•The article reviews spin crossover polymer composite materials.•Recent development in spin crossover polymers.•Elaboration of soft materials based on spin crossover compounds.
We review the ...synthesis, properties and applications of spin crossover polymer composites, polymers and some related ‘soft’ materials. These materials have received recently much attention because they provide an efficient way for the processing of spin crossover complexes in various shapes at various size scales and can give rise also to unique physical properties. First, we discuss in detail the state of the art of the elaboration of spin crossover polymer composites, using either inorganic complex precursors in solution or preformed spin crossover powder. A particular attention is paid on the influence of the polymer matrix on the spin crossover properties and on the use of ‘active’ polymers for development of synergies between the properties of the matrix and the load. Polymer composite devices for applications in the fields of artificial muscles, energy harvesting and thermochromic sensors are also highlighted. Then, more recent works, in which organic polymeric chains are used as ligands for the transition metal ions are presented. Finally, we overview various related ‘soft’ spin crossover compounds including spin crossover dendrimers, gels, liquid crystals and Langmuir Blodgett films with particular emphasis on compounds with supramolecular interactions of alkyl chains.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Spin crossover particles of formula Fe{(Htrz)2(trz)}0.9(NH2‐trz)0.3(BF4)1.1 and average size of 20 nm ± 8 nm are homogeneously dispersed in poly(vinylidene fluoride‐co‐trifluoro‐ethylene), ...P(VDF‐TrFE), and poly(vinylidene fluoride) (PVDF) matrices to form macroscopic (cm‐scale), freestanding, and flexible nanocomposite materials. The composites exhibit concomitant thermal expansion and discharge current peaks on cycling around the spin transition temperatures, i.e., new “product properties” resulting from the synergy between the particles and the matrix. Poling the P(VDF‐TrFE) (70–30 mol%) samples loaded with 25 wt% of particles in 18 MV m−1 electric field results in a piezoelectric coefficient d33 = −3.3 pC N−1. The poled samples display substantially amplified discharges and altered spin transition properties. Analysis of mechanical and dielectric properties reveals that both strain (1%) and permittivity (40%) changes in the composite accompany the spin transition in the particles, giving direct evidence for strong electromechanical couplings between the components. These results provide a novel route for the deployment of molecular spin crossover materials as actuators in artificial muscles and generators in thermal energy harvesting devices.
P(VDFTrFE) and poly(vinylidene fluoride) (PVDF) composites of spin transition nanoparticles are synthesized to obtain flexible, freestanding, macroscopic objects displaying original electromechanical properties. The synergy between the components leads to concomitant thermal expansion and electrical discharge peaks at the spin transition providing scope for the deployment of spin crossover materials as actuators in artificial muscles and generators in thermal‐energy‐harvesting devices.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The thermally induced spin‐crossover (SCO) phenomenon in transition metal complexes is an entropy‐driven process, which has been extensively studied through calorimetric methods. Yet, the excess heat ...capacity associated with the molecular spin‐state switching has never been explored for practical applications. Herein, the thermal damping effect of an SCO film is experimentally assessed by monitoring the transient heating response of SCO‐coated metallic microwires, Joule‐heated by current pulses. A damping of the wire temperature, up to 10%, is evidenced on a time scale of tens of microseconds due to the spin‐state switching of the molecular film. Fast heat‐charging dynamics and negligible fatigability are demonstrated, which, together with the solid‐solid nature of the spin transition, appear as promising features for achieving thermal energy management applications in functional devices.
Thermal management of metallic Joule‐heated microwires is experimentally demonstrated using a molecular spin‐crossover film as a thermal damper system. A transient attenuation of the wire temperature, up to 10%, is evidenced due to the excess heat capacity associated with the spin‐state switching, providing a first milestone toward the assessment of the potential of spin‐transition materials for thermal management applications.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Thin films of the molecular spin‐crossover complex Fe(HB(1,2,4‐triazol‐1‐yl)3)2 undergo spin transition above room temperature, which can be exploited in sensors, actuators, and information ...processing devices. Variable temperature viscoelastic mapping of the films by atomic force microscopy reveals a pronounced decrease of the elastic modulus when going from the low spin (5.2 ± 0.4 GPa) to the high spin (3.6 ± 0.2 GPa) state, which is also accompanied by increasing energy dissipation. This technique allows imaging, with high spatial resolution, of the formation of high spin puddles around film defects, which is ascribed to local strain relaxation. On the other hand, no clustering process due to cooperative phenomena was observed. This experimental approach sets the stage for the investigation of spin transition at the nanoscale, including phase nucleation and evolution as well as local strain effects.
A spatially resolved investigation of the spin transition in thin films by atomic force microscopy enables detection of an ≈30% variation of the elastic modulus between the two spin states. Film defects are shown to stabilize the high spin state locally due to strain relaxation. This visualization of local spin‐crossover properties has particular relevance for applications in nanoscale devices.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Molecular spin‐crossover (SCO) complexes are phase‐change materials that develop large spontaneous strains across the thermally induced phase transition, which can be advantageously used in designing ...soft actuators. Herein, a bilayer bending cantilever, made of thermoplastic polyurethane (TPU) with embedded Fe(NH2trz)3(SO4) SCO particles (25 wt%), is presented. The proposed actuator is fabricated by blade casting an SCO@TPU layer on a conducting Ag@TPU film to convert electrothermal input into mechanical response. Experiments are conducted to characterize the curvature of bilayer beams, which is then further analyzed using the Euler–Bernoulli beam theory. The beam curvature change, free transformation strain, and effective work density associated with the SCO are 0.11 mm−1, 1.6%, and 1.25 mJ cm−3, respectively. Further, the open‐ and closed‐loop response of the actuator is investigated using a custom‐built setup. The open‐loop identification suggests that the actuator gain increases monotonously when the control current increases. This natural adaptive character can explain the drastically diminished response time in closed‐loop proportional–integral–derivative control experiments (2–3 s). Finally, tracking experiments are carried out to evaluate the robustness of the actuator with and without payloads. The results for 30 240 endurance cycles reveal a mean positioning error of 0.8%.
