Characterization of thermal performance, flux

Solar Energy Materials and Solar Cells 182 (2018) 76–91

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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat

Characterization of thermal performance, flux transmission performance and optical properties of MAX phase materials under concentrated solar irradiation J. Sarwara,d, T. Shroufa, A. Srinivasab, H. Gaob, M. Radovicc, K. Kakosimosa,

T

⁎

a

Sustainable Energy and Clean Air Research Laboratory, Department of Chemical Engineering, Texas A&M University at Qatar, PO Box 23874, Doha, Qatar Department of Mechanical Engineering, Texas A&M University, College Station, United States c Department of Material Science & Engineering, Texas A&M University, College Station, United States d Department of Mechanical Engineering, University of Engineering & Technology, Lahore, Pakistan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Concentrating solar power Thermal performance Solar receiver MAX phase materials Durability

In this study, thermal performance and optical properties of MAX phase materials subjected to high concentrated flux are characterized. A new indoor facility is developed that allows for investigation of the independent effect of irradiance and temperature on the thermal performance of the material. Two MAX namely, Titanium Aluminum Carbide (Ti2AlC) and Chromium Aluminum Carbide (Cr2AlC) are examined in this study. Both materials are exposed to high concentrated homogenized flux in the range of 527.2 kWm−2 – 917 kWm−2 for 1000 s and 3000 s using a high flux solar simulator while their temperatures are maintained at 60 °C ± 5 °C via water-cooled heat flux gage. Materials’ surface characterization before and after irradiation is carried out using X-ray diffraction, scanning electron microscopy and X-ray fluorescence analysis. It is found that both materials have excellent resistance to high concentrated flux, but that Ti2AlC shows higher light scattering due to the oxidation of its surface. It is also found that the variations in the optical properties over time do not depend on the selected incident flux level. The thermal performance of Ti2AlC and Cr2AlC was found to varies in the 0.56 – 0.68 and 0.60 – 0.67 range, respectively, for selected flux levels. Flux transmission performance of both materials is not affected by exposure to high concentrated flux.

1. Introduction Concentrating solar power (CSP) utilizes concentrated solar irradiance to generate fuels like hydrogen or electricity without any greenhouse gas emissions. The flexibility of a CSP system to adjust its output, according to demand variations, makes it one of the most promising sustainable technologies to tackle the climate change problem. The International Energy Agency predicts a global compound average annual growth rate of ~ 23.4% until 2040 [1]. This increase can only be achieved by manufacturing efficient and durable components for the CSP systems [2] as it will ensure reliable energy conversion with minimal degradation of optical and thermal properties over the lifetime of a CSP system. The solar receiver is one of the most critical components of a CSP system. Solar receivers are made of ceramics or refractory metallic alloys [3]. A brief summary of solar receiver materials and their

geometrical configuration is presented in Table 1. The thermal (absorber) efficiency of a receiver is defined as the delivered energy over the incident energy [4] and depends on the material, geometry, aperture size and on the working material (e.g. thermofluid). Because thermal efficiency includes the effect of all these factors, it is considered to be an important criterion for the design of a solar receiver and several works has been carried out to improve it (see Table 1). For example, an ellipsoidal cavity-receiver with specularly reflecting inner walls has been proposed to improve the thermal efficiency of the receiver which is independent of the aperture size [5]. The effect of the aperture size of a cylindrical solar receiver has been investigated [6] for efficient solar energy harvesting. The variable aperture size required to maintain a constant temperature and the higher thermal efficiency for a thermochemical application is reported [7]. The variations in the thermal efficiency of a tubular receiver by changing the molar flow rate, mass flow rate and the enthalpy of a working material have also

Abbreviations: CCD, Charged Coupled Device; Cr2AlC, Chromium Aluminum Carbide; CSP, Concentrating Solar Power; FTP, Flux transmission performance; HFSS, High Flux Solar Simulator; SEM, Scanning Electron Microscopy; Ti2AlC, Titanium Aluminum Carbide; TP, Thermal performance; XRD, X-ray diffraction; XRF, X-ray Fluorescence ⁎ Corresponding author. E-mail address: [email protected] (K. Kakosimos). https://doi.org/10.1016/j.solmat.2018.03.018 Received 24 June 2017; Received in revised form 5 March 2018; Accepted 8 March 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.


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Nomenclature

Greek letters

SI Units

α ρ λ

A B C I P PT PFT qi qrear t x

Scale parameter Shape parameter Pixel value Intensity Thermal or flux transmission performance Thermal performance Flux transmission performance Incident flux kWm−2 Flux available at the rear of the material kWm−2 time s variable

Absorptance Reflectance Wavelength nm

Superscripts i j n

index Index Number of steps

Subscript ref refl

reference reflected

Table 1 A brief summary of solar receiver materials and their geometrical configurations. Concentrating Solar Power Receivers Material

Type

Temperature

Remarks

Ref

Volumetric receiver (VR) Porous ceramic foam with 92% alumina

Porous ceramic absorber

> 1000 °C

[4]

Pure silica honeycomb (slab 1), SiC particles (slab 2) Alumina-silica

two-slab

> 800 C°

The “Porcupine.”

940 °C − 1700 °C

Wire mesh receiver with SiC as absorber Incoloy 800, a nickel alloy Recrystallized SiC Tubular receiver Nickel based alloy 316 Stainless steel tubing with black paint Slip-cast fused silica block

Open/pressurized VR with metal or ceramic absorber

800–850 °C

High structural strength, Resistant to thermal shock, 824 kW/m2 peak flux with no cracking or degradation 78% − 90% thermal efficiency Decreased radiative losses 80–87%Absorber efficiency, Can endure up to 4 MW/m2, Resistant to thermal stresses development 82% overall efficiency 90% receiver efficiency 67% − 79% thermal efficiency

Gas based Liquid-based

≤ 800 °C 560 °C

High resistance to thermal shock Receiver efficiency: 90.04%, 100 kWt capacity

[21] [22]

Direct Absorption Liquid Receiver

≤ 700 °C

Thermal efficiency 80–90%, 600 kW/m2 maximum flux,

[23]

Spiral solar obstructed

≈ 650 °C

[24]

Centrifugal

> 900 °C

Solid particle

> 900 °C

Thermal efficiency≈ 60%, Optical efficiency ≈ 87% Particle absorptivity ≈ 90% Receiver efficiency of 75% ( ± 4%) High temp CSPs Receiver efficiency > 70%, not suitable for extended moisture exposure, can tolerate up to1000 kW/m2

External

565 °C

Thermal efficiency ≤ 88.8%,

[27]

