Low-temperature combustion synthesis of CuCr2O4 ... - Springer

J Sol-Gel Sci Technol (2012) 61:281–288 

DOI 10.1007/s10971-011-2625-2 

ORIGINAL PAPER 

Low-temperature combustion synthesis of CuCr 2 O 4 spinel powder 

for spectrally selective paints 

Qingfen Geng • Xin Zhao • Xianghu Gao • 

Shengrong Yang • Gang Liu 

Received: 22 September 2011 / Accepted: 2 November 2011 / Published online: 10 November 2011 

Ó Springer Science+Business Media, LLC 2011 

Abstract CuCr 2 O 4 spinel powder with high quality black 

hue, investigated as solar-absorbing pigment for spectrally 

selective paint, was synthesized by an environmental 

friendly sol–gel combustion process using citric acid as the 

fuel and metal nitrates as oxidizers. Single-phase CuCr 2 O 4 

spinel crystals were obtained after heat treatment of the asburnt 

powder at a low temperature (600 °C) and the 

average crystallite size of the CuCr 2 O 4 powders increased 

with the calcining temperature. Morphological analysis of 

powders calcined at various temperatures was done by field 

emission scanning electron microscopy. CuCr 2 O 4 powder 

calcined at 700 °C was chosen as pigment to fabricate 

thickness sensitive spectrally selective paint coatings by 

simple spray-coating technique. For the sake of comparison, 

the as-burnt powder composed of mixed metal oxides 

(i.e., CuO and Cr 2 O 3 ) was also used as pigment. The results 

reveal that the spinel CuCr 2 O 4 based paint coatings exhibit 

Q. Geng S. Yang 

State Key Laboratory of Solid Lubrication, Lanzhou Institute 

of Chemical Physics, Chinese Academy of Sciences, 

Lanzhou 730000, People’s Republic of China 

Q. Geng 

Graduate University of Chinese Academy of Sciences, 

Beijing 100049, People’s Republic of China 

X. Zhao X. Gao G. Liu (&) 

Research and Development Center for Eco-material 

and Eco-chemistry, Lanzhou Institute of Chemical Physics, 

Chinese Academy of Sciences, Lanzhou 730000, People’s 

Republic of China 

e-mail: gangliu@licp.cas.cn 

much higher spectral selectivity (a s = 0.88–0.91, e 100 = 

0.27–0.35) which is depending on the coating thicknesses 

than that of coatings using as-burnt powder as pigment 

(a s = 0.83–0.88, e 100 = 0.60–0.66). The CuCr 2 O 4 -based 

paint coatings showed no visible degradation after 600 h of 

condensation test and the performance criterion value is 

0.04, indicating that the coatings have excellent long term 

stability. 

Keywords CuCr 2 O 4 powder Sol–gel combustion 

Spectrally selective paint coatings Durability 

1 Introduction 

Spectrally selective coatings used in solar collector are 

known to improve the efficiency of solar-thermal conversion. 

A desirable selective coating is characterized by 

maximum absorption (a s ) over the solar spectrum 

(0.3–2.5 lm) and low thermal emittance (e T ) in the IR 

region (2.5–20 lm) [1]. Up to now, the best-known spectrally 

selective coatings used in the solar energy industry 

include: Sunselect (Interpane) [2, 3], which is prepared by 

sputtering; evaporated titanium nitride films (TINOX); and 

electro-deposited black chrome coatings. Typical a s values 

for all these highly spectrally selective coatings are above 

0.92, while the e T values fall in the range 0.03–0.07. 

However, electroplating method would cause environment 

pollution while sputtering and evaporation method need 

massive funds input. In recent years, some transition-metal 

oxides spinel films [4–7] and carbon–silica composite film 

[8] have been deposited on aluminium substrates by sol– 

gel dip-/spin-coating method and have good spectral 

selectivity. As all of these films need to be calcined at 

500 °C or above, the mechanical strength of aluminium 

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282 J Sol-Gel Sci Technol (2012) 61:281–288 

substrate would decrease greatly, which can hinder its 

practical application in solar collectors. Spectrally selective 

paint coatings are highly desirable as their mass production 

is inexpensive and convenient although their e T is usually 

very high (e T [ 0.25), which is caused by the infraredabsorbing 

polymers used as binder in the paint. The related 

spectrally selective paint consists of solar absorbing pigment 

and an organic binder. 

