A study of the physical properties of carbon nanofiber reinforced polypropylene composites

[Resumo]Os polímeros termoplásticos son coñecidos, en xeral, pola súa ampla empregabilidade en extrusión e moldeamento, cunha gran variedade de aplicacións tales como a embalaxe, os téxtiles e os compoñentes para a industria do automóbil. Unha tentativa de aumentar a súa aplicabilidade implica a incorporación de partículas nanométricas con propiedades eléctricas e mecánicas intrínsecas no interior da matriz termoplástica. Entre os diversos tipos de aditivos, as nanofibras de carbono, CNFs, demostran un potencial elevado debido ás súas características eléctricas e mecánicas, similares aos nanotubos de carbono, CNTs, pero cun custo asociado máis reducido. Estas características, relacionadas coa relativa facilidade de incorporación e dispersión en polímeros, aumentaron o interese nas CNFs como novos materiais capaces de achegaren solucións nalgunhas aplicacións innovadoras para os materiais compósitos. As CNFs poden ser preparadas con diámetros de dimensións nanométricas e dan lugar a materiais cunha razón lonxitude/diámetro bastante elevada. Neste traballo foron utilizadas nanofibras Pyrograf [Applied Sciences Inc. (ASI), Ohio, EUA], procesadas por evaporación química de fase vapor, CVD, que revelan unha morfoloxía similar a unha serie de vasos amoreados. Asociadas á súa elevada área superficial específica, as CNFs tenden a formar agregados que poderán reducir as propiedades dos nanocompósitos onde estas son inseridas, especialmente de non se producir a súa dispersión de maneira correcta e uniforme. O éxito para a aplicación das CNFs está estreitamente vinculado ao coñecemento do seu grao de dispersión nas propiedades finais dos nanocompósitos. Con base nesta premisa, realizouse un estudo intensivo en compósitos de CNTs e CNFs de base polimérica, motivado pola importancia da influencia dalgúns factores chave: morfoloxía, dispersión e distribución das nanofibras na matriz polimérica, interacción polímero-nanofibra e relación coas propiedades do posprocesamento. Co fin de definir de forma clara e sistemática a relación entre o procesamento, a morfoloxía e as propiedades finais, é importante identificar inicialmente as vantaxes e desvantaxes da contribución de cada compoñente (CNFs, matriz polimérica e método de procesamento) nas propiedades eléctricas, térmicas e mecánicas. Este estudo aséntase na introdución de diferentes tipos de nanofibras de carbono nunha matriz de polipropileno, PP, procesados nunha extrusora de duplo fuso. Esta técnica emprégase amplamente na industria de transformación de plásticos, o que permitirá un “scale-up” e a consecuente produción en masa que promoverá unha boa relación calidade/prezo destes materiais compósitos. O principal obxectivo deste traballo ten que ver coa investigación e o control do efecto das diferentes estruturas intrínsecas das CNFs nas propiedades morfolóxicas, térmicas, mecánicas, eléctricas, reolóxicas e electromecánicas dos nanocompósitos de CNF/PP. Inicialmente, catro tipos de CNFs (Pyrograf®-III VGCNF) foron sistematicamente incorporados na mesma matriz de polipropileno a través da extrusión de duplo fuso e mesturados so un efecto de corte relativamente elevado. Axiña ficou caracterizada a relación entre a morfoloxía e as propiedades térmicas, mecánicas e eléctricas. A morfoloxía foi analizada por microscopia electrónica de varrido, SEM, e a través dunha análise de escala de cincentos, GSA, con base en microscopia óptica de transmisión, LOM. As propiedades térmicas foron caracterizadas a través de ensaios de termogravimetría, TGA, e calorimetría diferencial de varrido, DSC. Como resultado desta caracterización, a evolución da morfoloxía e a distribución das CNFs na matriz polimérica foi correlacionada. Posteriormente, as propiedades reolóxicas e eléctricas dos nanocompósitos illantes e condutores foron estudadas e correlacionadas coa finalidade de avaliar a análise reolóxica como ferramenta útil que permita diferenciar comportamentos condutores dos non-condutores neste tipo de sistemas. Finalmente, a última parte deste estudo dedicouse á caracterización das propiedades electromecánicas destes compósitos para aplicacións como materiais transdutores. Este estudo establece os