High thermal radiation of Ca-doped lanthanum chromite

Abstract

Calcium-doped lanthanum chromite, La_1−xCa_xCrO₃, was prepared using a solid-state reaction method, and the effect of varying the Ca content (0 ≤ x ≤ 0.5) was investigated in relation to its crystalline structure, surface morphology, solar absorption and thermal radiation. This found that the crystalline structure is slightly distorted by Ca²⁺ doping, with an accompanying increase in the valence state of Cr ions and oxygen vacancies enhancing both the solar absorption and thermal emittance. Overall, the La_1−xCa_xCrO₃ system displays relatively high thermal radiation properties, with an optimal composition of La_0.5Ca_0.5CrO₃ exhibiting a solar absorption of 95% and a thermal emittance of 0.94. When used as a light absorber coupled to a thermoelectric module this proved capable of generating electricity and hot water, thereby demonstrating the suitability of this energy-saving material for use in solar thermal radiation applications.

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Introduction

The continued depletion of fossil fuel reserves over recent years has seen non-conventional energy resources playing a more a prominent role in everyday life; and as such, there has been increasing interest in solar energy given that it represents a virtually inexhaustible energy resource. Existing technology for converting solar energy to more usable forms can be divided into two distinct categories: solar thermal conversion,^1–6 which produces thermal energy, and photovoltaic (PV) conversion,^7–9 which produces electrical energy. Of these, solar thermal has become the more popular approach to solar energy conversion, being in concentrating solar power (CSP) systems that are expected (with appropriate support) to provide 10% of the world's electricity by 2050.

The efficiency of solar thermal power depends on the solar absorber materials, which are typically classified as selective or non-selective depending on their absorptance to emittance ratio. Selective absorber materials such as carbide and boride ceramics^10–15 offer excellent solar thermal conversion thanks to a high solar absorption and low emissivity, whereas the outstanding radiative heat transfer of non-selective absorber materials is can be attributed to a high solar absorption and emissivity.^6–18 High temperature emissivity measurements, however, have typically been carried out in a vacuum rather than an actual working environment in the presence of air; and as such, are not really relevant to absorbers that are currently subjected to extreme variations in operating temperature from more than 500 °C to less than 0 °C at night. A few excellent reviews of mid-temperature (100–400 °C) and high-temperature (400–700 °C) absorber materials have been published over the years,^19,20 but a survey of these reveals that only a few non-selective coatings have been developed that are suitable for high-temperature applications. Adding to the problem is the fact that these absorbers also need to meet certain requirements with regards to corrosion resistance and an ability to withstand severe abrasion by sand and dust. Thus, the last few years have seen high-temperature absorbers being increasingly sought after as a means of improving solar thermal conversion efficiency.

Ceramics, which are amongst the cheapest and most versatile of engineering materials, can offer the necessary combination of good thermo-physical properties, high workability, and stability at high temperature in either oxidizing or reducing atmospheres. As an alternative ceramic material, which has been used successfully as a cathode material,^21–24 alkaline-earth doped LaCrO₃ offers high thermal and chemical stability under an oxidizing atmosphere, as well as the ability to withstand high temperatures.^25–28 For example, Hilpert studied LaCrO₃-based interconnects exposed to oxidizing and reducing atmospheres and found that the oxygen non-stoichiometry of doped chromites has a large impact on the nature of the dopants and the defect structure of the chromites.²⁴ Aliovalent doping also affects the properties of a material by manipulating its oxygen vacancy concentration.^29–31 Using this, Khomchenko et al. have demonstrated that the ferroelectric and magnetic properties of aliovalent-doped Bi_0.95Ca_0.05Fe_1−yB_yO₃ (B  =  Ti and Mn; y = 0, 0.05, 0.1) perovskites can be tailored through an elimination of oxygen vacancies by B-site substitution.³² It is also worth noting that impurities and the addition of free carriers caused calcium-doped LaCrO₃ to become a p-type semiconductor,^33–35 which has a significant effect on its spectral properties. However, there have as yet been few reports pertaining to the spectral absorption properties of the La_1−xCa_xCrO₃ system.