In the last decades, spinel-type transition metal oxides 

have also been widely used as solar-absorbing materials for 

photo-thermal conversion of solar energy [9–14]. CuCr 2 O 4 

spinel, a narrow band gap semiconductor, is one of the 

most commonly used solar-absorbing pigments in the literature 

[15–17] for its optical properties and smaller oil 

absorption values than carbon soot and perylene black 

pigments [16]. 

The most common method for obtaining CuCr 2 O 4 pigment 

is the conventional solid-state reaction of oxides 

precursors [18, 19], which usually needs lengthy heat 

treatment at high temperature and extensive ball milling. 

This method generally introduces additional impurities and 

defects. Many wet chemistry techniques have also been 

attempted to prepare CuCr 2 O 4 includes such as coprecipitation 

[20], sol–gel process [21, 22], oxalate method using 

ammoniac copper oxalate chromate [23] and sol–gel 

combustion synthesis [24], etc. Among these methods, the 

sol–gel combustion synthesis shows promising potential 

for the synthesis of pigments, owing to its good chemical 

homogeneity aided by the solubility of the salt in water, 

high purity, low processing temperature, and possibility of 

controlling size and morphology of the particles. Besides 

that, sol–gel combustion synthesis is more environmental 

friendly than co-precipitation method in terms of releasing 

no waste liquid during the whole synthesis process. 

CuCr 2 O 4 powder was usually used as catalyst for 

chemical reactions [19, 22] and ceramic pigment for its 

chromatic characteristics [20, 21]. However, in the present 

work we synthesize CuCr 2 O 4 spinel by a nitrate-citrate sol– 

gel combustion process and study its solar-absorbing 

characteristic. The observation of XRD patterns reveals 

that single-phase tetragonal spinel CuCr 2 O 4 is obtained at a 

calcining temperature as low as 600 °C. The as-prepared 

CuCr 2 O 4 powder is then used as solar-absorbing pigment 

to fabricate spectrally selective solar absorber coatings by 

simple spray-coating technique. Considering that metal 

oxides, such as CuO, Cu 2 O, Cr 2 O 3 ,Fe 3 O 4 and Fe 2 O 3 , has 

been widely used as solar-absorbing pigments in spectrally 

selective paint in the literature [25–28] for their semiconductor 

properties, in this work we compare the performance 

of paint coatings using the as-burnt powder 

(composed of CuO and Cr 2 O 3 ) and CuCr 2 O 4 powder calcined 

at 700 °C as pigment, respectively. Apart from low 

price and good optical properties, the other sales argument 

for solar absorber coatings concerns their long term stability 

during serve time. In this study, durability tests 

established by the IEA-SHC Task X for low temperature 

absorbers were carried out to see whether paint coatings 

prepared in this work have excellent stability during collector’s 

lifetime. 

2 Experimental 

2.1 Sol–gel combustion synthesis of spinel CuCr 2 O 4 

Analytical-grade Cu(NO 3 ) 2 3H 2 O, Cr(NO 3 ) 3 9H 2 O, 

Polyethylene Glycol 200 (PEG 200) and citric acid were 

used as raw materials to prepare CuCr 2 O 4 powder. Appropriate 

amount of PEG 200, Cu(NO 3 ) 2 3H 2 O and Cr(NO 3 ) 3 

9H 2 O in stoichiometric ratio of Cu : Cr = 1 : 2 were 

dissolved in ultrapure water. Citric acid was then added into 

the prepared aqueous solution to chelate Cu 2 and Cr 3 . 

Stoichometric compositions of the metal nitrates and citric 

acid are calculated based on the components’ total oxidizing 

and reducing valences, which serve as the numerical 

coefficients for the stoichiometric balance, so that the 

equivalence ratio U c is unity and the energy released is 

maximum [29]. The ideal reaction between citric acid and 

metal nitrates for CuCr 2 O 4 synthesis is presented as Eq. 1: 

9CuðNO 3 Þ 2 

3H 2 O þ 18CrðNO 3 Þ 3 

9H 2 O þ 20C 6 H 8 O 7 

NH 3 H 2 O 

H 2 O ! 9CuCr2 O 4 þ 36N 2 " 

þ 120CO 2 "þ289H 2 O 

ð1Þ 

Thus, citric acid was added as a fuel to the above 

solution with an 18:5 molar ratio of citric acid to nitrate 

ions. The pH of the solution was adjusted to pH 7.0 by 

slowly dropping ammonia and continued stirring for 0.5 h. 

