from the precursor mixture ground for 2.0 h and heattreated at 1100 ◦ C were similar to those of Pr–ZrSiO4 (De la Luz, Prades, Beltrán, & Cordoncillo, 2013) ceramic pigments (i.e., L*/a*/b* = 84/3/63). In addition, our pigments featured much better colors that those of other yellow pigments, such as Ni-geikielite pigments (Llusar et al., 2015) (i.e., L*/a*/b* = 83.6/4.6/44.6), Cr-doped rutile pigments (Zou, 2013) (i.e., L*/a*/b* = 75.46/15.24/39.46), (Cr, Sb)-co-doped rutile pigments (Calatayud, Pardo, & Alarcón, 2016) (i.e., L*/a*/b* = 75/15/39) and (Ni, Sb)-co-doped rutile pigments (Dondi et al., 2006) (i.e., L*/a*/b* = 79/−3/50). These results indicate that the mechanical activation process can effectively improve the color properties of (Ni, Sb)-co-doped rutile pigments. It is important for ceramic pigments to have good thermal and chemical durability. These characteristics are mainly determined by the crystal structure of the ceramic pigments. The thermal and chemical durability of pigments increase when the pigments are highly crystalline and arrange compactly. The durability of pigments with excessively small grain sizes is typically low. Table 5 shows the color parameters and color difference (E*ab ) after thermal and chemical resistance testing. Figs. 12 and 13 show photographs of the pigments before and after thermal and chemical resistance tests, respectively. The color difference (E*ab ) was calculated by Bae, Tamura, and Imanaka (2017): 2

2

2 1/2

E∗ab = [(L∗) + (a∗) + (b∗) ]

.

(1)

The small color difference (E*ab ) shows that the pigment possesses a high durability. The color differences after the heattreatment at 1100 ◦ C, and 5 wt% HCl, 5 wt% HNO3 , and 5 wt% NaOH were 1.87, 1.36, 1.72, and 0.97, respectively. This good resistance can be attributed to the stable crystal structure (rutile) and high crystallinity of the pigments. These results indicate that the (Ni, Sb)-co-doped rutile pigment, synthesized via a mechanical activation-assisted solid-state reaction, possesses good thermal and chemical durability.

Please cite this article in press as: Zhang, S., et al. Synthesis and characterization of (Ni, Sb)-co-doped rutile ceramic pigment via mechanical activation-assisted solid-state reaction. Particuology (2018), https://doi.org/10.1016/j.partic.2017.12.016

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Fig. 11. Photographs of pigments formed from mixture of raw material ground for different durations and heat-treated at 1100 ◦ C, (a) 0, (b) 0.5, (c) 1.0, (d) 1.5, and (e) 2.0 h. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

Fig. 12. Photographs of samples, (a) without treatment and (b) 1100 ◦ C heat-treatment for 1.0 h.

Fig. 13. Photographs of samples, (a) without treatment, (b) soaked in 5 wt% HCl for 1.0 h, (c) soaked in 5 wt% HNO3 for 1.0 h, and (d) soaked in 5 wt% NaOH for 1.0 h.

Conclusions (Ni, Sb)-co-doped rutile yellow ceramic pigments were prepared via a mechanical activation-assisted solid-state process. The precursor mixture including TiO2 , Sb2 O3 , and Ni(NO3 )2 ·6H2 O and the separate precursors were ground for different durations in a stirrer bead mill. A rutile phase could be formed from an unground precursor mixture calcined at 1000 ◦ C. For the mixture of the precursors ground separately for 2.0 h calcination was achieved at 900 ◦ C. However, the mixture of precursors ground for 2.0 h could be calcined at 800 ◦ C. The mechanical activation was effective for promoting the reaction of the raw material mixtures for subsequent solid-state reactions at a lower calcination temperature. Mechanical activation decreased the crystallinity of the Sb2 O3 /anatase precursor phases and favored the formation of new phases (i.e., brookite and rutile), accelerated doping of nickel (Ni2+ ), and antimony (Sb5+ ) ions into TiO2 , lowered the formation temperature of rutile, and promoted crystal growth of rutile. In addition, the UV–vis absorption spectra and color parameters showed that the

yellow color of the pigment became more intense as the grinding time was prolonged, indicating that mechanical activation improved the color properties of the (Ni, Sb)-co-doped rutile pigment. The pigment from a raw material mixture ground for 2.0 h and heat-treated at 1100 ◦ C possessed good color properties (i.e., L*/a*/b* = 85.85/−2.54/62.83). Hence, our (Ni, Sb)-co-doped rutile pigments are promising candidates for environmentally friendly ceramic yellow pigments owing to their bright yellow hue and good chemical/thermal durability, high cost-effectiveness, and composition based on low-toxic elements.

Acknowledgements This work was financially supported by the Fundamental Research Funds for the Central Universities (No. 2015ZM102), Guangdong Yangfan Innovative & Entrepreneurial Research Team Program (No. 2016YT03C327) and Major Scientific and Technological Projects of Guangdong Province (No. 2015B090927002), China.

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Please cite this article in press as: Zhang, S., et al. Synthesis and characterization of (Ni, Sb)-co-doped rutile ceramic pigment via mechanical activation-assisted solid-state reaction. Particuology (2018), https://doi.org/10.1016/j.partic.2017.12.016