Fluidized bed

> 1000 °C

20–40%, Chemical application of concentrated solar energy, High heat transfer to fluidized particles

[1,28,29]

Particle Receiver 310 S (ASTM) steel, Particle properties: 83% Al2O3, 5% SiO2, 6% Fe2O3 Small bauxite particles, stainless steel and Nickel-based alloy Inconel 617 Bauxite particles, Refractory ceramic fiber Cylindrical Receiver 316 H Stainless steel, Black Paint absorber Opaque metallic tube solar absorber. Cylindrical cavity: alkaline-earth silicate

[15] [16] [17–20]

[25] [26]

experimental investigations have been carried out to study the thermal performance of a paint coated refractory alloy (Inconel 625) when exposed to a high concentrated flux and temperature [11,13,14]. It is reported that the high concentrated flux not only degrades its optical properties but reduces its thermal performance as well. It is worth noting that this work relies on calculating flux transmitted to the rear surface using analytical methods instead of measuring it using a heat flux gage. Also, the independent effect of temperature and flux on its thermal performance is not quantified. The selection of a durable solar receiver material depends on a broad range of mechanical, thermal, physical, optical and chemical properties [3] that may or may not be available in one particular material. For example, when compared to metallic alloys, ceramics exhibit advantageous properties such as high specific stiffness, stability at a high temperature, and resistance to chemical attacks and oxidation. Although ceramics can, theoretically, accommodate higher flux

been investigated and reported [8,9]. Solar receivers, employing refractory alloys or ceramics, typically operate at high temperatures, which reduces a component's life and reliability. These high temperatures can lead to an aggressive degradation of the thermal properties due to thermal fatigue of the material [10], and also degrade the optical and thermal properties of a coating [11]. The combination of high temperatures and an oxidative environment accelerates material degradation and leads to cracks in a receiver's coating [12]. A high concentrated flux also influences thermomechanical and optical properties (e.g. reflectance and absorptance) of a material over time. The repeated high concentrated flux cycles affect the material's ability to absorb and transfer heat which can be investigated by measuring the thermal performance [11]. The thermal performance defined as the ratio of the flux incident at the material surface to the flux transmitted to the rear of the material while exposed to a variable high concentrated flux [11]. A series of numerical and 77


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and flat surfaces

densities and thermal gradients [18], their inherent brittleness makes them susceptible to fracture and catastrophic failure at an early stage of their service life [11]. In addition, typical ceramics demonstrate lower thermal conductivity and thermal shock resistance when compared to refractory alloys. This, together with difficult and costly machining of ceramics materials limits their application in CSP systems. A family of ternary carbides and nitrides, commonly refer to as MAX phase, have a unique combination of ceramics- or metal-like properties that make them excellent candidate materials for solar receivers [30–35]. They are particularly interesting because of their excellent thermal stability up to ~1500 °C, exceptional thermal shock resistance, and damage tolerance. Furthermore, their good mechanical properties at elevated temperatures, and high stiffness [30,31,36], but more importantly because of their metal-like optical and transport (electrical and thermal) properties [30,37,38], and good machinability [33]. All MAX phases have Nano-layered hexagonal atomic structure and general chemical formula Mn+1AXn where M is an early transition metal (Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, etc.), A is mostly an element form groups 13–16, X is either carbon or nitrogen and n = 1–3. For example, when n = 1 then A-layer elements are separated by two M-layer elements (M2AX) such as Ti2AlC, V2AlN, Ti2SiC and Cr2AlC etc. Similarly, when n = 2 then A-layer is separated by three M-layers (M3AX2) such as Cr3AlN2, Ti3SiC2 etc. and so on [33]. Among 70 + MAX phases known today, two Al. containing carbon-based MAX phases, namely Titanium Aluminum Carbide (Ti2AlC) and Chromium Aluminum Carbide (Cr2AlC) has been identified as a most suitable material for a hightemperature solar receiver because of their excellent oxidation resistance in addition to other above-mentioned properties typical of MAX phases [39–42]. However, the combined thermal and optical properties of such materials have not been studied yet under high heat flux, regardless that both are critical for solar thermal applications. In this work, thermal, optical and surface characteristics of these two MAX phase materials, Ti2AlC and Cr2AlC, are investigated subjected to high concentrated flux. Herein, we describe a newly developed indoor thermal performance characterization facility to examine the independent effect of concentrated flux on thermal performance of these materials, under controlled temperature and ambient conditions. Then, we investigate the change in the optical properties of the materials accounting also for the light scattering. Finally, we study the potential changes in the materials’ composition and morphology after exposure to high heat flux.

2.2. Thermal and flux transmission performance Thermal performance is considered as one of the crucial criteria for the quantification of material degradation in solar applications. It represents a material's apparent property to transfer incident radiative heat and is expressed as the ratio of the transmitted heat flux (e.g. available at the rear of the material) to the incident heat flux (on the front surface of the material). Therefore, thermal performance is calculated via following equation [11]:

PT (t ) =

qrear (t ) qi (t )

(1)

where PT (t ) is the thermal performance of a material. The flux incident on the material is represented by qi (t ) while the flux transferred to the rear of the material is represented by qrear (t ) . Depending upon the reflectivity/absorptivity of the material, part of the incident flux is absorbed in the material while rest is reflected. Therefore, material's flux transmission performance, which is defined as the ratio of transmitted heat flux to the absorbed heat flux, can be expressed as:

PFT (t ) =

qrear (t ) αqi (t )

(2)

