Phase Change Materials Samples Research Articles

Inverse identification of thermal behaviour of a paraffin-based phase change material in complete and partial phase change cycles

From the macroscopic point of view, phase change hysteresis (PCH) means that a phase change process (e.g. solidification) does not follow the same temperature-enthalpy path as the opposite phase change process (melting). Although the PCH is observed in most phase change materials (PCMs), it is often neglected in computational models, resulting in discrepancies when compared to experimental data. The PCH can particularly be an issue in the modelling of latent heat thermal energy storage systems, where incomplete (partial) phase transitions are quite common. Lab-scale experimental methods for characterisation of PCMs, such as differential scanning calorimetry or the temperature-history method, employ only small PCM samples, and the obtained results are often insufficient for predicting the thermal behaviour of large volumes of PCMs. The present study explores numerical modelling approaches to the PCH, addressing both complete and partial melting-to-solidification cycles. A set of validation experiments was performed, focusing on phase transitions in a paraffin-based PCM enclosed in a rectangular cavity. An inverse identification method was used to minimise the root mean square error (RMSE) of temperatures in the PCM using the particle swarm optimisation method. A two-curve approach showed the highest accuracy in complete phase change cycles, with a 62% improvement in the RMSE when compared to the manufacturer data. As for cycles with partial phase changes, a curve-scale approach showed superior behaviour, reducing the RMSE as much as 99%. Conversely, another investigated approach – a curve-track model – exhibited inferior performance, making it less suitable for the modelling of partial phase changes.

Nanoscale Stabilization Mechanism of Sodium Sulfate Decahydrate at Polyelectrolyte Interfaces.

Sodium sulfate decahydrate (SSD) is a low-cost phase-change material (PCM) for thermal energy storage applications that offers substantial melting enthalpy and a suitable temperature range for near-ambient applications. However, SSD's consistent phase separation with decreased melting enthalpy over repeated thermal cycles limits its application as a PCM. Sulfonated polyelectrolytes, such as dextran sulfate sodium (DSS), have shown great effectiveness in preventing phase separation in SSD. However, there is limited understanding of the stabilization mechanism of SSD by DSS at the atomic length and time scales. In this work, we investigate SSD stabilization via DSS using neutron scattering and molecular dynamics (MD) simulations. Neutron scattering and pair distribution function analysis revealed the structural evolution of the PCM samples below and above the phase change temperatures. MD simulations revealed that water from the hydrate structure migrates from the hydrate crystal to the SSD-DSS interfacial region upon melting. The water is stabilized at this interface by aggregation around the hydrophilic sulfonic acid groups attached to the backbone of the polyelectrolyte. This architecture retains water near the dehydrated sodium sulfate, preventing phase separation and, consequently, stabilizing SSD rehydration. This work provides atomistic insight into selecting and designing stable and high-performance PCMs for heating and cooling applications in building technologies.

Thermodynamic and thermal degradation kinetics analysis of coconut shell biomass based phase change material

Phase change materials (PCMs) stores and releases thermal energy in the form of latent heat during phase transition. Though PCMs are durable in nature, they suffer commercial application owing to low thermal conductivity. Inclusion of metal and carbon based nanoparticles are typically adopted to overcome the complication of poor conducting nature of organic PCMs. In this experimental research we develop a bio based nanoparticle using coconut shell in an environmental friendly manner to enhance the thermal conductivity of organic PCM polyethylene glycol 1000. Bio nanoparticle (BNP) improves the thermal conductivity of the developed nanocomposite PCM by 73.1% with 0.9 wt% of coconut shell BNP hence we evaluate the thermodynamic and thermal kinetics parameter of the nanocomposite PCM sample with 0.9 wt% biochar based nanoparticle. In addition the authors have analysed the thermal decomposition kinetics of the optimized PCM composite using Coats and Redfern method to exhibit the reaction mechanism, thermodynamic and kinetic parameter.

