Healing Temperature Research Articles

Self-healing by design: universal kinetic model of strength recovery in self-healing ceramics

ABSTRACT We propose a new theoretical kinetic model of strength recovery by oxidation-induced self-healing of surface cracks in composites containing a healing agent (HA). The kinetics is a key parameter in the design of structural components that can self-heal the damage done in service. Based on three-dimensional (3D) observations of crack-gap filling, two crack-gap filling models, i.e., a bridging model and a tip-to-mouth filling model, are incorporated in the proposed kinetic model. These crack-gap filling models account for the microstructural features of the fracture surfaces, crack geometry, and oxidation kinetics of the healing-agent. Hence, the minimum and maximum remaining flaw sizes in the healed crack gaps are estimated for various healing temperatures, times, and oxygen partial pressure conditions. Further, the nonlinear elastic fracture mechanics suitable for small-sized remaining flaws, together with a statistical analysis of the original Weibull-type strength distribution, enables the prediction of upper and lower strength limits of the healed composites. Three sintered alumina matrix composites containing silicon carbide (SiC)-type HAs with various volume fractions and shapes, together with monolithic SiC ceramics, are considered. The strength of the healed-composite predicted by our model agrees well with the experimental values. This theoretical approach can be applied to HAs other than SiC and enables the design of self-healing ceramic components for various applications.

Weibull statistics of the lap-shear strength of a symmetric interface of amorphous poly(ethylene terephthalate)

For the first time, the distribution of the lap-shear strength (σ) developed upon the contact of two amorphous bulk samples of poly(ethylene terephthalate) (PET) in the surroundings of the PET bulk glass transition temperature has been analysed as a function of healing temperature and healing time by using a Weibull statistical model. The Weibull parameters, Weibull modulus (m) and scale factor (σ0) of partially self-healed symmetric amorphous PET–PET interface have been estimated by analysing some tens of the lap-shear strength experimental data sets. It has been found that the σ0 values are very close to the corresponding mean σ values averaged for each test series, meaning that the Weibull analysis gives physically meaningful results. It has been shown that the majority of the σ experimental data sets can be described properly with the standard Weibull distribution function proposed for brittle solids. The m values for the PET–PET interface have been compared with those reported recently for the polymer–polymer interfaces of non-crystallizable amorphous polymers. The impact of the potential ability of an amorphous polymer to crystallize on the scatter in the measured interface lap-shear strength has been discussed.

Development of self-contained microcapsules for optimised catalyst position in self-healing materials

Self-contained microcapsules for use in self-healing epoxy resin are successfully synthesized by suspension polymerization process. The microencapsulation of an epoxy resin using Polymethylmethacrylate (PMMA) as a shell material and the location of scandium triflate (Sc(OTf)3) as the catalyst into microcapsules shell during the microencapsulation processes is presented (PMMA/Sc(OTf)3-walled microcapsules). Spherical microcapsules of 80 μm in diameter with a liquid core content of 30 wt% (determined by HPLC) are produced. Catalyst location on microcapsules are assessed qualitatively by SEM-EADS and quantitatively by TGA showing high yields (⁓70 wt%). The evaluation of the healing efficiency was assessed in terms of fracture toughness recovery. PMMA/Sc(OTf)3-walled microcapsules showed an increased healing efficiency than that of conventional PMMA-walled capsule. The healing efficiency of the PMMA-walled capsules was 46.7 and 55.1% when the system healed at 80 and 120 °C, respectively. However, in the case of PMMA/Sc(OTf)3-walled microcapsules healing efficiency increased to 57.5 and 79.1% for the same healing temperatures.

