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Combustion synthesis of a blue Co-doped zinc aluminate near-infrared reflective pigment
Dyes and Pigments 142 (2017) 24e31
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Combustion synthesis of a blue Co-doped zinc aluminate near-infrared reflective pigment curariu a, Radu Laza u a, *, Robert Ianos¸ a, **, Eliza Muntean a, ***, Cornelia Pa Cornelia Bandas b, Gabriel Delinescu a a
Politehnica University of Timis¸oara, Faculty of Industrial Chemistry and Environmental Engineering, P-t¸ a Victoriei No. 2, Timis¸oara, 300006, Romania National Institute for Research & Development in Electrochemistry and Condensed Matter, Department of Condensed Matter, Plautius Andronescu Street, No. 1, Timis¸oara, 300224, Romania b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2016 Received in revised form 7 March 2017 Accepted 7 March 2017 Available online 8 March 2017
A near-infrared reflective blue pigment with a low cobalt content, Zn0.9Co0.1Al2O4, was prepared by solution combustion synthesis, which proved to be time-effective and energy-efficient. No additional annealing was required as the elevated temperature developed during the exothermic self-propagating combustion reaction (1428 C) enabled the direct formation of a Zn0.9Co0.1Al2O4 solid solution having an appealing blue color: L* ¼ 67.9, a* ¼ 3.7, b* ¼ 39.0. The resulting blue pigment had an average particle size of 6.1 mm and a BET surface area of 6.5 m2/g. The total solar reflectance of the pigment (63%) recommends the use of Zn0.9Co0.1Al2O4 as a near-infrared reflective blue pigment for cool coatings. The ability of the pigment to reflect the near-infrared radiation was confirmed by measuring the surface temperature of an aluminum sheet painted with an acrylic coating containing 30 % wt. pigment, that had a total solar reflectance of 59%. The excellent properties of Zn0.9Co0.1Al2O4, color and solar reflectance, together with the low cobalt content and the advantages of self-propagating combustion reaction recommend the use of combustion synthesis for the preparation of blue Zn0.9Co0.1Al2O4 near-infrared reflective pigments. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Combustion synthesis Cool pigments Total solar reflectance Blue Cobalt Spinel
1. Introduction Over the last few years because of global warming, the heat island effect in cities has become a critical problem. The World Meteorological Organization ranked 2015 as the warmest year on record, with 2016 on track to become even warmer [1]. This phenomenon manifests through daytime average air temperatures up to 5.6 C higher in urban areas compared to the surroundings [2]. Considering that approximately 53% of the total solar radiation reaching the surface of the earth is near-infrared (NIR) radiation [3], the heat absorbed by buildings and urban infrastructure contributes to microclimate change. One approach for decreasing the surface temperature of buildings relies on using so-called thermal infrared reflective pigments
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (R. Ianos¸), [email protected] (E. Muntean), [email protected] (R. Laz au). http://dx.doi.org/10.1016/j.dyepig.2017.03.016 0143-7208/© 2017 Elsevier Ltd. All rights reserved.
or cool pigments, which are inorganic pigments with high solar reflectance [4e6]. A solar reflective coating has high reflective properties in the NIR region (700e2500 nm), thus it can prevent the increase of the surface temperature and has an important role in energy conservation [4,7]. Studies have demonstrated that a solar reflective coating can reduce the surface temperature by more than 10 C than a standard coating of the same color [8,9]. This surface temperature reduction leads to a more comfortable interior climate and electricity savings with the air conditioning during the hot season [10]. Although white TiO2 exhibits the highest NIR reflection efficiency, the market requires colored pigments for aesthetic reasons. Due to their remarkable thermo-chemical, physical and optical properties Co2þ-doped ZnAl2O4 based pigments [11e13], commonly used as ceramic pigments in glazes, are potential candidates as NIR reflective pigments [14]. Typical methods used for the preparation of Zn1xCoxAl2O4 are: ceramic [15], coprecipitation [16], Pechini [17], sol-gel [18e22], hydrothermal [23], flame spray-pyrolysis [24]. However, all these methods are time and energy consuming (elevated temperature/pressure, long
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soaking time, expensive raw materials) and are contradicting the very purpose of using NIR reflective pigments: reducing the energy consumption. Given the limitations of current methods, this paper suggests the preparation of blue Co2þ-doped ZnAl2O4 NIR-reflective pigments with low cobalt content, using a light-fingerprint method known as combustion synthesis. Compared to the other synthesis routes, combustion synthesis is faster and more energy-efficient since it does not require annealing of the obtained powders. The entire amount of energy required for the formation of desired compound is self-generated during the exothermic redox reaction occurring between metal nitrates and fuels. The existing reports on combustion synthesis of Zn1xCoxAl2O4 [12,25,26] are focused on the single-fuel approach. In this paper we report the preparation of blue Zn0.9Co0.1Al2O4 pigment using a different approach, which relies on the use of urea-glycine fuel-mixture instead of a single fuel. A low dopant proportion was chosen for reasons related to cobalt toxicity. The color and the near infrared reflective properties of the pigment are discussed in relation to the synthesis procedure.
2. Material and method 2.1. Sample preparation In order to obtain 0.050 moles of Zn0.9Co0.1Al2O4 blue pigment (P) via combustion synthesis, a mixture of metal nitrates and fuels was used (0.045 moles Zn(NO3)2$4H2O - Merck, 0.005 moles Co(NO3)2$6H2O - Reactivul, 0.100 moles Al(NO3)3$9H2O - Fluka, 0.056 moles glycine C2H5NO2 - Fluka and 0.375 moles urea CH4N2O - Merck). The raw materials were dosed according to reaction equation (1), using glycine as fuel for zinc and cobalt nitrates and urea for aluminum nitrate.
