Electrocaloric materials for future solid-state refrigeration technologies

Keywords: hysteresis ; ferroelectric ; piezoelectric Reference LC-CHAPTER-2006-001 Record created on 2006-08-03, modified on 2017-05-10

/pdf/hysteresis-in-piezoelectric-and-ferroelectric-materials-2cpzvj201f.pdf

Hysteresis in Piezoelectric and Ferroelectric Materials

The use of carbon nanotubes for piezoresistive strain sensors has acquired significant attention due to its unique electromechanical properties. In this comprehensive review paper, we discussed some important aspects of carbon nanotubes for strain sensing at both the nanoscale and macroscale. Carbon nanotubes undergo changes in their band structures when subjected to mechanical deformations. This phenomenon makes them applicable for strain sensing applications. This paper signifies the type of carbon nanotubes best suitable for piezoresistive strain sensors. The electrical resistivities of carbon nanotube thin film increase linearly with strain, making it an ideal material for a piezoresistive strain sensor. Carbon nanotube composite films, which are usually fabricated by mixing small amounts of single-walled or multiwalled carbon nanotubes with selected polymers, have shown promising characteristics of piezoresistive strain sensors. Studies also show that carbon nanotubes display a stable and predictable voltage response as a function of temperature.

/pdf/a-review-carbon-nanotube-based-piezoresistive-strain-sensors-tulig9lhts.pdf

A Review: Carbon Nanotube-Based Piezoresistive Strain Sensors

We report here ``strain glass,'' a new glassy phenomenon in ferroelastic-martensitic system of Ni-rich intermetallic ${\mathrm{Ti}}_{50\ensuremath{-}x}{\mathrm{Ni}}_{50+x}$ ($xg1$), where local strain is frozen in disordered configuration below a critical temperature ${T}_{g}$. The ac elastic modulus shows a minimum at ${T}_{g}$, which exhibits logarithmic frequency dependence following Vogel-Fulcher relationship, and the corresponding internal friction shows a frequency-dependent peak located at a lower temperature. In situ high-resolution transmission electron microscopy observations reveal uncorrelated nanoclusters of martensiticlike phase, randomly frozen in the otherwise untransformed parentlike matrix. Being parallel to spin glass and relaxor, strain glass may shed new light on the fundamental physics of glass and lead to the discovery of novel properties.

/pdf/evidence-for-strain-glass-in-the-ferroelastic-martensitic-4pha9xvj11.pdf

Evidence for Strain Glass in the Ferroelastic-Martensitic System Ti 50-x Ni 50+x

Carbon nanotube (CNT) based sensors are often fabricated by dispersing CNTs into different types of polymer. In this paper, a prototype carbon nanotube (CNT) yarn strain sensor with excellent repeatability and stability for in situ structural health monitoring was developed. The CNT yarn was spun directly from CNT arrays, and its electrical resistance increased linearly with tensile strain, making it an ideal strain sensor. It showed consistent piezoresistive behavior under repetitive straining and unloading, and good resistance stability at temperatures ranging from 77 to 373 K. The sensors can be easily embedded into composite structures with minimal invasiveness and weight penalty. We have also demonstrated their ability to monitor crack initiation and propagation.

https://iopscience.iop.org/article/10.1088/0957-4484/21/30/305502/pdf

Carbon nanotube yarn strain sensors

Polyisoprene—multi-wall carbon nanotube composites for sensing strain

Carbon nanotubes offer attractive possibilities for developing new sensors because of superior mechanical and electrical properties. So far most studies relate the mechanical deformation to the change of nano-scale electrical properties. We present an attempt to use the multi-wall carbon nanotubes (MWCNT) to develop a new material for sensing macro-scale strain. Polymer composites containing dispersed nano-size particles, for example, polyisoprene - multi-wall carbon nanotube composites (PMCNTC) were prepared by treatment of the composite matrix with chloroform providing an increase of mobility and better dispersion of the nano-particles within the matrix. MWCNT with a small amount of solvent was carefully ground in a china pestle before adding to the polyisoprene matrix. Both the polyisoprene matrix solution and concentrated MWCNT product were mixed in a mixer with small glass beads at room temperature for 15 min. The product was dried at 40 °C for over 12 h and vulcanized under high pressure at 160 °C for 20 min. PMCNTC shows attractive tensile and compressive strain sensing properties. A mechanism of sensing effects is being investigated.

Polyisoprene – Multi-Wall Carbon Nanotube Composites for Sensing Strain

We present the first low frequency (0.1-50 Hz) measurements of the complex elastic constant on C 60 single crystals. We have measured a negative dip anomaly of 25% at Ti=261 K, which did not show up in previous elastic studies at 10 4 Hz. The slow relaxation observed near Ti is supposed to be due to the very slow dynamics of the order parameter fluctuations. In addition it is shown that the elastic anomalies observed at 10 4 Hz and 160 K originate from the same physical process as the «glass» transition at 80 K observed in thermal expansivity

Low frequency elastic properties of the structural and freezing transitions in single-crystal C60.

The low-frequency elastic properties of incommensurate ferroelastic crystals [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{CuCl}}_{4}$ are studied in the vicinity of the incommensurate phase transitions using parallel-plate-stress and three-point-bending methods. The temperature dependences of the effective elastic constants (relative Young's modulus) along all principal crystal directions and the shear elastic compliance ${\mathit{S}}_{55}$=1/${\mathit{C}}_{55}$ were obtained. Similarly to the elastic properties at ultrasonic frequencies (10 MHz), the elastic anomalies at low frequencies (5 Hz) near the paraelastic-incommensurate transition (at ${\mathit{T}}_{\mathit{i}}$) are well explained in the framework of the plane-wave approximation. In the transition region from the incommensurate to the commensurate improper ferroelastic phase we apply soliton theory. From the experimental data it follows that the anomalous behavior of ${\mathit{S}}_{55}$ near the phase transition from incommensurate to commensurate improper ferroelastic phase has the same features as the dielectric constant in the vicinity of incommensurate-commensurate improper ferroelectric phase transition, including the Curie-Weiss-type anomaly for ${\mathit{S}}_{55}$(T) near ${\mathit{T}}_{\mathit{c}}$, thermal hysteresis phenomena, and similar changes in the character of the anomaly under applied static stress ${\mathrm{\ensuremath{\sigma}}}_{5}$. The last results are explained within the framework of soliton theory proposed by Holakovsky and Dvorak [J. Phys. C 21, 5449 (1988)]. \textcopyright{} 1996 The American Physical Society.

Low-frequency elastic properties of the incommensurate ferroelastic [N( CH 3 ) 4 ] 2 CuCl 4

The complex dielectric constant of Rb0.75(ND4)0.25D2AsO4 along the tetragonal crystallographic c axis has been measured as a function of temperature for frequencies between 100 Hz and 1 MHz. The dielectric relaxation is described using a Frohlich-type distribution function. The expressions for epsilon ' and epsilon are evaluated on this basis and fitted to the experimental data. The characteristic central frequency fepsilon obeys an Arrhenius law and the relaxation process is polydispersive. Below the freezing temperature Tf=68.9 K the dielectric intensity epsilon (0)- epsilon ( infinity ) is decreasing with decreasing temperature and its temperature dependence can be described by the replica-symmetry-breaking solution of the infinite range Ising spin glass model in the presence of random fields.

Dielectric study of Rb0.75(ND4)0.25D2AsO4

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