Temperature induced self-healing capability transition phenomenon of bitumens

The temperature may induce the inconsistent order of self-healing capability between two bituminous materials, which could be called as the temperature induced self-healing capability transition phenomenon. To confirm this phenomenon, two base bitumens and four different styrene–butadiene-styrene polymer modified bitumen (SPMB) binders are selected to systematically investigate their self-healing capability through establishing the healing rate temperature (HR-T) curves and healing index temperature (HI-T) curves. Fluorescence microscopy (FM) is applied to study the internal morphology of different SPMB binders. The phase transition (PT) temperature range is determined by the differential scanning calorimeter (DSC) test. Within the PT temperature range, three kinds of special temperatures are discovered, i.e. healing peak temperature (HPT), healing critical temperature range (HCTR) and healing cross temperature (HCT). The HPT and HCTR are both lower than the high temperature PT range, followed by the softening point, and can be applied as the optimum healing temperature to guide the formulation of energy efficient pavement induction healing strategies. The HCT clearly divides the HR-T curves or HI-T curves of bitumens into different parts, thus leading to an opposite order of self-healing capability between two bitumens, which confirms the self-healing capability transition phenomenon. Therefore, the self-healing capability can not be evaluated comprehensively and accurately at only one given temperature. Most notably, it can be inferred that the temperature induced phase structure and property transition of bitumen is the essential reason for the self-healing capability transition phenomenon, which is significantly affected by the bitumen type, polymer type and content.

Self‐Healing Engineering Ceramics with Oxidation‐Induced Crack Repair

Engineering ceramics capable of restoring damage‐induced surface cracking may achieve enhanced reliability and longer lifetime at high temperature applications compared with nonhealing ceramics. Selection of healing agents dispersed in ceramic matrix composites which upon oxidation with environmental oxygen may rapidly fill the volume of a surface crack and recover compromised strength are reviewed. Novel approaches envisage acceleration of crack healing to reduce healing period as well as healing temperature. Potential fields of application for self‐healing ceramics operating at elevated service temperatures are presented.

A Robust and High Self‐Healing Efficiency Poly(Urea‐Urethane) Based on Disulfide Bonds with Cost‐Effective Strategy

AbstractThe development of self‐healing poly(urea‐urethane)s with remarkable mechanical properties as well as self‐healing capability is an important challenge through a low‐cost approach. In this study, cystine dimethyl ester (CDE) is synthesized cost‐effectively using cystine from bio‐based sources as raw material, and a novel self‐healing poly(urea‐urethane) is successfully prepared using synthesized CDE as a disulfide‐containing chain extender through a two‐step approach. The as‐prepared disulfide‐containing poly(urea‐urethane) (SPU) presents an excellent self‐healing capability with self‐healing efficiency of 95.3% at a moderate healing temperature (≈60 °C). Meanwhile, the SPU exhibits robust mechanical properties possessing a maximum tensile strength of 25.80 MPa and a high elongation at break of over 460% simultaneously. The compromise between self‐healing capability and mechanical performance can be responsible for the exchange of dynamic disulfide bonds and the existence of quadruple hydrogen bonding. This strategy provides an available idea to develop robust self‐healing polymer materials cost‐effectively.

Controlling Healing and Toughness in Polyurethanes by Branch-Mediated Tube Dilation.

In this work, we propose the use of regular branching of polyurethanes as a way to regulate chain dynamics and govern crystallization in highly dense hydrogen-bonded systems. As a result, robust and healable polyurethanes can be obtained. To this end, we synthesized a range of aliphatic propane diol derivatives with alkyl branches ranging from butyl (C4) to octadecanyl (C18). The series of brush polyurethanes was synthesized by polyaddition of the diols and hexamethylene diisocyanate. Polyurethanes with very short (C < 4) and very long (C = 18) brush lengths did not lead to any significant healing due to crystallization. An intermediate amorphous regime appears for polymers with middle branch lengths (C = 4 to 8) showing a fine control of material toughness. For these systems, the side chain length regulates tube dilation, and significant macroscopic healing of cut samples was observed and studied in detail using melt rheology and tensile testing. Despite the high healing degrees observed immediately after repair, it was found that samples with medium to long length brushes lost their interfacial strength at the healed site after being heated to the healing temperature for some time after the optimal time to reach full healing. Dedicated testing suggests that annealed samples, while keeping initial tackiness, are not able to completely heal the cut interface. We attribute such behavior to annealing-induced interfacial crystallization promoted by the aliphatic branches. Interestingly, no such loss of healing due to annealing was observed for samples synthesized with C4 and C7 diols, which is identified as the optimal healing regime. These results point at the positive effect of branching on healing, provided that a critical chain length is not surpassed, as well as the need to study healing behavior long after the optimal healing times.