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measured using a thermal vision FLIR T 640 camera operating at a speed of 15 frames/second. The phase composition of the resulted powder was assessed based on the X-Ray diffraction pattern recorded using a Rigaku ULTIMA IV X-Ray diffractometer (radiation CuKa). Crystallite size was calculated by Scherrer's equation, using the specific diffraction peaks for 220 and 311 hkl planes. The heating behavior of the powder was pursued by thermal analysis using a Netzsch STA 449 C instrument. TG and DSC curves were recorded in air atmosphere, using alumina crucibles, in the temperature range 25e1000 C, at a 10 C/minute heating rate. The specific surface area of the pigment was measured by the BET (Brunauer, Emmet, Teller) method, using a Micromeritics ASAP 2020 instrument, after previously degassing the sample for 5 h at 300 C, under 5 mmHg vacuum. The BET measurement was carried using N2 as adsorption gas, at liquid nitrogen temperature. Particle size was assessed using a Malvern Mastersizer 2000 laser particle size analyzer. Particle morphology was observed by transmission electron microscopy, using a FEI Titan G2 80-200 instrument. The obtained blue pigment (denoted as sample P) was dispersed into a water-based acrylic copolymer emulsion, using a sinusdissolver. The mass % composition of the prepared paint was: 38.7% acrylic copolymer, 30% pigment, 30% water and 1.3% additives. The paint was applied onto a 60 60 mm aluminum sheet using a block applicator with 300 mm gap. After drying, the thickness of the coating (denoted as sample C) was measured using a Byko-test 1500 film gauge. The film homogeneity was investigated by SEM using a FEI Quanta FEG 250 microscope. The prepared coating was subjected to IR irradiation, using an infrared light bulb (250 W, 1100 nm dominant wavelength) placed 20 cm above the sample. The temperature evolution of the coated surface was recorded by thermal imaging using a FLIR T640 IR
81Zn(NO3)2(aq) þ 9Co(NO3)2(aq) þ 100C2H5NO2(aq) þ 180Al(NO3)3(aq) þ 675CH4N2O(c) þ 675/2O2(g) ¼ 90Zn0.9Co0.1Al2O4(c) þ 875CO2(g) þ 1600H2O(g) þ 1085N2(g) (1)
After dosing, the raw materials were homogenized and dissolved solely in their own crystallization water by heating at 80 C and magnetic stirring at 300 rpm for 30 min. The porcelain dish containing the resulted solution was placed on the heating mantle, which was previously heated at 450 C. While the solution temperature was increasing, part of the contained water was evaporated and the viscosity of the solution increased. After a while, the ignition of the self-propagated combustion reaction took place. [HAZARD NOTE - the combustion process is highly exothermic and thus, must only be carried out on the scales indicated in the experimental method and with extreme care, by experimental workers skilled in the routine use of such procedures.] In less than a minute the sample reached incandescence and a large volume of colorless gases evolved (CO2, H2O, N2). Under the effect of the combustion front propagation, the raw materials were transformed in a blue voluminous powder. The such-prepared sample was subjected to wet milling for 30 min in a planetary ball mill, using agate jars and grinding balls. The sample was afterwards washed with distilled water and dried at 110 C for 3 h. 2.2. Sample characterization and testing The temperature evolution during the combustion process was
camera operating at 15 frames-second. For comparison, two other aluminum sheets of the same size were coated using a white TiO2based paint (denoted as sample W) and a carbon-black based paint (denoted as sample B) and their thermal response was assessed under the same conditions. The color (assessed by CIEL*a*b* coordinates) and total solar reflectance (ASTM E892-87, SPECTRALON standard) of the pigment (P) and coating (C) were measured by diffuse reflectance spectroscopy in the wavelength interval 300e2500 nm (5 nm step), using a Perkin Elmer Lambda 950 UV-VIS-NIR spectrophotometer equipped with an integrating sphere. D65 illuminant was used and the observer's angle was set to 10 . 3. Results and discussion The self-propagating combustion process took place very rapidly, as the temperature variation curve shows (Fig. 1). The combustion process ignited at 284 C. Once triggered, the highly exothermic redox reaction between metal nitrates and the fuel mixture evolved in a self-propagated manner and the temperature increased rapidly up to a maximum combustion temperature of 1428 C. The temperature decreased afterwards quite fast down to 650 C, and then the sample slowly cooled down.
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Fig. 1. Sample temperature evolution during the combustion reaction (1). Fig. 2. The XRD pattern of the pigment obtained from the combustion reaction.
The whole process took only 30 s, from the ignition (284 C) until the combustion reaction was over (650 C). In the absence of thermodynamic data for Zn0.9Co0.1Al2O4 solid solution, standard enthalpy of reaction, standard Gibbs free energy and adiabatic temperature for ZnAl2O4 were calculated, using the available thermodynamic data [27,28]. Due to the low cobalt proportion in the pigment composition (1.4 atomic % Co) and based on the fact that Zn0.9Co0.1Al2O4 represents a solid solution derived from ZnAl2O4 by partial substitution of Zn2þ with Co2þ, one may expect the standard enthalpy of reaction, standard Gibbs free energy and adiabatic temperature to show minor variations for Zn0.9Co0.1Al2O4 compared to ZnAl2O4. The measured combustion temperature (1428 C) is lower than the adiabatic temperature (2234 C), which is mainly explained by the heat loss caused by the reaction taking place under non-adiabatic conditions. On the other hand, the strong exothermic nature of the combustion reaction is confirmed by the highly negative value of the standard enthalpy of reaction (DrH0298 ¼ 4244 kJ/mole of ZnAl2O4). The negative value of the standard Gibbs free energy (DrG0298 ¼ 6671 kJ/mole of ZnAl2O4) indicates the spontaneous nature of the process, which is thermodynamically favored. Considering that the reaction presented in equation (1) illustrates the combustion process, a total of 39.6 moles of combustion gasses (CO2, H2O and N2) evolve for each mole of Zn0.9Co0.1Al2O4. As the combustion temperature reaches 1428 C during the reaction, the actual volume of released gases is highly increased. This is the reason why, the powder resulted from the process is very voluminous and spongy. The X-Ray diffraction pattern shown in Fig. 2 indicates the Zn0.9Co0.1Al2O4 solid-solution being the single crystalline phase present, with an average crystallite size of 32 nm. The formation of the Co-doped spinel structure directly from the combustion reaction without any subsequent annealing, is due to the high temperature developed during the combustion reaction. The thermal investigation of the obtained powder suggests that the combustion reaction was complete. The total mass loss calculated from the TG curve is less than 0.5% (Fig. 3), which proves there
are no raw materials left unreacted. Moreover, the DSC curve shows no endothermic or exothermic effect whatsoever, which denotes the absence of any physical or chemical transformations in the studied temperature interval. Despite the short duration of the combustion reaction, the high temperature enhances the sintering process. Therefore the primary particles with diameter no larger than 80 nm (Fig. 4) are highly sintered and form micronic-sized clusters, which is in
Fig. 3. TG-DSC curves of the combustion-synthesized blue pigment.