Optimizing supercooling and phase stability by additives in calcium chloride hexahydrate for cyclical latent heat storage

Calcium chloride hexahydrate (CaCl2·6H2O) based phase change material (PCM) is promising for latent heat storage due to its favorable thermal properties, but supercooling and thermal cyclic instability limits its wider application. We examine the role of nucleation agents, in particular strontium chloride hexahydrate (SrCl2·6H2O), as an additive to the PCM, on the degree of supercooling and the latent heat. Utilizing a water bath and a cooling chamber, we systematically evaluate the degree of supercooling in various PCM samples, while X-Ray diffraction and microscopic analysis are employed to characterize the role of the additive. The results suggest that particle shapes of the additives and their crystallographic phase contribute significantly to supercooling suppression. Optimizing a tradeoff between supercooling and latent heat reduction for long-term nucleation performance, we identify designer PCM composition with an optimal 3 wt% of SrCl2·6H2O for an automated thermal cycling system by differential scanning as well as drop calorimetry. Incorporating 2 wt% potassium chloride (KCl) shifts the composition at the peritectic point away from the unfavorable CaCl2·4H2O phase, enhancing the freeze-thaw cycle stability of CaCl2·6H2O. Notably, the latent heat variation is ∼0.08% over 1000 freeze-thaw cycles.

Synthesis and Characterization of Shape-Stable Bio-Char Composite PCM Material for Solar Desalination Application

In the present study, three different D-Mannitol based activated bio-char composite shape-stable phase change materials (PCM) were developed for medium temperature energy storage applications like solar desalination, solar thermal storage and waste heat recovery with varying bio-char compositions. The activated bio-char (A-BC) composite-based samples were prepared from sawdust by using the pyrolysis method. The phase composition, morphology, latent heat capacity and phase change temperature of the fabricated samples were assessed by XRD, SEM, differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The fabricated bio-char composite materials exhibited good thermal properties and structural stability compared to pure PCM. The phase change material with a blend of minimum 15 wt.% activated bio-char and 85% D-Mannitol was observed to be leak resistant. FTIR and XRD results confirm that the chemical properties of the bio-char composite remain the same as the pure PCM, which confirms that there are no chemical interactions between the PCM and bio-char. PCM sample with 15% activated bio-char exhibited an enhanced thermal stability compared to the pure PCM.

Investigation of T-history Method Application for Sub-zero Phase Change Material Thermal Properties

The T-history method, first proposed by Zhang & Jiang, is a simple method used to determine the latent heat, specific heat, thermal conductivity, melting point, and degree of supercooling of a phase-change material (PCM). The method is based on the measurement of the temperature of the material over time. It assumes that the PCM sample and the reference material exchange thermal energy with the environment in a similar manner, because it is based on the lumped capacitance method, and the temperature distribution inside the sample is uniform. There are multiple advantages that the T history method has over conventional methods; however, the T history method also has some downfalls, which have been improved over the years by different authors. This study experimentally investigates the reliability of this technique for PCM with a phase change temperature below 0 ° C. The technique is used to first determine the properties of denoised water with a phase change temperature of 0 ° C, using ethylene glycol with a mixture ratio of 50/50 as reference material. Second, by determining the properties of KCl with a concentration of 19.5% salt to water for a phase change temperature of -10 ° C. Two analytical approaches were used, one by immersing the samples in antifreeze during the charging process and the other by exposing the samples to ambient air during the discharging phase. The experimental results were further validated using the known literature to determine the latent heat of KCl. This study found that the latent heat of KCl measured using the technique has a 6.2% difference from the known results determined by the DSC. This study recommends the use of the T history method when designing latent heat thermal energy storage systems (LHTESS), because the sample size is an important characteristic for accurate thermophysical properties during the design process.

The “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” experiment

Present as well as future challenges of space exploration point to the need for improved thermal control systems. The “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” experiment, which is approved by ESA for execution on board the International Space Station, aims to contribute directly to current knowledge and basic understanding of heat and mass transport in phase change materials (PCMs) that incorporate a free surface in reduced gravity. The experiment will apply fixed temperatures to opposite ends of PCM samples held in cuboidal and cylindrical containers in order to drive controlled melting and solidification cycles that will be observed by means of optical cameras. The recorded images will be complemented by thermal measurements at key positions along the samples, which will allow different thermocapillary flow regimes to be distinguished according to their temporal dynamics. It is anticipated that thermal Marangoni (thermocapillary) convection will increase the heat transfer rate in these PCM devices by a significant factor (on the order of two or more) compared to melting governed by thermal diffusion (conduction). If the PCM designs prove robust, the experiment results can be expected to lead to substantial improvements in future designs for passive PCM applications in space missions.

