Electropolishing of NiTi shape memory alloys in methanolic H2SO4

The electropolishing of NiTi shape memory alloys was surveyed electrochemically. Anodic polarization of NiTi up to 8 V was performed in various aqueous and methanolic H₂SO₄ solutions. The passivity could be overcome in methanolic solutions with 0.1 mol dm−3≤CH₂SO₄≤7 mol dm−3. The dissolution kinetics was studied in dependence of the polarization potential, the H₂SO₄-concentration, the water concentration and the temperature. For lower concentrations of sulfuric acids (CH₂SO₄≤0.3 mol dm−3) electropolishing conditions were not observed for potentials up to 8 V. The dissolution remained under Ohmic control. In the concentration range from 1 to 7 mol dm⁻³ a potential independent limiting current was registered depending linearly on the logarithm of concentration. The best results were obtained with a 3 mol dm⁻³ methanolic sulfuric acid at 263 K which yielded an electropolishing current of 500 A m⁻² at a potential of 8 V. Surface roughness as well as current efficiency showed an optimum under these conditions.     

Review on the temperature memory effect in shape memory alloys

Shape memory alloys (SMAs) are well known for their unique shape memory effect (SME) and superelasticity (SE) behavior. The SME and SE have been extensively investigated in past decades due to their potential use in many applications, especially for smart materials. The unique effects of the SME and SE originate from martensitic transformation and its reverse transformation. Apart from the SME and SE, SMAs also exhibit a unique property of memorizing the point of interruption of martensite to parent phase transformation. If a reverse transformation of a SMA is arrested at a temperature between reverse transformation start temperature (A _s) and reverse transformation finish temperature (A _f), a kinetic stop will appear in the next complete transformation cycle. The kinetic stop temperature is a ‘memory’ of the previous arrested temperature. This unique phenomenon in SMAs is called temperature memory effect (TME). The TME can be wiped out by heating the SMAs to a temperature higher than A _f. The TME is a specific characteristic of the SMAs, which can be observed in TiNi-based and Cu-based alloys. TME can also occur in the R-phase transformation. However, the TME in the R-phase transformation is much weaker than that in the martensite to parent transformation. The decrease of elastic energy after incomplete cycle on heating procedure and the motion of domain walls have significant contributions to the TME. In this paper, the TME in the TiNi-based and Cu-based alloys including wires, slabs and films is characterized by electronic-resistance, elongation and DSC methods. The mechanism of the TME is discussed.     

SHS of shape memory CuZnAl alloys

Aiming at preparation of shape memory alloys (SMAs), we explored the SHS of Cu_{1 − x} Zn_{1 − y} Al_{1 − z} alloys (0.29 < x < 0.30, 0.74 < y < 0.75, and 0.83 < z < 0.96). The most pronounced shape memory effect was exhibited by the alloys of the following compositions (wt %): (1) Cu(70.6)Zn(25.4)Al(4.0), (2) Cu(70.1)Zn(25.9)Al(4.0), and (3) Cu(69.9)Zn(26.1)Al(4.0). The effect of process parameters on the synthesis of CuZnAl alloys was studied by XRD, optical microscopy, and scanning electron microscopy (SEM). The grain size of CuZnAl was found to depend on the relative amount of the primary CuZn and AlZn phases. Changes in the transformation temperature and heat of transformation are discussed in terms of ignition intensity and compaction. Mechanism of the process depends on the level of the temperature attained relative to the melting point of components. At the melting point of AlZn, the process is controlled by the solid-state diffusion of AlZn into a product layer. The ignition temperature for this system depends on the temperature of the austenite-martensite transformation in CuZnAl alloys. The composition and structure of the products was found to markedly depend on process parameters. The SHS technique has been successfully used to prepare a variety of SMAs.      