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Fig. 4. TEM image and elements mapping of the pigment obtained from the combustion process.
accordance to the results of the laser grain size investigation (Fig. 5). At the same time, the TEM image of the pigment shows coalescent particles, proving an advanced sintering process. The mapping of elements (Fig. 4) shows a homogeneous distribution of Zn, Al, Co and O. Particle size is one of the parameters that influences the reflection capacity of the pigments. In order to enhance reflectance, the particle size must be higher than half of the reflected wavelength [29]. The electromagnetic radiation mainly responsible for the heating effect has wavelengths between 700 and 1100 nm [29,30], where 26% of the total solar energy is concentrated, meaning approximately 50% of the NIR radiation [4]. As a result, in order to maximize the reflectance in the 700e1100 nm range, the particle size should be greater than 0.35e0.55 mm. Particle size
investigation proved that 10% of the particles are smaller than 1.0 mm, 50% of the particles are smaller than 6.1 mm (D0.5) and 90% of the particles are below 20.0 mm (D0.9). Such a particle size distribution recommends the obtained powder for use as a thermoreflective pigment (Fig. 5). From the nitrogen adsorption e desorption analysis the pigment shows a type IV isotherm, with H3 hysteresis (Fig. 6), which are specific for porous particles agglomerations. This correlates with the synthesis conditions in general, and with the high volume of
Fig. 5. Particle size distribution of the obtained pigment.
Fig. 6. Adsorption-desorption isotherms of the obtained pigment.
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generated gases in particular. The BET specific surface area of the pigment is 6.5 m2/g and the pore volume is 0.033 cm3/g. The powder obtained at the end of the combustion process shows an even blue color, without any dark or black areas. According to Gaudon et al. [17], the development of the blue color is conditioned by thermal treatment (annealing) at high temperatures, which ensures the inclusion of the Co2þ cations in tetrahedral spaces to substitute Zn2þ. Co2þ inclusion in octahedral coordination in zinc aluminate structure leads to a pink color [16]. The inclusion of Co2þ in both tetrahedral and octahedral coordination leads to color alteration. As a result, in order to obtain pure blue color, it is essential to ensure tetrahedral coordination to Co2þ. From this point of view, the blue color obtained directly from the reaction is explained by the high homogeneity of the raw materials mixture and also by the high temperature (1428 C) developed during the self-propagating combustion reaction (Fig. 1). The diffuse reflectance spectrum of the obtained pigment (Fig. 7) is similar to those of the blue pigments containing Co2þ in tetrahedral coordination. Blue oxide compounds colored with cobalt owe their color to Co2þ with an [18Ar]3d7 electronic structure. As a result of the energy level splitting, the following terms are obtained in the case of Co2þ free ion: 4F, 4P, 2H, 2G, 2F and 2P [31]. These terms are in their turn subjected to further splitting depending on the symmetry of the ligands crystalline field. According to the literature data [32], the electronic transitions responsible for the electronic spectra of the d7 ions in tetrahedral coordination are: 4T2g(F) ) 4A2g(F) e at 3330-2000 nm, 4T1g(F) ) 4A2g(F) e at 2174-1282 nm and 4T1g(P) ) 4 A2g(F) e at 755-615 nm [32]. This last transition has as result the band triplet at 545, 585, and 620 nm, which is typical for Co2þ ions in tetrahedral sites and such pigments feature a specific blue coloration. The strong absorption band near the UV region, up to approximately 425 nm is due to the ligand-to-metal charge transfer (OeCo/Zn). As far as concerns the NIR region, the lack of absorption bands between 780 and 1100 nm indicate the obtained pigments as suitable candidates for heat reflecting coatings, since this region in the solar radiation is responsible for surface heating. The two lowintensity absorption bands located at 405 and 480 nm, also noticed
Fig. 7. Diffuse reflectance spectra of the pigment (P) and prepared coating (C).
Fig. 8. Graphical representation of (F(R)hn)2 depending on hn for pigment P.
in the case of CoxZn1xAl2O4 [20] and CoAl2O4 [33], could be assigned to 2T(G) ) 4A2g(F) forbidden transition, typical for Co2þ ions in tetrahedral coordination [33,34]. The Tauc plot (Fig. 8) derived from the diffuse reflectance spectroscopy measurements indicates a direct band gap of 4.04 eV, which is actually lower than the value (4.98 eV) reported by Anand et al. [12] for the same stoichiometry. The CIEL*a*b* parameters indicate the obtained pigment situates in the blue-green field. The value of b* is highly negative, whilst the value of a* is only slightly negative, which suggests a blue color with a slight green hue (Fig. 9). Comparing our results with the literature data (Table 1), one may notice that for the same Co2þ content, the pigment P obtained by us (Zn0.9Co0.1Al2O4) is bluer (with a more negative b* value) than the pigment prepared by the polymeric precursor method annealed at 900 C [20]. Moreover, pigment P (Zn0.9Co0.1Al2O4) is bluer than other pigments in the ZnO-CoO-Al2O3 system prepared with a higher Co2þ content. Despite the low chromophore content, the C* value (chroma) is higher for pigment P (Zn0.9Co0.1Al2O4), which indicates a higher saturation of the blue shade and thus, superior optical properties. Ensuring similar or even improved color features whilst using a low cobalt content (only 1.4 atomic % Co) represents a serious advantage of the combustion method, especially since higher chromophore content may cause environmental issues due to the toxicity of the cobalt ion. As for the thermo-reflective properties, pigment P shows a total solar reflectance of 63%, which together with the reflectance measured at 1100 nm (RP,1100 ¼ 58%) recommend the use of Zn0.9Co0.1Al2O4 as a cool pigment. It worth mentioning that the solar reflectance measured at 1100 nm is a lot higher in the case of pigment P (Zn0.9Co0.1Al2O4) than for CoAl2O4 obtained via Pechini method (R1100 z 34%) [14], especially when Zn0.9Co0.1Al2O4 contains less Co2þ, but at the same time shows a higher proportion of blue (lower b*) and higher color saturation (Table 1). It is important to highlight that no additional annealing was involved for obtaining the blue Zn0.9Co0.1Al2O4 pigment (P), as it is usually the case in most of the other synthesis methods (Table 1). In our case, the development of the spinel structure and the formation