where PFT (t ) is the material's flux transmission performance and α is the absorptance of the material. An indoor facility has been developed for the characterization of the thermal and flux transmission performance that allows the independent characterization of the effect of irradiance and temperature under wellcontrolled conditions. The facility consists of a high flux solar simulator, referred as HFSS hereafter, capable of projecting a Gaussian distributed concentrated irradiation of adjustable flux [43]. The flux can be adjusted by altering the supplied current to the HFSS, as it is illustrated in Fig. 1. Since, HFSS emits Gaussian distributed flux [43], a light collimator [44] is necessary to homogenize the incident radiation on the material. The light collimator, referred as homogenizer hereafter, is placed at the focal point of the solar simulator and is aligned with its optical axis. The sample is, then, placed in front of the homogenizer. An alumina casing has been fabricated to hold the sample in position in front of the homogenizer and to protect the surrounding instrumentation, such as a radiometer, and a three-axis movement controller, from spilled concentrated light. At the rear face of the sample, the heat flux is measured using a water cooled Gardon-type circular foil radiometer. A Vatell TG1000-0 is used as a heat flux gage, with a colloidal graphite coating, having a repeatability of 1%, a sensitivity of up to 2 mV/(W/cm2), and a transducer calibration accuracy of ± 3% [45]. A very thin layer of a highly conductive thermal paste, (ceramic thermal compound, Céramique™ 2) improves contact and heat transfer between the sample and the heat flux gage. Both the sample holder and the heat flux gage are aligned with the optical axis of the solar simulator by a three-axis movement controller which facilitates measurement of the spatial distribution of the flux, as well. The movement controller consists of three Newmark precision slides: NLS8–500–101, NLS8–300–102, and NLS8–200–101, for the x, y, and z-axes respectively, with a resolution of 0.08 µm and a 1 µm repeatability [46]. A k-type thermocouple is attached to the sample, through the alumina casing, to record its temperature. The heat flux and temperature measurements are acquired, at a sampling rate of 20 Hz, using two compact real Input/Output National Instruments data acquisition modules: NI-9211 and NI-9213 with a 0.06% typical gain error at the 40–70 °C temperature range [47,48]. All data are collected and processed using LabVIEW software (version 11.0). The heat flux gage is water cooled using a Julabo FC1600T recirculating cooler with a temperature stability of ± 0.2 °C [49]. Water

2. Methodology 2.1. Materials Commercial Ti (−325 mesh, 99.5%), Cr (−325 mesh, 99.5%) Al (−325 mesh, 99.5%), TiC powders (2–3 µm average particle size, 99.5% purity) and Cr3C2 (−325 mesh, 99.5%) powders, all from Alfa Aesar, MA, USA, were used as starting raw materials to synthesize Ti2AlC and Cr2AlC samples. For Ti2AlC, Ti, Al, and TiC powders were mixed in 1.05:1.05:0.95 M ratio, and pressure-less reaction sintered at 1400 °C for 4 h in flowing argon to form highly porous Ti2AlC samples that were further drill milled into a Ti2AlC powder. Ti2AlC were further densified at 1325 °C and 100 MPa in ultra-high purity argon (UHP Ar) for 25 mins into 20 mm dia. and 5 mm high discs using pulsed electric current sintering (PECS).1 For Cr2AlC, Cr, Al, and Cr3C2 powders were mixed in 1:2.4:1 M ratio and reaction sintered into 20 mm dia. and 5 mm high fully dense discs at 1275 °C and 20 MPa for 30 mins in UHP Ar using PECS. Fabricated discs were mechanically polished using SiC grinding papers and diamond suspensions with the final polishing using 60 nm OP-S colloidal silica for 15 min in order to obtain mirror smooth 1 This method is commonly but inaccurately referred to as Spark Plasma Sintering (SPS).

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Fig. 1. Schematic of the indoor facility for the characterization of thermal performance.

2.4. Material Characterization

cooling of the gage is not only important for the accurate measurement of the heat flux (i.e. qrear ) but it also facilitates the temperature control of the sample itself. So, incident heat flux is controlled from the HFSS and the temperature via the heat flux gage thus, allowing investigation of the independent effects of irradiance and temperature on materials’ thermal and flux transmission performance. The indoor experimental setup for thermal and flux transmission performance characterization is shown in Fig. 2 which is, to the best of our knowledge, the first setup of its kind to investigate thermal performance of a material under controlled irradiance and temperature in ambient laboratory conditions.

Different analytical methods were used for the investigation of the effect of the concentrated irradiance on the selected samples. To determine the elemental composition of the samples, X-ray fluorescence analyses (XRF) of the sample surfaces exposed to irradiation was carried out using a Rigaku ZSX Primus II wavelength Dispersive XRF. The qualitative analysis was performed via a built-in EZ scan tool that employs a semi-quantitative fundamental parameter calculations method [50]. X-ray diffraction (XRD) using a Rigaku Ultima IV Multipurpose XRD system equipped with a Scintillation counter as a detector and a silicon sample holder [51] were used to determine phase composition of the samples before and after exposure to irradiation. The XRD was carried out in continuous mode with a scan speed of 2° per minute and step size of 0.02°. The materials’ surface morphology was obtained using an FEI Quanta 400 Environmental Scanning Electron Microscope utilizing an Everhart-Thornley detector. The secondary electron mode was used to take the image at the accelerating voltage of 30 keV. Each characterization analysis was conducted on fresh samples and after every exposure step.

2.3. Flux characterization Before carrying out thermal performance experiments, it is necessary to characterize both the Gaussian distributed irradiation and the homogenized one. These measurements were conducted in two phases and in the absence of a sample. In the first step, the homogenizer is removed and the heat flux gage is positioned at the focal point of the HFSS using the three axis movement controller. The heat flux gage was moved transversely to the optical axis of the HFSS in steps of 1 mm and a total range of ± 10 mm. At each step, the flux was recorded for 60 s to obtain its mean and standard deviation values. A more detailed description of the process is provided elsewhere [6]. In the second step, the same procedure was repeated but with the homogenizer in place and the heat flux gage right after it. The homogenized irradiation was measured, and it also used later as the incident flux (i.e. qi of Eq. (1)). Using the built-in current controller of the HFSS, six flux values were selected (in the range of 849.5 kWm−2 – 1429.5 kWm−2 peak flux) and their spatial intensity distribution was logged in both phases. These results are presented in Section 3.

2.5. Optical properties characterization One of the most important optical property of materials used in solar (light) applications is reflectance (absorptivity). The reflectance is defined as the ratio of the radiant flux reflected by a surface to the incident flux on the same surface. Depending upon the polar, azimuth or solid incident angles, nine goniometric reflectances have been defined and reviewed [52]. Among these reflectance types, the biconical reflectance is shown in Fig. 3. Generally, spectrophotometers are used to measure the spectral reflectance, transmittance, and absorptance of a material by employing

Fig. 2. Indoor experimental setup of thermal performance characterization.