A novel type of shape memory polymer blend and the shape memory mechanism

A novel styrene-butadiene-styrene tri-block copolymer (SBS) and poly(?-caprolactone) (PCL) blend were introduced for its shape memory properties. Compared to the reported shape memory polymers (SMPs), this novel elastomer and switch polymer blend not only simplified the fabrication process but also offer a controllable approach for the study of mechanisms and the optimization of shape memory performances. Microstructures of this blend were characterized by differential scanning calorimetry (DSC), AFM microscope observation and tensile test. DSC results demonstrated the immiscibility between SBS and PCL. AFM images and stress-strain plot further confirmed the two-phase morphology within the blend. It was found that the SBS and PCL continuous phases contributed to the shape recovery and shape fixing performances, respectively. A detailed shape memory mechanism for this type of SMP system was then concluded and an optimized SMP system with both good recovery and fixing performances was designed from this mechanism.     

In-situ formation of TiN-TiO2 composite layer on NiTi shape memory alloy via fluidized bed reactor

In this work, the NiTi alloy was oxynitrided in a fluidized bed reactor to attain an in-situ TiN-TiO₂ protective composite layer. Samples were treated at 540 ± 10 °C for various holding times ranging between 0 h and 8 h. Microstructural evolution on the surface was analyzed by scanning electron microscopy, X-ray diffraction, hardness test, electrochemical behavior, Ni ion release, and bioactivity. Quantitative phase analysis from X-ray diffraction pattern of the treated sample for 8 h showed that TiN (71.3%) and TiO₂ (23.0%) were dominant phases on surface. Hardness results revealed as the oxynitriding time increased from 0 h to 8 h, hardness values increased from 263.4 HV_0.1 to 1227.4 HV_0.1. Scanning electron microscopy observation and energy dispersive X-ray spectroscopy mapping micrographs showed that the grown of TiN with dendritic branches was hindered by Ni-rich regions. Electrochemical measurements using polarization and electrochemical impedance spectroscopy analysis revealed corrosion resistance of the oxynitrided samples was increased by ~170% from 173.3 kΩ cm² for the bare NiTi alloy to 473.1 kΩ cm² for the treated NiTi sample for 8 h. It was found that concentration of the released Ni ions decreased from 0.070 (bare NiTi) mg/l to 0.028 mg/l (treated for 8 h) after oxynitriding treatment. Enhanced biocompatibility of the surface treated sample for 8 h was explained by formation of thick and homogenous TiN-TiO₂ composite layer. Finally, bioactive behavior of the oxynitrided samples were studied using simulated body fluid.     

On the development of metallic inert anode for molten CaCl2–CaO System

Some fundamental aspects related to inert anode development in molten CaCl₂–CaO were investigated based on thermodynamic analysis, electrochemistry of metals and solubility of oxide measurements. The Gibbs free energy change of several key anodic reactions including electro-stripping of metals, electro-formation of metallic oxides, electro-dissolution of metallic oxides as well as oxygen and chlorine evolution was calculated and documented, for the first time, as a reference to develop metallic inert anode in chloride based melts. The anodic behaviors of typical metals (Ni, Fe, Co, Mo, Cu, Ag, and Pt) in the melt were investigated. The results confirmed the thermodynamic stability order of metals in the melts and revealed that surface oxide formation can increase the stability of the electrodes in CaO containing melt. Furthermore, solubility of several oxides (NiO, Fe₂O₃, Cr₂O₃, Co₃O₄, NiFe₂O₄) in pure CaCl₂ or CaCl₂–CaO melts was measured to evaluate the stability of oxide coating or a cermet inert anode in the melt. It was found that the solubility of NiO decreased with increasing CaO concentration, while that of Fe₂O₃ increased. Ni coated with NiO film had much higher stability during anodic polarization.     

In-plane deformation of shape memory polymer sheets programmed using only scissors

This paper describes an unconventional, yet simple method to program sheets of shape memory polymer into a variety of two dimensional (2D) structures. The final shape is “encoded” by physically cutting an initial design out of a pre-strained film. The orientation of the initial cut-out relative to the direction of strain and the subsequent relaxation of strain via heating defines the final shape. The appeal of the approach described here is that an easy, low-cost cutting method can achieve a similar shape memory effect attained by more complex processing techniques. Unlike conventional methods, where the final shape of a shape memory polymer must be defined a priori, the direction of cutting of the polymer defines its final shape without any complex pre-programmed strain profiles. A geometric model relating the resolved 2D polymer shape to the initial shape and strain orientation reveals linear correlation between the model-predicted and experimentally-observed shapes. In addition to demonstrating the principle with simple rectangular shapes, we suggest geometries related to encryption and high aspect ratio fibers.     