We therefore herein report on the preparation of a series of La_1−xCa_xCrO₃ oxides by a solid-state reaction method, which was monitored through thermogravimetric and differential thermal analysis. The effect of Ca doping on the solar absorbance and thermal emittance is also explored, along with any associated change in the crystal structure or surface morphology, with a view to the material's potential application as a solar absorber material.

Experimental

Materials and methods

Samples of La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5) were synthetized by means of a standard solid-state reaction. For this, analytical-grade dry powders of La₂O₃, CaCO₃, and Cr₂O₃ were first mixed to an appropriate weight ratio, as calculated from the stoichiometric composition, and then ball milled with ethanol. The resulting mixtures were dried, placed in a crucible, and then calcined overnight at 900 °C. The calcined powders were subsequently reground, mixed with polyvinyl alcohol, and then pressed at 10 MPa into discs measuring 60 mm in diameter and 3 mm in thickness. Finally, these discs were sintered at 1400 °C for 10 h at a rate of 3 °C min⁻¹, giving a final sample weight of ∼15 g. The resulting La_1−xCa_xCrO₃ samples were named as LC0, LC1, LC2, LC3, LC4 and LC5 according to the amount of Ca dopant added (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5).

Characterization

The calcination of the ceramic was assessed through a combination of thermogravimetric analysis and differential scanning calorimetry (TGA-DSC, NETZSCH STA 449C) in order to measure the weight loss, the change in thermal properties and the temperature of reaction. Heating and cooling rates were maintained at 20 °C min⁻¹, with a maximum temperature of 1300 °C under an air atmosphere being used throughout. A powder X-ray diffraction (XRD) system (ARL X'TRA, Thermo Scientific) with a Cu K_α radiation source (λ = 0.15406 nm) was used to characterize the crystalline structure and identify the dominant phase present in the powders after calcination. The Rietveld refinement analysis was performed with General Structure Analysis System (GSAS) EXPGUI.^36,37 The surface morphology of the samples was observed directly by scanning electron microscopy (SEM; SU8010, Hitachi) and the surface roughness was obtained by 3D Measure Laser Microscope (OLS4000, Japan). The thermal expansion coefficient (TEC) between room temperature and 500 °C was measured using an RPZ-01 dilatometer (Luoyang, China) at a heating rate of 5 °C min⁻¹.

The composition of the samples was determined from Fourier transform infrared reflection spectra obtained with a Spectrum 400-FTIR (FT-IR, Frontier, PerkinElmer LLC) using the KBr pellet technique. Each spectrum represents an average of 32 scans obtained at ambient temperature in a wavelength range of 4000 to 400 cm⁻¹. The surface composition was analyzed by X-ray photoelectron spectroscopy (XPS), using a scanning XPS microprobe (PHI5000 VersaProbe, ULVAC-PHI). A carbon contaminant (C 1s = 284.6 eV) was used to calibrate all binding energy data, and the XPS spectrum was fitted by XPSPeak software. Reflectance spectra in the 0.2–2.5 μm wavelength range were measured using a UV-vis-NIR spectrophotometer (UVPC measurement software, Shimadzu; Varian Cary 5000, Varian), with BaSO₄ as the reflectance sample. The diffused spectra in the 2.5–15 μm wavelength range were obtained with FT-IR spectrometer (Frontier, PerkinElmer LLC) equipped with a gold-coated integrating sphere and a heating accessory.

With reflection measurement methods the emittance (absorptance) is obtained indirectly from the measured reflectance based on the relation for opaque materials, ε = α = 1 − R: ε, α, and R being the emittance, absorptance, and reflectance, respectively. The optical absorbance and thermal emissivity can be derived from the reflectance spectrum according to eqn (1) and (2), respectively.³⁸

here λ is the wavelength; R(λ) is the reflectance; P_SUN(λ) refers to the normal solar irradiance at wavelength λ, which is defined according to ISO standard 9845-1 (1992), AM 1.5; and P_B(λ) is given by Planck's law, which is calculated at 373.15 K according to eqn (3):³⁹where C₁ = 3.743 × 10⁻¹⁶ W m², C₂ = 1.4387 × 10⁻² m K, and T is the absolute temperature.