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Fig. 9. Position of the pigment (P) and prepared coating (C) colors in the CIEL*a*b* diagram. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
of desired blue color is solely due to the heat released during the combustion reaction. The superior optical properties of the obtained pigment, together with the multiple benefits of the self-
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propagating combustion reaction justify the combustion method as being the right choice for the synthesis of Zn1xCoxAl2O4 e based blue pigments. The SEM images (Fig. 10.) show a homogeneous distribution of the pigment within the dry coating, which is thick and uniform enough to completely mask the substrate. After drying, the coating C measured a thickness of 140 mm. The diffuse reflectance spectrum of the coating is similar to that of the pigment (Fig. 7). A significant difference in the case of the coating is the presence of an additional absorption band in the range of 2200e2500 nm. According to the literature data, this band is assigned to the acrylic copolymer used [36], whose composition may be also changed or adjusted in order to maximize the thermoreflective properties. From the colorimetric point of view (Table 1), coating C shows superior features compared to pigment P: b* is more negative, whilst a* is closer to 0. This means a higher blue proportion and lower green component. In terms of chromaticity this translates in an increase of chroma C* from 39.2 to 45.3. The total solar reflectance of the coating (TSRC ¼ 59%) is lower than in the case of pigment P (TSRC ¼ 63%), which may be explained by the presence of the acrylic copolymer whose composition has not been optimized for thermo-reflective coatings. Even so, the coating obtained within the present study is significantly bluer (Table 1) and shows at 1100 nm a higher reflectance (R1100 ¼ 59%) than the CoAl2O4/TiO2 coating prepared by Hedayati et al. (R1100 z 42%) [14]. The temperature of the blue-coated sample (C) climbed up to 53 C within the first 6 min of IR irradiation and stabilized at 56 C after a much slower temperature increase (Fig. 11). By comparison [37], a carbon black-coated sample heated up to 72 C and the TiO2 white-coated sample heated up to 47 C, under the same experimental conditions. The ability of the Zn0.9Co0.1Al2O4 pigment to
Table 1 Comparison of the pigment/coating CIEL*a*b*, C*, h parameters with similar data. Stoichiometry
Preparation route
L*
a*
b*
C*
h ( )
Reference
Zn0.9Co0.1Al2O4 (P) Zn0.9Co0.1Al2O4 Zn0.8Co0.2Al2O4 Zn0.75Co0.25Al2O4 Zn0.5Co0.5Al2O4 CoAl2O4 Zn0.9Co0.1Al2O4 (C) CoAl2O4
Combustion synthesis, no annealing Polymeric precursor, 900 C Gelatine method, 1000 C/4 h Combustion synthesis, 900 C/1 h Combustion synthesis, no annealing Pechini, 1200 C/2 h Water-based coating, 30 wt pigment P Acrylic resin tinted with TiO2
67.9 45.8 58.7 46.8 45.2 52.0 64.5 61.0
3.7 2.3 3.9 12.7 6.8 1.2 1.7 4.2
39.0 26.4 33.4 20.4 24.3 35.2 45.3 29.6
39.2 26.5 33.6 24.0 25.2 35.2 45.3 29.8
264.6 265.0 276.7 238.1 254.4 268.0 267.9 262.0
This study [20] [35] [26] [15] [14] This study [14]
Fig. 10. SEM images of the coating (C) applied onto aluminum sheet.
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Fig. 11. Surface temperature variation during IR radiation exposure.
reflect the NIR thermal radiation recommends this as a cool pigment. 4. Conclusions A blue zinc spinel pigment with low cobalt content (1.4 at. % Co) was obtained by combustion synthesis, starting from an aqueous solution of metal nitrates, urea and glycine. The high exothermicity of the self-propagating combustion reaction, assessed by the standard enthalpy of reaction (DrH0298 ¼ 4244 kJ/mole of ZnAl2O4), supplied the necessary energy for the formation of the desired Zn0.9Co0.1Al2O4 solid solution and thus, no additional annealing was required. Although the measured combustion temperature (1428 C) was lower than the calculated adiabatic temperature (2234 C), it was high enough for Co2þ inclusion in the spinel crystalline lattice and development of the blue color (L* ¼ 67.9, a* ¼ 3.7, b* ¼ 39.0). The average particle size of the pigment was 6.1 mm and the BET specific surface area was 6.5 m2/g. The ability of the Zn0.9Co0.1Al2O4 pigment to reflect the NIR thermal radiation was confirmed by its high total solar reflectance (63%) and also by measuring the temperature of an aluminum sheet coated with an acrylic paint containing 30 mass % pigment. The intense blue color obtained using low cobalt content, high total solar reflectance, and the absence of any additional annealing represent solid arguments for using combustion synthesis for the preparation of blue Zn0.9Co0.1Al2O4 near-infrared reflective pigment. Acknowledgements This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS e UEFISCDI, project number PN-II-RU-TE-2014-4-1587. References [1] WMO Statement on the Status of the Global Climate in 2014, WMO-No. 1152,
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The synthesis, characterization and optical properties of silicon and praseodymium doped Y6MoO12 compounds: environmentally benign inorganic pigments with high NIR reflectance. Dyes Pigments 2011;88:109e15. [31] Shriver EF, Atkins W, Langford CH. Inorganic chemistry. Oxford University Press; 1990. [32] Greenwood NN, Earnshaw A. Chemistry of the elements. Oxford: Pergamon Press; 1984.
