Effect of calcination temperature and dolomite or Al2O3 doping on properties of NIR–reflective CoFe2O4 black pigment

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ScienceDirect Materials Today: Proceedings 17 (2019) 1595–1601

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MRS-Thailand 2017

Effect of calcination temperature and dolomite or Al2O3 doping on properties of NIR–reflective CoFe2O4 black pigment Nuchjarin Sangwong, Mantana Suwan, Sitthisuntorn Supothina* National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), 114 Thailand Science Park Phahonyothin Rd., Klong Luang, PathumThani 12120, Thailand.

Abstract CoFe2O4 NIR-reflective black pigment has been synthesized by solid-state reaction of cobalt oxide (Co3O4) and hematite (Fe2O3) at a temperature range of 900-1200°C to investigate the effect of reaction temperature on pigments’ properties. Various amounts of mineralizers such as CaO and MgO derived from dolomite mineral (CaMg(CO3)2) and Al2O3 were incorporated to investigate their effects on crystalline phase and physical properties of the pigments. Stoichiometric proportion of raw materials including Co3O4, hematite, dolomite and Al2O3 with a chemical formula of (CoO)1-2x(CaMg(CO3)2)x⋅(Al2O3)y(Fe2O3)1-y, where x, y = 0.25 and 0.5, were homogeneously mixed by wet-ball milling in acetone for at least 5 h. After filtering and drying, the mixture were calcined in ambient atmosphere at 900, 1000, 1100 and 1200oC for 3 h. The synthesized products were washed with de-ionized water for 3 times, dried at 105˚C, ground into fine powder and finally sieved through a 325-mesh screen. XRD analysis of the undoped products revealed the presence of pure spinel CoFe2O4 at calcination temperatures of 1000oC and higher. At 900oC, trace amount of unreacted Fe2O3 was observed. The CIE 1976 L*a*b* colorimetric analysis revealed a gradual change of color hue from dark brown for the precursor (L*=35.99) to black at 1100 ̊C (L*=19.59), and turned to dark grey at 1200 ̊C (L*=26.74). The NIR reflectance calculated at a wavelength rage of 700 to 2500 nm following the ASTM E903 standard gradually decreased from 26.2% after calcination at 900 ̊C to 24.4 and 17.5% after calcination at 1000 and 1100 ̊C, respectively, and slightly increased to 19.6% after calcination at 1200 ̊C. Doping with Al2O3 resulted in a slight shift of XRD peaks towards higher diffraction angles upon the increase of Al2O3 content as a result of substitution of Fe3+ site with a smaller Al3+. On the other hand, dolomite doping resulted in a gradual shift towards lower diffraction angles upon the increased dolomite content due to substitution of Co2+ site with Mg2+ and Ca2+. The Al2O3 and dolomite doping resulted in a change of color hue from deep black to lighter black, and became dark brown at high dolomite doping content. The NIR reflectance gradually increased from 17.5% for the undoped sample to 22.5% for the sample doped with 0.5 molar ratio of Al2O3 (i.e. CoO⋅(Al2O3)0.5(Fe2O3)0.5), and to 24.5 and 45.6% for the samples doped with dolomite at molar ratio of 0.25 (i.e. (CoO)0.5(CaO:MgO)0.25⋅Fe2O3) and 0.5 (i.e.

* Corresponding author. Tel.: +66 2564 6500; fax: +66 2564 6447. E-mail address:[email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.

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(CaO:MgO)0.5⋅Fe2O3), respectively. It has been demonstrated that NIR reflectance of the CoFe2O4 black pigment can be enhanced by doping with mineralizers with only a slight decrease in blackness. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: Cobalt ferrite; Spinel; Solid-state synthesis; Near-infrared reflectance; Cool pigment