Nanoindentation of shape memory polymer networks

This work examines the small-scale deformation and thermally induced recovery behavior of shape memory polymer networks as a function of crosslinking structure. Copolymer shape memory materials based on diethylene glycol dimethacrylate and polyethylene glycol dimethacrylate with a molecular weight of 550 crosslinkers and a tert-butyl acrylate linear chain monomer were synthesized with varying weight percentages of crosslinker from 0 to 100%. Dynamic mechanical analysis is used to acquire the bulk thermomechanical properties of the polymers, including the glass transition temperature and the elastic modulus over a wide temperature range. Instrumented nanoindentation is used to examine ambient temperature deformation of the polymer networks below their glass transition temperature. The glassy modulus of the networks measured using nanoindentation is relatively constant as a function of crosslinking density, and consistent with values extracted from monotonic tensile tests. The ambient temperature hardness of the networks increases with increasing crosslinking density, while the dissipated energy during indentation decreases with increasing crosslinking density. The changes in hardness correlated with the changes in glass transition but not changes in the rubbery modulus, both of which can scale with a change in crosslink density. Temperature induced shape recovery of the indentations is studied using atomic force microscopy. For impressions placed at ambient temperature, the indent shape recovery profile shifts to higher temperatures as crosslink density and glass transition temperature increase.     

New directions in the chemistry of shape memory polymers

The rapidly expanding field of shape memory polymers (SMPs) is driven by a growing number of potential applications, such as biomaterials, optics, and electronics. The basic concept involves polymers that can be trapped in a thermodynamically-unfavorable shape, then triggered by an external stimulus to return to their original shape, doing useful work in the process. Part of the attraction of using SMPs is that the energy released during actuation is stored in the polymer itself, rather than requiring an external force to change shape. This approach is beneficial for applications where external actuation is impossible or inconvenient. Polymers are also advantageous over shape memory metal alloys or ceramics in that there are endless combinations of functional groups and material properties to suit a variety of purposes, based on the monomers and polymerization conditions chosen. This advantage of SMPs is of particular interest in the development of materials with additional, desirable physicochemical attributes that are not necessarily coupled to the shape memory (SM) behavior itself. The SM behavior is quantitatively measured to facilitate comparison of various polymer systems, and researchers have used a number of defining parameters to guide the development and characterization of materials with extremely precise and reliable SM responses. In this review, recent trends in the structural or chemical characteristics of SMPs are explored, with an emphasis on how the molecular structure and functionality of each polymer affects its mechanical response.     

Thermomechanical characterization of a shape memory polymer based self-repairing syntactic foam

While the current self-healing approaches such as micro-capsules, hollow fibers, thermally reversible covalent bonds, ionomers, incorporation of thermoplastic particles, etc., are very effective in self-healing micro-length scale damage, self-healing of structural scale or macro-length scale damage remains one of the grand challenges facing the self-healing community. We believe that self-healing of structural damage may need multiple steps, at least two steps: close then heal (CTH), similar to the biological healing of wounds in the skin. In a previous study [1], it has been proven that the confined shape recovery functionality of a shape memory polymer (SMP) based syntactic foam can be utilized to repair structural damage such as impact damage repeatedly, efficiently, and almost autonomously. The purpose of this study is to investigate the effect of various design parameters on the closing efficiencies of both the pure SMP and the SMP based syntactic foam. A systematic test program is implemented, including glass transition temperature (T_g) determination by dynamic mechanical analysis (DMA), isothermal compressive constitutive behavior at various temperatures, and stress-controlled uniaxial compression programming and shape recovery. During thermomechanical cycle testing, two stress levels are utilized for programming and three confinement conditions (fully confined, partially confined, and free) are investigated for shape recovery. It is found that the programming stress is restored under confined recovery conditions, which helps in fully closing the crack; the foam shifts the T_g higher and increases the stiffness at temperatures above the T_g; higher programming stresses lead to slightly higher shape fixity but lower shape recovery in free recovery cases; a higher programming stress also results in a higher peak stress for confined recovery conditions; while the peak stress recovered is controlled by thermal stress, the final stress recovered is controlled by the programming stress, which is stored and recovered using an entropic mechanism. This study lays a solid foundation for using shape memory polymer based composites to self-repair macro-length scale damage.     