R. Ianos¸ et al. / Dyes and Pigments 142 (2017) 24e31 [33] Alvarez-Docio CM, Reinosa JJ, del Campo A, Fern andez JF. 2D particles forming a nanostructured shell: a step forward cool NIR reflectivity for CoAl2O4 pigments. Dyes Pigments 2017;137:1e11. [34] Zayat M, Levy D. Blue CoAl2O4 particles prepared by the solegel and citrateegel methods. Chem Mater 2000;12:2763e9. [35] Costa AF, Pimentel PM, Melo DMA, C^ amara MSC, Chantelle L, Melo MAF, et al. ^mica Preparation of cobalt aluminates pigments for use in ceramic glazes. Cera
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2016;62:179e85. [36] Synnefa A, Santamouris M, Apostolakis K. On the development, optical properties and thermal performance of cool colored coatings for the urban environment. Sol Energy 2007;81:488e97. but¸ u R, P [37] Ianos¸ R, Muntean E, Ba a R, Laza acurariu C, Bandas C. Combustion synthesis of pink chromium-doped alumina with excellent near-infrared reflective properties. Ceram Int 2017;43:2568e72.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Combustion synthesis of a blue Co-doped zinc aluminate near-infrared reflective pigment curariu a, Radu Laza u a, *, Robert Ianos¸ a, **, Eliza Muntean a, ***, Cornelia Pa Cornelia Bandas b, Gabriel Delinescu a a
Politehnica University of Timis¸oara, Faculty of Industrial Chemistry and Environmental Engineering, P-t¸ a Victoriei No. 2, Timis¸oara, 300006, Romania National Institute for Research & Development in Electrochemistry and Condensed Matter, Department of Condensed Matter, Plautius Andronescu Street, No. 1, Timis¸oara, 300224, Romania b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2016 Received in revised form 7 March 2017 Accepted 7 March 2017 Available online 8 March 2017
A near-infrared reflective blue pigment with a low cobalt content, Zn0.9Co0.1Al2O4, was prepared by solution combustion synthesis, which proved to be time-effective and energy-efficient. No additional annealing was required as the elevated temperature developed during the exothermic self-propagating combustion reaction (1428 C) enabled the direct formation of a Zn0.9Co0.1Al2O4 solid solution having an appealing blue color: L* ¼ 67.9, a* ¼ 3.7, b* ¼ 39.0. The resulting blue pigment had an average particle size of 6.1 mm and a BET surface area of 6.5 m2/g. The total solar reflectance of the pigment (63%) recommends the use of Zn0.9Co0.1Al2O4 as a near-infrared reflective blue pigment for cool coatings. The ability of the pigment to reflect the near-infrared radiation was confirmed by measuring the surface temperature of an aluminum sheet painted with an acrylic coating containing 30 % wt. pigment, that had a total solar reflectance of 59%. The excellent properties of Zn0.9Co0.1Al2O4, color and solar reflectance, together with the low cobalt content and the advantages of self-propagating combustion reaction recommend the use of combustion synthesis for the preparation of blue Zn0.9Co0.1Al2O4 near-infrared reflective pigments. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Combustion synthesis Cool pigments Total solar reflectance Blue Cobalt Spinel
1. Introduction Over the last few years because of global warming, the heat island effect in cities has become a critical problem. The World Meteorological Organization ranked 2015 as the warmest year on record, with 2016 on track to become even warmer [1]. This phenomenon manifests through daytime average air temperatures up to 5.6 C higher in urban areas compared to the surroundings [2]. Considering that approximately 53% of the total solar radiation reaching the surface of the earth is near-infrared (NIR) radiation [3], the heat absorbed by buildings and urban infrastructure contributes to microclimate change. One approach for decreasing the surface temperature of buildings relies on using so-called thermal infrared reflective pigments
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (R. Ianos¸), [email protected] (E. Muntean), [email protected] (R. Laz au). http://dx.doi.org/10.1016/j.dyepig.2017.03.016 0143-7208/© 2017 Elsevier Ltd. All rights reserved.
or cool pigments, which are inorganic pigments with high solar reflectance [4e6]. A solar reflective coating has high reflective properties in the NIR region (700e2500 nm), thus it can prevent the increase of the surface temperature and has an important role in energy conservation [4,7]. Studies have demonstrated that a solar reflective coating can reduce the surface temperature by more than 10 C than a standard coating of the same color [8,9]. This surface temperature reduction leads to a more comfortable interior climate and electricity savings with the air conditioning during the hot season [10]. Although white TiO2 exhibits the highest NIR reflection efficiency, the market requires colored pigments for aesthetic reasons. Due to their remarkable thermo-chemical, physical and optical properties Co2þ-doped ZnAl2O4 based pigments [11e13], commonly used as ceramic pigments in glazes, are potential candidates as NIR reflective pigments [14]. Typical methods used for the preparation of Zn1xCoxAl2O4 are: ceramic [15], coprecipitation [16], Pechini [17], sol-gel [18e22], hydrothermal [23], flame spray-pyrolysis [24]. However, all these methods are time and energy consuming (elevated temperature/pressure, long
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soaking time, expensive raw materials) and are contradicting the very purpose of using NIR reflective pigments: reducing the energy consumption. Given the limitations of current methods, this paper suggests the preparation of blue Co2þ-doped ZnAl2O4 NIR-reflective pigments with low cobalt content, using a light-fingerprint method known as combustion synthesis. Compared to the other synthesis routes, combustion synthesis is faster and more energy-efficient since it does not require annealing of the obtained powders. The entire amount of energy required for the formation of desired compound is self-generated during the exothermic redox reaction occurring between metal nitrates and fuels. The existing reports on combustion synthesis of Zn1xCoxAl2O4 [12,25,26] are focused on the single-fuel approach. In this paper we report the preparation of blue Zn0.9Co0.1Al2O4 pigment using a different approach, which relies on the use of urea-glycine fuel-mixture instead of a single fuel. A low dopant proportion was chosen for reasons related to cobalt toxicity. The color and the near infrared reflective properties of the pigment are discussed in relation to the synthesis procedure.
2. Material and method 2.1. Sample preparation In order to obtain 0.050 moles of Zn0.9Co0.1Al2O4 blue pigment (P) via combustion synthesis, a mixture of metal nitrates and fuels was used (0.045 moles Zn(NO3)2$4H2O - Merck, 0.005 moles Co(NO3)2$6H2O - Reactivul, 0.100 moles Al(NO3)3$9H2O - Fluka, 0.056 moles glycine C2H5NO2 - Fluka and 0.375 moles urea CH4N2O - Merck). The raw materials were dosed according to reaction equation (1), using glycine as fuel for zinc and cobalt nitrates and urea for aluminum nitrate.