1. Introduction A mega trend of urbanization and an increasing demand for energy consumption of residential buildings has driven research and development in the field of energy-efficient materials for building envelop, especially roofings. As high as 60% of solar radiation can be absorbed and transferred into the building via roof and ceiling, resulting in accumulated heat during a daytime and in the evening. It has been reported that energy utilization in the buildings accounts for a substantial portion of total energy consumption [1-3]. Energy-efficient buildings are constructed by using thermal-insulating building envelopes such as insulating walls and roofs which can retard the heat that is absorbed and transferred into the buildings. Nevertheless, the heat absorbed by insulated materials will be gradually released into the building during a nighttime that can increase working load of air-conditioning systems. A new alterantive of using solar-reflective coating has been used to reflect solar radiation, particularly in the near-infrared (NIR) region, thus reducing the heat to be absorbed at the first place. The coating is formulated by adding NIRreflective pigments of desired color in addition to typical colorants. It is well known that white pigments such as titanium dioxide (TiO2) has high NIR reflectance of about 90% while black pigments such as carbon black has low NIR reflectance of almost 0. Despite of their high solar radiation absorption (i.e. low reflectance), dark-shaded coatings are preferred due to their aesthetic appearance. Thus, the replacement of traditional dark pigments which have high NIR absorption by NIR-reflective dark pigment in paint formulation will lead to the coatings of similar color but higher NIR reflectance. It has been reported that by replacing traditional dark pigmentswith NIR-reflective dark pigments, total solar reflectance of the formulated dark paint increased from 5% to 30% [4]. Therefore, it becomes important to develop various shades of dark pigments with high NIR reflectance. NIR-reflective pigments of various colors have been synthesized [5-11]. However, there are only few literatures reporting the synthesis of dark NIR-reflective pigments [12-14]. Liu et al. synthesized dark brown Mg-doped ZnFe2O4 pigment by using a sol-gel method [12]. Doping of Mg in zinc ferrite resulted in a change of color hue from brick red to dark brown with color parameter L* of approximately 48-49 and NIR reflectance in a range of 51-58% depending on the amount of Mg dopant. Sanada et al. employed a solid-state reaction to synthesize a series of brownish-black, greenish-black, black blue and black NIR-reflective pigments with L* of approximately 23-45 and reflectance of 18.0-51.4% in the Vis-NIR regions were obtained from (CoO)x(MgO)1-x·(Al2O3)y(Fe2O3)1-y. The most black pigment (L* = 23.5) has Vis-NIR reflectance of 18.0% [13]. Recently, Oka and Masui synthesized high NIRreflective Ca2Mn1-xTixO4 black pigment by using a solid-state reaction with L* in a range of 23.7-28.5 and NIR reflectance in a range of 50.1-72.5 [14]. The Ca2Mn0.85Ti0.15O4 pigment was found to be an optimum composition which displayed L* and R values of 23.8 and 66.2%, respectively. In this present work, a CoFe2O4-based NIR-reflective black pigment has been synthesized by means of solid-state reaction of cobalt oxide (Co3O4) and hematite (Fe2O3) at a temperature range of 900-1200 °C. Mineralizers such as CaO, MgO and Al2O3 have been incorporated to alter phase composition as well as its physical properties. In order to reduce the synthesis cost and thus promote a widespread utilization, inexpensive hematite and dolomite (CaMg(CO3)2) minerals were employed as the sources of Fe2O3, and CaO and MgO, respectively. 2. Experimental procedure Cobalt ferrite (CoFe2O4) black pigment was synthesized by solid-state reaction of cobalt oxide (Co3O4, SigmaAldrich) and hematite (Fe2O3, NIC Interchem Co.Ltd.). Stiochiometric amount of Co3O4 and Fe2O3 were homogeneously mixed by wet ball milling in acetone for at least 5 h. After filtering and drying at 105oC, the mixture

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was calcined in an electrical furnace under ambient atmosphere at 900, 100, 1100 and 1200oC for 3 h. The resultant products were washed with de-ionized water for 3 times, dried at 105˚C, ground into fine powder and finally sieved through a 325-mesh. Effects of adding foreign oxides, including dolomite (CaMg(CO3)2, C.K. Ceramics Co. Ltd.) and Al2O3 (C.K. Ceramics Co. Ltd.) on phase composition, color hue and reflectance were investigated by adding them into the raw material mixture to obtain a chemical formula of (CoO)1-2x(CaMg(CO3)2)x⋅(Al2O3)y(Fe2O3)1-y, and synthesized with the same procedure. X-ray diffraction analysis was performed on the Rigaku TTRAX III for phase identification. Morphology and microstructure of the samples were observed with a field-emission scanning electron microscope (FE-SEM, HITACHI SU8230) performed at an operating voltage of 10 kV. Diffused reflectance spectra were acquired on a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent Technologies) at the wavelength range of 300 to 2500 nm. The NIR reflectance was calculated in the wavelength range of 700 to 2500 nm following the ASTM standard (number E903-96). The CIE 1976 L*a*b* colorimetric method was employed for color measurement using the Spectrophotometer (Konica Minolta, CM-2600d). In this method, the L* axis represents the lightness (0 = absolute black, 100 = absolute white), the a* axis represents the green (negative value) to red (positive value), and the b* axis represents the blue (negative value) to yellow (positive value). 3. Results and discussion Figure 1 shows XRD patterns of the undoped samples synthesized by calcination of the Co3O4 and Fe2O3 at 900, 1000, 1100 and 1200˚C for 3 h. The undoped samples show strong reflection peaks which can be well matched with the standard pattern of face-centered-cubic CoFe2O4 with a spinel structure (JCPDF# 22-1086). The reflection peaks of CoFe2O4 are sharp and more intense after calcination at higher temperature due to an increase of crystallinity. However, at calcination temperature of 900 C ̊ , weak reflection peaks of Fe2O3 (JCPDF# 99-0060) corresponding to unreacted and hematite are observed. The CIE 1976 L*a*b* color coordination, NIR reflectance as well as photo images of the undoped pigments are summarized in Table 1. Upon increasing calcination temperature, the color hue changes from dark brown for the precursor (L*=35.99) to black at 1000 and 1100 ̊C (L*=22.62, 19.59), and turns to dark grey at 1200 ̊C (L*=26.74). Value of NIR reflectance gradually decreases from 22.7 for the precursor to 26.2, 24.4 and 17.5% after calcination at 900, 1000 and 1100 ̊C, respectively due to the color change to darker hue. It slightly increases to 19.6% after calcination at 1200 ̊C due to a slight color change from dark to dark grey. Based on these results, a modest calcination temperature of 1100 ̊C was chosen to futher investigate the effect of adding CaMg(CO3)2 and Al2O3 on the pigment properties. JCPDF 22-1086>> CoFe2O 4 JCPDF 99-0060>> Fe2O 3 Hematite o JCPDF 80-1532>> CoCo2O 4 Cobalt Oxide