Influence of fast neutron and gamma irradiation on the thermoelectric properties of n-type and p-type SiGe alloy

The thermoelectric materials that work in radioisotope thermoelectric generators or space reactor power sources will be exposed to neutron irradiation and gamma irradiation for a long time. Herein, the influences of neutron irradiation doses between 1013 n/cm2 and 1015 n/cm2, and gamma irradiation of 104 Gy, on the transport properties of n- and p-type SiGe alloys were investigated. It was found that, neutron irradiation causes defects, and leads to a dramatic decrease in carrier mobility for both n- and p-type SiGe. As a result, the thermoelectric properties of both n- and p-type SiGe decline steeply after neutron irradiation. However, the thermoelectric properties of SiGe alloys can be restored by heat treatment at proper temperature. The n-type SiGe is more sensitive to neutron irradiation than p-type SiGe. After 1015 n/cm2 neutron irradiation, the electrical conductivity, power factor and ZT of n-type SiGe decrease to 0.014%, 5% and 3.8% that of an un-irradiated sample; while the electrical conductivity, power factor, and ZT of p-type SiGe decrease to 0.14%, 12% and 5.2% that of an un-irradiated sample. The healing temperature is 1273 K for n-type SiGe, while 623 K for p-type SiGe. Gamma irradiation has no obvious effect on the transport properties of SiGe alloys.

Hydrogen bonding derived self-healing polymer composites reinforced with amidation carbon fibers

Self-healing polymer materials (SHPM) have aroused great interests in recent years. Ideal SHPM should have not only simple operations, but also high elongations at break, tensile strain and self-healing properties at room temperature. Herein, the amidated carbon fibers (CFs) reinforced self-healing polymer composites were designed by hydrogen bonding interaction between functionalized CFs and hyperbranched polymers. The amidated CFs were prepared by transformation of hydroxyl to acylamino through a one-step amidation. By introducing amidated CFs, amidated CFs self-healing polymer composites (called AD-CF) exhibited many desirable characteristics compared to pure polymer composites, such as a better elasticity, lower healing temperatures, and higher self-healing efficiencies. The stress–strain test was selected to carefully study the self-healing property of the AD-CF. The observed same recovery condition, i.e. without any mechanical breakdown after the 10 sequential cycles of cutting and healing indicates no aging of the AD-CF. The ability of AD-CF to exhibit a soft state and rapid self-healing at room temperature makes it possible for much wider applications.

Detailed Analysis of the Influencing Parameters on the Self-Healing Behavior of Dynamic Urea-Crosslinked Poly(methacrylate)s.

For this paper, the self-healing ability of poly(methacrylate)s crosslinked via reversible urea bonds was studied in detail. In this context, the effects of healing time and temperature on the healing process were investigated. Furthermore, the impact of the size of the damage (i.e., area of the scratch) was monitored. Aging processes, counteracting the self-healing process, result in a decrease in the mechanical performance. This effect diminishes the healing ability. Consequently, the current study is a first approach towards a detailed analysis of self-healing polymers regarding the influencing parameters of the healing process, considering also possible aging processes for thermo-reversible polymer networks.