contidos e as estruturas máis axeitadas das CNFs para as propiedades físicas desexadas dos nanocompósitos de polipropileno baseados nas nanofibras de carbono. En particular, os nanocompósitos procesados con CNFs con elevados contidos de grafito na parede exterior e con tratamentos térmicos a 1500 ºC posúen resposta eléctrica e exhiben uns baixos límites de percolación e uns requisitos próximos dos escudos de protección electromagnéticos e de radiofrecuencia, EMI/RFI. Por outra parte, os nanocompósitos procesados con CNFs, en que a camada exterior foi removida por efecto pirolítico, revelaron posuír un bo desempeño mecánico; porén, estes mostraron ser illantes eléctricos para a mesma cantidade de carga no interior da matriz de PP. As análises termogravimétricas amosan un aumento da estabilidade térmica dos nanocompósitos poliméricos co aumento da concentración de CNF na matriz de PP. Os ensaios por DSC amosaron un incremento da estabilidade térmica dos nanocompósitos poliméricos co aumento da concentración de CNF na matriz de PP. O DSC sinalou un aumento do grao de cristalinidade coa inclusión de nanofibras de carbono, independente do tipo e concentración das CNF. O grao de dispersión avaliado mediante o uso de LOM e GSA amosou unha correlación entre a concentración de carga e a variancia, un parámetro que permite cuantificar a dispersión: canto máis elevada sexa a variancia, mellor será a dispersión dos aglomerados de CNF na matriz polimérica. Detectouse unha diferenza nas propiedades viscoelásticas entre os nanocompósitos condutores e illantes. As propiedades reolóxicas permitiron establecer unha relación viscosidade–propiedades eléctricas dos compósitos. Por outra banda, os nanocompósitos condutores de CNF/PP amosaron unha resposta piezorresistiva máis elevada e unha meirande variación da resistividade eléctrica na presenza dunha acción mecánica externa, para concentracións preto do límite de percolación eléctrico. Finalmente, foi caracterizada a dependencia do factor de Gauge, GF, parámetro asociado á resposta piezorresistiva, en función da deformación mecánica e da velocidade de deformación. Neste sentido, ficou demostrado que os nanocompósitos condutores obtidos no ámbito deste estudo poderán ser utilizados como sensores.[Resumen] Los polímeros termoplásticos se conocen, en general, por su amplio uso en extrusión y moldeo, con una gran variedad de aplicaciones tales como envasado, textiles y componentes para la industria automovilística. Un intento de ampliar su rango de aplicación consiste en incorporar partículas nanométricas con propiedades eléctricas y mecánicas intrínsecas, en el interior de la matriz termoplástica. Entre los diversos tipos de cargas, las nanofibras de carbono, CNFs, demuestran un elevado potencial debido a sus propiedades eléctricas y mecánicas similares a los nanotubos de carbono, CNTs, pero a un menor coste. Estas propiedades, junto con la facilidad que presentan para ser incorporadas y dispersas en los polímeros, han aumentado el interés de las CNFs como nuevos materiales capaces de ofrecer soluciones a algunas aplicaciones innovadoras en el ámbito de los materiales compuestos. Las CNFs se pueden producir con diámetros con dimensiones nanométricas, dando lugar a materiales con una razón de longitud/diámetro bastante elevada. En este trabajo se han empleado nanofibras Pyrograf®-III [Applied Sciences Inc. (ASI), Ohio, EUA], procesadas por deposición química de vapor, CVD, y que muestran una morfología parecida a la de vasos apilados. Debido a su elevada área superficial específica, las CNFs tienden a formar agregados que pueden reducir las propiedades de los nanocompuestos en los que se encuentran, especialmente si la dispersión de las mismas no se produce de forma correcta y uniforme. El éxito para la aplicación de las CNFs depende de modo muy directo del conocimiento de los niveles de dispersión de las mismas en las propiedades finales de los nanocompuestos. Teniendo en cuenta este hecho, se ha producido un estudio intensivo de compuestos de base polimérica con CNTs y CNFs, motivado por la importancia de la influencia de algunos factores clave: morfología, dispersión y distribución de las nanofibras en la matriz polimérica, interacción polímero con nanofibra y relación con las