Relative intensity (a.u.)

*

o

1200 C 3 h.

o

1100 C 3 h.

o

1000 C 3 h.

* * o

25

30

*o

35

o

o

*

40

*

o

45

*

50

*

900 C 3 h.

o

55

*

o

* * o Precursor

60

65

70

2 Thata (degree) Fig. 1. XRD patterns of the undoped pigments synthesized at 900, 1000, 1100 and 1200°C.

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N. Sangwong et al./ Materials Today: Proceedings 17 (2019) 1595–1601 Table 1. CIE 1976 L*a*b* color coordination, NIR reflectance (700-2500 nm) and photographs of the undoped CoFe2O4 pigments synthesized at 900-1200oC. Synthesis temperature (oC)

L*

a*

b*

%R

Precursor

35.99

22.15

12.09

22.7

900

23.67

0.71

-0.72

26.2

1000

22.62

0.45

-0.93

24.4

1100

19.59

0.04

-1.03

17.5

1200

26.74

0.14

-1.01

19.6

Photograph

Figure 2 shows XRD patterns of the doped pigments after calcination at 900, 1000, 1100 and 1200oC. It reveals that the addition of Al2O3 and dolomite into the CoFe2O4 did not change its spinel crystal structure. However, doping Fe2O3 with Al2O3 resulted in a slight gradual shift of XRD peaks towards higher diffraction angles upon the increase of Al2O3 content compared to that of the parent phase to form the Al2O3-doped compound with a chemical formula of CoO (Al2O3)y(Fe2O3)1-y, where y = 0.25 and 0.5. The slight shift of XRD reflections can be attributed to substitution of a smaller Al3+ (ionic radius = 0.535 Å [15]) for a slightly larger Fe3+ (ionic radius = 0.55 Å [15]) in octahedral site of the spinel structure. Such ionic substitution would lead to a slight reduction of interplanar spacing, and thus a slight shift towards higher 2θ value. Weak reflection peaks of CoAl2O4 (JCPDF# 44-0160) and corundum Al2O3 (JCPDF# 99-0036) are observed as minor phases. They becomes more intense as the Al2O3 content increases from 0.25 to 0.5 mole. The CoAl2O4 is a competing reaction to the CoFe2O4, and the Al2O3 could be the unreacted raw material. A similar effect but with the opposite XRD peak shifting is observed for dolomite doping. That is, there is a gradual shift towards lower diffraction angles upon the increased dolomite content. In this case, a doped compound with chemical formula of (CoO)1-2x(MgO:CaO)x⋅Fe2O3 (x = 0.25 and 0.5) is obtained. The XRD peak shift can be ascribed to substitution of Mg2+ (ionic radius = 0.57 Å [15]) and/or Ca2+ (ionic radius = 1.0 Å [15]) for a Co2+ (ionic radius = 0.58 Å [15]) in tetrahedron site of spinel structure. Based on the ionic radii of these three divalent ions, we believe that larger proportion of Ca2+ compared to Mg2+ substituted for the Co2+ site. The substitution in this tetrahedron site will be further investigated by an Extended X-ray Absorption Fine Structure (EXAFS) technique. Weak reflection peaks of CaFe2O4 (JCPDF# 72-1199) and Ca2Fe2O6 (JCPDF# 47-1744) are observed as minor phases. The reflection peaks of Ca2Fe2O6 becomes much sharper and more intense when the dolomite molar ratio increases from 0.25 and 0.5. The average crystallite sizes of these pigments calculated from the XRD line broadening of the (311) peak using the Scherer equation are summarized in Table 2. Upon increasing the Al2O3 or dolomite content, the crystallite size decreased from 76.7 nm for the undoped pigment to 49.0 and 46.9 nm when doped with Al2O3 or dolomite, respectively (x or y = 0.5). Figure 3 shows UV-Vis-NIR diffuse reflectance spectra of the pigments after calcination at 1100 oC for 3 h. In general, Al2O3 doping gives rise to an enhancement of reflectance mainly in the NIR region, whereas dolomite doping leads rise to an enhancement of reflectance in both the visible and NIR regions. In addition, the Al2O3 and dolomite doping results in a change of color hue from deep black to a lighter black as summarized in Table 2. The increase of Al2O3 content, the L* value slightly increase due to a brigther color hue. For example, the NIR reflectance increases from 17.5% for the undoped sample to 20.5 and 22.5% for the sample doped with 0.25 and 0.5 molar ratio of Al2O3 (i.e. CoO⋅(Al2O3)0.25(Fe2O3)0.75 and CoO⋅(Al2O3)0.5(Fe2O3)0.5), respectively. However, the incorporation of dolomite into CoFe2O4 has more dramatic effect on the reflectance and color hue of the resultant pigments depending on the amount of the doping level. Higher dolomite doping level results in a significant increase of reflectance to 24.5 and 45.6% for molar ratio of 0.25 and 0.5 (i.e. (CoO)0.5(MgO:CaO)0.25⋅Fe2O3 and (MgO:CaO)0.5⋅Fe2O3), respectively. Its color changes from black (undoped sample) to lighter black (at x=0.25) and finally to dark brown (at x=0.5) which leads to a gradual increase of the NIR reflectance.