Nanoindentation of NiTi shape memory thin films at elevated temperatures

The shape memory effect and nanoindentation response of various phases of sputtered NiTi shape memory thin films were investigated as a function of temperature. The phase transformation temperatures of NiTi films were observed to be sensitive to a compositional shift. The mechanical properties of NiTi thin films also presented a significant response to phase transformations. At the same load, the maximum indentation depth for austenite is smaller than for martensite, indicating that martensite is softer than austenite. A martensite thin film was converted to austenite via in situ heating nanoindentation and displayed the mechanical properties similar to the austenite film at room temperature. These results underscore the validity of elevated temperature nanoindentation methods as a means of interrogating the mechanical properties of materials that undergo thermally-induced phase transformations. The details of the load–displacement curves are also described.     

Study on the thermal-induced shape memory effect of pyridine containing supramolecular polyurethane

In this paper, a series of pyridine containing supramolecular polyurethanes (PUPys) were synthesized from BINA, HDI and BDO. Then the structure, morphology and thermal-induced shape memory effect (SMEs) of PUPys were investigated systematically. Results show that strong hydrogen bonding is formed in the urethane group as well as in the pyridine ring; and phase separation consisting of soft phase and hard phase occurs in the PUPy. In addition, it is found that the lower limit of BINA content for PUPys exhibiting good SMEs is 30 wt%. PUPys with higher BINA content show higher shape fixity, higher shape recovery and better strain stability. Moreover, the shape recovery force increases with the decreasing of BINA content. Finally, the temperature-dependent FT-IR spectra support that the hydrogen bonding in the pyridine ring serves as the molecular switch; while the hydrogen bonding in the urethane groups acts as the physical netpoints for the utilization of PUPys as SMMs.     

Synthesis and shape memory effects of Si-O-Si cross-linked hybrid polyurethanes

A series of novel Si-O-Si cross-linked organic/inorganic hybrid polyurethanes (HYPUs) with shape memory effect were prepared from isophorone diisocyanate (IPDI), poly(ethylene oxide) (PEO), and a newly synthesized hybrid diol (HD) containing hydrolysable Si-OEt groups. After hydrolyzation and condensation of Si-OEt groups, the resulted films were characterized using wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and shape recovery test to get the insight into the relationship between shape memory behaviors and polymeric structures. The glass transition temperatures (T_g) and storage modulus increased with Si-O-Si cross-linking increasing in the hybrid polyurethanes. The hybrid polyurethanes can recover to their original shapes almost completely in less than 40 s in atmosphere and in less than 10 s in water, respectively, when heated at 25 °C above T_g. The shape memory mechanism is coming from the freezing at low temperature and activation at high temperature of micro-Brownian movement of amorphous molecular chains since the temperature ranges at which the sharpest changes of recovered curvature happened are found to be around their glass transition range. The high ratio of storage modulus below and above accounted for the temporary shape fixing at low temperature. The samples with more Si-O-Si cross-linking have higher storage modulus at high temperature, resulting in faster shape recovery speed but lower temporary shape fixing.     

Facile tailoring of thermal transition temperatures of epoxy shape memory polymers

A critical parameter for a shape memory polymer (SMP) lies in its shape memory transition temperature. For an amorphous SMP polymer, it is highly desirable to develop methods to tailor its T_g, which corresponds to its shape memory transition temperature. Starting with an amine cured aromatic epoxy system, epoxy polymers were synthesized by either reducing the crosslink density or introducing flexible aliphatic epoxy chains. The thermal and thermomechanical properties of these epoxy polymers were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). All the crosslinked epoxy polymers with T_g's above room temperature were found to possess shape memory properties. Overall, our approach represents a facile method to precisely tune the T_g of epoxy SMP polymers ranging from room temperature to 89 °C.     