UV-curable self-healing polyurethane coating based on thiol-ene and Diels-Alder double click reactions

Endowing UV-curable coatings with self-healing properties by incorporating dynamic bonds into their cross-linking network is of vital significance. However, in practical applications, self-healing polymers are often plagued with problems such as a high healing temperature and long healing time. To address this challenge, we developed a new type of UV-curable polyurethane coating by the combination of Diels-Alder and thiol-ene click reactions. The Diels-Alder bonds in the polyurethane samples developed in this work furnished perfect mechanical properties and self-healing ability. Self-healing of polyurethane based on Diels-Alder bonds usually occurs at 120 °C. Thiol-ene click reaction is a robust, novel polymerization method, and it improved the structural performance of the polymer with a more homogenous structure being formed than that achieved with traditional UV-curing. Herein, a novel UV-curable self-healing coating, which is referred to as a PU-SH system, was designed by integrating thiol-ene into polyurethane resins containing Diels-Alder bonds. Results of analyses by assorted techniques (including Fourier-transform infrared spectroscopy, different scanning calorimetry, rheology, optical microscopy, and 3D optical interferometric profilometry) strongly indicated that the self-healing ability of the prepared coatings is superior to those of previously studied traditional UV-curing coatings, which are referred to as PU systems. The resultant coatings of the PU-SH system showed a remarkable reduction in the healing temperature from 120 °C for the PU system to 90 ˚C. The rapid scratch-healing properties of the UV-curable coatings, taken together with the moderate healing temperature makes the material attractive for a variety of applications, such as in flexible electronic screens, car paint film, and aircraft interior finishes.

Mechanical experiment evaluation of the microvascular self-healing capability of bitumen using hollow fibers containing oily rejuvenator

Smart microvascular self-healing bituminous composites are considered a promising approach to prolong pavement service life. The objective of this work was to investigate the microvascular self-healing capability of bituminous material by using hollow fibers with oily rejuvenator through a direct-tension mechanical experiment. The hollow fibers with oily rejuvenator were prepared using a wet-spinning approach by applying polyvinylidene fluoride material. The fixed-length hollow fibers with sealed ends were mixed in bitumen to form composite samples. Through a repetitive tensile method, the tension strength data were used to calculate the self-healing efficiency autonomously by considering crack closure and healing at various healing temperatures and times. The scanning-electron-microscope morphology showed that the polyvinylidene fluoride hollow fibers had a tight interfacial structure with bituminous material without debonding. X-ray computed tomography results indicated that the fibers were distributed homogeneously in bitumen. The fiber content and fiber orientation affected the self-healing capability of bituminous samples at 0 °C. To simplify the influence of the above two factors of the fibers, only one fiber was placed in bitumen samples parallel to the tension direction. With an increase in temperature from 0 to 30 °C, the self-healing capability of the bitumen samples increased dramatically. This phenomenon is attributed to the accelerated penetration speed of rejuvenator in the bitumen. An increase in time increased the self-healing capability of the bitumen samples. The rejuvenator may have sufficient time to leak from the hollow fibers and penetrate the bitumen. The results provide a guide to the microstructural design of the vascular fibers and the application of hollow fibers in self-healing asphalt pavement.

Experimental assessment of self-healing characteristics in AA2014matrix with nitinol wire and solder alloy as healing agents

This work investigates the self-healing characteristics of hybrid metal matrix composites which contains Ni-Ti based Shape memory alloy wire and solder alloy as healing agents reinforced in aluminium alloy AA2014 matrix. Inspired by nature, the idea for smart material attempts to retain shape of the deformed structure through SMA wire bridging and binds the crack interface through solder alloy that are already reinforced inside the matrix. The objective of this paper is to find out the best suited healing temperature favorable to attain higher healing assessment efficiency. The completely damaged and partially damaged composite samples produced by flexural test were thermally treated at different temperatures (i.e. at 250 °C, 370 °C, 480 °C, 600 °C, 615 °C). The results shown that about 80% of recovery in deformed shape for partially damaged samples, 70% of reduction in crack width efficiency for fully damaged samples and 52% of recovery in flexural strength for partially damaged sample was achieved at 600 °C healing temperature.