Photo-thermal measurements were conducted using a solar simulator (model 94043A, 4 × 4 inch source diameter, Newport) fitted with an AM 1.5 direct filter as the light source. The optical power density of the incident light concentrated by the Fresnel lens was measured by a hand-held optical power/energy meter (model 1918-R, Newport) equipped with a thermopile detector (Model 818P-020-12, Newport). Thermal radiation pictures of the samples under different intensities of solar radiation were obtained by FLIR cameras (Automation & Science). For electrical measurements, a thermoelectric generator module (TEG1-127-1.0-2.0, Thermonamic Electronics Corp., Ltd., China) was attached between the ceramic absorber and a custom-made water cooler using thermally conductive adhesives. The current–voltage curves were then measured with a computer-controlled source meter (Keithley 2440 5A).

Results and discussion

TG-DSC analysis

The thermogravimetric (TG) and differential scanning calorimetry (DSC) curves obtained for La_1−xCa_xCrO₃ (x = 0, 0.2, 0.4) sintered in an air atmosphere are shown in Fig. 1a and b. By combining these TG/DSC results with the XRD data in Fig. 1c, the process of synthesis can be divided into six separate stages. In Stage I, which ranges from room temperature to 300 °C, the slight mass loss evident in Fig. 1a is indicative of the loss of surface water. In Fig. 1b, the three endotherms at 338, 500 and 720 °C correspond to the decomposition of oxides. This is attributable to the fact that La₂O₃ is mostly converted to La(OH)₃ during ball milling, and is turn decomposed to LaOOH when heated to between 300 and 500 °C in Stage II. At the same time, La₂O₃ also reacts with CO₂ to form La₂(CO₃)₃, which decomposes to La₂O₂CO₃ at a temperature between 500 and 720 °C in Stage III. At temperatures greater than 720 °C, La₂O₂CO₃ decomposes to La₂O₃, which then reacts with Cr₂O₃ in Stage IV.⁴⁰ The weight loss prior to 650 °C is mainly attributed to the change of La(OH)₃, whereas the loss after 650 °C can be attributed to the known decomposition of CaCO₃.⁴¹ Finally, the endothermal peak evident at 1080 °C in Stage V is validated by the XRD data in Fig. 1c as being associated with the formation of an ABO₃-type perovskite oxide, which can be described by the following reaction formula, eqn (4)–(9):

La(OH)₃ → LaOOH + H₂O↑ (4)

2LaOOH → La₂O₃ + H₂O↑ (5)

La₂(CO₃)₃ → La₂O₂CO₃ + 2CO₂↑ (6)

La₂O₂CO₃ → La₂O₃ + CO₂↑ (7)

La₂O₃ + Cr₂O₃ → 2LaCrO₃ (8)

Fig. 1 TG/DSC curves of La_1−xCa_xCrO₃ (where x = 0, 0.2, and 0.4) in air: (a) thermogravimetric curves, (b) differential scanning calorimetry curves, (c) XRD spectra of precursor oxides sintered at different temperatures.

Structure and morphology

Structural distortion of the crystal lattice is known to vary with an increasing amount of Ca²⁺ dopant, with the degree of distortion being quantified by a tolerance factor (t) that describes the geometric distortion of ABO₃-type perovskite as:where r_A, r_B, and r_O are the ionic radii of the ions. Since the ionic radius of atoms in the crystal structure also varies with changes in their coordination number or valence, t was calculated using the coordination numbers of A-site ions (La³⁺, 1.061 Å; Ca²⁺, 0.99 Å), Cr ions (0.52 Å), and O ions (1.40 Å, 6-fold coordinated) of 12, 6, and 6, respectively. The calculated values for the tolerance factor of the La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5) samples range from 0.906 to 0.894; and as the t value of a perovskite structure typically lies between 0.75 and 1.1, this confirms that Ca-doped lanthanum chromites still retain an ABO₃ perovskite structure.

