Thermal Hysteresis Loop Research Articles

Modelling Post-molding Warping

Abstract The post-molding warping of injection molded parts is strongly influenced by thermal expansion history. We show that a thermal expansion hysteresis loop occurs, so that heating and cooling curves can sometimes be different, due to microstructural changes. Following an extensive investigation of thermal expansion for a polypropylene, we propose a model to predict the specific volume change during post thermal treatments. The model predictions are in good agreement with the experimental measurements.

Room temperature study of the optical switching of a spin crossover compound inside its thermal hysteresis loop

We have studied the low-spin to high spin state phase transition induced by a single or a sequence of nanosecond laser pulses within the thermal hysteresis loop of the [Fe(NH2-trz)3](NO3)2-H2O spin crossover compound. We demonstrate that the final state that is photoinduced can be finely controlled by changing the central wavelength and the energy of the laser pulses. A simple model accounts for the observed phenomena and paves the way for the practical applications to optical data storage at room temperature of spin state transition compounds.

Spin transition induced by crystal – liquid crystal transition

Cobalt(II) compounds [Co(C10C8C2-terpy)2](BF4)2 (1) and [Co(C12C10C2-terpy)2](BF4)2 (2) (CxCyCz-terpy = 4'–5'''-Alkyl–1'''-alkoxy-2,2':6',2''-terpyridine) with branched alkyl chains, based on a terpyridine frame, were synthesized. The cobalt(II) compounds 1 and 2 exhibit the spin transition between low-spin and high-spin with thermal hysteresis loop at the liquid crystal transition temperature (T1/2↑ = 275 K and T1/2↓ = 250 K for 1 and T1/2↑ = 307 K and T1/2↓ = 296 K for 2), and the spin transition is induced by crystal – liquid crystal transition. The bifunctional materials show synchronizing both spin transition and liquid crystal transition.

Supramolecular lattice-solvent control of iron(ii) spin transition parameters

We report on the synthesis of spin transition compounds 1, 2 of formula [Fe(L)2](A)2 (where L = 2′,6′-bis(pyrazol-1-yl)-3,4′-bipyridine, A = ClO4−—compound 1; A = BF4−—compound 2) and compound 3 of formula [Fe(L)(LH)](BF4)3·H2O·CH3CN (where LH = 3-(2,6-bis(pyrazol-1-yl) pyridine-4-yl)-pyridinium(+)). Compounds 1, 2 and 3 were characterized by single-crystal X-ray diffraction, ESI-ToF mass spectrometry, 1H NMR and elemental analysis. The single-crystal X-ray diffraction study of the counter anion analogues 1 and 2 reveals almost identical molecular structures without any significant presence of intermolecular interactions. However, in the case of compound 3, the crystal structure reveals supramolecular interactions involving molecular cations, BF4− anions and, most importantly, lattice solvent molecules. The presence of solvent water molecules induces the presence of two different types of hydrogen bonding: (i) water molecules interacting with the fluorine atoms of BF4− anions and (ii) water molecules interconnecting protonated and nonprotonated nitrogens of pyridine-3-yl substituents of neighboring complex cations. These overall hydrogen bonding pattern between the neighboring iron(II) complex cation moieties is responsible for the formation of a one dimensional (1D) hydrogen bonded zig-zag chain. The magnetic investigations elucidate high temperature spin transition behavior for both anion analogues 1 and 2, while compound 3 exhibits a lattice-solvent dependency of the temperature-driven spin transition accompanied with stepwise solvent liberation above room temperature. After complete solvent removal the solvent-free compound 3d, [Fe(L)(LH)](BF4)3, shows an abrupt spin transition accompanied with thermal hysteresis loop; T1/2(↑) = 240 K and T1/2(↓) = 231 K, ΔT1/2 = 9 K. The Ising-like model that includes two vibrational modes has been applied in a direct fitting of magnetic data. The model recovers the temperature evolution of the χT product functions for all compounds under study, involving also compound 3d with the thermal hysteresis.