Bio-based epoxidized natural rubber/chitin nanocrystals composites: Self-healing and enhanced mechanical properties

A kind of epoxidized natural rubber (ENR)/CNCs composites without using conventional crosslinking agents was successfully fabricated in this paper, in which a supramolecular network based on hydrogen bonds is formed. FTIR confirmed that parts of hydroxyl groups of CNCs reacted with the epoxy group of ENR and hydrogen bonds were developed between CNCs and ENR. This made CNCs involve in the formation of supramolecular network, effectively improving the mechanical properties and endowing the ENR/CNCs composites with superior self-healing capacity simultaneously. The tensile strength of ENR/CNCs composites with 20 wt% CNCs rises to 1.19 MPa, almost 2 times than that of the neat ENR. Importantly, the ENR/CNCs composite with a filler loading of 20 wt% exhibits impressive self-healing efficiency of 83% of the its original strength and 95% of the original extensibility without any external stimulus at room temperature. Healing behavior is strongly influenced by CNCs contents, healing temperature, healing time and its times.

Comprehensive effect of the mechanical properties, laminated structure and healing conditions on the self-healing behaviors of laminated Al2O3-MgO ceramic composites

The comprehensive effect of the mechanical properties, laminated structure and healing conditions on the self-healing behaviors of laminated Al2O3-MgO ceramic composites was researched. The flexural strengths before and after healing at the temperatures of 715, 815 and 915 °C for 30 min, 60 min and 90 min were measured. According to recovering rate the best healing condition was 900 °C for 60 min. The crack-healing ability of the laminated Al2O3-MgO ceramic composites was improved due to the enhancement of the original flexural residual effect and the oxide content effect by the elimination of pores and the filling of cracks. In addition, the factors influencing the original flexural residual effect and the oxide content effect were analyzed. Meanwhile, the effects of the particle size and the healing temperature on the healing ability were analyzed qualitatively based on the healing of a single particle model.

Crack Healing and Mechanical Properties Recovery in SA 508⁻3 Steel.

Internal cracks could be healed under the process of hot plastic deformation. In this study, mechanical properties recovery after crack healing in SA 508–3 steel were investigated. Microstructures of the crack healing zones were observed using an optical microscope (OM) and electron back scattered diffraction (EBSD) technology, and the recovery degrees of mechanical properties in the crack healing zones with the healing temperature and a reduction ratio were tested systematically. The results showed that the internal cracks in SA 508–3 steel disappeared and were replaced by newly formed grains, achieved by recrystallization and abnormal grain growth. The tensile properties of crack healing zones could be fully restored, while their impact and low cycle fatigue properties could only be partially achieved. The recovery degrees of mechanical properties in crack healing zones increased with increasing the healing temperature and reduction ratio in the temperature range of 950–1050 °C. When the temperature was above 1150 °C, the impact properties began to deteriorate because of grain coarsening and larger MA (martensite–austenite) constituents. The microstructural evolution of the crack zone in the SA 508–3 steel was sketched.

Reversible Self-Healing for Preserving Optical Transparency and Repairing Mechanical Damage in Composites.

This research concentrates on the healing of optical properties, roughness, contact angle hysteresis, and shallow scratches in polymer/nanoparticle composites. A series of ternary composite blends [epoxy/halloysite nanotubes (HNTs)/cellulose acetate butyrate (CAB)] with various CAB concentrations were fabricated and subjected to a series of mechanical damages. The optimized concentration of a nanoparticle is 1.0 vol %, and the CAB concentration is 3.0 vol % based on the mechanical reinforcement and wear resistance. Nanoscale scratching, microlevel falling-sand test, and macrolevel Taber abrasions were utilized to damage the surfaces. The induced damage (roughness and surface scratch up to hundreds of nanometers in depth) healed upon heating. At any temperatures above the softening transition of the semi-interpenetrating network structure of the polymer composites, CAB migrates into the microcracks, and the essential mechanical parameters (modulus, strength, strain to failure) are recovered; in our particular epoxy/HNTs/CAB system, optical transparency is also recovered efficiently. CAB also moves to the macroscopic air/specimen interface and favorably modifies the surface properties, reducing the roll-off angles of water droplets from ∼90° to ∼20°. Through an appropriate choice of CAB additives with different molecular weights, the healing temperature can be tailored.