Fig. 2a shows the XRD spectra of the La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5) samples sintered at 1400 °C for 10 h, in which the gradual shift in the main diffraction peaks toward a higher diffraction angle indicates a reduction in the crystal volume, and means that Ca²⁺ had already begun to substitute for La³⁺ and reduce the interplanar distance of the LaCrO₃ lattice.^42,43 Refinements were performed based on the following structures: LaCrO₃ (ICSD 59595), La_0.9Ca_0.1CrO₃ (ICSD 81985), La_0.8Ca_0.2CrO₃ (ICSD 81989), and La_0.7Ca_0.3CrO₃ (ICSD 81990), and the profile was fitted using pseudo-Voigt functions to fit the lattice and profile parameters GU, GV, GW, LX, and LY. The XRD pattern of LaCrO₃ was selected for Rietveld fitting in Fig. 2b, which shows the typical best fit that was observed and calculated, the difference in the diffraction profiles of the powders, and the expected Bragg reflections. The Rietveld refinements for La_1−xCa_xCrO₃ (0.1 ≤ x ≤ 0.5) are shown in ESI (Fig. S1†). These results identified an orthorhombic structure with a Pnma space group at room temperature, and at all compositions. The refined lattice parameters and unit cell volume obtained through structural refinements of the samples are shown in Table 1, which reveals an obvious variation in both between different samples. For example, the lattice parameters (a, b, c) and unit cell volume decrease with an increasing concentration of Ca²⁺ substituting for La³⁺ in LaCrO₃ that can be attributed to the smaller ionic radii of Ca²⁺ ions. Aliovalent doping, however, can also generate oxygen vacancies and cause a transition in the ions valence state. This means that owing to the effects of charge compensation, trivalent chromium ions in oxides will convert to either a tetravalent or hexavalent form that will affect the angle and distance of Cr³⁺–O–Cr⁴⁺ bonds. This, in turn, may have an effect on the absorption properties.

Fig. 2 (a) XRD patterns of La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5) powders. Inset schematic is of the crystal structure of LaCrO₃. (b) Rietveld refinements (line) of the observed XRD patterns (+) for LaCrO₃. Vertical bars below each pattern show the position of all possible reflection peaks, while the lowest curve depicts the difference between the observed and calculated intensity.

Table 1 Summary of the results of the least-squares refinements of the XRD diffraction data collected from La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5) powders

La1−xCaxCrO3	Lattice parameters			Cell volume (Å)^3	wRp	Rp
	a (Å)	b (Å)	c (Å)
x = 0	5.482	7.762	5.518	234.774	0.16	0.13
x = 0.1	5.471	7.743	5.499	232.981	0.16	0.13
x = 0.2	5.452	7.712	5.475	230.219	0.14	0.11
x = 0.3	5.437	7.682	5.491	229.352	0.17	0.14
x = 0.4	5.428	7.668	5.484	228.274	0.16	0.14
x = 0.5	5.414	7.667	5.410	224.579	0.20	0.15

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The SEM images in Fig. 3 clearly show the microstructure of the La_1−xCa_xCrO₃ system, which it can be seen that the oxides are composed of nearly spherical particles and the average particle size increases with Ca content. This seems to be indicative of an incorporation of Ca into La-sites, thereby promoting grain growth and increasing the free energy of the system.⁴⁴ What is more, the few isolated pores that are visible are sufficient to produce “light traps” that are capable of scattering light at different angles through a combination of reflection, refraction, and scattering. This ultimately increases the optical path of the oxide, and therefore improves its spectral absorption. Wen et al. have suggested that there is a certain correlation between emissivity and surface roughness;⁴⁵ other studies have confirmed this.^46,47 Thus, in order to study the surface condition of each sample, their surface roughness (R_a) was determined by using a 3D laser microscope (OLS4000, Japan) to scan the surface. Fig. S2† shows the micrographs (grey scale) and surface roughness statistical graphs (colour) for each sample, from which we see that the La_1−xCa_xCrO₃ oxides (0 ≤ x ≤ 0.4) have a very similar R_a of 0.18 μm. From Fig. 6, it is also evident that the absorption spectrum of LaCrO₃ differs quite significantly from that of Ca-doped LaCrO₃ (0.1 ≤ x ≤ 0.4) across a range of different wavelengths. In contrast, La_0.5Ca_0.5CrO₃ gives a much higher R_a of 0.29 μm and a little higher absorptance; the increased surface roughness produced by excessive Ca doping has little effect on the absorption properties in the case of La_0.5Ca_0.5CrO₃. It can therefore be concluded that Ca doping can mainly affect the spectral properties.