The temperature of solution was raised to 70 °C and 

continued stirring till the solution turned into high-viscous 

gel. The gel was then kept at 135 °C for a sufficient period 

of time to allow the Cu-Cr-citric xerogel to form. Then, the 

xerogel was ignited in air using a few drops of absolute 

ethanol as initiating combustion agent. The xerogel burnt 

in a self-propagating combustion manner and large volume 

of fume evolved. Finally, a voluminous porous powder 

(denoted as as-burnt powder in this context) was obtained 

and the powder was calcined in air at temperatures ranging 

from 500 °C to 1,000 °C for 2 h. 

2.2 Fabrication of the paint coatings 

Thickness sensitive spectrally selective (TSSS) paint was 

prepared using a standard procedure [16, 17]. Black spinel 

powder (the as-prepared CuCr 2 O 4 powder) pigment 

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J Sol-Gel Sci Technol (2012) 61:281–288 283 

dispersion was prepared first, by mixing the pigment with 

the commercially available polyurethane modified by 

epoxy resin and solvent in certain proportions and grinding 

in a ball mill for 12 h. The concentration (i.e., partial 

pigment-to-volume concentration ratio, PVC) of the pigment 

in dispersions was kept at a suitable value at which 

the paint show excellent film-forming property and meanwhile 

the proportion of binder is not too high to affect the 

thermal emittance of the paint coatings. After the dispersion 

process, suitable amount of curing agent was added to 

the mixture system to form paint. TSSS paint was sprayed 

on aluminium substrates whose mean solar reflectance (r s ) 

and thermal emittance (e 100 ) are 0.821 and 0.039, respectively. 

The thickness of coatings varies with the spraying 

time. 

2.3 Characterization techniques 

The thermal decomposition behavior of the xerogel was 

examined using a NETSCH STA 449 C simultaneous 

thermal analyzer at a heating rate of 10 °C min -1 . Air at 

20 mL min -1 was used as purge gas. The phase identification 

of the xerogel precursor, the as-burnt and calcined 

powders was performed using X-ray diffraction (XRD) on 

a Rigaku D/max 2400/PC diffractometer with Cu Ka 

radiation (k = 1.5406 Å). The average crystallite size of 

the powders was measured by X-ray line-broadening 

technique employing the Scherrer’s equation. Field emission 

scanning electron microscope (FE-SEM, JSM 6701F) 

was used to investigated the morphology of obtained 

powders. Prior to analysis, a thin layer of Au was evaporated 

onto the specimens for electrical conductivity. FT-IR 

spectra were recorded on a Bruker TENSOR 27 FT-IR 

spectrometer in KBr pellets with a resolution of 4 cm -1 

using 32 scans for each sample. The thickness of the 

coatings was measured by QuaNix 4500 coating thickness 

gauge. To perform the condensation tests Q8UV3 weathering 

chamber was used. 

The optical measurements in the wavelength range 

0.3–2.5 lm were made in a Perkin Elmer Lambda 950 UV/ 

VIS/NIR spectrometer equipped with an integrating sphere 

(module 150 mm). The total reflectance was measured 

relative to a BaSO 4 reference. The infrared near normal 

specular reflectance was measured between 2.5 and 20 lm 

with a Bruker TENSOR27 FT-IR spectrometer, equipped 

with an integrating sphere (A562-G/Q) using a gold plate 

as reference. The solar absorptance a s is theoretically 

defined as a weighted fraction between absorbed radiation 

and incoming solar radiation. It was calculated according 

to Eq. 2 [30]. Where k is wavelength, R (k) reflectance and 

S (k) direct normal solar irradiance. It is defined according 

to ISO standard 9845-1, normal radiance, AM1.5. 