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emitted light, neutral density filters, and a CCD camera with a data acquisition system. More specifically, light is guided, from the light source, with a flexible optical fiber of 0.06 in. diameter and a numerical aperture of 0.55 [59]. The light is then focused by an aspheric lens of 0.554 numerical aperture and a focal length of 40 mm [60]. The Lambertian surface was placed in at a distance of 152 mm from the lens. Its surface is a plasma coated with Al2O3 (Aluminum oxide) to achieve near Lambertian diffuse reflections [61]. A 12-bit 1392 × 1040 Basler scout scA 1400–17 fm CCD camera was used to take the images of the reference and reflected light onto the Lambertian surface. A Schneider Makro Apo Componon HM 60 mm f/4 lens with two Andover neutral density filter having optical densities of 2 and 0.3 were attached to the camera. These filters were selected to achieve maximum gray scale value of 3750 i.e. 91.5% of a full bit depth of 4095. The exposure time was selected to 80 ms. The Pylon View software [62] was used to capture and visualize the image. The captured image is at an angle. Therefore, scaling and rectification were carried out by a perspective correction method. Furthermore, image transformation to 12-bit format was carried out using an in-house MATLAB code. The details about perspective correction and image transformation are explained elsewhere [43]. Prior to any light exposure of the CCD camera, a dark current signal was recorded by placing a black, non-transmitting lid on the camera. The recorded value was subtracted from the captured images prior to perspective correction. The reference measurement was undertaken by capturing the image of the emitted light, at color temperatures of 3000 K, 4500 K, and 6500 K, on to the Lambertian surface. The reflected measurements were undertaken by emitting the light onto the material and then capturing the image of the reflected light onto the Lambertian surface, under same conditions. For each reference and reflected image, the pixel data is transformed into a two-dimensional numerical array [62]. The reflectance and absorptance of the material is found via the following equations:

Fig. 3. Biconical reflectance [52].

an integrating sphere. This method is fast but has limited applications for measurement of scattering due to their inability of providing a spatial distribution of the scattered light [53,54]. Recently, several new measurement methods have been developed to measure the spatial and spectral distribution of the reflected light based on cameras equipped with a charged couple device (CCD) [53,55,56]. It is considered that the reflectance measurements based on a CCD camera are robust and faster than the traditional goniometric or integrating sphere methods. Therefore, a new experimental method was developed, based on the biconical reflectance, for the measurement of the absolute reflectance and the spatial distribution of the reflected light. This method is referred as “Imaging Reflectometer,” hereafter, while the underlying principle pertains to the comparison of the sample's reflected image on a Lambertian surface with a reference image [57]. Furthermore, the developed method is validated by comparing the reflectance results obtained via a conventional spectrometer method [58]. More details for both methods are provided in the following Sections 2.5.1 and 2.5.2. Either optical measurement was conducted on the fresh samples and after every exposure to the concentrated flux.

2.5.1. Imaging reflectometer The schematic of the experimental setup of the imaging reflectometer for the measurement of the reflectance and absorptance is shown in Fig. 4. The essential components of the imaging reflectometry are a tunable LED based illuminator, MSB-MX-25 (Dicon LED), with a flexible optical fiber, a diffusely reflecting Lambertian surface, a lens to focus the

ρ=

n

n

n

n

ij ∑i ∑ j Crefl ij ∑i ∑ j Cref

α = 1−ρ

Fig. 4. Schematic (right) and actual set-up (left) for the reference (top) and reflected (bottom) light measurement using imaging reflectometry.

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(3) (4)


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when exposed to an average concentrated flux of 430 kWm−2 [27]. The review of the literature shows that selected homogenized irradiance levels are suitable for solar thermal and solar thermochemical applications using any volumetric, tubular, particle or cylindrical receiver configurations. For the current work,all samples were exposed to 12 different exposure scenarios of homogenized fluxes whose sequence, intensity, and duration are presented in Table 2.

ij where ρ is the reflectance of the material, Xrefl is the reflected pixel

ij Xref

value of the image while is the reference pixel value of the image. The absorptance of the material is represented by α. 2.5.2. Reflectance via spectrometer The schematic of reflectance measurement using the total internal reflection method is shown in Fig. 5. A USB-650-VIS-NIR spectrometer coupled with an integrating sphere [63] are used to measure the spectral distribution of incident and reflected irradiance with 3 ms integral time and a wavelength range of 350 – 1000 nm. The details of the light source, lens have been presented in previous paragraphs. The light path for both reference and reflected measurement was kept same i.e. 150 mm. Multiple measurements (up to 10) were collected, for each color temperature of the emitted light (i.e. 3000 K, 4500 K and 6500 K), at an interval of 30 s to reconstruct the light spectrum. The area under the curve of the spectrum of the reference and reflected light was obtained by standard integrating numerical tools. The reflectance of the material is found via the following equation:

ρ=

780 nm I (λ ) ∂λ nm refl 780 nm I (λ ) ∂λ 380 nm ref

3.2. Validation The reflectance of the selected as-processed samples was obtained using both reflectance experimental setups described earlier. The spectral distributions of the reference and reflected flux are shown in Figs. 7(a) – (c) while the 2D-spatial distributions of the reference and reflected flux obtained via imaging reflectometer are shown in Figs. 7(d) – (f). The average reflectance was determined to be 35.01 ± 0.01 for as-processed Cr2AlC sample and 27.56 ± 0.01 for as-processed Ti2AlC sample, under same conditions. Instead, using imaging reflectometer, the reflectance of Cr2AlC and Ti2AlC was found equal to 36.1 ± 0.01 and 28.3 ± 0.02 respectively. For both materials, imaging reflectometry results in ~3% higher values for reflectance than those obtained using integrating sphere setup. This variation is reasonably small and attributed to the limitation of the integrating sphere to collect most of the scattered light. As evident from Figs. 7(e) and (f), the spatial distribution of the reflected flux from asprocessed samples has shown scattering of light. Note that the collection area of the integrated sphere is limited to 3.14 × 10−4 m2 while the imaging reflectometer can collect light over an area of 6.25 × 10−2 m2. Nevertheless, this small variation still validates both experimental setups for the measurement of the reflectance. Later, the impact of the different exposure scenarios to reflectance will be presented.

∫380 ∫

(5)

where ρ is the reflectance of the material, Irefl is the intensity of the spectral distribution of the reflected light while Iref is the intensity of the spectral distribution of the reference light. 3. Results 3.1. Flux characterization The incident flux, with and without homogenization, for the selected intensity (HFSS current) are shown in Fig. 6. The peak irradiance varies in the range of 894.5 kWm−2 – 1429.5 kWm−2 while corresponding homogenized irradiance ranges between 579.3 kWm−2 and 917.1 kWm−2. At all irradiance levels homogenization was considered to be very good, while around 33.6 ± 1.9% loss was estimated due to absorption in the homogenizer, internal reflections and scattering. The peak and homogenized irradiances are selected such that these incident flux irradiance levels can be used for volumetric, tubular, particle and cylindrical receivers. For example, for a particle receiver, a peak temperature of 730 °C can be achieved when an absorber is subjected to a peak and average flux of 824 kWm−2 and 500 kWm−2 [4]. For a high temperature solar tubular receiver such as SOLHYCO, a linear relationship between the surface temperature of a 50 mm by 50 mm Inconel 625 plate and incident uniform high flux irradiance shows that a maximum temperature of 833.6 °C (1106.75 K) can be achieved when the tube is exposed to a uniform flux irradiance of 692 kWm−2 [64]. An outlet temperature of 900 °C can be achieved for a centrifugal particle receiver when exposed to an input flux of 670 kWm−2 [25]. Furthermore, an external cylindrical receiver can achieve 565 °C temperature

