| Nanofibers with large specific surface area and excellent mechanical properties have significant values in the fields of filtration,biological medicine,protective clothing,electronic devices and others.Electrospinning nanofiber yarns not only have the characteristics of nanofibers,but also exhibit better mechanical properties than nanofiber membranes,and can meet the requirements of three-dimensional weaving.The nanofiber yarn can be obtained by twisting the aligned nanofiber bundles or by directly continuously twisting and drawing the nanofibers.The former can obtain yarn with high quality,but the yarns cannot be successfully prepared;the latter has some problems including low output and poor stability in the spinning process.Graphene composite nanofibers can be stacked into membranes;on the other hand,graphene/polymer composite films are obtained by casting.The graphene composite materials obtained by the two methods cannot be reprocessed or lose the advantage of nanofibers with large specific surface area.Flexible strain sensors can be widely used in health monitoring,intelligent robot,smart clothing,and building structure monitoring.Due to poor flexibility,low stretchability and sensitivity of strain sensors based on metal and semiconductor,they are difficult to fulfill the requirements in the high strain fields.At present,the flexible strain sensors are mainly prepared using nano-conductive materials/polymer composites.However,there are great challenges in simultaneously obtaining high sensitivity,strain range,stability and linear relationship between strain and relative change in resistance.In order to solve the above problems,this study used graphene and polyacrylonitrile(PAN)as raw materials and the multi-needle liquid-bath electrospinning method to prepare continuous PAN/graphene composite nanofiber bundles,and discussed the effects of the content of graphene,dispersion and spinning parameters on the structure and performance of nanofiber bundles.The PAN/graphene composite nanofiber yarns were prepared by twisting,and the effects of the number of plies and degree of twists on the structure and performance of the yarns were discussed.The high conductive carbon/graphene composite nanofiber yarn(CNY)was successfully prepared by stabilization and carbonization.The flexible strain sensors based on CNYs were designed,and the effects of the number of CNYs and thickness of thermoplastic polyurethane(TPU)on the sensing performance of flexible strain sensor were explored.The carbonized nanofiber weaving fabrics were obtained by weaving using pure PAN nanofiber yarn and traditional PAN staple yarn,stabilization and carbonization,and the strain sensor was prepared using the carbonized fabrics.The effects of the weft density and the thickness of substrate on the sensing performance were investigated.The main findings were obtained as the following aspects:(1)Preparation of successful PAN nanofiber bundles and yarns.The PAN nanofiber bundles were successfully prepared under the external voltage of 17 kV,flow rate of 4.0 ml/h,spinning distance of 9 cm and winding rate of 110 m/h.Under this condition,the average diameter of nanofibers was 258 nm,and the alignment degree of nanofibers along the axial direction of bundles was 70.9%.The maximum breaking stress and elongation of nanofiber bundles were 0.596 cN/dtex and 4.74%,respectively.When the PAN nanofiber bundles were combined with different number of plies,the diameters and twisting angles of yarns increased with increasing the number of plies,and the uniformity of yarn was also significantly improved.When the number of plies and twists were 3 and 1500 tpm,the maximum breaking strength and initial modulus obtained were 34.9 MPa and 391.3 MPa,respectively.When the number of plies and twists were 5 and 2000 twists per meter(tpm),the maximum breaking elongation obtained was 26.1%.With the augment of the temperature and time for setting twist,the effect of setting twist was improved.The optimal temperature and time were 90℃ and 30 min,respectively.The breaking stress and elongation of yarn obtained by five plies and 1000 twists per meter were 32.8 MPa and 20.8%,increasing by 18.3%and 23.2%in this state,respectively.Meanwhile,the crystallinity of nanofibers increased from 34.5%to 39.9%.(2)Preparation of PAN/graphene composite nanofiber bundles.The dispersibility of graphene in spinning solution was optimal when the ultrasound times were 9 minutes before and after adding PAN.PAN/graphene composite nanofiber bundles were prepared using the multi-needle electrostatic device with an auxiliary electrode,and the optimal spinning parameters were that the spinning voltage,auxiliary voltage,winding speed and graphene content were 20 kV,15 kV,110 m/h and 1%,respectively.Under the above conditions,the diameter of the composite nanofiber was 210.6 nm;the alignment degree is 74.34%;the conductivity of the nanofiber bundle was 2.438×10-7 S/cm;the breaking stress,elongation and initial modulus were 0.47 cN/dtex,4.11%and 30.21 cN/dtex,respectively.Adding less than 1%of graphene was beneficial to improve the mechanical and conductive properties of composite nanofibers.