N. Sangwong et al./ Materials Today: Proceedings 17 (2019) 1595–1601

JCPDF 44-0160>> CoAl2O4 Δ JCPDF 47-1744>> Ca Fe O 2 2 6 JCPDF 72-1199>> CaFe2O4 JCPDF 99-0036>> Al2O3

Δ

Δ

Δ

(440)

(511)

Δ Δ

Δ

(422)

Δ

(400)

(222)

(200)

Relative intensity (a.u.)

(311)

*

Δ

Δ

1599

x=0.5,y=0

x=0.25,y=0

*

*

*

*

x=0,y=0.5

x=0,y=0

(440)

(511)

(422)

(400)

(311)

(222)

(200)

x=0,y=0.25

JCPDS 22-1086 CoFe2O4

25

30

35

40

45

50

55

60

65

70

2 Thata (degree) Fig. 2. XRD patterns of (CoO)1-2x(MgO:CaO)x⋅(Al2O3)y(Fe2O3)1-y pigments synthesized at 1100oC.

Reflectance (%)

100

(CoO)1-2x(MgO:CaO)x(Al2O3)y(Fe2O3)1-y

x=0.5;y=0

o

1100 C_3h.

80 x=0.25;y=0

60

x=0;y=0.5 x=0;y=0.25 x=0;y=0

40

20

0 500

1000

1500

2000

2500

Wavelength (nm)

Fig. 3. Reflectance spectra of the (CoO)1-2x(MgO:CaO)x⋅(Al2O3)y(Fe2O3)1-y pigments synthesized at 1100°C. Table 2. CIE 1976 L*a*b* color coordination, NIR reflectance (700-2500 nm), crystallite and particle sizes and photographs of the (CoO)1-2x(MgO:CaO)x⋅(Al2O3)y(Fe2O3)1-y pigments synthesized at 1100oC. x/y (molar ratio)

L*

a*

b*

%R

Crystallite size (nm)

Particle size (µm)