Bilayer bending cantilevers, made of thermoplastic polyurethane (TPU) with embedded Fe(NH2trz)3(SO4) spin‐crossover (SCO) particles and silver flakes, are built to convert electrothermal input into mechanical response. The beam curvature change and response time obtained in closed‐loop proportional–integral–derivative control experiments and the robustness of the actuators demonstrate the potential of the proposed actuator for shape control applications, such as soft morphing.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In this work, we propose shape-morphing electro-thermal actuators, which can realize various pre-programmed movements, such as large-strain bending or helical curling. The bimorph actuators consist ...of an electrically conducting polymer composite layer, used for Joule heating, and a spin-crossover polymer composite layer, which affords for a large thermal strain. By using needle-shaped spin crossover particles and adjusting their orientation with respect to the two axes of the T-shaped actuator, the deformation of each device can be encoded in a predictable manner. Based on mechanical property measurements and associated finite element analysis we show that the observed movements are governed primarily by the anisotropy of the transformation strain generated by the spin crossover particles. We envision that this type of self-folding structures can be advantageously used in remote and small-scale mechanical applications.
•Design of bimorph film based on spin crossover polymer composite.•Mechanical measurements revealed the anisotropy of the transformation strain.•Shape-morphing electro-thermal actuators were build.•Finite Element Analysis (FEA) simulations can predict the thermomechanical properties.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Closed-loop control by means of a simple PID makes possible to diminish the open-loop response time by a factor of 10.•The mean response time in closed-loop equal to about 1.5 s, when no load is ...embedded.•Load robustness is emphasized with similar response time for loads up to 350.•Loading corresponds to ca. 5 times the own weight of the actuator.•Very satisfying sine wave tracking is observed for a typical sine wave period of 20 s.
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Bending actuators represent a popular class of artificial muscles for which efficient closed-loop control is a remarkable challenge, due to the complexity of physico-chemical phenomena occurring during dynamic contraction. In this study, we investigate an electro-thermally actuated, bilayer bending actuator based on spin crossover molecules. While these artificial muscles can open-loop contract in several tens of seconds, we show that a simple closed-loop PID-control is able to reduce the response time to ca. 1.5 s, even with loads up to 5 times the actuator weight (i.e. a maximal load of about 343 mg). We demonstrate also satisfactory sine wave tracking performance. The relevance of this linear control approach applied to a nonlinear actuator is the consequence of the high sensibility of the actuator to the current, responsible for the Joule effect. Notably, the actuator roughly behaves like a second-order linear system, whose time “constants” decrease with current. Consequently, the natural speed improvement peculiar to the PID-controller is further amplified by this nonlinear effect, without any loss in stability of the closed-loop system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•The volume change of spin crossover complex is exploited to generate motion.•Spin crossover complexes bilayer actuators are fabricated by a blade casting technique.•Both open- and ...closed-loop controls of the actuators were investigated.•Speed and amplitude of the actuator bending movement, on heating/cooling, depend on spin crossover particle filler concentrations.
Macroscopic, soft bilayer spin crossover@polymer actuators were fabricated by a blade casting technique using Fe(NH2trz)3SO4 spin crossover particles embedded in a P(VDF-TrFE) matrix. Incorporation of silver flakes in the second layer allowed for electro-thermal actuation via Joule heating. The speed and amplitude of the actuator bending movement, resulting from the expansion/contraction of the spin crossover particles on heating/cooling, were tested for different particle concentrations (15, 25, 33 and 50 wt%) under both open- and closed-loop controls. The capability of the actuators to embark charge and to follow a sinusoidal position was also investigated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We used a spray-coating process to cover silicon microcantilevers with ca. 33 wt% Fe(Htrz)2(trz)(BF4)@P(VDF70-TrFE30) nanocomposite thin films of 1500 nm thickness. The bilayer cantilevers were then ...used to investigate the thermomechanical properties of the composites through a combined static and dynamic flexural analysis. The out-of-plane flexural resonance frequencies were used to assess the Young’s modulus of the spray-coated films (3.2 GPa). Then, the quasi-static flexural bending data allowed us to extract the actuation strain (1.3%) and an actuation stress (7.7 MPa) associated with the spin transition in the composite.
A series of spin-crossover (SCO) coordination nanoparticles (ca. 60 nm) with the general formulae Fe(Htrz)1+y-x(trz)2-y(NH2trz)x(BF4)y·nH2O (x = 0, 0.1, 0.2 and 0.3) were synthesized in concentrated ...solutions without using any surfactant or polymer. The nanoparticle powders were investigated by transmission electron microscopy, powder X-ray diffraction, magnetometry, calorimetry, Raman/IR spectroscopies, elemental analysis and 57Fe Mössbauer spectrometry. Remarkably, the latter revealed a large decrease of the lattice stiffness when incorporating a small amount of amino-triazole ligand, reflected by the drop of the Debye temperature from 285 K (x = 0) to 205 K (x = 0.3). This collapse of the lattice cohesion was attributed to a reorganization of the supramolecular interactions between the Fe-triazole chains. This effect on the SCO properties is also discussed.
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