3.3. Transmitted fluxes Both Ti2AlC and Cr2AlC samples were exposed to the homogenized fluxes for 1000 s and 3000 s following the sequence listed in Table 2 while their temperature was maintained at 60 °C ± 5 °C. As it is shown in Fig. 8, the irradiance available at the rear of Ti2AlC and Cr2AlC has increased with the increase in incident irradiance. 3.4. Optical properties characterization Reflectance for both samples was measured initially in as-processed state and after every exposure scenario described in Table 2. Fig. 9 illustrates the reflected images (spatial and intensity distribution) of the Cr2AlC sample after exposure at variable flux at selected exposure times of 3000 s (end of first test) and 24,000 s (end of all tests). The intensity of the reflected flux decreases, which results in reduced reflectance but no scattering, was observed. This behavior shows that the smoothness of the surface of the Cr2AlC is not considerably affected by the high concentrated flux or contaminants to show discernible scattering.

Fig. 5. Schematic of the reference (left) and reflected (right) light measurement setup via the total internal reflection method.

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Fig. 6. An example of the non-homogenized incident irradiation (Gaussian) profile (left) and the selected six homogenized profiles (right). Bars reflect standard deviation of measurements.

Table 2 Peak irradiance of Gaussian profile, incident homogenized irradiance and sequence of experiments. Cr2AlC Sequence

1 2 3 4 5 6 7 8 9 10 11 12

Ti2AlC

Flux Peak (kW m−2)

homogenized

1057.3 1152.3 1057.3 894.5 894.5 823.1 823.1 1195.2 1429.5 1152.3 1195.2 1429.5

711.3 768.8 711.3 579.3 579.3 527.2 527.2 821.5 917.1 768.8 821.5 917.1

± ± ± ± ± ± ± ± ± ± ± ±

5.1 6.6 5.1 3.3 3.3 3.3 3.3 6.2 8.2 6.6 6.2 8.2

Exposure time

Flux

Exposure time

Peak (kW m−2)

Homogenized

(s) 3000 3000 1000 1000 3000 1000 3000 3000 3000 1000 1000 1000

1152.3 1057.3 894.5 823.1 1195.2 1429.5 1195.2 1152.3 1057.3 894.5 823.1 1429.5

768.8 711.3 579.3 527.2 821.5 917.1 821.5 768.8 711.3 579.3 527.2 917.1

(s) ± ± ± ± ± ± ± ± ± ± ± ±

6.6 5.1 3.3 3.3 6.2 8.2 6.2 6.6 5.1 3.3 3.3 8.2

1000 1000 1000 1000 1000 3000 3000 3000 3000 3000 3000 1000

exposure as shown in Fig. 11. However, there is a sudden decrease in the reflectance from 28.3% to 17.2% after first exposure of 1000 s. Further exposure to the high concentrated variable flux, the reflectance of the Ti2AlC decreases to 15.7%. The spatial distribution of the reflected flux, (refer to Fig. 12), shows scattering of light which indicates that the smoothness of the surface of the sample is affectedby the concentrated irradiance. The scattering of the light has increased over time while intensity of the reflected light has decreased. Also, a reflectance variation of ~ ± 1.2% was found (shown by error bars in Fig. 11) after exposure to 711.3 kWm−2 and 768.8 kWm−2 which indicates that the Ti2AlC is sensitive to the spectral distribution of the incident light. It also indicates that the surface of Ti2AlC is reacting with the surrounding environment which results in contamination of the surface that consequently, increases scattering [65]. Nevertheless, the standard deviation of reflectance of the population and the sample is found as 3.3 and 0.3 respectively. This behavior will be further discussed in the subsequent sections. The variation in absorptance of both materials calculated using Eq. (4) is presented in Table 3. The absorptance of Ti2AlC is increased from

Fig. 10 shows the measured reflectance of the sample after every exposure scenario listed in Table 2 and reflectance variation over time. It can be seen that reflection of the Cr2AlC decreases from 36.1% to 32.9% after first exposure testing (percentage decrease of 6.3%), while after exposure for 24,000 s (all scenarios) it has decreased to 31.1% (percentage decrease of 13.8%) It can be also observed that change in reflectance does not depends on the incident flux level, Fig. 10. For example, the percentage decrease in the reflectance of Cr2AlC is found to be 0.6% after exposure for 1000 s to incident flux level of 579.3 kWm−2, while percentage decrease in the reflectance is found to be only 0.2% when exposed under high incident flux level of 821.5 kWm−2 for the same exposure time. The standard deviation of reflectance of the whole population for Cr2AlC is 1.2% while standard deviation of reflectance of the sample is 0.2%. It is also observed that the rate of change of reflectance decreases over time. For example, after first exposure, the reflectance of the material changes by 7.0 × 10−4 s−1 while after all scenarios, the rate of change of reflectance is found as 7.6 × 10−5 s−1. In case of Ti2AlC, reflectance also decreases over time and after each 82


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Fig. 7. Reference and reflected spectrum at light color of (a) 6500 K (b) 4500 K, and (c) 3000 K. The processed reflected image for (d) the reference light source, (e) fresh Cr2AlC reflection, and (f) fresh Ti2AlC reflection.

has increased after each exposure and a difference due to increase in thermal performance at each irradiance level is shown in Fig. 13(b) and (c). This difference is calculated using Eq. (6) depending upon the sequence of experiments mentioned in Table 2:

∆P = Pi1000 (t ) − Pi3000 (t ) Or ∆P = Pi3000 (t ) − Pi1000 (t )

(6)