0/0

19.59

0.04

-1.03

17.5

76.7

0.79 ±0.20

0/0.25

20.74

0.18

-0.90

20.5

75.1

0.81 ±0.26

0/0.5

21.86

-0.10

-1.00

22.5

49.0

0.79 ±0.19

0.25/0

23.09

0.66

-0.88

24.5

51.8

1.84 ±0.33

0.5/0

32.06

18.86

13.23

45.6

46.9

1.88 ±0.45

Photograph

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Particle morphology of the synthesized pigments examined by FE-SEM is shown in Figure 4, and the particle size obtained by SEM image analysis is summarized in Table 2. It is very clear from SEM images that particle morphology of the Al2O3-doped pigments are similar to that of the undoped one. In contrast, dense aggregated larger particles are observed for the dolomite-doped pigments. The particle diameter of Al2O3-doped pigments is in a range of 0.79 ±0.19 - 0.81 ±0.26 µm which is comparable to the size of the undoped sample (0.79 ±0.20 µm). A significant particle growth is observed in the case of dolomite-doped pigments. The average particle sizes are 1.84 ± 0.33 and 1.88 ± 0.45 µm for the pigments doped with 0.25 and 0.5 mole of dolomite, respectively. The effect of dolomite on particle growth can be explained based on its low melting point of only about 730 oC, and it can be a fluxing agent during solid-state reaction.

(b)

(c)

(d)

(e)

(a)

Fig. 4. FE-SEM images of (CoO)1-2x(MgO:CaO)x⋅(Al2O3)y(Fe2O3)1-y pigments after calcination at 1100 oC. (a) x=0, y=0, (b) x=0, y=0.25, (c) x=0, y=0.5, (d) x=0.25, y=0 and (e) x=0.5, y=0.

4. Conclusions CoFe2O4 NIR-reflective black pigments with L* and R in a range of 19.59-22.62 and 17.5-24.4%, respectively, were synthesized by solid-state reaction of cobalt oxide (Co3O4) and hematite (Fe2O3) at temperatures 1000 and 1100°C. Calcination at higher temperature (1200°C) resulted in dark grey color hue and NIR reflectance of 19.6%. Doping the CoFe2O4 pigment with Al2O3 resulted in XRD peak shift towards higher diffraction angles as a result of substitution of Fe3+ site with a smaller Al3+, and a slight change of color hue to lighter black pigment with NIR reflectance of 22.5% for the CoO⋅(Al2O3)0.5(Fe2O3)0.5 pigment. On the other hand, dolomite doping resulted in a gradual shift towards lower diffraction angles due to substitution of Co2+ site with larger Mg2+, and Ca2+ of comparable ionic size to the Co2+. The color hue changed from black to lighter black for the (CoO)0.5(MgO:CaO)0.25⋅Fe2O3 pigment with NIR reflectance of 24.5%. The dolomite also acted as fluxing agent during solid-state reaction resulting in particle growth which was not observed in the case of Al2O3 doping.

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Acknowledgements This work has been finanically supported by the National Metal and Materials Technology Center,the National Science and Technology Development Agency (NSTDA) (Thailand), Grant No. MT-B-59-CER-07-313-I. References [1] S. Hassid, M. Santamouris, M. Papanikolaou, A. Linardi, N. Klitsikas, C. Georgakis, D. Assimakopoulos, Energ. Building 32 (2000) 131–141. [2] M. Santamouris, M. Papanikolaou, G. Mihalakakou, Climatic Change 80 (2007) 265–276. [3] E. Stathopoulou, G. Mihalakakou, M. Santamouris, H. Bagiorgas, J. Earth Syst. Sci. 117 (2008) 227–236. [4] E.S. Cozza, M. Alloisio, A. Comite, G. Di Tanna, S. Vicini, Sol. Energy 116 (2015) 108–116. [5] K.J. Sreeram, C.P. Aby, B.U. Nair, T. Ramasami, Sol. Energ. Mat. Sol. Cell 92 (2008) 1462–1467. [6] T. Thongkanluang, T. Kittiauchawal, P. Limsuwan, Ceram. Int. 37 (2011) 543–548. [7] V.S. Vishnu, M.L. Reddy, Sol. Energ. Mat. Sol. Cell 95 (2011) 2685–2692. [8] J. Zou, P. Zhang, C. Liu, Y. Peng, Dyes Pigments 109 (2014) 113–119. [9] S. Jose, M.L. Reddy, Dyes Pigments 98 (2013) 540–546. [10] J.M. Calatayud, P. Pardo, J. Alarcón, Dyes Pigments 134 (2016) 1–8. [11] P. Meenakshi, M. Selvaraj, Sol. Energ. Mat. Sol. Cell 174 (2018) 530–537. [12] L. Liu, A. Han, M. Ye, W. Feng, Sol. Energy 113 (2015) 48–56. [13] K. Sanada, Y. Morisawa, Y. Ozaki, J. Ceram. Soc. Japan 122 (2014) 322–328. [14] R. Oka, T. Masui, RSC Adv. 6 (2016) 90952–90957. [15] R.D. Shannon, Acta Crystallogr. Sect. A: Found. Crystallogr. 32 (1976) 751-767.