propiedades del postprocesado. Con el objetivo de definir de modo claro y sistemático la relación entre procesado, morfología y propiedades finales, es importante identificar primero las ventajas y desventajas de la contribución de cada componente: CNFs, matriz polimérica y el método de procesado, en las propiedades eléctricas, térmicas y mecánicas. Este estudio muestra la introducción de diferentes tipos de nanofibras de carbono en una matriz de polipropileno, PP, procesadas en una extrusora doble husillo. Esta técnica es muy empleada en la industria de transformación de plásticos, lo que permitirá una producción a gran escala, proporcionando así una buena relación entre calidad / precio para este tipo de materiales compuestos. El principal objetivo de este trabajo se centra en la investigación y control del efecto de las diferentes estructuras intrínsecas de las CNFs en las propiedades morfológicas, térmicas, mecánicas, eléctricas, reológicas y electromecánicas de los nanocompuestos de CNFs / PP. Inicialmente, cuatro tipos de CNFs (Pyrograf®-III VGCNFs) fueron sistemáticamente incorporados en la misma matriz de polipropileno a través de extrusión doble husillo y mezclados a tasas de corte relativamente elevadas. A continuación, se caracterizó la relación entre morfología, y las propiedades térmicas, mecánicas y eléctricas fue caracterizada. La morfología fue analizada por microscopía electrónica de barrido, SEM, y a través de un análisis de la escala de grises, GSA, con base en microscopía óptica de transmisión, LOM. Las propiedades térmicas fueron caracterizadas a través de ensayos termogravimétricos, TGA y de calorimetría diferencial de barrido, DSC. Como resultado de esta caracterización, se correlacionó la evolución de la morfología y la distribución de las CNFs en la matriz polimérica. Posteriormente, se estudiaron y correlacionaron las propiedades reológicas y eléctricas de los nanocompuestos aislantes y conductores con el objetivo de evaluar el análisis reológico como herramienta útil que permita diferenciar comportamientos conductores de aislantes en este tipo de sistemas. Por último, la parte final de este estudio está dedicada a la caracterización de las propiedades electromecánicas de estos compuestos para aplicaciones como materiales transductores. Este estudio establece los contenidos y las estructuras más adecuadas de las CNFs para las propiedades físicas deseadas en nanocompuestos de polipropileno basados en nanofibras de carbono. En particular, los nanocompuestos procesados con CNFs con elevados contenidos de grafito en la pared externa y con tratamientos térmicos de 1500 ºC demuestran tener respuesta eléctrica, mostrando bajos límites de percolación y requisitos próximos a los requeridos por los escudos de protección electromagnética y de radio-frecuencia (EMI / RFI). Por otro lado, los nanocompuestos procesados con las CNFs en las que la pared exterior fue retirada por efecto pirolítico, mostraron poseer un buen comportamiento mecánico, aunque al mismo tiempo demostraron ser aislantes eléctricos, para la misma cantidad de carga en el interior de la matriz de PP. Los análisis termogravimétricos muestran un aumento de la estabilidad térmica de los nanocompuestos poliméricos con el aumento de la concentración de CNFs en la matriz de PP. El análisis por DSC ha mostrado un aumento del grado de cristalinidad con la introducción de las nanofibras de carbono, independientemente del tipo y concentración de CNFs. El grado de dispersión evaluado mediante el uso de LOM y GSA demostró la existencia de una correlación entre la concentración de la carga y la varianza, un parámetro que permite cuantificar la dispersión: cuanto más elevada es la varianza, mejor resultará la dispersión de los aglomerados de CNFs en la matriz polimérica. Se ha detectado una diferencia considerable en las propiedades viscoelásticas entre los nanocompuestos conductores y aislantes. Así mismo las propiedades reológicas permitieron establecer una relación entre la viscosidad y las propiedades eléctricas de los compuestos. Por otro lado, los nanocompuestos conductores de CNFs / PP han mostrado una respuesta piezoresistiva más elevada y por tanto una mayor variación de la resistividad eléctrica en presencia de una acción mecánica externa, a concentraciones próximas al del límite de percolación eléctrica. Por último, se caracterizó la dependencia del factor de Gauge, GF, parámetro asociado con la respuesta piezoresistiva, en función de la deformación mecánica y de la velocidad de deformación, fue caracterizada. De este modo, queda demostrado que los nanocompuestos conductores obtenidos en el ámbito de este estudio, pueden ser utilizados como futuros sensores.