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measured using a thermal vision FLIR T 640 camera operating at a speed of 15 frames/second. The phase composition of the resulted powder was assessed based on the X-Ray diffraction pattern recorded using a Rigaku ULTIMA IV X-Ray diffractometer (radiation CuKa). Crystallite size was calculated by Scherrer's equation, using the specific diffraction peaks for 220 and 311 hkl planes. The heating behavior of the powder was pursued by thermal analysis using a Netzsch STA 449 C instrument. TG and DSC curves were recorded in air atmosphere, using alumina crucibles, in the temperature range 25e1000 C, at a 10 C/minute heating rate. The specific surface area of the pigment was measured by the BET (Brunauer, Emmet, Teller) method, using a Micromeritics ASAP 2020 instrument, after previously degassing the sample for 5 h at 300 C, under 5 mmHg vacuum. The BET measurement was carried using N2 as adsorption gas, at liquid nitrogen temperature. Particle size was assessed using a Malvern Mastersizer 2000 laser particle size analyzer. Particle morphology was observed by transmission electron microscopy, using a FEI Titan G2 80-200 instrument. The obtained blue pigment (denoted as sample P) was dispersed into a water-based acrylic copolymer emulsion, using a sinusdissolver. The mass % composition of the prepared paint was: 38.7% acrylic copolymer, 30% pigment, 30% water and 1.3% additives. The paint was applied onto a 60 60 mm aluminum sheet using a block applicator with 300 mm gap. After drying, the thickness of the coating (denoted as sample C) was measured using a Byko-test 1500 film gauge. The film homogeneity was investigated by SEM using a FEI Quanta FEG 250 microscope. The prepared coating was subjected to IR irradiation, using an infrared light bulb (250 W, 1100 nm dominant wavelength) placed 20 cm above the sample. The temperature evolution of the coated surface was recorded by thermal imaging using a FLIR T640 IR
81Zn(NO3)2(aq) þ 9Co(NO3)2(aq) þ 100C2H5NO2(aq) þ 180Al(NO3)3(aq) þ 675CH4N2O(c) þ 675/2O2(g) ¼ 90Zn0.9Co0.1Al2O4(c) þ 875CO2(g) þ 1600H2O(g) þ 1085N2(g) (1)
After dosing, the raw materials were homogenized and dissolved solely in their own crystallization water by heating at 80 C and magnetic stirring at 300 rpm for 30 min. The porcelain dish containing the resulted solution was placed on the heating mantle, which was previously heated at 450 C. While the solution temperature was increasing, part of the contained water was evaporated and the viscosity of the solution increased. After a while, the ignition of the self-propagated combustion reaction took place. [HAZARD NOTE - the combustion process is highly exothermic and thus, must only be carried out on the scales indicated in the experimental method and with extreme care, by experimental workers skilled in the routine use of such procedures.] In less than a minute the sample reached incandescence and a large volume of colorless gases evolved (CO2, H2O, N2). Under the effect of the combustion front propagation, the raw materials were transformed in a blue voluminous powder. The such-prepared sample was subjected to wet milling for 30 min in a planetary ball mill, using agate jars and grinding balls. The sample was afterwards washed with distilled water and dried at 110 C for 3 h. 2.2. Sample characterization and testing The temperature evolution during the combustion process was
camera operating at 15 frames-second. For comparison, two other aluminum sheets of the same size were coated using a white TiO2based paint (denoted as sample W) and a carbon-black based paint (denoted as sample B) and their thermal response was assessed under the same conditions. The color (assessed by CIEL*a*b* coordinates) and total solar reflectance (ASTM E892-87, SPECTRALON standard) of the pigment (P) and coating (C) were measured by diffuse reflectance spectroscopy in the wavelength interval 300e2500 nm (5 nm step), using a Perkin Elmer Lambda 950 UV-VIS-NIR spectrophotometer equipped with an integrating sphere. D65 illuminant was used and the observer's angle was set to 10 . 3. Results and discussion The self-propagating combustion process took place very rapidly, as the temperature variation curve shows (Fig. 1). The combustion process ignited at 284 C. Once triggered, the highly exothermic redox reaction between metal nitrates and the fuel mixture evolved in a self-propagated manner and the temperature increased rapidly up to a maximum combustion temperature of 1428 C. The temperature decreased afterwards quite fast down to 650 C, and then the sample slowly cooled down.
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Fig. 1. Sample temperature evolution during the combustion reaction (1). Fig. 2. The XRD pattern of the pigment obtained from the combustion reaction.
The whole process took only 30 s, from the ignition (284 C) until the combustion reaction was over (650 C). In the absence of thermodynamic data for Zn0.9Co0.1Al2O4 solid solution, standard enthalpy of reaction, standard Gibbs free energy and adiabatic temperature for ZnAl2O4 were calculated, using the available thermodynamic data [27,28]. Due to the low cobalt proportion in the pigment composition (1.4 atomic % Co) and based on the fact that Zn0.9Co0.1Al2O4 represents a solid solution derived from ZnAl2O4 by partial substitution of Zn2þ with Co2þ, one may expect the standard enthalpy of reaction, standard Gibbs free energy and adiabatic temperature to show minor variations for Zn0.9Co0.1Al2O4 compared to ZnAl2O4. The measured combustion temperature (1428 C) is lower than the adiabatic temperature (2234 C), which is mainly explained by the heat loss caused by the reaction taking place under non-adiabatic conditions. On the other hand, the strong exothermic nature of the combustion reaction is confirmed by the highly negative value of the standard enthalpy of reaction (DrH0298 ¼ 4244 kJ/mole of ZnAl2O4). The negative value of the standard Gibbs free energy (DrG0298 ¼ 6671 kJ/mole of ZnAl2O4) indicates the spontaneous nature of the process, which is thermodynamically favored. Considering that the reaction presented in equation (1) illustrates the combustion process, a total of 39.6 moles of combustion gasses (CO2, H2O and N2) evolve for each mole of Zn0.9Co0.1Al2O4. As the combustion temperature reaches 1428 C during the reaction, the actual volume of released gases is highly increased. This is the reason why, the powder resulted from the process is very voluminous and spongy. The X-Ray diffraction pattern shown in Fig. 2 indicates the Zn0.9Co0.1Al2O4 solid-solution being the single crystalline phase present, with an average crystallite size of 32 nm. The formation of the Co-doped spinel structure directly from the combustion reaction without any subsequent annealing, is due to the high temperature developed during the combustion reaction. The thermal investigation of the obtained powder suggests that the combustion reaction was complete. The total mass loss calculated from the TG curve is less than 0.5% (Fig. 3), which proves there
are no raw materials left unreacted. Moreover, the DSC curve shows no endothermic or exothermic effect whatsoever, which denotes the absence of any physical or chemical transformations in the studied temperature interval. Despite the short duration of the combustion reaction, the high temperature enhances the sintering process. Therefore the primary particles with diameter no larger than 80 nm (Fig. 4) are highly sintered and form micronic-sized clusters, which is in
Fig. 3. TG-DSC curves of the combustion-synthesized blue pigment.
