Graphite Nanoplatelets Research Articles

Synthesis and characterization of enhanced conductivity acrylonitrile‐butadiene‐styrene based composites suitable for fused filament fabrication

AbstractThis work reports on the design and preparation of composites based on a high impact acrylonitrile‐butadiene‐styrene polymer matrix filled either with aluminium microparticles or graphite nanoplatelets. A surfactant and tin were further added to improve the dispersion and adhesion of the fillers in an attempt to improve the conductivity of the materials. The composites were obtained via twin‐screw extrusion and their performance in fused filament fabrication was successfully tested and compared with other conventional ways of manufacturing plastic parts. Furthermore, the composites were characterized in terms of structure, composition, and thermal/electrical properties. Scanning electron and atomic force microscopies, as well as energy dispersive X‐ray spectrometry, allowed to confirm the presence of added particles and their dispersion within the polymer matrix. Differential scanning calorimetry and conductivity measurements completed the study and revealed enhanced conductivity in the composites as well as a huge decrease in electrical resistivity of the graphite nanoplatelets‐filled nanocomposites, thus resulting in semiconductor materials.

Multifunctional Porous Films Based on Polylactic Acid/Polycaprolactone Blend and Graphite Nanoplateles

The aim of this work was to develop polylactic acid (PLA) based porous films with suitable properties to be used as sensors and absorbers. The formulation was designed to make the films porous, ductile, and conductive and characterized by surface functionalization. The phase inversion method was used to impart porosity to the polymer film. Polycaprolactone (PCL) and graphite, in the form of graphite nanoplatelets (GNP), were added to increase the elongation at break of the polymer matrix and to make the systems conductive, respectively. To optimize the formulation, the effect of polymer concentration in the initial solution on the viscosity, porosity, and morphology was investigated. FE-SEM measurements showed that the phase inversion method allowed limiting the aggregation of both PCL domains and GNP, whose flakes were found to adhere well to the polymer matrix and to have a nucleating effect, as evidenced by DSC measurements. A simple aminolysis reaction was performed using a diamine, i.e., ethylenediamine, to generate surface amino functionalities. In particular, the extent of functionalization as a function of reaction time was investigated by using FT-IR spectroscopic measurements. By combining these results with the stability of the polymer film, which tends to lose its structural integrity due to the erosion caused by the aminolysis reaction, the optimal contact time with the aqueous solution of ethylenediamine was found. Mechanical measurements showed that the presence of PCL increased the elongation at break of films by 3 times compared to neat PLA. Finally, the films developed starting from the optimized formulations based on PLA/PCL blends, containing GNP and surface functionalized, proved to be effective electrodes for the voltammetric determination of ascorbic acid. In addition, it is of utmost relevance that both amino functionalization and GNP conferred high capacity to the films to retain fluorescein, a model dye.

Crystallization and Melting Behavior of UHMWPE Composites Filled by Different Carbon Materials

In order to understand the effect of different carbon materials on the crystallization and melting behavior of ultrahigh molecular weight polyethylene (UHMWPE), UHMWPE composites were prepared by different carbon materials through solution mixing in this paper. UHMWPE was oxidized to improve the interfacial interaction between UHMWPE and carbon materials. The UHMWPE composites and oxidized UHMWPE composites were prepared using granular graphite particle (GP), graphite nanoplatelets (GNP), and flaky graphene oxide (GO) as fillers. The effect of the type and the content of carbon materials and the oxidization of UHMWPE on crystallization and melting temperatures, crystallinity, and crystal form of UHMWPE and oxidized UHMWPE composites was investigated by differential scanning calorimetry, X-ray diffraction, scanning electron microscope, X-ray photoelectron spectrum, and Fourier transform infrared spectroscopy. The results indicated that there are coexistence of the heterogeneous nucleation and the hindering effect of crystal growth by carbon materials for UHMWPE crystallization. The different influence of carbon materials on the crystallization and melting behavior of UHMWPE was discussed by the heterogeneous nucleation of carbon materials and the restriction of the macromolecular chain motion of UHMWPE by carbon materials.