[Abstract] Thermoplastic polymers are known, in general, for their broad use in the melt extrusion and moulding plastic industry with a large variety of applications such as packaging, textiles and automotive components. One attempt to increase their application range is to incorporate into them, nanoscale fillers with intrinsically high electrical and mechanical performance. Among nanoscale modifiers, carbon nanofibers, CNFs are very suitable due to show similar mechanical and electrochemical properties compared to the carbon nanotubes, CNTs, but at a lower cost. These facts, together to the relatively easier incorporation and dispersion into polymers also raised the interest in CNFs as adequate new materials to provide solutions to some challenges in composite applications. CNFs can be prepared with diameters in the nanometer scale, resulting in high aspect ratios, AR. Pyrograf®-III nanofibers [Applied Sciences Inc. (ASI), Ohio, USA], used in this study in particular, are a sort of vapor-grown carbon nanofibers, VGCNFs, fabricated by chemical vapor deposition, CVD, with stacked-cup morphology. Associated with their large surface area, CNFs tend to come in aggregated structures which may negatively affect the composites properties if not dispersed correctly. Success strongly depends on knowing the influence of degree of dispersion of CNFs in the final properties. In this regard, an intensive research in CNTs and CNFs based polymer composites has been motivated by the importance of several key-factors: morphology, dispersion and distribution of nanofillers in the host polymer, polymer-nanofiller interactions and finding out processing-structure-final properties relationships. In order to define clear and systematic relationships between processing, final structure and final properties, is important to identify first the advantages and disadvantages that each kind of CNFs, polymer and processing method may contribute to the final electrical, thermal and mechanical performance. This study introduces different carbon nanofiber-based polypropylene, PP, nanocomposites processed by twin-screw extrusion, a common-use industrial technique which allows a large-scale production and provides a good relation between quality and cost of the composite materials. The main focus is to investigate and tailor, if possible, the effect of the content and the different intrinsic CNFs´ structures on morphology and thermal, mechanical, electrical, rheological and electro-mechanical performance of the final nanocomposites. First, four different Pyrograf®-III VGCNFs were systematically incorporated in the same polypropylene matrix by twin-screw extrusion under relatively high shear mixing conditions. Then, the relations between morphology, thermal and mechanical and electrical analysis were studied. Morphology was analyzed by scanning electron microscopy, SEM, and by statistical greyscale analysis, GSA, based on transmitted light optical microscopy, LOM. Thermal properties were characterized by thermogravimetric analysis, TGA, and differential scanning calorimetry, DSC. As a result of this first approach, the morphology together with distribution in the polymer was evaluated. After, rheological and electrical properties in electrical and non-electrical conducting composites were evaluated and correlated with the aim of examining if rheological analysis, besides being a method to assess processing performance, allows distinguishing electrical conducting from electrical isolating response in this kind of systems. The last part of this study was dedicated to investigate the electromechanical performance of these composites as piezoresistive transductors. This study establishes that the desired physical properties of carbon nanofiber based-polypropylene nanocomposites fabricated by shear extrusion can be tailored when