Where i represents homogenized flux level and 1000 or 3000 represents exposure duration. The maximum difference of thermal performance for Cr2AlC is obtained after first exposure. That difference decays over time in case of Cr2AlC but there is no particular trend observed for difference of thermal performance of Ti2AlC. This behavior further suggests that the surface of Ti2AlC is more severely reacting with the surrounding environment under concentrated flux. The thermal performance of both materials when compared to different incident flux levels has shown decrease in its thermal performance with the increase in incident irradiance (Fig. 13(a)). In case of Cr2AlC, its thermal performance has varied in the range of ~0.60 - ~0.67 while for Ti2AlC, the variation in thermal performance is found to be in the range of ~0.56 ~0.68. The trend lines plotted in Fig. 13(a) also show that there is a positive shift of 0.016 ± 0.005 in the thermal performance of Cr2AlC after first and second set of exposures under same flux level. The trend line for Ti2AlC has shown the both negative and positive shift in the range of 0.015 ± 0.005 after first and second set of exposures under same flux level. The results of thermal performance suggest that the thermal performance of both materials is affected by optical properties of their surfaces. To include the effect of surfaces’ optical properties and for the quantification of materials’ ability to transmit absorbed flux, the flux transmission performance is calculated using Eq. (2) and result is presented in Fig. 14. A linear regression model [66] with a R2 value of 0.16 and root mean squared error of 0.027 best fit the flux transmission performance of Ti2AlC as shown in Fig. 14 (Trend line – Ti2AlC). A bestfit linear regression model [66] for the flux transmission performance of Cr2AlC is also created as shown by trend line in Fig. 14. This regression model best fits the data with a R2 value of 0.28 and a root mean squared error of 0.025. This result and trend line shows that the flux transmission performance of Cr2AlC has decreased having a range of 0.1 ± 0.03. The flux transmission performance of Ti2AlC has increased in the range of 0.07 ± 0.03. Further statistical analysis to obtain the distribution of the results and its discussions is presented in subsequent

Fig. 8. Irradiance available the back of Cr2AlC and Ti2AlC.

71.7% (corresponding reflectance of fresh sample is 28.3%) to 84.3% (corresponding reflectance of 15.7% after exposure of all scenarios). The increase in absorptance of Cr2AlC is found as 63.9% for fresh sample which increases to 69% after exposure of all scenarios. This shows that the absorptance of both materials have increased. For Cr2AlC, the absorption is increased by ~8% while corresponding increase in the absorption of Ti2AlC is 17.5%. 3.5. Thermal and flux transmission performance Thermal performance of both materials calculated using Eq. (1) is shown in Fig. 13 with the trend lines obtained using a linear regression model [66]. The root mean squared error for linear regression of Cr2AlC for first exposure and second exposure is found as 0.0281 and 0.0094 respectively. The root mean squared error for linear regression of Ti2AlC is found as 0.0561 and 0.0484 for first and second exposure respectively. It is found that the thermal performance of both materials 83


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Fig. 9. Reflectance behavior of Cr2AlC (a) as processed sample (b) after 3000 s exposure (scenario 1), and (c) at the end of testing and after 24,000 s exposure.

Fig. 10. Cr2AlC sample: (a) reflectance vs irradiance level for different durations of exposure (b) overall reflectance over experimental time noting the irradiance flux, as well.

Fig. 11. Ti2AlC sample: (a) reflectance vs irradiance level for different durations of exposure (b) overall reflectance over experimental time noting the irradiance flux, as well.

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Fig. 12. Reflectance behavior of Ti2AlC (a) as-processed (b) after 1000 s exposure (scenario 1), and (c) at the end of testing and after 24,000 s exposure.

‘Discussions’ section.

3.6.3. Scanning electron microscopy The microstructure of Cr2AlC and Ti2AlC surfaces before and after exposure to the high concentrated flux were studied by SEM and selected but typical microstructures are shown Figs. 17 and 18. Both materials have shown some deposition on their surfaces but in case of Ti2AlC, the deposition is observed on whole surface which indicates that it is highly oxidized. Although both materials were irradiated at high concentrated flux but no crack in the surface were observed.

3.6. Material properties variations 3.6.1. X-ray fluorescence The XRF analysis of both Ti2AlC and Cr2AlC surfaces has shown a difference in their chemical composition before and after exposure to high concentrated irradiance (refer to Table 3). The composition of primary components of MAX phase materials such as Titanium, Chromium, Aluminum and Carbon are reduced while Oxygen and Silicon composition is increased. The most noticeable increment in Oxygen as a result of oxidation of Ti2AlC and Cr2AlC [39,67] and formation of alumina in both cases, and possibly TiO2 in the case of Ti2AlC. Increase on sodium, and especially silicon content on the surfaces can be attributed most likely to the surface contamination of the samples during exposure in ambient air.

4. Discussions The thermal performance and the flux transmission performance of the MAX phase materials are related to each other and depend on the material's surface changes and optical properties. The following discussion will consider the most important variable that affects the thermal performance and the flux transmission performance of the selected MAX phase materials and predicts their reliability and life cycle for solar thermal applications. First, the results are summarized before proceeding to the discussions. Results of this study clearly show that flux transmitted to the back of the both materials increases after each exposure of 1000 s or 3000 s at same irradiance level as shown in Fig. 8. The optical properties of both materials have been measured after each exposure and they show that the reflectance of both materials has decreased after each exposure. In the case of Cr2AlC, the reflectance has decreased from 36.1% to 31.1%, but no scattering of the light is observed as shown in Fig. 9. In the case of Ti2AlC, this decrease is more pronounced, i.e. from 28.3% to 15.7%, and significant scattering of the light can be also observed, Fig. 12. This behavior of increase is scattering for Ti2AlC and no discernible scattering for Cr2AlC is consistent with the materials’ surface characterization results using XRF, XRD and SEM. The surface of Ti2AlC has

3.6.2. X-ray diffraction XRD for Cr2AlC before and after exposure of high concentrated irradiance shown that the a pattern that is typical for standard Cr2AlC [68], as shown in Fig. 15. Besides Cr2AlC, results in Fig. 15 suggests presence of Cr7C3 in the sample, both before and after exposure, which is common immunity in this material. More importantly, aluminum or chromium oxides were not detected on the surface after exposure. In the case of Ti2AlC before and after exposure of concentrated irradiance, XRD pattern were also found to correspond well with the standard ones for Ti2AlC [69], (refer to Fig. 16). However, very weak peaks corresponding to Al2O3 and Ti2O can be also identified in the XRD pattern after exposure to concentrated irradiance, suggesting slight oxidation of the surface. The latter is in good agreement with XRF results in Table 4. Table 3 Absorptance of Ti2AlC and Cr2AlC. Time (s)

Irradiance 527.2 (kWm−2)

1000 3000 1000 3000

Ti2AlC 83.1 ± 83.7 ± Cr2AlC 68.0 ± 68.0 ±

579.3 (kWm−2)

711.3 (kWm−2)

768.8 (kWm−2)

821.5 (kWm−2)

917.1 (kWm−2)

0.3 0.3

83.1 ± 0.5 83.7 ± 0.4

83.6 ± 1.1 83.6 ± 0.4

82.8 ± 0.4 84.3 ± 0.9

83.3 ± 0.4 83.3 ± 0.5

84.3 ± 0.4 83.3 ± 0.2

0.3 0.8

67.8 ± 0.2 67.8 ± 0.2

67.6 ± 0.2 67.0 ± 0.1

68.4 ± 0.1 67.1 ± 0.1

68.8 ± 0.2 68.1 ± 0.2

68.9 ± 0.1 68.2 ± 0.3

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Fig. 13. Thermal performance of Cr2AlC and Ti2AlC.