R 2:5 

0:3 

a s ¼ 

S R ðkÞð1 

RðkÞÞdk 

2:5 

0:3 SðkÞdk 

ð2Þ 

Thermal emittance (e T ) is a weighted fraction between 

emitted radiation and the Planck black body distribution r 

(k,T). It was calculated according to the following Eq. 3. 

R 20 

2:5 

e T ¼ 

rð R k; TÞð1 

RðkÞÞdk 

20 

2:5 rðk; TÞdk 

ð3Þ 

Thermal emittance values of samples in this work were 

obtained at 100 °C and were denoted as e 100 . 

3 Results and discussion 

3.1 Characterizations of the CuCr 2 O 4 powder 

To investigate the decomposition mechanism of the xerogel 

precursor and thereby the formation of the metal oxides, 

the xerogel precursor was submitted to thermal 

analysis. The TG/DTA traces for the nitrate-citric xerogel 

are reported in Fig. 1. It could be seen that there are two 

exothermic peaks which appear at 220 and 303 °C, 

respectively. The first sharp exothermic peak at about 

220 °C with a concurrent large weight loss of *55% can 

be ascribed to the reaction of nitrates with citric acid 

because large amounts of gases such as H 2 O, CO 2 and N 2 

are liberated during the combustion, whereas the decomposition 

of PEG 200 could be responsible for the second 

exothermic peak which is relatively wide at about 303 °C 

with a weight loss of *6% which is coincide with the mass 

fraction of PEG 200 in the xerogel. Thereafter, there was 

no evidence of heat change taking place up to 1,000 °C in 

the DTA curve and the weight of the sample almost 

remained constant above 400 °C, so the phase transition, if 

takes place, should be observed by XRD technique. The 

Fig. 1 TG and DTA plots for xerogel precursor 

123


combustion process of xerogel precursor can be considered 

as a thermally induced anionic redox reaction of the 

xerogel wherein the citrate ions act as reductant and nitrate 

ions act as oxidant. 

Chemical and structure changes, and the desired crystal 

phases present in the materials during combustion 

and calcinations processes could be observed by characterization 

using different kinds of spectrometers. This may 

be helpful in understanding the combustion reaction 

mechanism. 

Figure 2 shows the FT-IR spectra of CuCr 2 O 4 precursor 

and powder calcined at different temperatures. For the 

xerogel, the broad absorption band around 3,200 cm -1 and 

band at 660 cm -1 are associated with the O–H stretching 

vibration of water and citric acid. It was reported previously 

[31, 32] that the stretching vibrations for free carboxyl 

groups in citric acid were located between 1,760 and 

1,700 cm -1 and splits into two bands since citric acid had 

two types of carboxyl groups, one middle carboxyl group 

and two terminal carboxyl groups. In Fig. 2, only antisymmetric 

carbon–oxygen stretching vibration located at 

1,620 cm -1 [33] are found. Thus, it can be concluded that 

in the xerogel citric acid is behaving as tridentate ligand 

having only carboxylate ions and having no carbonyl 

groups. The bands at 1,380 and 830 cm -1 indicate the 

existence of nitrate ions [34]. After combustion the 

absorption bands related to NO - 3 , O–H and –COO - almost 

disappear, while a set of new bands appear at 620, 545, and 

414 cm -1 , which are characteristic bands of Cr–O in 

Cr 2 O 3 . The band located at 500 cm -1 , which is the characteristic 

band of Cu–O in CuO, is not obvious because it is 

covered up partially by the band at 545 cm -1 . The disappearance 

or decrease of the characteristic bands of the 

–COO - group and NO - 3 ion in the FT-IR spectra of asburnt 

powder confirms that the –COO - - 

groups and NO 3 

ions take part in the reaction during combustion. The 

powder calcined at 500 °C shows almost the same FT-IR 

spectrum as that of as-burnt powder. When calcined at 

600–1,000 °C, powders only reveal the well resolved 

absorption bands at 610 and 510 cm -1 corresponding to 

the stretching vibration of Cr–O and Cr–O-Cu bond of 

CuCr 2 O 4 [35]. 

To aid further interpretation of the reaction process, the 

TG/DTA and FT-IR spectra analysis is supplemented by 

XRD analysis. Figure 3 shows the XRD patterns for 

CuCr 2 O 4 precursor and powder calcined at different temperatures. 