the adequate CNFs structure and content are chosen. In particular, nanocomposites fabricated with CNFs with highly graphitic outer wall layer and with heat treatments of 1500 ºC revealed to have electrical response, exhibiting low thresholds and values close to meet the electromagnetic / radio-frequency interference (EMI / RFI) shielding requirements. On the other hand, nanocomposites fabricated with CNFs with pyrolitical stripped outer layer showed good mechanical performance, but they revealed non-electrical conducting response at the same contents of CNFs. Thermogravimetric analysis showed shift to higher temperatures of the main thermal degradation of the polymer with the addition of CNFs. DSC indicated a strong enhancement of the degree of crystallinity with the inclusion of the filler, independently on the filler content and type. The degree of dispersion evaluated using LOM and GSA shows a correlation between the filler concentration and the variance, a parameter which measures quantitatively the dispersion: the lower the variance, the better the cluster´s dispersion, for all composites. Besides, it was also observed a large difference in viscoelastic properties between electrical conducting and isolating composites. In this regard, rheological analysis demonstrated to establish direct viscoelastic – electrical properties relationships. On the other hand, the electrical conducting CNFs / PP composites showed to have better piezoresistive performance or variation of the electrical resistivity in the presence of external deformations at concentrations close to the electrical thresholds. Finally, the dependence of Gauge Factor, GF, parameter associated with piezoresistive response, as a function of the deformation and velocity of deformation was calculated. In this way, it was demonstrated that the electrical conducting composites obtained in this study can also be used as self-sensing materials.[Resumo] Os polímeros termoplásticos são conhecidos, em geral, pela sua ampla empregabilidade em extrusão e moldação, com uma grande variedade de aplicações tais como embalagem, têxteis e componentes para a indústria automóvel. Uma tentativa de aumentar a sua aplicabilidade envolve a incorporação de partículas nanométricas com propriedades eléctricas e mecânicas intrínsecas, no interior da matriz termoplástica. Entre os diversos tipos de aditivos, as nanofibras de carbono, CNFs, demonstram um potencial elevado devido às suas características eléctricas e mecânicas, similares aos nanotubos de carbono, CNTs, mas com um custo associado mais reduzido. Estas características, associadas à relativa facilidade de incorporação e dispersão em polímeros, aumentaram o interesse nas CNFs como novos materiais capazes de promover soluções em algumas aplicações inovadoras para os materiais compósitos. As CNFs podem ser preparadas com diâmetros com dimensões nanométricas, resultando em materiais com uma razão comprimento/diâmetro bastante elevada. Neste trabalho foram utilizadas nanofibras Pyrograf®-III [Applied Sciences Inc. (ASI), Ohio, EUA], processadas por evaporação química de fase vapor, CVD, revelando uma morfologia similar a um serie de copos empilhados. Associados à sua elevada área superficial especifica, as CNFs tendem a formar agregados que poderão reduzir as propriedades dos nanocompósitos onde estas são inseridas, especialmente se a dispersão das mesmas não ocorrer correcta e uniformemente. O sucesso para a aplicação das CNFs está fortemente dependente do conhecimento do grau de dispersão das mesmas nas propriedades finais dos nanocompósitos. Tendo por base esta premissa, um estudo intensivo em compósitos de CNTs e CNFs de base polimérica tem sido realizado, motivado pela importância da influência de alguns factores chave: morfologia, dispersão e distribuição das nanofibras na matriz polimérica, interacção polímero-nanofibra e relação com as propriedades pós-processamento. Com o intuito de definir de forma clara e sistemática a relação entre o processamento, morfologia e propriedades finais, é importante identificar inicialmente as vantagens e as desvantagens da contribuição de cada componente: CNFs, matriz polimérica e o método de processamento, nas propriedades eléctricas, térmicas e mecânicas. Este estudo reporta a introdução de diferentes tipos de nanofibras de carbono numa matriz de polipropileno (PP), processados numa extrusora de duplo fuso. Esta técnica é amplamente utilizada na indústria de transformação de plásticos, o que permitirá um “scale-up” e consequente produção em massa, promovendo uma boa relação entre a qualidade/preço destes materiais compósitos. O principal ob