Fig. 16. X-ray diffraction of Ti2AlC as-processed and after exposure and other constituents.

Table 4 The XRF data of Ti2AlC and Cr2AlC before and after exposure, and the % difference of components.

Fig. 14. Flux transmission performance of Cr2AlC and Ti2AlC.

Component

Ti2AlC Ti Al C O Si Na Fe, Zr, P, Ca, S, Cr2AlC Cr Al C O Si Na Fe, Zr, P, Ca, S, O,

Fig. 15. X-ray diffraction of Cr2AlC as-processed and after exposure and other constituents.

shown more than 62 times increase in a contaminant (Si) as compared to Cr2AlC, Table 4, which results in noticeable scattering for Ti2AlC. The decrease in reflectance consequently results in an increase in the absorption of both materials and results are tabulated in Table 3. The 86

Before exposure Weight fraction (%)

After exposure

Difference XRF

%

67.66 19.55 8.27 4.22 0.07 0.08 < 0.1

62.15 19.07 2.68 44.34 3.47 0.54 < 0.3

− 5.51 − 0.48 − 5.58 40.12 3.40 0.46

− 8.15 − 2.47 − 67.54 950.79 5158.28 595.28

73.50 16.80 7.04 2.04 0.20 0.13 < 0.2

68.73 12.87 6.86 2.87 0.36 0.28 < 0.4

− 4.78 − 3.95 − 0.18 0.83 0.16 0.15

− 6.51 − 23.50 − 2.57 40.38 82.46 113.34

Weight fraction (%)


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Fig. 17. Scanning electron micrographs of Cr2AlC (i) as-processed (b) after exposure of variable high flux for 24,000 s (iii) Energy dispersive spectrometry result after exposure of variable high flux for 24,000 s.

Fig. 18. Scanning electron micrographs of Ti2AlC (i) as-processed (ii) after exposure of variable high flux for 24,000 s (iii) Energy dispersive spectrometry result after exposure of variable high flux for 24,000 s.

be ~39% after first exposure for 1000 s. Further exposure of variable high concentrated flux for 21,000 s has shown a non-uniform reduction in the rate of change of reflectance which has also affected its thermal performance as shown in Fig. 13(c). Overall, thermal performance of both materials has increased over time due to increase in their absorptance. Further analysis to estimate their flux transmission performance has shown negligible increase or decrease in their ability to transmit flux to the rear of the material. This result shows that exposure to high concentrated flux has only affected their surface and any damage/reaction at the surface has not penetrated in the material. Statistical analysis has been carried out to understand the variations in the thermal performance, the flux transmission performance and the reflectance of both materials. It is found that the variations in these properties can be described using Weibull distribution, which is very useful to investigate the reliability of these materials and their lifetime modeling [70]. The probability plot for Weibull distribution of reflectance, thermal performance, and flux transmission performance, obtained using a standard numerical tool [71], for both materials is shown in Fig. 19. The Weibull distribution best fits the data and satisfies the one-sample Kolmogorov-Smirnov test [72] for the null hypothesis. The shape and scale parameters of the Weibull distribution along with p-value for null hypothesis are shown in Table 5. The asymmetry of the data around the reflectance mean (skewness) is calculated using the standard tool as well, and results are tabulated in Table 5. The skewness shows that the data is more spread out to the left

thermal performance of both materials is increased after each exposure as it is shown in Figs. 13 (b) and 13 (c) by the difference of thermal performance after each exposure. The thermal performance of both materials is decreased with the increase in incident irradiance level from 527.2 kWm-2–917.1 kWm−2 as shown in Fig. 13(a). Similar behavior has also been reported for other solar receiver materials such as refractory Inconel 625 alloy coated with Pyromark 2500 as absorptive coating [11]. On the other hand, the flux transmission performance of Cr2AlC has decreased while flux transmission performance of Ti2AlC has increased as shown by trend lines in Fig. 14 obtained via linear regression. Both materials’ surface characterization is performed using XRF, XRD, and SEM before and after exposure to high concentrated fluxes and results are shown in Section 3.6. The results indicate slight oxidation of the surface during exposure, but only in the case of Ti2AlC. The results have shown that the thermal and flux transmission performance of both materials is effected by the change in optical and surface properties of the materials. The decrease in reflectance (increase in absorptance) has enhanced the thermal performance of both materials. However, the change in reflectance of both materials is not uniform. For example, the percentage decrement in the reflection of Cr2AlC was 6.3% after first exposure but, further exposure of variable high concentrated irradiance for 21,000 s, it decreases for only ~5%. Similar behavior is observed in the thermal performance of Cr2AlC, which is shown by the decay in the difference of thermal performance in Fig. 13(b). In the case of Ti2AlC, percentage decrement was found to 87


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Fig. 19. Probability plot for Weibull distribution (a1) Cr2AlC – reflectance (a2) Cr2AlC – flux transmission performance (a3) Cr2AlC – thermal performance (b) Ti2AlC – reflectance (a2) Ti2AlC – flux transmission performance (a3) Ti2AlC – thermal performance.

Table 5 The parameters of Weibull distribution along with skewness of the data and corresponding p-value. Weibull distribution

Scale parameter

Shape parameter

Skewness

P-value

ρ – Cr2AlC PFT - Cr2AlC PT - Cr2AlC ρ – Ti2AlC PFT - Ti2AlC PT - Ti2AlC

32.3218 0.8597 0.6485 16.6037 0.7862 0.6523

59.6376 38.9786 43.5609 53.2499 47.3921 24.0802

0.1341 − 0.7336 − 0.8739 − 0.6013 − 0.8472 − 0.6475

0.8264 0.9972 0.9796 0.9199 0.5607 0.4950

f (x ) =

B x A −1 −( x ) B ⎛ ⎞ e A A≥0, B ≥ 0 A ⎝ A⎠

(7)

Where A and B are the scale and shape parameters. Following integration of the probability density function equation, a cumulative density function is obtained to calculate the probability of the survival of both materials. It is found that the probability of the change of the reflectance properties of both materials (probability of failure) is after first exposure only. After that, the mean probability of survival is 0.90 under current experimental conditions. Nevertheless, to investigate the reflectance reduction behavior due to the surface changes, change in reflectance of both materials with exposure time were fitted using power fitting analogous to Arrhenius function, (refer to Fig. 20). The curves fitted well the experimental data having the uncertainty of ± 0.7% for Cr2AlC and ± 1.7% for Ti2AlC. As shown in Fig. 20(b), the decaying reflectance behavior does not follow the linear pattern for both materials, but rather concave Arrhenius-type behavior. This indicates that there are several factors

of the mean for all except reflectance of Cr2AlC which is spread out to the right of the mean but close to the normal distribution (skewness of 0.1341). Since the data follows Weibull distribution, therefore, following probability density function best represents two-parameter Weibull distribution [73]

Fig. 20. (a) Reflectance function of Cr2AlC and Ti2AlC (b) logarithmic behavior of reflectance.