The xerogel powder is a mixture of amorphous 

substances and citrate crystals. The XRD patterns of the asburnt 

powder and powder calcined at 500 °C reveal that 

they are mixed phases of CuO (ICCD-PDF No. 89-5898) 

and Cr 2 O 3 (ICCD-PDF No. 84-0312). Therefore, the heat 

released in the process of exothermic decomposition has 

been observed in DTA curve is sufficient for complete 

conversion of the metal compounds to metal oxides and the 

fast decomposition of the organic constituents helps in 

generating a loose powder. However, single phase spinel 

CuCr 2 O 4 can not be synthesized directly by the sol–gel 

combustion process for that temperature reached during 

combustion do not suffice for the complete formation of 

the CuCr 2 O 4 spinel phase, which is possibly caused by the 

intrinsic characteristic of the reaction system and the 

container type according to Kiminami [36]. Figure 3 

depicts that single-phase CuCr 2 O 4 is finally formed at 

600 °C, which is almost 400 °C lower than that of the solid 

state reaction method [18]. The patterns of the CuCr 2 O 4 

powder calcined at 600 °C are in accordance with the data 

of ICDD (PDF No. 87-0432) and are indexed on the basis 

of a tetragonal unit cell with the following lattice constants 

Fig. 2 FT-IR spectra for xerogel, as-burnt powder and calcined 

powders 

Fig. 3 XRD patterns for xerogel, as-burnt powder, calcined powders 

and ICCD-PDF No. 87-0432 for tetragonal spinel CuCr 2 O 4 

123


a = b = 0.6011 nm and c = 0.7735 nm. In addition, the 

c/a ratio 1.2868 is agree roughly with that from ICDD PDF 

No. 87-0432 (c/a = 1.2908). Above 700 °C, more defined 

peak shapes with stronger intensities are observed, indicating 

crystallite growth of the CuCr 2 O 4 powder as the 

temperature increases. 

Figure 4 depicts the FE-SEM micrographs of the powders 

calcined at 500, 700, 900, and 1,000 °C. During the 

combustion process, a large amount of gaseous material is 

released leading to numerous microscopic pores that keep 

the as-burnt powder (whose FE-SEM morphology is not 

shown) loosely agglomerated. Calcined in air at elevated 

temperatures, the pores become smaller and then disappear. 

The micrograph of 500 °C calcined powder in Fig. 4a 

shows the formation of powder consisting of highly 

agglomerated particles with an average particle size of 

80 nm. At higher calcining temperature, the particles tend 

to fuse together to form octahedral-like structure. The 

average particle size dramatically increases from 80 nm to 

1.0 lm on increasing the calcinations temperature from 

500 to 1,000 °C. It is observed that particles at 1,000 °C 

possess very regular octahedron-shaped morphology with 

well crystallization. The fine structure at high temperature 

of 1,000 °C, may be attributed to the large microscopic air 

gas, existing between the particles leading to a decrease in 

diffusion rate that hinders the particle growth [37]. 

The as-burnt powder and 500 °C calcined powder are 

black-gray in color while all of the as-synthesized CuCr 2 O 4 

powders calcined above 600 °C show deep black hue. It 

can be inferred that there is no obvious difference between 

the solar absorptances of CuCr 2 O 4 powders calcined at 

different temperatures, so the difference in photothermal 

efficiencies of these powders will mainly depend on there 

thermal emittances. Figure 6 shows the graph of reflectance 

as a function of the wavenumber for the as-burnt 

powder and powders calcined at different temperatures, 

from which the thermal emittances of these powders can be 

calculated. To eliminate the factitious operation error, 

sampling preparation is important. When measure the 

reflectance spectra, we fill every powder sample in the 

sample container and make the powder is flush with the 

edge of the sample container by means of a piece of glass. 

It can also be seen from Fig. 6 that the as-burnt powder has 

the lowest reflectance in the range from 2.5 to 20 lm (i.e., 

form 4,000 to 500 cm -1 ) and the reflectances of the calcined 

powders increase with increasing the calcining temperature. 