A review on flexible electrochemical biosensors to monitor alcohol in sweat

The continued focus on improving the quality of human life has encouraged the development of increasingly efficient, durable, and cost-effective products in healthcare. Over the last decade, there has been substantial development in the field of technical and interactive textiles that combine expertise in electronics, biology, chemistry, and physics. Most recently, the creation of textile biosensors capable of quantifying biometric data in biological fluids is being studied, to detect a specific disease or the physical condition of an individual. The ultimate goal is to provide access to medical diagnosis anytime and anywhere. Presently, alcohol is considered the most commonly used addictive substance worldwide, being one of the main causes of death in road accidents. Thus, it is important to think of solutions capable of minimizing this public health problem. Alcohol biosensors constitute an excellent tool to aid at improving road safety. Hence, this review explores concepts about alcohol biomarkers, the composition of human sweat and the correlation between alcohol and blood. Different components and requirements of a biosensor are reviewed, along with the electrochemical techniques to evaluate its performance, in addition to construction techniques of textile-based biosensors. Special attention is given to the determination of biomarkers that must be low cost and fast, so the use of biomimetic materials to recognize and detect the target analyte is turning into an attractive option to improve electrochemical behavior.Authors acknowledge the Portuguese Foundation for Science and Technology (FCT), FEDER funds by means of Portugal 2020 Competitive Factors Operational Program (POCI) and the Portuguese Government (OE) for funding the project PluriProtech—“Desenvolvimentos de soluções multicamada para proteção ativa contra ameaças NBQR”, ref. POCI-01-0247-FEDER-047012. Authors also acknowledge strategic funding of UID/CTM/00264/2020 of 2C2T and by the “plurianual” 2020–2023 Project UIDB/00264/2020

Supercapacitors based on AC/MnO2 deposited onto dip-coated carbon nanofiber cotton fabric electrodes

This work introduces the preparation of flexible carbon composite electrodes based on the top-down approach starting from the dip-coating of carbon nanofibers (CNFs) onto a cotton fabric. On these so-obtained conductive cotton fabrics, further layers of activated carbon and manganese oxide (MnO2) materials were subsequently added to enhance the electrochemical performances of negative and positive electrodes. At the end, two different types of asymmetric supercapacitors (SCs) were assembled with those textile electrodes by using porous paper and Nafion-Na ion-exchange membranes as separators. The different SCs were electrochemically characterized by means of cyclic voltammetry (CV), galvanostatic charge/discharge (G–CD) and electrochemical impedance spectroscopy (EIS). These hybrid carbon-based textile SCs exhibited capacitance performance of 138 and 134 F g–1 with the porous paper and Nafion membrane, respectively, and low self-discharge rates. Furthermore, in this study is considered the combination of two methods (cycling and floating) for studying the long-term durability tests of SCs. In particular, the floating methodology utilizes much more harsh conditions than the common cycling based on G-CD tests at high currents usually discussed in literature. The solid-state (Nafion membrane) hybrid device demonstrated very long durability with 10 K cycles and additional 270 h at a constant voltage of 1.6 V. In summary, the hybrid SCs fabricated with low cost materials and simple methodologies reported in this study showed very promising results for flexible energy storage applications.This work was partly financed by FEDER funds through the Competitivity Factors Operational Programme - COMPETE and by national funds through FCT – Foundation for Science and Technology (project POCI-01-0145-FEDER-007136). A.J. Paleo acknowledges the European COST Action CA15107- Multi-Functional Nano-Carbon Composite Materials Network (MultiComp) for its support with a Short Term Scientific Mission (STSM) grant at CNR-ITAE of Messina