Energizing organic phase change materials using silver nanoparticles for thermal energy storage

Medium temperature phase change materials (PCMs) are of great interest for thermal devices due to their energy storage capability. In the current study, organic PCMs with silver nanoparticles are experimentally investigated and energized to improve the energy storage ability. Organic PCM composites with silver metal nanoparticles of 20–40 nm in size at different concentrations of 0.2 %, 0.4 %, 0.6 %, 0.8 % and 1.0 % were prepared and characterized to explore the thermophysical properties. Scanning electron microscopy observations provides a higher magnification level to ensure the presence of Ag nanoparticles in the PCM matrix. Optical investigation confirms that the permeability of PCM with Ag nanoparticle decreases from 22.7 % to 15.9 % at a concentration of 0.8 %, which increases the absorptivity; this phenomenon is due to the dark colour of the silver nanoparticle. Thermal conductivity of Ag dispersed composite improved the thermal conductivity of the base material PCM from 0.218 W/mK to 0.44 W/mK with a mass fraction of 0.8 % of nanoparticles, resulting in an improvement of 101 %, which can be attributed to the well-developed thermal networks with the PCM matrix. The latent heat of the composite PCM differs by 1 % from the base PCM, which is important for effective energy storage due to better intermolecular attraction. The Ag dispersed composites were also thermally stable up to a temperature of 220 °C and their thermal stability was slightly improved by the nanoparticles. Composite PCM samples with better thermal conductivity were tested for 500 thermal cycles and its chemical stability and thermal properties were equated with PCM. In addition, a numerical heat transfer analysis is performed under varied heat sources (60 °C, 70 °C & 80 °C) to compare the thermal performance between PCM and the synthesized nano PCM. In correspondence to the experimental characterization and numerical investigation the silver nanoparticle dispersed PCM exhibits outstanding energy storage & release routine, ensuring the beneficial usage of developed composite PCM in TES applications like battery thermal management and solar photovoltaic thermal systems.

A sustainable biochar-based shape stable composite phase change material for thermal management of a lithium-ion battery system and hybrid neural network modeling for heat flow prediction

Electric vehicles are gaining global popularity lately, and along with it, efficient battery thermal management systems (BTMS) are also gaining traction amongst the research community. Phase change material (PCM) based BTMS can be an attractive solution due to its high energy density and isothermal energy exchange. However, pure PCM has some drawbacks like low thermal conductivity, volume expansion, and leakage during phase change. A shape stable composite PCM (SSCPCM) resolves these drawbacks but is an expensive solution due to the high cost of conventional supporting materials. Biochar (BC) based SSCPCM developed in this study can be a cheap and sustainable solution for BTMS. The characterization studies are carried out for combination of pure PCM (Myristyl Alcohol) and biochar at various loadings of 6 %, 12 %, 18 %, and 24 % by weight, using Fourier transform infrared analysis (FT-IR), X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), particle size analysis (PSA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The shape stability studies reveal that the PCM with minimum of 24 % biochar (PCM-BC24) is shape stable. The thermal conductivities and effusivities of pure and shape-stable composite PCMs are also studied. Enhancement in thermal conductivity of PCM-BC24 is studied with the addition of multi-walled carbon nanotubes (MWCNT) in 0.5 % and 1 % concentrations. FT-IR and XRD results reveal that the interaction of biochar and MWCNT with pure PCM is purely physical and chemically stable. The degree of supercooling is reduced by 19.26 %, 23.49 %, and 25.17 % for PCM-BC24 with 1 % MWCNT compared to pure PCM at a heating rate of 5.0, 7.5, and 10 °C/min, respectively. Thermal conductivity and effusivity of PCM-BC24 with 1 % MWCNT are 458.72 % and 146.25 % higher than the pure PCM, respectively. The TGA results reveal that PCMs thermal stability is not adversely affected by the addition of BC & MWCNT. Furthermore, an ANFIS model is developed to predict the heat flow (mW/mg) values of PCM samples and found that the generalized bell-shaped input and linear output membership function is best suitable with a coefficient of determination (R2) of 0.971.