Preparation of microcapsule and its influence on self-healing property of asphalt

A kind of microcapsule was prepared, and a new type of self-healing asphalt was manufactured by mixing asphalt, microcapsule and curing agent with proper ratio. The ductility(15 °C and 10 °C), softening point and penetration of asphalt were tested both before and after adding microcapsule and curing agent into the asphalt. The results showed that the three indexes were kept in normal ranges after adding microcapsule and curing agent. However, the ductility(10 °C) was lower than the technical standard when the additive amount of microcapsule was 5.0wt%. Whereafter, the influence of additive amount, self-healing temperature and time on the asphalt self-healing property were all investigated by ductility self-healing tests (10 °C). The outcome indicated that the addition of microcapsule into the asphalt promote its self-healing properties obviously. The self-healing rate of blank asphalt in 20 °C and for 3 h was 20%, and when the microcapsule additive amount was 3.0wt%, the rate was improved to 40.0%. The highest self-healing rate was 52.0%. The elevation of temperature and extension of time were both beneficial for the promotion of the self-healing properties of asphalt. However, when healing temperature and healing time were improved to 30 °C and 7 h, the improvement of self-healing rate was not obvious.

Healing mechanism of multi-vacancy defective graphene under carbon irradiation

The irradiation healing process of defective graphene was studied by reactive molecular dynamics simulation of injecting C atoms on a multi-vacancy graphene sheet. We studied the effect of environment temperature and incident energy of injected atoms on healing process of defective graphene. Our simulations show that a relatively high temperature (about 1600 K) is prerequisite for perfect healing of defective graphene. Moreover, an appropriate incident energy for injected atoms (0.16 eV/atom for ∼1800 K) is also necessary for perfect healing, even under a suitable temperature for perfect healing. Defect structures, such as carbon chains and blister-like structures, will occur and hinder the healing process, if the adsorption process (determined basically by incident energy) is faster than the reorganization process (dominated by temperature). In addition, the temperature dependence of reorganization capability of graphene was further studied by molecular dynamics simulation of relaxing an intact graphene sheet with adsorbed atoms. The analysis of the evolution of various micro-structures, emerged during the reorganization simulations, is helpful for deeply understanding the healing mechanism of defective graphene sheet under carbon irradiation.

Study on fatigue self-healing properties of basalt fiber asphalt mixture

Asphalt concrete pavement will age gradually and produce fatigue damage under the repeated load and environment. Under the repeated action of driving load and the continuous influence of external environment, it will gradually age and produce fatigue damage. It is found that the asphalt mixture has self-healing characteristics, and it is of positive significance to improve the service life of asphalt pavement by giving full play to the self-healing characteristics of asphalt mixture. With the extensive application of basalt fiber asphalt mixture, its self-healing characteristics have become an urgent problem to be studied. In this paper, according to the four-point bending fatigue test of basalt asphalt mixture, the slope ratio of fatigue curve is selected as the recovery. The effects of basalt fiber asphalt mixture on its self healing ability under three factors, such as damage degree, healing time and healing temperature, were analyzed by orthogonal test, and compared with matrix asphalt mixture and SBS modified asphalt mixture. The results showed that the self healing capacity of asphalt mixture was inversely proportional to the damage degree and time. When the damage degree is more than 50%, the healing performance of the matrix asphalt mixture decreases obviously, and the SBS modified asphalt mixture almost does not heal, while the healing performance of the basalt fiber asphalt mixture is the best.