Experimental study on the hybrid carbon based phase change materials for thermal management performance of lithium‐ion battery module

The widespread use of lithium-ion battery (LIB) urgently needs a thermal management system with excellent performance to manage it. Phase change material (PCM) has been adapted for thermal management of LIB because it can absorb a mass of energy without additional power. In this research, the thermal conductivity of PCM is improved, and its cooling effect is detected. The addition of hybrid thermal conductivity filler is beneficial to its uniform dispersion and improvement of aggregation phenomenon on the surface of materials. The results show that the PCM with hybrid filler has better thermal conductivity, which is increased by 264%. In different proportions of composite materials, the composite phase change material with 1% few-layer graphene and 2% graphite nanoplatelet can play a better cooling effect in LIB module thermal management, and it respectively decreases the maximal temperature of the battery module by 7.17°C, 10.52°C, and 16.33°C at 1C, 2C, and 3C conditions, and the maximal temperature difference does not exceed the safety range of 5°C.

Transforming Conventional Construction Binders and Grouts into High-Performance Nanocarbon Binders and Grouts for Today’s Constructions

The transformation of conventional binder and grout into high-performance nanocarbon binder and grout was evaluated in this investigation. The high-performance nanocarbon grout consisted of grey cement, white cement, lime, gypsum, sand, water, and graphite nanoplatelet (GNP), while conventional mortar is prepared with water, binder, and fine aggregate. The investigated properties included unconfined compressive strength (UCS), bending strength, ultrasound pulse analysis (UPA), and Schmidt surface hardness. The results indicated that the inclusion of nanocarbon led to an increase in the initial and long-term strengths by 14% and 23%, respectively. The same trend was observed in the nanocarbon binder mortars with white cement, lime, and gypsum in terms of the UCS, bending strength, UPA, and Schmidt surface hardness. The incorporation of nanocarbon into ordinary cement produced a high-performance nanocarbon binder mortar, which increased the strength to 42.5 N, in comparison to the 32.5 N of the ordinary cement, at 28 days.

Influence of Carbon Micro- and Nano-Fillers on the Viscoelastic Properties of Polyethylene Terephthalate.

In this research study, three carbon fillers of varying dimensionality in the form of graphite (3D), graphite nano-platelets (2D), and multiwall carbon nanotubes (1D) were incorporated into a matrix of poly (ethylene terephthalate), forming carbon-reinforced polymer composites. Melt compounding was followed by compression moulding and then a quenching process for some of the samples to inhibit crystallization. The samples were analysed using dynamic mechanical thermal analysis (DMTA) and scanning electron microscopy (SEM), considering the dimensionality and loading of the carbon fillers. The dynamic mechanical analysis revealed a similar decline of storage moduli for all composites during the glassy to rubbery transition. However, storage moduli values at room temperature increased with higher loading of nano-fillers but only to a certain level; followed by a reduction attributed to the formation of agglomerates of nanotubes and/or rolled up of nano-platelets, as observed by SEM. Much greater reinforcement was observed for the carbon nanotubes compared to the graphite and or the graphite nano-platelets. The quenched PET samples showed significant changes in their dynamic mechanical properties due to both filler addition and to cold crystallization during the DMTA heating cycle. The magnitude of changes due to filler dimensionality was found to follow the order: 1D > 2D > 3D, this carbon filler with lower dimensionality have a more significant effect on the viscoelastic properties of polymer composite materials.

Soft Composites Filled with Iron Oxide and Graphite Nanoplatelets under Static and Cyclic Strain for Different Industrial Applications.

Simultaneously exhibiting both a magnetic response and piezoelectric energy harvesting in magneto-rheological elastomers (MREs) is a win–win situation in a soft (hardness below 65) composite-based device. In the present work, composites based on iron oxide (Fe2O3) were prepared and exhibited a magnetic response; other composites based on the electrically conductive reinforcing nanofiller, graphite nanoplatelets (GNP), were also prepared and exhibited energy generation. A piezoelectric energy-harvesting device based on composites exhibited an impressive voltage of ~10 V and demonstrated a high durability of 0.5 million cycles. These nanofillers were added in room temperature vulcanized silicone rubber (RTV-SR) and their magnetic response and piezoelectric energy generation were studied both in single and hybrid form. The hybrid composite consisted of 10 per hundred parts of rubber (phr) of Fe2O3 and 10 phr of GNP. The experimental data show that the compressive modulus of the composites was 1.71 MPa (virgin), 2.73 (GNP), 2.65 MPa (Fe2O3), and 3.54 MPa (hybrid). Similarly, the fracture strain of the composites was 89% (virgin), 109% (GNP), 105% (Fe2O3), 133% (hybrid). Moreover, cyclic multi-hysteresis tests show that the hybrid composites exhibiting higher mechanical properties had the shortcoming of showing higher dissipation losses. In the end, this work demonstrates a rubber composite that provides an energy-harvesting device with an impressive voltage, high durability, and MREs with high magnetic sensitivity.