(3)Preparation of PAN/graphene composite nanofiber yarns.The composite nanofiber bundles containing graphene content of 1%were twisted.When the number of plies was constant,the diameter of the yarn firstly decreased and then increased with the increase of twists;the breaking stress and elongation firstly augmented and then decreased,and the initial modulus reduced.When the number of twists was constant,with the increase of the number of plies,the diameter of the yarn increased;the breaking stress and elongation firstly increased and then decreased,and the initial modulus decreased.When the number of plies and twists were four and 1500 tpm,the maximum breaking stress were 16.54 MPa;the optimal breaking elongation was 26.42%when the number of plies and twists were three and 2000 tpm.(4)Stabilization and carbonization of PAN/graphene composite nanofibers.With the increase of temperature and time in the process of stabilization,the degree of stabilized reaction was accelerated.The cyclization and aromatization in the process of stabilization were promoted by adding a small amount of graphene into the composite nanofibers,which was conducive to the formation of layered structure and crystalline structure.However,these reactions were limited due to the excessive addition of graphene.The optimal stabilized condition for carbon/graphene composite nanofibers based on PAN was 270 C and 1.5 h.The structure of carbon/graphene composite nanofibers was compact,and the defect and diameter of nanofibers decreased during carbonization.Adding a small amount of graphene into the nanofibers was beneficial to the formation of orderly graphitized structure,and the conductivity of carbon nanofiber yarn was improved,and the maximum value was obtained when the content of graphene was 1%.With Increasing carbonized temperature and time,the conductivity improved.The conductivity was 66.44±13.16 S/cm when the graphene content of 1%,and the stabilized condition was 270 C and 1.5 h,and the carbonized temperature and time were 1100 C and 3 h,respectively.With the increase of graphene content,the breaking stress firstly increased and then decreased.The maximum value was obtained when the graphene content was 1%,and the modulus gradually decreased.The breaking stress and modulus firstly increased and then decreased with the increase of carbonized temperature and time.The compoaite nanofiber yarns with 1%graphene content was treated for 3 h under 1000 C,the maximum stress and modulus obtained were 59.49 MPa and 14.63 GPa,respectively.(5)The strain sensors based on CNY and TPU were prepared.With increasing the number of CNYs,the stability of strain sensor firstly increased and then decreased,and the average gauge factor gradually decreased.As the thickness of TPU matrix decreasing,the average gauge factor of the strain sensors increased,but the stability reduced.When the number of carbon composite nanofiber yarn and the thickness of strain sensor were 4 and 185 μm,respectively,the sensor with both high sensitivity and stability was obtained,and the comprehensive performance was optimal.When the strain was 2%,the average gauge factor was larger than 400,and the coefficient of variation was only 1.9%during the cyclic loading-unloading process.The stretching rates had no effect on the performance of the sensor.The sensor could accurately detect weak changes caused by ultralight mass,sound waves and muscle tightness,and the excellent sensing performance could be obtained under both static and dynamic conditions.(6)The fabrics fabricated using carbonized PAN nanofiber yam(PNY)were used to prepare the flexible strain sensors with high performance.The optimal sensing performances were obtained using the fabric with a single PNY as weft and weft density for 100/10 cm after stabilization and carbonization.When the thickness of TPU substrate was 372 μm,the good linear relationship between the relative change in resistance and strain was obtained,and the linear coefficient was 0.993.The excellent sensitivity and stability were presented under different tensile strain ranges and stretching rates,and the average gauge factor was 77.3 at the strain range of 12%.The stability of the sensor increased with increasing the thickness of TPU film,and the average gauge factor firstly increased and then decreased with increasing the thickness of TPU film.As the linear density of yarn increasing,the compactness of fabric increased,and the average gauge factor increased.However,the strain range decreased.When the compactness of the fabric was small,the distribution of the fracture points of yarn on the fabric was dispersed.The number of fracture points and fracture gaps gradually increased with the increase of strain,showing the excellent linear relationship and strain range.Strain sensor could accurately monitor the large motions of human body joints and small changes;the sensor showed high sensitivity and stability under the dynamic and static loads.The conductive carbon nanofiber yarn based on PAN and fabrics were prepared through the multi-needle liquid-bath electrospinning method,twisting,weaving,stabilization and carbonization.The flexible strain sensor based on these carbonized materials exhibited the outstanding comprehensive sensing performances. |
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