Effective thermal conductivity measurement of PCM composites during phase transition by using the transient plane source method

The thermal conductivity of phase change materials (PCMs) during phase transitions is of great significance to the evaluation of the thermal performance of energy-saving buildings. Based on the transient plane source method, a method for measuring the effective thermal conductivity of PCM composites was proposed, by which the variations in effective thermal conductivity with liquid fraction and temperature during phase transitions can be obtained. The correctness of the proposed method was verified through matched simulation experiments. Three kinds of PCM samples were prepared for experimental measurement on the basis of the direct incorporation method, and the volume proportions of decanoic acid to gypsum were set to 10%, 12%, and 15%. The phase transition regions and latent heat values of these PCM samples were measured using the differential scanning calorimetry technique. Results showed that the effective thermal conductivity of the PCM first increased and then decreased during the phase transition. As the fraction of PCM content increased, the phase transition regions gradually expanded, and the effective thermal conductivity changed more drastically. The good measurement repeatability demonstrated the feasibility of this method for measuring the effective thermal conductivity of composites during phase transition.

Metrology based on transient heat transfer theory for evaluating phase change materials with differential scanning calorimetry

Correct sensible heat curve (virtual baseline) during phase change is a must for accurate evaluation of latent heat. In this article, the dynamically moving interface (DMI) model for any phase change material (PCM) measured with a differential scanning calorimeter (DSC) is devised not only for elucidating the complex mechanisms of transient heat transfer in the five stages from solid phase to liquid phase, but also for constructing a correct sensible heat curve during phase change. We derive all the governing relations in exact or analytic form to describe the corresponding heat currents in the five stages of transient heat transfer together with the correct sensible heat curve during phase change inside any tiny PCM sample. The error budget analysis for the expanded uncertainty of specific latent heat about the tiny indium sample is detailed after the full assessment to specific latent heats of several PCMs.

Numerical investigation on improving the heat storage and transfer performance of ceramic /D-mannitol composite phase change materials by bionic graded pores and nanoparticle additives

To speed up the thermal response rate of the latent heat storage system, this research draws on the ideas of bionics and proposes two methods to enhance the performance of heat storage and heat transfer during phase change process. First, a biomimetic, double-gradient porous ceramic was applied to assemble phase change material (PCM) D-mannitol. The optimized gradient pore structure ensures that the composite possesses higher effective thermal conductivity, and better uniformity of phase interface evolution, with reasonable heat storage density. Numerical simulation predicts a 226 % increase in effective thermal conductivity comparing with the pure D-mannitol. Then, two kinds of carbon-based nanoparticles were added to further reinforce the heat transfer performance. Results found that graphite nanoparticles provide the most significant enhancement in the effective thermal conductivity of the composite material under the premise of ensuring a higher heat storage density. In conclusion, the effective thermal conductivity of the final composite achieves 3.33-fold increase due to the collaboration of the double gradient pore framework and the additive graphite nanoparticles. Accordingly, the overall heat transfer rate could be raised by 4.2 times, comparing with the pure PCM sample. This work demonstrates that the bidirectional gradient pore skeleton has significant advantages in heat storage and transfer over the single pore and unidirectional gradient pore.

Terahertz Nanoimaging and Nanospectroscopy of Chalcogenide Phase-Change Materials

Chalcogenide phase-change materials (PCMs) exhibit optical phonons at terahertz (THz) frequencies, which can be used for studying basic properties of the phase transition and which lead to a strong dielectric contrast that could be exploited for THz photonics applications. Here, we demonstrate that the phonons of PCMs can be studied by frequency-tunable THz scattering-type scanning near-field optical microscopy (s-SNOM). Specifically, we perform spectroscopic THz nanoimaging of a PCM sample comprising amorphous and crystalline phases. We observe phonon signatures, yielding strong s-SNOM signals and, most important, clear spectral differences between the amorphous and crystalline PCM, which allows for distinguishing the PCM phases with high confidence on the nanoscale. We also found that the spectral signature can be enhanced, regarding both signal strength and spectral contrast, by increasing the radius of the probing tip. From a general perspective, our results establish THz s-SNOM for nanoscale structural and chemical mapping based on local phonon spectroscopy.