Stretchable piezo‐electric energy harvesting device with high durability using carbon nanomaterials with different structure and their synergism with molybdenum disulfide

AbstractRecently, polymer composite based stretchable piezo‐electric energy harvesting device have attracted a great attention in composite industry. At laboratory scale, these piezo‐electric devices have ability to generate impressive voltage (~8 V) with high durability (>0.5 million cycles) depending upon nature of the material used in electrode and substrate. In this work, composites were prepared by mixing room temperature vulcanized silicone rubber (RTV‐SR) and nanofillers such as multi‐wall carbon nanotube (MWCNT), nano‐carbon black (nano‐CB), graphite nanoplatelets (GNP), in single or hybrid with molybdenum disulfide (MoS2). These fillers differentiate on basis of their structure ranging from 0‐dimensional (0‐D) to 3‐D. The effects of these structures on mechanical properties are demonstrated. Among them, MWCNT with 1‐D, tube shape morphology, high aspect ratio of 65 shows dominating mechanical properties even at 2 per hundred parts of rubber (phr) loading followed by nano‐CB, GNP at 8 phr and MoS2 at 2 phr. A synergistic effect in mechanical properties between hybrid fillers such as tensile strength and fracture strain was demonstrated. In the end, the composites were tested for piezo‐electric energy generation and durability cycles for the best candidate such as MWCNT was tested for up to 0.5 million cycles was demonstrated.

Epoxy composites filled with graphite nanoplatelets modified by FeNi nanoparticles: Structure and microwave properties

This paper is concerned with investigation of the structure and microwave properties of epoxy resin composites with graphite nanoplatelets (GNPs) decorated by FeNi nanoparticles prepared by the salt impregnation. It was confirmed by SEM and XRD that the method gives nanopowder where the metal componentFe20Ni80 is in the form of nanoparticles (20–40 nm in diameter) which are distributed over the surface and edges of the GNPs. The graphite phase in the investigated nanopowder is predominant, and Fe20Ni80 component mainly contains fcc FeNi3. Measurements of magnetic properties confirmed that Fe20Ni80 are small, randomly oriented assembly of spherical (or close to spherical) particles on the GNPs surface. The saturation magnetization of GNP-Fe20Ni80 particles is 25 emu/g and it weakly depends on temperature. Electrical resistivity measurements have shown that decoration of GNPs leads to essential increase of conductivities well as improvements EMR absorption properties in high frequency range (26–60 GHz) in (GNP-Fe20Ni80)/with in compression GNP/epoxy under the same volume content of 1.45 vol.%(GNP-Fe20Ni80)/L285 composites demonstrated superior broadband absorption properties with microwave absorption efficiency higher than 97% in the whole investigated frequency region.The effective absorption bandwidths are as high as 12.2 GHz were observed at the frequency range 41.9 and 53.2 GHz and 13.3 GHz at the frequency range 51.1 GHz–64.4 for this composite. It is assumed that decoration of GNPs surface by nanoscale Fe20Ni80 particles leads to a formation of the multiple dielectric and magnetic loss mechanisms, such as interfacial polarization, dipole polarization, space-charge polarization, eddy current loss, Debye dipolar relaxation, natural resonance and exchange resonance, which improve the microwave absorption properties of the investigated composites.