It should be noted that the thermal emittance of 

1,000 °C calcined powder has not been measured for that 

this temperature is too high to highlight the superiority of 

the combustion method to solid state reaction, which means 

that if we use 1,000 °C calcined powder as pigment for the 

spectrally selective paint coatings, the cost will be high. 

The calculated thermal emittances for the as-burnt powder 

and powders calcined at 500, 600, 700 and 900 °C are 0.92, 

0.89, 0.83, 0.80, and 0.74, respectively. From these data, 

we can found that the thermal emittance decreases with 

increasing calcining temperature, which is reflected by the 

increase in reflectance as shown in Fig. 5. The increase in 

reflectance may be caused by the perfecting of crystals 

when elevating the calcining temperature. 

Fig. 4 FE-SEM morphologies 

of powders calcined at a 500, 

b 700, c 900 and d 1,000 °C for 

2h 

123


Fig. 5 Reflectance spectra of the as-burnt powder and powders 

calcined at different temperature 

Based on the discussion above, we should choose 

900 °C calcined CuCr 2 O 4 powder as pigment to obtain 

spectrally selective paint coatings with high photothermal 

efficiency. However, considering the same reason given 

above related to 1,000 °C calcined CuCr 2 O 4 powder we 

choose 700 °C but not 900 °C calcined CuCr 2 O 4 powder as 

pigment to fabricate TSSS paint coatings in this work. In 

the XRD patterns part, we discussed the phase change 

during the process of raising the calcining temperature. The 

results revealed that CuCr 2 O 4 powder can only be obtained 

when the temperature up to 600 °C. The XRD patterns of 

as-burnt powder and 500 °C calcined powder showed 

obvious diffraction peaks of metal oxides (i.g. CuO and 

Cr 2 O 3 ). In the literature, metal oxides were usually used as 

pigments to fabricate TSSS paints, so for the sake of 

comparison the as-burnt powder is also used as pigment. 

To fabricate the TSSS paint coatings, we use room temperature 

curable polyurethane modified by epoxy resin as 

binder and usage of which would make mass production 

convenient and to make it possible to repaint on site for 

already installed collectors or solar facade. 

3.2 Characterizations of the paint coatings 

3.2.1 Spectral selectivity 

The reflectance spectra of paint coatings using as-burnt 

powder (sample A) and 700 °C calcined CuCr 2 O 4 powder 

(sample B) as pigment are shown in Figs. 6 and 7, 

respectively and the values of a s and e 100 are recorded in 

Table 1. The a s of paint coating based on as-burnt powder 

ranges from 0.83 to 0.88 while e 100 ranges from 0.60 to 

0.66, which indicating the low spectral selectivity of the 

paint coatings. In the case of 700 °C calcined CuCr 2 O 4 

Fig. 6 Reflectance spectra in the range from 0.25 to 20 lm of paint 

coatings with different thicknesses using the as-burnt powder as 

pigment (sample A1 to A5) 

Fig. 7 Reflectance spectra in the range from 0.25 to 20 lm of paint 

coatings with different thicknesses using 700 °C calcined powder at 

as pigment (sample B1 to B5) 

powder based paint coatings, the values for these two 

parameters are a s = 0.88–0.91 and e 100 = 0.27–0.35, 

indicating the spectral selectivity is much higher than that 

of sample A. From Table 1 we can also found that both a s 

and e 100 increase with the film thickness. For the convenience 

of comparing sample A and B, we choose two 

samples of similar thickness, which is highlighted in bold 

in Table 1. The results confirm that the spectral selectivity 

of sample B5 is much better than sample A5. So, it can be 

concluded that spinel CuCr 2 O 4 powder is more suitable 

than mixture of CuO and Cr 2 O 3 to fabricate TSSS paint 

coatings. 

123


Table 1 a s and e 100 calculated from reflectance spectra in Figs. 6 and 

7 

Sample d (lm) a s e 100 

A1 3.43 0.83 0.60 

A2 3.95 0.84 0.61 

A3 4.03 0.84 0.62 

A4 4.29 0.86 0.64 

A5 4.72 0.88 0.66 

B1 2.80 0.88 0.27 

B2 3.28 0.89 0.30 

B3 3.32 0.89 0.32 

B4 4.53 0.90 0.34 

B5 4.76 0.91 0.35 

Comparing the thermal emittances of paint coatings and 

that of as-synthesized powders (discussed above), it can be 

found that although the infrared-absorbing resin binder had 

been involved in the system the thermal emittances of paint 

coatings are much lower than that of powders. It can be 

explained by the low thermal emittance of metallic substrates 

employed to fabricate TSSS paint coatings. 