Fig. 3 SEM images of La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5).

The FTIR spectrum of La_1−xCa_xCrO₃ in Fig. 4 reveals several main absorption peaks between 400 and 4000 cm⁻¹, with the absorption bands at 449, 594, and 636 cm⁻¹ assigned to the O–Cr–O, Cr–O, and Cr–O stretching mode vibrations, respectively. The higher frequency band extends from 800 to 475 cm⁻¹ with a center of 540 cm⁻¹ due to a stretching vibration, whereas the low frequency band extends from 475 to 300 cm⁻¹ with a center of 400 cm⁻¹ due to a bending vibration of metal oxygen bonds. Meanwhile, the weak peak at 920 cm⁻¹ is attributed to the La–O stretching vibration.^48–50 The disappearance of the peaks at 594 cm⁻¹ and 636 cm⁻¹ with increasing Ca²⁺ concentration can therefore be explained by a change in the force constants and bond distances that cause lattice distortion. In the perovskite structure, the A-site cation is surrounded by twelve oxygen ions, whereas the B-site cation is bonded to six oxygen ions in the octahedral interstices of the oxygen sub-lattice. The vibrational modes associated with the motion of these atoms would therefore be expected to be sensitive to the force constants, and hence the bond distances.³⁴ When the oxide is irradiated by a certain frequency of infrared light its dipole moments increase or decrease in response to the forces acting on dipoles in the periodic field, and it is these vibrating dipole moments that cause infrared absorption.

Fig. 4 Infrared transmission spectra of La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5).

Surface composition

From the compositional surface analysis of each sample performed by X-ray photoelectron spectroscopy (XPS), the binding energies (Table 2) were determined from a best fit of the representative peaks for the La 3d, O 1s, and Cr 2p levels. The fitted XPS spectra for the most relevant regions of the sample series (x = 0, 0.5) are displayed in Fig. 5, which reveals that there is no significant difference between samples in the La 3d zone (Fig. 5a); all display the typical double-peak profile of La 3d_3/2 at around 854.7 and 850.9 eV, with La 3d_5/2 components at around 834.1 and 837.8 eV. Interestingly, these energies are all quite close to the values recorded for La³⁺ ions in an oxidizing environment.⁵¹ In the O 1s zone (Fig. 5b), there are two main peaks clearly discernible: a low-binding-energy peak at around 529.0 eV that is typically attributed to lattice oxide species, and a broader peak at around 531.2 eV that can be attributed to CO₃²⁻, OH–, O–, O₂²⁻, and O²⁻ species absorbed on the surface.⁵² Note also that as the concentration of Ca is increased, the peak corresponding to lattice oxygen (O²⁻) disappears, which indicates the formation of oxygen vacancies. In the Cr 2p spectra (Fig. 5c), the undoped La_1−xCa_xCrO₃ (x = 0) system exhibits a clear peak at about 576 eV (Cr³⁺/Cr⁴⁺), with no peak being detected around 579 eV (Cr⁶⁺).⁵³ However, as the concentration of Ca is increased, the appearance of a peak at around 579 eV is indicative of an increase in Cr⁶⁺ concentration to maintain electrical neutrality. This increase in Cr⁶⁺ ions in turn leads to the generation of more oxygen vacancies, as evidenced by the ratio of absorbed oxygen in the O 1s zone. This is significant, as the oxidation state of Cr and O ions has an effect on the solar absorption properties of the La_1−xCa_xCrO₃ system.

Table 2 Binding energies of the main peaks of core electrons, as extracted from the XPS spectra of La_1−xCa_xCrO₃ (x = 0, 0.5)

Sample	XPS binding energy (eV) of main peaks (relative percentage in parenthesis)
	La 3d5/2	O 1s	Cr 2p3/2
LaCrO3	834.1	529.2 (58.2)	575.4 (67.9)
	837.8	531.6 (41.8)	576.4 (32.1)
La0.5Ca0.5CrO3	834.3	528.9 (39.5)	575.4 (37.9)
			576.9 (43.0)
	837.7	530.6 (60.5)	579.5 (19.1)

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Fig. 5 X-ray photoelectron spectra of (a) La 3d, (b) O 1s, and (c) Cr 2p of La_1−xCa_xCrO₃ (x = 0, 0.5).