A novel approach to predict the recovery time of shape memory polymers

In this paper a novel approach is presented for prediction of the recovery time for a shape memory polymer. The Transient Stress Dip Tests of Fotheringham and Cherry are used to determine the two parameters of a Kelvin-Voigt element. The characteristic retardation time of this element can then be calculated to predict the recovery time. It is shown that this approach is successful in predicting the recovery times for a shape memory polymer drawn and recovered under a range of temperatures. Furthermore it is shown that the ratio of the recovery stress to the draw stress is independent of the drawing conditions to a very good approximation.     

Effects of moisture on the thermomechanical properties of a polyurethane shape memory polymer

The glass transition temperature of an ether-based polyurethane shape memory polymer (SMP) has been found to decrease significantly after immersion in water. In order to get a better understanding of the mechanism behind this phenomenon, a systematic study on the effects of moisture on the glass transition temperature and thermomechanical properties of this SMP was carried out. The results reveal that the hydrogen bonding between N-H and CO groups is weakened by the absorbed water. Furthermore, the water absorbed into the SMP can be separated into two parts, i.e. the free water and the bound water. Each part inside the SMP was quantified. Bound water significantly reduces the glass transition temperature in an almost linear manner and has a significant influence on the uniaxial tensile behavior, while the effect of free water is negligible. In addition, the recovery stress and recovery strain in constrained/free recovery induced by water were investigated and compared with that induced thermally.     

Performance of self-healing epoxy with microencapsulated healing agent and shape memory alloy wires

We report the first measurements of a self-healing polymer that combines a microencapsulated liquid healing agent and shape memory alloy (SMA) wires. When a propagating crack ruptures the embedded microcapsules, the liquid healing agent is automatically released into the crack where it contacts a solid catalyst embedded in the matrix. The SMA wires are then activated to close the crack during the healing period. We show that dramatically improved healing performance is obtained by the activation of embedded SMA wires. We conclude that improved healing is due to a reduction of crack volume as a result of pulling the crack faces closed, and more complete polymerization of the healing agent due to the heat produced by the activated SMA wires.     

Electroactive shape memory performance of polyurethane/graphene nanocomposites

A series of electroactive shape memory polyurethane (SMPU) nanocomposites were synthesized from poly(tetramethylene ether) glycol (PTMG), 4,4-methylenebis(phenyl isocyanate) (MDI) and 1,3-butandiol (1,3-BD) with the addition of various amounts of thermally reduced graphenes (TRG) which were chemically modified with allyl isocyanate (iTRG). The effects of iTRG on electroactive shape recovery behaviors as well as the conventional direct heat actuated SMPU material have been studied in terms of morphological, thermal, mechanical, electrical properties and thermomechanical cyclic behavior. It was found that significant increases in electrical conductivity and temperature were obtained high iTRG contents (>2%) to electrically actuate the nanocomposite, along with large increases in glass transition temperature (T_g) and initial modulus with a dramatic drop in elongation at break.     

Cyclic stress-strain behavior of shape memory polymer based syntactic foam programmed by 2-D stress condition

Cyclic stress-strain responses of a shape memory polymer based syntactic foam and pure SMP were investigated after a two dimensional (2-D) programming process. Three types of cyclic loading patterns were used: multiple compression-tension cycles at 5% maximum strain, one compression-tension cycle at 40% maximum strain, and one coupled thermo-mechanical cycle at 40% maximum strain. The results indicate that the foams initially harden after the first cycle and then soften in subsequent cyclic loadings, while the pure SMP shows a monotonous softening. The hysteresis loops tend to shrink and approach asymptotically to a steady state before fatigue failure for both the foam and the pure SMP. The programmed foam and the programmed pure SMP are less sensitive to their strain histories, and dissipate more energy as compared with the non-programmed counterparts, showing better fatigue properties at the same strain level. Also, the programmed foam exhibits better mechanical properties under one cyclic thermo-mechanical loading, possessing a good adaptability to ambient temperature changing. Furthermore, the programmed foam demonstrates an increase in yield strength during compression and a decrease in residual tensile stress during tension. These particularly facilitate the programmed foam to be used as a self-healing sealant in expansion joints.