Effect of expanded graphite on the thermal conductivity of sodium sulfate decahydrate (Na2SO4·10H2O) phase change composites

Inorganic salt hydrate phase change materials (PCMs) are of interest for near-room temperature thermal energy storage (TES) systems, but their low thermal conductivity, ~ 0.5 W/m-K, limits their performance. In this work, we report the thermal conductivity and bulk density of composites containing sodium sulfate decahydrate (SSD) Na2SO4·10H2O with three types of graphite: expanded graphite (EG), milled EG (MG), and graphite nanoplatelets (GnP). The effect of these thermophysical properties on TES performance is presented. The composites were made using a readily scalable one-pot synthesis procedure with graphite received as-is. A 583% increase in thermal conductivity (4.1 W/m-K) was achieved with 25 wt% EG. However, as EG fraction increases, bulk density decreases and thermal conductivity plateaus. This ultimately resulted in lower thermal performance at higher EG fractions despite higher thermal conductivity. This highlights the tradeoff between PCM composite properties and performance, and why thermal conductivity is insufficient to describe PCM thermal performance. GnP is added to EG-SSD to increase bulk density and energy storage density, but these density improvements do not offset lower thermal conductivity and thus thermal performance declined. Similarly, MG-SSD composites had a higher bulk density and energy storage density, but lower thermal conductivity and thermal performance than EG-SSD composites at similar compositions. Atomistic molecular dynamics simulations were performed to understand the structure-property relationship of graphite-SSD interfaces. The simulations support the hypothesis that atomic level contact resistance between graphite and SSD increases thermal resistance at the interfaces resulting in effectively lower bulk thermal conductivity in MG-SSD compared to EG-SSD.

Polyamide composites containing graphene nanoplatelets produced via thermomechanical exfoliation

Composites are prepared via melt blending polyamide (PA) with flake graphite and graphene nanoplatelets (GNPs); the latter are produced via thermomechanical exfoliation (TME) processing of graphite. The electrical percolation threshold of the GNP composites is around 30 wt%, with a peak conductivity of 10−1 S/m at 40 wt% loading, whereas the graphite composites do not show typical percolation behaviour. In the presence of the GNPs and graphite the thermal conductivity of the composites increases, with the graphite composites exhibiting a conductivity of 2.74 W/mK at 40 wt% loading. Both graphite and GNP containing composites exhibit an increase in flexural modulus of up to 140% with respect to the neat matrix, at 40 wt% filler loading; the impact strength of the PA/GNP composites increased by up to 200% at 20 wt% loading, compared to the neat PA. Ternary composite blends containing PA with a maleated ethylene-octene copolymer (EOC), and 30 wt% GNPs result in further improvements of the toughness of the composite, with a peak impact strength of 150 J/m (650% increase over the neat PA).

Effect of graphite nanoplatelets surface area on mechanical properties of room‐temperature vulcanized silicone rubber nanocomposites

AbstractGraphite nanoplatelet (GNP) flakes have attracted attention for the production of polymeric nanocomposites owing to their excellent mechanical, thermal, and electrical properties. In this work, the impact of three grades of GNP flakes with different surface area on the properties of room‐temperature vulcanized silicone rubber (RTV‐SR) nanocomposites was analyzed. The surface area was determined by the Brunauer–Emmett–Teller (BET) method using N2 adsorption isotherms. The BET analysis showed that the GNP grades had BET surface areas of 422, 295, and 123 m2/g. Nanocomposites based on the three grades of GNP flakes with different surface area and RTV‐SR were prepared using the solution‐casting process. Mechanical analysis of the samples showed that the compressive modulus increased from 3.8 MPa (0 phr) to 4.4, 4.34, and 5.26 MPa at the filler loading of 15 phr of GNPs with 422, 295, and 123 m2/g, respectively. In addition, the fracture strain of the nanocomposites increased from 116% (0 phr) to 228%, 198%, and 151% in the samples with 422, 295, and 123 m2/g, respectively, at the filler loading of 15 phr GNP.

Experimental and Theoretical Analysis of Mechanical Properties of Graphite/Polyethylene Terephthalate Nanocomposites.