R. Ianos¸ et al. / Dyes and Pigments 142 (2017) 24e31
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Fig. 4. TEM image and elements mapping of the pigment obtained from the combustion process.
accordance to the results of the laser grain size investigation (Fig. 5). At the same time, the TEM image of the pigment shows coalescent particles, proving an advanced sintering process. The mapping of elements (Fig. 4) shows a homogeneous distribution of Zn, Al, Co and O. Particle size is one of the parameters that influences the reflection capacity of the pigments. In order to enhance reflectance, the particle size must be higher than half of the reflected wavelength [29]. The electromagnetic radiation mainly responsible for the heating effect has wavelengths between 700 and 1100 nm [29,30], where 26% of the total solar energy is concentrated, meaning approximately 50% of the NIR radiation [4]. As a result, in order to maximize the reflectance in the 700e1100 nm range, the particle size should be greater than 0.35e0.55 mm. Particle size
investigation proved that 10% of the particles are smaller than 1.0 mm, 50% of the particles are smaller than 6.1 mm (D0.5) and 90% of the particles are below 20.0 mm (D0.9). Such a particle size distribution recommends the obtained powder for use as a thermoreflective pigment (Fig. 5). From the nitrogen adsorption e desorption analysis the pigment shows a type IV isotherm, with H3 hysteresis (Fig. 6), which are specific for porous particles agglomerations. This correlates with the synthesis conditions in general, and with the high volume of
Fig. 5. Particle size distribution of the obtained pigment.
Fig. 6. Adsorption-desorption isotherms of the obtained pigment.
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generated gases in particular. The BET specific surface area of the pigment is 6.5 m2/g and the pore volume is 0.033 cm3/g. The powder obtained at the end of the combustion process shows an even blue color, without any dark or black areas. According to Gaudon et al. [17], the development of the blue color is conditioned by thermal treatment (annealing) at high temperatures, which ensures the inclusion of the Co2þ cations in tetrahedral spaces to substitute Zn2þ. Co2þ inclusion in octahedral coordination in zinc aluminate structure leads to a pink color [16]. The inclusion of Co2þ in both tetrahedral and octahedral coordination leads to color alteration. As a result, in order to obtain pure blue color, it is essential to ensure tetrahedral coordination to Co2þ. From this point of view, the blue color obtained directly from the reaction is explained by the high homogeneity of the raw materials mixture and also by the high temperature (1428 C) developed during the self-propagating combustion reaction (Fig. 1). The diffuse reflectance spectrum of the obtained pigment (Fig. 7) is similar to those of the blue pigments containing Co2þ in tetrahedral coordination. Blue oxide compounds colored with cobalt owe their color to Co2þ with an [18Ar]3d7 electronic structure. As a result of the energy level splitting, the following terms are obtained in the case of Co2þ free ion: 4F, 4P, 2H, 2G, 2F and 2P [31]. These terms are in their turn subjected to further splitting depending on the symmetry of the ligands crystalline field. According to the literature data [32], the electronic transitions responsible for the electronic spectra of the d7 ions in tetrahedral coordination are: 4T2g(F) ) 4A2g(F) e at 3330-2000 nm, 4T1g(F) ) 4A2g(F) e at 2174-1282 nm and 4T1g(P) ) 4 A2g(F) e at 755-615 nm [32]. This last transition has as result the band triplet at 545, 585, and 620 nm, which is typical for Co2þ ions in tetrahedral sites and such pigments feature a specific blue coloration. The strong absorption band near the UV region, up to approximately 425 nm is due to the ligand-to-metal charge transfer (OeCo/Zn). As far as concerns the NIR region, the lack of absorption bands between 780 and 1100 nm indicate the obtained pigments as suitable candidates for heat reflecting coatings, since this region in the solar radiation is responsible for surface heating. The two lowintensity absorption bands located at 405 and 480 nm, also noticed
Fig. 7. Diffuse reflectance spectra of the pigment (P) and prepared coating (C).
Fig. 8. Graphical representation of (F(R)hn)2 depending on hn for pigment P.