Diffused spectrum and thermal properties

In Fig. 6, it can be seen that the absorption spectra of LaCrO₃ differs quite significantly from that of Ca-doped LaCrO₃ across a range of different wavelengths. From the calculated values of α_s and ε displayed in Table 3 it is clear that Ca doping enhances both the solar absorption and thermal emission, with both reaching an equilibrium value at x = 0.2 of as much as 95% and 0.94, respectively. The mixed oxide created upon substitution of La with an alkaline earth metal induces the generation of Cr⁴⁺ or Cr⁶⁺ species,^54–56 which means that Ca²⁺ doping introduces an impurity energy level of Cr⁴⁺ in the LaCrO₃ forbidden gap. This generates Cr³⁺–O–Cr⁴⁺ polar hopping with an activation energy of 0.32 eV and an optical band gap of 2.15 eV, as per the de Broglie relation:
where h = 6.62606896 × 10⁻³⁴ J s, c is the speed of light, and E_a is the activation energy. From this, the maximum excitation wavelength of LaCrO₃ and Cr³⁺–O–Cr⁴⁺ polar hopping are calculated to be 0.576 and 3.874 μm, respectively,^57,58 from which it can be concluded that Ca²⁺ doping broadens the scope of spectral absorption.

Fig. 6 Room temperature absorption spectra in the 0.2–15 μm wavelength range for La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5).

Fig. 7 Temperature dependence of radiative heating time for (a) LaCrO₃ and (b) La_0.5Ca_0.5CrO₃.

Table 3 Solar absorptivity and infrared emissivity of the La_1−xCa_xCrO₃ system

Spectral region (μm)	0.2–3	3–5	8–14	2.5–15
	Solar absorption (α)	Infrared emissivity (ε)
LaCrO3	77.4%	0.46	0.89	0.72
La0.9Ca0.1CrO3	91.7%	0.92	0.95	0.93
La0.8Ca0.2CrO3	94.0%	0.92	0.94	0.93
La0.7Ca0.3CrO3	93.8%	0.92	0.93	0.93
La0.6Ca0.4CrO3	93.9%	0.92	0.93	0.92
La0.5Ca0.5CrO3	94.5%	0.93	0.94	0.94

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The law of conservation of energy rules that the amount of incident energy must be equal to the sum of the absorbed, reflected and transmitted energy for an object in a vacuum at constant temperature. In other words, as there is no other source of energy input or output, any energy absorbed by an object must increase its thermal energy; so in order for the temperature of the object to remain constant, it must radiate the same amount of energy as it absorbs. A perfect emitter is therefore also a perfect absorber, and by this analogy, those samples with a higher absorptance in the infrared spectrum can be assumed to have better thermal radiation properties. From Fig. 5 and 8, we see that Ca-doped LaCrO₃ exhibits a higher infrared absorptance than LaCrO₃. This clearly demonstrates that doping can alter the optical energy band and influence the spectral properties of a material,^59–62 which in the case of Ca²⁺ doping, is due to an introduction of an impurity energy level of Cr⁴⁺ or Cr⁶⁺ in the LaCrO₃ forbidden gap.

Fig. 8 (a) Temperature dependence of radiative heating time for water using LaCrO₃ and La_0.5Ca_0.5CrO₃ as source of radiation under a solar intensity of 3 W cm⁻². (c) Current–voltage and power–voltage curves of the device under irradiation at various incident fluxes for La_0.5Ca_0.5CrO₃. (b and d) Schematic diagrams showing the self-assembly of a solar thermal measurement device.

In order to better illustrate the high solar absorptance created by Ca doping, the thermal irradiation performance of LC0 and LC5 was investigated further by recording their irradiation temperature for a given mass under a concentrated light source at different time intervals. As shown in Fig. 8, the temperature of the LC0 was found to initially increase rapidly with solar intensity, but reached a plateau within the first 50 s; the equilibrium value increasing only with the intensity of irradiation, not duration. In comparison, LC5 reached a steady-state condition more quickly and had a higher equilibrium value for any given intensity, and so clearly has the better solar absorption and thermal irradiation properties when used as a light absorber.