Mechanism for optical switching of the spin crossover [Fe(NH2-trz)3](Br)2·3H2O compound at room temperature

This paper reports on phase transition photo-induced by a nanosecond laser pulse in the molecular spin crossover material [Fe(NH(2)-trz)(3)] (Br)(2).3H(2)O (with NH(2)trz = 4-amino-1,2,4-triazole) around room temperature and in the close vicinity of the thermal hysteresis loop. The measurements are carried out using a time-resolved pump-probe experiment and by recording the reflectivity change at various temperatures and laser intensities. The dynamics of the optically induced reflectivity changes are presented and discussed. We propose a simple model that describes well the recorded phenomena. It takes into account the physical and optical properties of the sample that directly impact the amplitude and the dynamics of the laser-induced heating of the compound.

Electric-Field-Induced Charge-Transfer Phase Transition: A Promising Approach Toward Electrically Switchable Devices

Much research has been directed toward the development of electrically switchable optical materials for applications in memory and display devices. Here we present experimental evidence for an electric-field-induced charge-transfer phase transition in two cyanometalate complexes: Rb(0.8)Mn[Fe(CN)(6)](0.93).1.62H(2)O and Co(3)[W(CN)(8)](2)(pyrimidine)(4).6H(2)O, involving changes in their magnetic, optical, and electronic properties as well. Application of an electric field above a threshold value and within the thermal hysteresis region leads to a transition from the high- to the low-temperature phase in these compounds. A model is proposed to explain the main observations on the basis of a para-ferroelectric transition. Our observations suggest that this new concept of electrical switching, based on materials exhibiting charge-transfer phase transitions with large thermal hysteresis loops, may open up doors for novel electro-optical devices.

Spin Transition Induced by Mesophase Transition in the Cobalt(II) Compounds with Branched Alkyl Chains

Cobalt(II) compounds [Co(C10C8C2-terpy)2](BF4)2 (1) and [Co(C12C10C2-terpy)2](BF4)2 (2) (CxCyCz-terpy = 4′-5″′-Alkyl-1″′-alkoxy-2,2′:6′,2″-terpyridine) with branched alkyl chains, based on a terpyridine frame, was synthesized. The cobalt(II) compounds 1 and 2 exhibit the spin transition between low-spin and high-spin with thermal hysteresis loop at the liquid crystal transition temperature (T 1/2↑ = 275 K and T 1/2↓ = 250 K for 1 and T 1/2↑ = 307 K and T 1/2↓ = 296 K for 2), and the spin transition is induced by crystal-mesophase transition. The bifunctional materials show synchronizing both spin transition and liquid crystal transition.

Bidirectional photo-switching of the spin state of iron(II) ions in a triazol based spin crossover complex within the thermal hysteresis loop

We have investigated the effect of short laser pulses (532 nm, 4 ns, <30 mJ cm −2) on the spin state of iron(II) ions in the spin crossover compound {[Fe II(Htrz) 2(trz)](BF 4)} within the hysteresis region of the high-spin (HS) to low-spin (LS) first-order thermal phase transition. Using Raman spectroscopy we have evidenced quasi-complete HS → LS as well as LS → HS photo-conversions, which can be induced by a single laser shot in the descending (351 K) and ascending (378 K) branches of the hysteresis loop, respectively. No effect has been observed, however, close to the center of the hysteresis loop even for repeated exposures.

Effects of Metal Doping on the Spin-Crossover Properties of an Iron(II) Complex with Extended π-Conjugated Schiff-Base Ligand Having an N4O2 Donor Set

Abstract The spin-crossover (SCO) complex [Fe(qnal)2]·CH2Cl2 (1), (Hqnal: N-(8′-quinolyl)-2-hydroxy-1-naphthaldimine) with an N4O2 donor set, has been synthesized and characterized. Investigation of magnetic properties shows that the complex exhibits an abrupt and complete spin transition with a 5 K wide thermal hysteresis loop. The X-ray diffraction analysis of complex 1 reveals that the molecules are connected into a quasi one-dimensional chain through extended π–π interactions between aromatic rings of ligands. The effects of metal doping on SCO properties have been investigated in the mixed-metal system [Fe1−xMx(qnal)2]·CH2Cl2 (M = zinc(II) and nickel(II)). The results reveal that metal doping increases the gradual character of spin transition, and no marked differences found between zinc and nickel doping, which suggest the dominant effect of the doping-degree (concentration) rather than metal species on cooperativity. However, the metal doping shows different effects on critical temperature (T1/2), where a more pronounced descending of T1/2 is observed in response to increased Zn-doping than in Ni-doping, indicating the noticeable consequence of internal pressure due to the different radii of doping metal ions.