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kWm−2, because they are promising for solar receivers in concentrating solar power systems. Furthermore, their optical properties variations due to exposure of high concentrated flux and materials’ composition, morphology and elemental analysis have been carried out using an imaging reflectometer and XRD, XRF and SEM. Following conclusions can be drawn from this work:

involved in the reduction of reflectance of both materials. Fig. 20(a) shows that the reflectance decay is independent of incident high flux level, which indicates that incident flux level is higher than the threshold activation energy to initiate the oxidation of the surface of the materials due to the surrounding environment. The increase in scattering of the light after each exposure for Ti2AlC (Fig. 12) also shows that the material's surface smoothness is independent of the incident flux level. The change in surface smoothness consequently results in the reflectance decay. The decaying of the reflectance over time was further investigated using functions presented in Fig. 20(a) and Eq. (8).

1. The developed indoor facility is a useful setup for the investigations of thermal and optical performances of materials under controlled flux and temperature condition. 2. The decrement of reflectance of both materials with exposure time follows power law, i.e. the relative change in its reflectance decreases over time. The reflectance of Cr2AlC decreases from 36.1% to 31.1% after exposure to high concentrated flux for 24,000 s but under same conditions, the reflectance was estimated to further decrease by only 3.2% over the ten years. For Ti2AlC, the reflectance decreases from 28.3% to 15.7% after an exposure to variable high concentrated flux for 24,000 s. The reflectance of Ti2AlC was estimated to decreases by only 6.8% over ten years under similar experimental conditions. 3. Cr2AlC has shown minimal to no scattering of light while Ti2AlC has shown a scattering of light due to oxidation of its surface. 4. The surrounding environment is another parameter other than flux and temperature that affect the optical and thermal performance of the materials. 5. The thermal performance of both materials is affected by the change in absorptance/reflectance of the materials. Thermal performance of Cr2AlC varies in the range of 0.60 – 0.67 while the thermal performance of Ti2AlC varies in the range of 0.56 – 0.68. 6. Flux transmission performance is not affected by the exposure of high concentrated flux. 7. Ti2AlC and Cr2AlC have shown excellent resistance to high concentrated flux as no crack in both surfaces are observed using SEM.

ρTi2 AlC (t ) = 27.75t −0.05832 ρCr 2 AlC (t ) = 36.27t −0.01327

(8)

It was found that the reflectance of Cr2AlC decreases to by only 3.15% over ten years while the reflectance of Ti2AlC further decreases by only 6.83% over the same period under similar experimental conditions. Consequently, the absorptance of both materials is increased that results in an increment of thermal performance of these materials over time under same experimental conditions. To further investigate this behavior, XRF, XRD and SEM analysis has been carried out for both materials. For Ti2AlC, a protective layer of TiO2 [74] and Al2O3 [75] has been formed on the surface of the Ti2AlC which is due to the presence of air in the surrounding environment and low-temperature experimental conditions as explained by Basu et al. [67]. Since the experiments are performed at low temperature, therefore the formation of TiO2 and Al2O3 is limited as shown by XRF result i.e. Ti and Al reduction is not significant while C is reduced by ~67%. There is a significant increase in the of Na and Si due to surface contamination during exposure, Table 3. The reflectance of the material is reduced and due to this uneven formation of oxides layer, a scattering of light is observed as shown in the results. Nevertheless, initial results of Ti2AlC has shown promise for their application in high concentrated flux applications as there are no cracks on the surface are observed which shows that Ti2AlC is quite resistant to high concentrated flux. Also, thermal performance of Ti2AlC is comparable to already tested refractory alloy. Cr2AlC is considered resistant to thermal shock, but results have shown that it is quite resistant to high concentrated flux as well. There is no crack observed on the surface although it was exposed to high concentrated flux. However, unlike in the case of Ti2AlC, insignificant increase in oxygen content was detected on the surface of Cr2AlC samples after exposure by XRF (Table 3), and no formation of any oxide phase can be observed from XRD results (Fig. 15). Presence of Na, and Si after exposure can also be attributed to the surface contamination. In addition, any eventual formation of an oxidized layer on sample's surface cannot be observed from SEM images result. Those results are in good agreement with optical behavior as no scattering of the reflected light is observed that can be attributed to the formation of oxide layer. Nevertheless, Cr2AlC has shown the promising result for its potential application in high concentrated flux application due to its high thermal performance and resistance to high irradiance and thermal shock. Further tests are needed for both materials to investigate their thermal performance and optical behavior under controlled surrounding environment. Furthermore, tunable filters can also be used to investigate spectral reflectance of the materials. Such work has already been proposed [54].

Overall, this research concludes that the developed indoor setup is quite useful for indoor thermal and optical performance investigations. Based on the thermal performance, the flux transmission performance and resistance to the high concentrated flux, the selected MAX phase materials have shown promise for their application as a receiver material in solar thermal applications. Further studies are required to investigate the independent effect of high temperature and high temperature under controlled surrounding conditions. Furthermore, some other MAX phase materials such as Ti3SiC2 and Ti3AlC2 are also good candidates for solar thermal applications due to their excellent thermal conductivity and decomposition temperature higher than 2300 °C among 70 + MAX phases known today [33]. Acknowledgements This publication was made possible by a NPRP award [NPRP 6-1162-044] from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. The High Performance Computing resources and services used in this work were provided by the IT Research Computing group in Texas A&M University at Qatar. References

5. Conclusions

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In this work, a new indoor thermal performance characterization facility is introduced that allows investigation of the independent effect of irradiance and temperature on thermal performance and optical properties of materials. Two carbon based MAX phase materials namely Cr2AlC and Ti2AlC are selected to characterize their thermal performance under homogenized flux in the range of 527.2 kWm−2 – 917.1 89


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