3.2.2 Durability tests 

Durability of the paint coatings is an alternative important 

issue to be concerned in the view of the practical application. 

Factors like high temperature, high air humidity, 

airborne pollutants and sun radiation can cause the coating 

to deteriorate and hence affect the optical selectivity of the 

surface. The commonly used method to test the durability 

of a spectrally selective surface coating is to exposure it to 

elevated temperature and humidity in a climate chamber 

for a curtain time. The condensation test is the most 

important test to perform on the type of coatings in this 

work because previous experience shows that alumina can 

be sensitive to moisture [38]. The condensation test is 

performed according to the recommended accelerated test 

procedure described in IEA-SHC Task X [39]. Tested 

samples should be assessed after 80, 150, 300 and 600 h 

according to the following performance criterion (PC): 

PC =-Da s 0.25De T B 0.05, where Da s is the difference 

in normal solar absorptance before and after the test 

and De T is the difference in normal thermal emittance. This 

equation states that during the service life of 25 years, the 

system should not show a loss of the solar fraction greater 

than 5%. A PC value of more than 0.05 indicates that the 

surface is not condense proof and has poor resistance 

towards moisture and hence there is no use in continuing 

the test. In this work, the condensation test was performed 

by putting the samples aluminium substrate, sample B3 and 

sample B4 in the Q8UV3 accelerated weathering tester in 

Fig. 8 Reflectance spectra of samples (Al substrate, Sample B3 and 

sample B4) before and after 600 h of condensation test 

which the temperature is set to 40 °C and humidity 95%. 

After 600 h, the surfaces of sample B3 and B4 show no 

visible degradation while the aluminium substrate is 

heavily destroyed. It can be concluded that the surface 

coatings can serve as a barrier against a hostile environment. 

The reflectance spectra of the three samples before 

and after 600 h of condensation test are shown in Fig. 8. 

Both the PC 600 values for sample B3 and B4 calculated 

according to PC =-Da s 0.25De T are 0.04, which 

means that the paint coatings qualify for the test. As the 

stagnation temperature for glazed solar collectors can be up 

to 160–170 °C and for the unglazed solar thermal systems 

not surpass 120 °C [40], the surface coatings of solar collectors 

should have excellent thermal stability. The thermal 

stability has been discussed in our previous work in which 

the same resin was used as binders [9]. The result shows 

that the paint coating using polyurethane modified by 

epoxy resin as binder can be used in solar collector for at 

least 45 years. So, it can be inferred that the spectrally 

selective paint coatings prepared in this work qualify for 

the condensation test and thermal stability test and could be 

safely used in solar collectors for many years. 

4 Conclusions 

The sol–gel combustion method is shown to be attractive 

for preparing CuCr 2 O 4 spinel. The XRD analysis shows 

that the calcinations temperature up to 600 °C is necessary 

for obtaining spinel CuCr 2 O 4 . By comparing the thermal 

emittances of the as-burnt powder and powders calcined at 

various temperatures, powder calcined at 700 °C was 

selected as solar-absorbing pigment to fabricate thickness 

sensitive spectrally selective paint coatings on aluminium 

substrate by simple and cost effective spray-coating 

123


technology. The solar absorptance (a s ) and the thermal 

emittance (e T ) of the coatings were determined from the 

corresponding reflectance spectra in the 0.3–20 lm range. 

The coatings show good spectrally selective properties 

(a s = 0.88–0.91 and e 100 = 0.27–0.35). Besides ideal 

optical properties, the coatings also show excellent long 

term stability which is confirmed by results of accelerated 

tests established by IEA-SHC Task X, indicating that they 

could be widely used as surface coatings in solar collectors 

and solar facade. 

Acknowledgments This work was financially supported by the 

‘‘Western Light’’ Talents Training Program of CAS, the Solar Action 

Plan of CAS (Grant 1731012394) and National Natural Science 

Foundation of China (Grant 51003111). 

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