Structure and Magnetic Properties of Iron(II/III) Complexes with N2O22– Coordinating Schiff Base Like Ligands (Eur. J. Inorg. Chem. 4/2009)

AbstractThe cover picture shows the general formula of the iron complexes presented in the Microreview by B. Weber and E.‐G. Jäger on p. 465ff with single crystals of one of the complexes in the background. The application of this system ranges over model compounds for biological systems to molecular magnets or molecular switches. The examples show the reversible binding of the biomolecule nitric oxide, a spin‐crossover complex with a 70 K wide thermal hysteresis loop around room temperature and an example for spontaneous magnetic ordering below 10 K.

Graphical Abstract: Eur. J. Inorg. Chem. 4/2009

AbstractThe cover picture shows the general formula of the iron complexes presented in the Microreview by B. Weber and E.‐G. Jäger on p. 465ff with single crystals of one of the complexes in the background. The application of this system ranges over model compounds for biological systems to molecular magnets or molecular switches. The examples show the reversible binding of the biomolecule nitric oxide, a spin‐crossover complex with a 70 K wide thermal hysteresis loop around room temperature and an example for spontaneous magnetic ordering below 10 K.

Observation of an asymmetry in the thermal hysteresis loop at the scale of a single spin-crossover particle

We have studied the spin-crossover phenomenon of single isolated micro-particles using a Differential Interference Microscope (DIC) operating in transmission mode. For that we have selected the [Fe(NH 2-trz) 3]Br 2-2.5H 2O coordination polymer complex well-known to exhibit, at room temperature, a spin transition with a large thermal hysteresis loop. The comparison of the thermal hysteresis loop of several isolated micro-particles demonstrates some heterogeneity and also an obvious asymmetry of the cycle. The warming branch clearly appears more abrupt than the cooling branch. We also report some correlations between the threshold temperature and the size of the particles.

Mechanism of reversible spin transition with a thermal hysteresis loop in [Fe III(qsal) 2][Ni(dmise) 2] · 2CH 3CN: Selenium analogue of the precursor of an Fe(III) spin-crossover molecular conducting system

A novel Fe(III) spin-crossover complex, [Fe(qsal) 2][Ni(dmise) 2] · 2CH 3CN 1 [qsalH = N-(8-quinolyl)-salicylaldimine, dmise = 4,5-dithiolato-1,3-dithiole-2-selone] was prepared. The magnetic susceptibility measurements revealed 1 exhibited a cooperative spin transition with a thermal hysteresis loop of 15 K. The high and the low temperature structures of 1 indicated three-dimensional intermolecular π−π interactions play a key role in the cooperative spin transition, accompanying a reversible molecular slipping of π-dimer of Ni(dmise) 2 along the molecular long axis. The transfer integral calculation for 1 suggested the π-dimer of Ni(dmise) 2 is in the spin singlet state.

Optical and magnetic properties of the photo-induced state in the coordination network Na2Co4[Fe(CN)6]3.3·14H2O

The study of the metastable state, obtained by light irradiation of Na2Co4[Fe(CN)6]3.3·14H2O is reported using reflectivity and magnetic measurements. This compound is characterized by a charge transfer phenomenon occurring between a high-temperature phase (HT phase) formed by paramagnetic CoII–FeIII pairs and a low-temperature phase (LT phase) formed by diamagnetic CoIII–FeII pairs with the occurrence of a thermal-induced hysteresis loop. A metastable phase can be also formed at low temperature by irradiation following a photo-induced charge transfer between the LT phase and the photo-induced phase (PI phase) formed by CoII–FeIII pairs. The phenomenon corresponds to the well-known light-induced excited spin state trapping (LIESST) effect. The study of the dynamics in the PI phase showed that the relaxation curves follow monoexponential decays. This is in apparent contradiction with the presence of a thermal hysteresis loop. The thermodynamic parameters that govern the relaxation have been determined. The accuracy of these parameters has been checked by simulating the experimental T(LIESST) curve.