In this work, graphite nanoplatelets (GNP) were incorporated into poly (ethylene terephthalate) (PET) matrix to prepare PET-GNP nanocomposites using a melt compounding followed by compression moulding and then quenching process. Both static and dynamic mechanical properties of these quenched materials were characterized as a function of GNP contents using dynamic mechanical thermal analysis (DMTA) and tensile machine, respectively. The results demonstrated that the addition of GNP improved the stiffness of PET significantly. Additionally, the maximum increase in the storage modulus of 72% at 6 wt.% GNP. The incorporation of GNP beyond 6 wt.% into PET decreases the storage moduli, but they remain higher than pure PET. The observed reduction could be due to agglomeration, resulting in poorer dispersion and distribution of higher levels of GNP into the PET matrix. In contrast to the results for moduli, tensile strength and elongations at break reduce with increasing the GNP content. For example, tensile strength reduced from ∼46 MPa (neat PET) to ∼39 MPa (−15%) for the nanocomposites containing 2 wt.% GNP. This reduction is accompanied by a decline in elongation at break from ∼6.3 (neat PET) to ∼3.4 (−46%) for the same nanocomposites. Such reductions are followed by a gradual decrease in upon further addition of GNP. These reductions indicate that increasing GNP loadings, results in brittleness in nanocomposites. In addition, it was found that quenched PET and composite samples were not fully crystallized after processing and therefore (cold) crystallized during the first heating cycle DMTA, as indicated by a rise in storage moduli above the glass transition temperature during the DMTA first heat. Furthermore, mathematical models based on non-linear theories are developed to capture the experimental data. For this, a set of mechanical stress-strain data is used for model parameters’ identification. Another set of data is used for the model validation that demonstrates good agreements with the experimental study.

Ultra-high thermal conductive epoxy-based copper/graphite nanoplatelets materials for heat management application

The combination of polymer and copper (Cu) is common and economical in thermal interface materials (TIMs). However, it is remains challenging for traditional polymer-Cu composites to obtain high thermal conductivity (TC) (>10 Wm−1K−1) due to the poor filler connection. Herein, we introduced small amount of 2D-structured graphite nanoplatelets (GNPs) into bulk Cu flakes/epoxy composites via the thermal molding method. Surprisingly, we found an extraordinary synergistic effect, revealed highly thermally conductive percolation network through intercalation of GNPs between Cu flakes. A high isotropic TC 13.4 Wm−1K−1 is achieved with 5 wt% GNPs and 80 wt% Cu flakes, which superior than most reported Cu/polymer composites. Although a high electrical conductivity of 34000 S/m was obtained, a phonon-dominated thermal transport mechanism was observed due to the existence of GNPs bridges between Cu flakes. The resulting composite also demonstrate excellent thermal management ability, superior acid resistance and good mechanical property, which offers a promising composite in thermal management application.

Polymer nanocomposites based on graphite nanoplatelets (GNPs): a review on thermal-electrical conductivity, mechanical and barrier properties

Polymer nanocomposites based on graphite nanoplatelets (GNPs): a review on thermal-electrical conductivity, mechanical and barrier properties

Dynamic dielectric properties of isotactic polypropylene-g-maleic anhydride crosslinked by capped-end polyether diamine and filled with native or functionalized nano-graphite particles

Dynamic dielectric properties of an isotactic polypropylene matrix grafted with maleic anhydride (CA 100) and then crosslinked by polyether amine molecules and reinforced with different weight percentages of graphite nanoplatelets (GNPs), KNG180, were studied for the first time and compared to those obtained by DMA (Dynamic Mechanical Analysis). The main objective of this work was to investigate the reinforcement effect of GNPs focusing on the GNPs/matrix interfacial adhesion using dynamic dielectric relaxation spectroscopy in the frequency range from 0.1 Hz to 1 MHz and temperature range from 20 to 140 °C. The obtained interfacial polarization increments \({\Delta \varepsilon }_{\mathrm{MWS}}\) from MWS (Maxwell Wagners Sillars) relaxation showed a threshold value of 3% in weight of KNG180. This analysis suggests that interfacial compatibility between matrix and fillers in the case of nanocomposite KNG180 3 wt% is higher than those of other nanocomposites. A new plasma treatment was used to modify graphite nano-fillers to produce different types of nanocomposites. The 5 wt% plasma treated graphite nanocomposite shows a good dispersion of the nano-fillers but also a high value of \({\Delta \varepsilon }_{\mathrm{MWS}}\), which is an indication of high graphite/graphite interaction. This evolution could show that this material can be close to the formation of an electrical percolation network.

Investigations of electrical conductivity and damage healing of graphite nano-platelet (GNP)-taconite modified asphalt materials

This proof of concept study investigates the electrical conductivity and damage healing capability of graphite nano-platelet (GNP)-taconite modified asphalt materials. Different combinations of GNP and taconite concentrate are added to an asphalt binder, and the electrical conductivity is measured by a four-probe conductivity test. Similar combinations of GNP and taconite concentrate are used to prepare asphalt mixture specimens to investigate the healing capability. Semi-circular bend (SCB) tests are performed at low temperature in two stages. First, specimens are damaged by loading them into the post peak region using crack mouth opening displacement (CMOD) control. The specimens are then subjected to microwave heating, after which they reloaded to measure the strength capacity. It is shown that the addition of GNP and taconite concentrate could effectively enhance the electrical conductivity of asphalt binders. The results also demonstrate the influence of humidity on the measured conductivity. The SCB fracture tests show that the application of microwave energy to the pre-damaged specimens restores their original load capacity, a clear indication of damage healing.