Fig. 8 gives an example of the practical application of LC5 in the solar thermal radiation field. For this, its thermal expansion (ΔL/L₀) and coefficient of thermal expansion were measured in air between 50 and 500 °C; the latter giving a value of 11.2 × 10⁻⁶/°C that indicates sufficient thermal stability to meet basic application needs. It is known that the infrared absorptance of LC5 is enhanced with increasing temperature in Fig. 9. According to Kirchhoff's law and the law of energy conservation, infrared emissivity of opaque materials is equal to their absorptance. Thus, LC5 has the highest infrared emissivity at 400 °C, which is consistent with the observation of samples irradiated under a solar intensity of 3 W cm⁻².

Fig. 9 Temperature dependence of the absorption spectra in the 2.5–15 μm wavelength range for La_0.5Ca_0.5CrO₃.

The experimental device used for heating water show in Fig. 8b consisted of an optical concentration system, a ceramic radiator (LC5) and water. Since pure water absorbs almost all solar energy in the near infrared and infrared spectrum, but little in the visible range, its volume (6 × 10⁻⁵ m³) causes a ∼15 °C temperature rise under steady-state conditions. Thus, in the case of LC5 (2.8 × 10⁻³ m² irradiative area, 10 g), the temperature of the water increases to 84 °C within 30 min, as illustrated in Fig. 8a. More interesting, however, is the fact that LC5 can also utilize the heat generated by coupling it to a thermoelectric module to produce electricity. As seen in Fig. 7d, the thermoelectric module was sandwiched between the LC5 and a custom-made water cooling system that was used to increase the temperature difference across the thermoelectric module. The current–voltage and power–voltage curves in Fig. 8c reveal both the open circuit voltage and short-circuit current increase with incident flux. Moreover, the near-linear relationship between current and voltage implies that the voltage generated is constant.⁶³ The peak output power (P_max) was calculated to be 0.18 W for an incident flux of 6 W cm⁻², which is sufficient output to easily drive a propeller motor (see the Video provided in the ESI†) and is comparable to the effect recently achieved by Sm_0.5Sr_0.5CoO_3−δ light absorbers.⁶⁴ Since La_0.5Ca_0.5CrO₃ offers the best combination of solar absorption and thermal emittance, it is considered to have great potential for use in solar thermal radiation applications.

Conclusions

A series of Ca-doped lanthanum chromites, La_1−xCa_xCrO₃ (0 ≤ x ≤ 0.5), were successfully synthesized by a traditional solid state reaction, with subsequent TG-DSC and XRD analysis identifying 1400 °C as an appropriate sintering temperature. XRD and FTIR analysis confirmed that Ca ions are doped into LaCrO₃, with the tolerance factor and cell parameters revealing a greater distortion of the crystal lattice with increasing dopant concentration. XPS analysis also identified that the chromium ions are in a mixed valence state, with the ratio of Cr³⁺ to Cr⁶⁺ decreasing with Ca²⁺ concentration. The porous surface morphology of the samples, as observed by SEM, also creates “light traps” that increase the optical path and absorption of sunlight. Calculation of the solar absorbance and infrared emissivity demonstrated that Ca doping broadens the spectral absorption of LaCrO₃ oxides; the optimal composition of La_0.5Ca_0.5CrO₃ exhibiting an excellent solar absorption of 95% and a thermal irradiation of 0.94. Moreover, as the infrared emission increases with temperature, using this material as a light absorber allows for both the generation of electricity and heating of water. On the basis of these findings, Ca-doped lanthanum chromites are expected to become an important energy-saving material in high temperature applications.

Acknowledgements

This work was supported by the funding from the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD), the Innovation Foundation for Graduate Students of Jiangsu Province (KYLX_0745), the Innovation Foundation for Graduate Students of Jiangsu Province (CXZZ13_0425), the independent research topic of State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201211), as well as the Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences (KLICM-2014-10).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16319b

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