in the case of CoxZn1xAl2O4 [20] and CoAl2O4 [33], could be assigned to 2T(G) ) 4A2g(F) forbidden transition, typical for Co2þ ions in tetrahedral coordination [33,34]. The Tauc plot (Fig. 8) derived from the diffuse reflectance spectroscopy measurements indicates a direct band gap of 4.04 eV, which is actually lower than the value (4.98 eV) reported by Anand et al. [12] for the same stoichiometry. The CIEL*a*b* parameters indicate the obtained pigment situates in the blue-green field. The value of b* is highly negative, whilst the value of a* is only slightly negative, which suggests a blue color with a slight green hue (Fig. 9). Comparing our results with the literature data (Table 1), one may notice that for the same Co2þ content, the pigment P obtained by us (Zn0.9Co0.1Al2O4) is bluer (with a more negative b* value) than the pigment prepared by the polymeric precursor method annealed at 900 C [20]. Moreover, pigment P (Zn0.9Co0.1Al2O4) is bluer than other pigments in the ZnO-CoO-Al2O3 system prepared with a higher Co2þ content. Despite the low chromophore content, the C* value (chroma) is higher for pigment P (Zn0.9Co0.1Al2O4), which indicates a higher saturation of the blue shade and thus, superior optical properties. Ensuring similar or even improved color features whilst using a low cobalt content (only 1.4 atomic % Co) represents a serious advantage of the combustion method, especially since higher chromophore content may cause environmental issues due to the toxicity of the cobalt ion. As for the thermo-reflective properties, pigment P shows a total solar reflectance of 63%, which together with the reflectance measured at 1100 nm (RP,1100 ¼ 58%) recommend the use of Zn0.9Co0.1Al2O4 as a cool pigment. It worth mentioning that the solar reflectance measured at 1100 nm is a lot higher in the case of pigment P (Zn0.9Co0.1Al2O4) than for CoAl2O4 obtained via Pechini method (R1100 z 34%) [14], especially when Zn0.9Co0.1Al2O4 contains less Co2þ, but at the same time shows a higher proportion of blue (lower b*) and higher color saturation (Table 1). It is important to highlight that no additional annealing was involved for obtaining the blue Zn0.9Co0.1Al2O4 pigment (P), as it is usually the case in most of the other synthesis methods (Table 1). In our case, the development of the spinel structure and the formation
R. Ianos¸ et al. / Dyes and Pigments 142 (2017) 24e31
Fig. 9. Position of the pigment (P) and prepared coating (C) colors in the CIEL*a*b* diagram. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
of desired blue color is solely due to the heat released during the combustion reaction. The superior optical properties of the obtained pigment, together with the multiple benefits of the self-
29
propagating combustion reaction justify the combustion method as being the right choice for the synthesis of Zn1xCoxAl2O4 e based blue pigments. The SEM images (Fig. 10.) show a homogeneous distribution of the pigment within the dry coating, which is thick and uniform enough to completely mask the substrate. After drying, the coating C measured a thickness of 140 mm. The diffuse reflectance spectrum of the coating is similar to that of the pigment (Fig. 7). A significant difference in the case of the coating is the presence of an additional absorption band in the range of 2200e2500 nm. According to the literature data, this band is assigned to the acrylic copolymer used [36], whose composition may be also changed or adjusted in order to maximize the thermoreflective properties. From the colorimetric point of view (Table 1), coating C shows superior features compared to pigment P: b* is more negative, whilst a* is closer to 0. This means a higher blue proportion and lower green component. In terms of chromaticity this translates in an increase of chroma C* from 39.2 to 45.3. The total solar reflectance of the coating (TSRC ¼ 59%) is lower than in the case of pigment P (TSRC ¼ 63%), which may be explained by the presence of the acrylic copolymer whose composition has not been optimized for thermo-reflective coatings. Even so, the coating obtained within the present study is significantly bluer (Table 1) and shows at 1100 nm a higher reflectance (R1100 ¼ 59%) than the CoAl2O4/TiO2 coating prepared by Hedayati et al. (R1100 z 42%) [14]. The temperature of the blue-coated sample (C) climbed up to 53 C within the first 6 min of IR irradiation and stabilized at 56 C after a much slower temperature increase (Fig. 11). By comparison [37], a carbon black-coated sample heated up to 72 C and the TiO2 white-coated sample heated up to 47 C, under the same experimental conditions. The ability of the Zn0.9Co0.1Al2O4 pigment to
Table 1 Comparison of the pigment/coating CIEL*a*b*, C*, h parameters with similar data. Stoichiometry
Preparation route
L*
a*
b*
C*
h ( )
Reference
Zn0.9Co0.1Al2O4 (P) Zn0.9Co0.1Al2O4 Zn0.8Co0.2Al2O4 Zn0.75Co0.25Al2O4 Zn0.5Co0.5Al2O4 CoAl2O4 Zn0.9Co0.1Al2O4 (C) CoAl2O4
Combustion synthesis, no annealing Polymeric precursor, 900 C Gelatine method, 1000 C/4 h Combustion synthesis, 900 C/1 h Combustion synthesis, no annealing Pechini, 1200 C/2 h Water-based coating, 30 wt pigment P Acrylic resin tinted with TiO2
67.9 45.8 58.7 46.8 45.2 52.0 64.5 61.0
3.7 2.3 3.9 12.7 6.8 1.2 1.7 4.2
39.0 26.4 33.4 20.4 24.3 35.2 45.3 29.6
39.2 26.5 33.6 24.0 25.2 35.2 45.3 29.8
264.6 265.0 276.7 238.1 254.4 268.0 267.9 262.0
This study [20] [35] [26] [15] [14] This study [14]
Fig. 10. SEM images of the coating (C) applied onto aluminum sheet.
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R. Ianos¸ et al. / Dyes and Pigments 142 (2017) 24e31
Fig. 11. Surface temperature variation during IR radiation exposure.
reflect the NIR thermal radiation recommends this as a cool pigment. 4. Conclusions A blue zinc spinel pigment with low cobalt content (1.4 at. % Co) was obtained by combustion synthesis, starting from an aqueous solution of metal nitrates, urea and glycine. The high exothermicity of the self-propagating combustion reaction, assessed by the standard enthalpy of reaction (DrH0298 ¼ 4244 kJ/mole of ZnAl2O4), supplied the necessary energy for the formation of the desired Zn0.9Co0.1Al2O4 solid solution and thus, no additional annealing was required. Although the measured combustion temperature (1428 C) was lower than the calculated adiabatic temperature (2234 C), it was high enough for Co2þ inclusion in the spinel crystalline lattice and development of the blue color (L* ¼ 67.9, a* ¼ 3.7, b* ¼ 39.0). The average particle size of the pigment was 6.1 mm and the BET specific surface area was 6.5 m2/g. The ability of the Zn0.9Co0.1Al2O4 pigment to reflect the NIR thermal radiation was confirmed by its high total solar reflectance (63%) and also by measuring the temperature of an aluminum sheet coated with an acrylic paint containing 30 mass % pigment. The intense blue color obtained using low cobalt content, high total solar reflectance, and the absence of any additional annealing represent solid arguments for using combustion synthesis for the preparation of blue Zn0.9Co0.1Al2O4 near-infrared reflective pigment. Acknowledgements This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS e UEFISCDI, project number PN-II-RU-TE-2014-4-1587. References [1] WMO Statement on the Status of the Global Climate in 2014, WMO-No. 1152,
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