High-performance cellulose nanofiber-derived composite films for efficient thermal management of flexible electronic devices

Heat dissipation material shows great promise as a thermal management tool for high-power-density flexible electronic devices. However, conventional materials (e.g., metals, carbon materials, and polymer-based composites) typically are difficult to meet the required thermal conductivity and mechanical properties at the same time. Additionally, performance enhancement of the heat-dissipating materials generally requires complicated procedures with high manufacturing costs. Here, we design a facile and robust synthesis procedure by processing all-natural cellulose nanofiber (CNF) and graphite nanoplatelet (GNP) to generate a high-performance sustainable thermal management material. The obtained CNF-GNP composite film contains extensive heat transfer highways and highly aligned hydrogen bonds in its brick-and-mortar microstructure. Consequently, the CNF-GNP-40 film exhibits high thermal conductivity (21.42 W m−1 K−1), high strength (115.8 MPa), high toughness (4.19 MJ m−3), low density (∼1.3 g cm−3), and super flexibility simultaneously. Besides, the composite film possesses excellent stability after multiple cycles of mechanical deformation and exposure to various environments. Taking the thermal management of wearable devices as an example, experimental results reveal that applying the composite film can effectively reduce the temperature of both device and skin by about 8 °C on average, making it a prospective thermal management material for flexible electronic devices.

For the improvement of mechanical and microstructural properties of UHPC with fiber alignment using carbon nanotube and graphite nanoplatelet

This paper investigates the influence of carbon nanotube (CNT) and graphite nanoplatelet (GNP) on the microstructure and mechanical characteristics of UHPC with steel fiber alignment. The content of CNT and GNP ranged from 0 to 0.3%, by mass of binder. Predominant fiber alignment was manipulated using a flow-induced casting method during UHPC placement. Experiment results indicated that the increase of CNT and GNP content from 0 to 0.3% led to 15%, 40%, and 50% improvement in compressive strength, flexural strength, and T150 (dissipated energy) of UHPC, respectively. Fiber alignment was shown to increase flexural strength and T150 by 30% and 35%, respectively, compared to UHPC with random finer orientation. Moreover, the synergy of nanomaterial and fiber alignment can lead to a maximum enhancement of 80% and 90% in flexural strength and T150, respectively. Microstructural analysis indicated that CNT and GNP can enhance cement hydration and enable the bridging of cracks at nano or microscale. Moreover, the use of CNT and GNP reduced the porosities of fiber-matrix interface from 6%-12.5% to 4%–7% and UHPC matrix from 5.5% to 4%. This consequently contributed to the significant improvement in mechanical properties of UHPC.

Largely improved thermal conductivity and flame resistance of phase change materials based on three-dimensional melamine foam/phosphorous cellulose/graphite nanoplatelets network with multiple energy transition abilities

Solid-liquid organic phase change materials (OPCMs) appeal tremendous attention in thermal energy storage and management for their various merits. However, low thermal conductivity (TC), inferior encapsulation ability and fatal fire hazard significantly impair the application accessibility of the OPCMs. This work designed and fabricated a new three-dimensional (3D) melamine foam (MF)/phosphorous cellulose (PC)/graphite nanoplatelets (GNPs) (MF/PC/GNP) network to compound with the OPCMs with all these problems simultaneously sovled. With the assistance of MF-modified ice-templated method and PC, the GNPs form a dense network to efficiently enhance the TC of the composite PCMs to 1.08 W/(m·K) at a relatively low content of 2.39 wt%, while the electric conductivity (EC) reaches 0.690 S/cm. In addition, the peak of heat release rate (PHRR) of the composite PCMs decreases from 725.62 kW/m2 of pure PEG to 624.04 kW/m2. Besides, the MF/PC/GNP network also endows the composite PCMs with multiple energy-transition aiblities.