Phase Change Materials and Their Benefits in ETICS

Phase change materials (PCMs) are materials with the ability of absorption of latent heat based on a phase change. PCMs are able to store and release a large amount of energy at certain temperatures melting or freezing. The aim of the research is to verify whether this phenomenon (material) can be used within an external thermal insulation composite system (ETICS). This is particularly the usage of PCMs in the base coat. The research is focused on two main areas. The first area concerns the water condensation on the surface of the ETICS and the associated phenomenon of algae attack. The second area concerns the warming of ETICSs with the use of dark color shades. Practical experiments showed a positive effect of PCMs on the heat-storage properties of the ETICS base coat. It was also experimentally verified that the PCM sample did not condense water vapor on the sample surface compared to the reference sample.

Experimental and numerical study of modified expanded graphite/hydrated salt phase change material for solar energy storage

Solar thermal storage system has strict requirements on the problems of hydrated salt phase change materials (PCMs) such as supercooling, phase separation, low thermal conductivity and cyclic stability. In addition, the long-term storage/release heat cycle of PCMs lead to the loss of crystallization water and shorten the service life. Herein, we demonstrate an improved model to predict the thermal conductivity of modified carbon-based matrix form-stable PCMs based on the relationship between porosity (ε) and modified expanded graphite (MEG) content. The proposed new model may predict the thermal conductivity of carbon-based matrix composites with relatively high precision. Furthermore, the hydrophilic and hydrophobic groups on the surface of EG sheet have been rearranged via titanate coupling agent, and then MEG is introduced as high thermal conductivity porous matrix. The modified aluminum potassium sulfate dodecahydrate (MAPSD) is used as the PCM. Five different EG contents of MEG/APSD composite PCM samples with the same density have been prepared by the method of “melting blend-solidification and form-stable”. In addition, 80 times storage/release heat experiments are carried out on the samples and the heat transfer process of the three-dimensional (3D) model is numerically simulated. It results show that the weight loss of the composites with MEG samples decreased by only 7.85%. The simulation results suggest that the temperature field and phase change interface vary uniformly. Moreover, the composites samples with MEG has the superior heat transfer performance and much higher heat storage/release rate. With sample density 900 kg/m3 and the MEG content of 20 wt%, the MEG/APSD composites has relatively good thermal performance and high cost performance. Therefore, it can be applied for industrial waste heat energy recovery applications, the solar domestic hot water system and mid-temperature thermal energy storage system.

Characterization of form-stable phase-change material for solar photovoltaic cooling

Solar PV panel cooling is essential to achieve maximum efficiency of PV modules. Phase-change material (PCM) is one of the prominent options to cool the panel and reduce the temperature, since PCMs have low thermal conductivity. Expanded graphite particles are used to enrich the structure and stability as well as to increase the thermal properties. In the present research work, polyethylene glycol (PEG) 1000 is used as a base material and expanded graphite for inclusive particle. A novel form-stable PEG1000/EG composite PCM mixture is prepared, using impregnation and dispersion method. Expanded graphite and PEG1000/EG sample phase compositions are investigated, using X-ray diffraction technique. No new peak is identified in the composite PCM sample. The surface morphology and structure of EG and PEG1000/EG are investigated, using scanning electron microscopy (SEM). Chemical stability analysis is done by Fourier-transform infrared spectroscopy. Thermal properties of the prepared composite PCMs are analysed by differential scanning calorimetry, thermogravimetric analysis (TGA) and KD2 pro analyser. Results show that addition of EG in various propositions (5%, 10% and 15%) enhances the thermal conductivity of PCM samples from 0.3654 to 1.7866 W mK−1, while melting point and latent heat of fusion of PCM samples are getting reduced. TGA thermographs are used to investigate the thermal stability of the composite PCM samples. TGA curves show that loss of mass happens above the operating temperature, and it is varied with different mass ratios of EG. Characterization of the prepared composite PCM samples is compared and found that PEG1000-85%/EG-15% is the best form-stable PCM, suitable for cooling the solar PV panel as well as to improve the electrical efficiency coupled with a decrease of temperature in the range of 35 °C to 40 °C.