Spin crossover complexes as building units of hydrogen-bonded nanoporous structures

The paper reports on the synthesis, crystal structure, differential scanning calorimetry (DSC) and magnetic properties of a spin crossover salt of formula [Fe(bpp)2](cis,cis-1,3,5-Hchtc)·5.5 H2O (1) (bpp = 2,6-bis(pyrazol-3yl)pyridine; cis,cis-1,3,5-Hchtc = monoprotonated cyclohexanetricarboxylate dianion). The salt shows a porous structure with water molecules sitting in the channels. The electronic configuration of the Fe2+ ions is high-spin (HS). Desolvation of 1 yields a material exhibiting an abrupt spin crossover with a thermal hysteresis loop located near room temperature (T1/2↑ = 319 K and T1/2↓ = 309 K). Rehydration of this desolvated salt yields an essentially low-spin (LS) material. This is the first example of a series of hydrogen-bonded porous structures that combine a Fe(bpp)2 spin crossover unit acting as a hydrogen-bond donor and polycarboxylate anions acting as hydrogen-bond acceptors with different connectivities.

Conductivity of the opal-VO2 composite at the semiconductor-metal phase transition

The Shape of the branches for the thermal hysteresis loop of the electrical conductivity for the opal-VO2 composite shows a size dependence near the semiconductor-metal phase transition. The steps observed on the heating and cooling branches of the loop are determined by collective phase transitions in separate monodisperse groups of VO2 nanocrystals within opal voids.

THERMAL HYSTERESIS IN RESISTIVITY AND MAGNETIZATION OF PrCa(Sr)MnO

Thermal hysteresis in resistivity and magnetization of Pr 0.65 Ca 0.35-x Sr x MnO 3(x = 0–0.35) manganites has been systematically studied to establish the interplay of their charge and spin-ordering. The increasing Sr concentration (x) transforms the charge-ordered (CO)/antiferromagnetic (AFM) insulating system (for x = 0) into a mixed-phased one (for x = 0.1) with sharp metal–insulator (MI) transition and finally leads to a ferromagnetic (FM) metallic (for x = 0.35) system. It has been found that the interplay of charge and spin-ordering is higher in mixed-phased state and the thermal hysteresis loop area is bigger. It increases with the increase of sharpness of MI transition. Interplay of charge and spin-ordering decreases with the increase of either CO/AFM insulating or FM metallic phase and is almost absent in completely CO/AFM insulating (with x = 0) or FM metallic (with x = 0.35) states bringing about zero thermal hysteresis loop.

Opposite effects of interactions and disorder on the switching properties of the spin transition compound [Fe II(L) 2][ClO 4] 2 · C 7H 8

We report on a detailed analysis of the thermal transition hysteresis loop and the relaxation curves of the recently investigated compound [Fe II(L) 2][ClO 4] 2 · C 7H 8 [V. Mishra et al., Inorg. Chem. (2008)]. We developed a variant of a previous kinetic cooperative model, modified for including short-range and long-range Gaussian distributions of energy parameters. The resulting set of coupled master equations was solved numerically for the dynamic and the quasi-static case. We have elucidated the various correlations between the distribution widths and the long-range interaction parameter, which basically have opposite effects. It is shown that presence of a steep hysteresis loop may hide a sizable short-range distribution, while the stretched character of the relaxation curves may also hide the presence of strong interactions. We also show that the coherent analysis of the set of relaxation curves at different temperatures does not allow to disentangle properly all the relevant parameters. We discuss these data with respect to the other photomagnetic experiments, T(LIESST) curve and light-induced thermal hysteresis (LITH) loop, and finally we try to address the question of the real nature of the cooperative relaxation process.

One laser shot induced complete phase transition in the spin crossover complex Fe(pyrazine)[Pt(CN) 4

A photo-induced phase transition in the spin crossover complex Fe(pyrazine)[Pt(CN) 4] has been triggered by a 4 ns green pulsed laser excitation within its thermal hysteresis loop. Complete spin conversion from the low spin to the high spin state has been repeatedly observed by irradiating a polycrystalline powder sample. In addition, the non-linear behaviour of the photoconversion efficiency and, in particular, the occurrence at ca. 5 mJ/cm 2 of an energy density threshold, below which no light-induced phase transition appears, is observed. This threshold is shown to be temperature dependent, which is confirmed by a simple theoretical description.

Elastic Model for Complex Hysteretic Processes in Molecular Magnets

In this paper, we present an original model to describe the role of the elastic interactions in cooperative spin crossover compounds that switch between the low-spin (diamagnetic) to high-spin (paramagnetic) states. The interactions are analyzed by the way of their effect on thermal hysteresis loop at different external pressures and on the first-order reversal curves distribution. A comparison with experimental data for the pure spin crossover compound [Fe(btr) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (NCS) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ]H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O is provided.