Surface free energy analysis for stable supercooling of sodium thiosulfate pentahydrate with microcosmic-visualized methods

A novel method of microcosmic-visualizing was employed in investigating the supercooling stability of a phase change material (PCM). The supercooling behavior of the PCM under circumstances of different surface free energy was studied both dynamically and statically, and a theory analysis of how the surface free energy of the substrate affects the stable supercooling of PCM was carried out. The results showed that, the PCM droplets phase changed after a 20-cm free fall on substrate with larger surface free energy, while remained supercooled on substrate with lower surface free energy. The PCM in PTFE container initiated crystallization at −8.99 °C, withstood over 400% on hours at liquid state compared with the sample in glass container, while a large supercooling degree of over 57 °C was achieved. It indicated that containers made of materials with lower surface free energy could be considerably beneficial in stabilizing supercooling of PCMs, which could give a guide to the practical application on long-term energy storage of PCMs. What is more, the surface free energy theory also works on the air-liquid interface. The oil-covered PCM sample withstood a temperature of 5 °C for 10 months without crystalized in a refrigerator, showing great potential on supercooling stabilization.

Thermophysical characterization and comparison of PCMs using DSC and T-History experimental setup

In this paper, the thermal characterization of the chosen phase change materials (PCMs) was done using a conventional DSC analysis initially. However, the thermophysical properties of a very small amount of measured samples differ from those of the bulk measurements, including heterogeneous additives. Inconsistent results were observed in the DSC analysis, particularly for the inorganic PCM samples. Hence, a T-History experimental characterization for PCM investigation was carried out. The results obtained using DSC and fabricated T-History evaluation were compared and discussed in detail. The validated results showed that the T-History method of characterization is better compared to DSC analysis, especially, in the case of inorganic salt-hydrated PCMs due to its consistent and reasonable accuracy in measurements. The validated results also suggest that the RT28 and HS29 PCMs could be employed either as building integrated components or in separate energy storage modules in order to enhance passive cooling in future building construction.

Cost-effective device to characterize phase change materials

Phase change materials (PCMs) have been an emerging research topic for the last few years. However, the lack of detailed and precise information regarding their thermophysical properties is a major concern for researchers. The current paper presents a simple method to determine the specific heat (), thermal conductivity and melting temperature () of a PCM using a novel low-cost device. Compared to the conventional existing methods, such as differential scanning calorimetry (DSC), T-history, panel tests and simple thermal analyses, this new method grants the following features: simplicity and ease of manufacturing and the ability to measure the thermophysical properties of heterogeneous PCM samples without a reference material. Using this novel device, the thermophysical properties of sodium sulfate decahydrate were measured and good agreement is obtained with values collected from the open literature using other devices.

Microstructural design and thermal characterization of composite diatomite-vermiculite paraffin-based form-stable PCM for cementitious mortars

The thermal performance of cementitious mortars can be improved with the use of form-stable phase change materials (PCMs) consisting of a solid-liquid paraffin mix impregnating diatomite in a diatomite-vermiculite mineral system. The present study includes designing, developing, and characterizing a novel PCM based on a combination of diatomite and calcined, expanded vermiculite, and an optimized paraffin system consisting of a mixture of solid paraffin and naphthenic oil. The diatomite is expected to store the paraffin and wrap the vermiculite particles, increasing the amount of stagnant air. X-ray fluorescence is used to assess the chemical composition of the diatomite and vermiculite. Differential scanning calorimetry (DSC) and Fourier transform infraredspectroscopy (FT-IR) measurements are used to characterize the paraffin mixes aimed at optimum performance for residential and commercial buildings. PCM samples made from paraffin and diatomite (PCM1), vermiculite (PCM2), and a mix of diatomite and vermiculite (PCM(1 + 2)) are characterized by scanning electron microscopy and FT-IR. Cement mortars are prepared in the volume ratios of 1:4 cement:sand (reference mortar), 1:4:1 cement:sand:diatomite, 1:4:1 cement:sand:vermiculite, and 1:4:1 cement:sand:PCM. The thermal properties of these materials are evaluated. The DSC results of the paraffin mixtures indicate that 20% naphthenic oil is the ideal concentration in the paraffin mixture to optimize the solid-liquid temperature (57.45 °C) and latent heat for building applications (159.42 J/g). All PCMs have reduced thermal conductivities compared to the reference mortar (1.47 W/m·K), with the lowest being associated with the PCM(1 + 2) sample (0.82 W/m·K). The PCM(1 + 2) sample also presents an improved combination of diffusivity, volumetric heat capacity, and thermal resistivity. The simultaneous addition of vermiculite and diatomite to the PCM composition improves the thermal properties, while maintaining the basic characteristics such as chemical and structural stability.