Electrical properties of melt-mixed polypropylene and as-grown carbon nanofiber composites: analysis of their interphase via the AC conductivity modeling

The morphology, crystallinity, and electrical conductivity (σ′ and σ″) as a function of frequency of polypropylene (PP) melt-extruded with different amounts of as-grown carbon nanofibers (CNFs) from 0 to 1.4 vol. % are examined. The PP/CNF composites present CNF aggregates randomly distributed within the PP and an insulator–conductor transition at CNF contents near 0.9 vol. %. The degree of crystallinity of PP/CNF composites with loadings of 1.4 vol. % increases ∼15% with respect to the neat PP (∼34%), with σ´ ∼ 8.6 × 10−5 S m−1 (σ″ ∼ 8.3 × 10−4 S m−1) at 2 MHz. In addition, the values of the electrical conductivity σint´ ∼2.9 × 10−6 S m−1 (σint″∼3.7 × 10−4 S m−1) at 2 MHz, as a result of the interphase (ϕint ∼0.05 vol. %) of the 1.4 vol. % PP/CNF composites, are estimated by the use of a modified generalized effective medium model (GEM). The analysis gathered in here indicates that the interphase between the polymer and the conducting particle may have a quantifiable effect on the electrical properties of carbon-based polymer composites, and this fact should not be neglected in the production of conducting polymer composites (CPCs) with enhanced electrical properties.This study was funded by FCT-Foundation for Science and Technology: “Plurianual” 2020–2023 Project UIDB/00264/2020

Electrical properties of polypropylene-based composites melt-processed with as-grown carbon nanofibers

Electrical conductivity, dielectric permittivity, electrical modulus, and electrical impedance of polypropylene (PP) composites melt-processed with different contents of as-grown carbon nanofibers (CNFs) are studied. As expected, the electrical conductivity of PP/CNF composites increased as the incorporation of CNFs is raised in the polymer, yielding a maximum of ∼ 6 ×10−6 S m−1 for PP/CNF 3 wt. % composites. That enhancement relates to a gradual improvement of the dielectric permittivity as the incorporation of CNFs rises into the PP up to a maximum of ∼ 13 for PP/CNF 3 wt. % composites at 1MHz, which is attributed to the rise of the interface polarization effect. Moreover, the Cole-Cole model is used through the electrical modulus to analyze the effect of CNF contents on the dielectric relaxation of PP/CNF composites from which is deduced that the incorporation of CNFs increases their heterogeneity and relaxation times. The analysis gathered here aims at contributing to the understanding of the electric features of polymer composites filled with a type of CNFs, which are not subjected to any thermal post-processing method after their synthesis by chemical vapor deposition (CVD).This research was funded by the project UID/CTM/00264/2021 of 2C2T under the COMPETE and FCT/MCTES (PIDDAC) co-financed by FEDER through the PT2020 program

Nonlinear Thermopower Behaviour of N-Type Carbon Nanofibres and Their Melt Mixed Polypropylene Composites

The temperature dependent electrical conductivity σ (T) and thermopower (Seebeck coeffi-cient) S (T) from 303.15 K (30◦ C) to 373.15 K (100◦ C) of an as-received commercial n-type vapour grown carbon nanofibre (CNF) powder and its melt-mixed polypropylene (PP) composite with 5 wt.% of CNFs have been analysed. At 30◦ C, the σ and S of the CNF powder are ~136 S m−1 and −5.1 µV K−1, respectively, whereas its PP/CNF composite showed lower conductivities and less negative S-values of ~15 S m−1 and −3.4 µV K−1, respectively. The σ (T) of both samples presents a dσ/dT 0 character, also observed in some doped multiwall carbon nanotube (MWCNT) mats with nonlinear thermopower behaviour, and explained here from the contribution of impurities in the CNF structure such as oxygen and sulphur, which cause sharply varying and localized states at approximately 0.09 eV above their Fermi energy level (EF)

Electronic features of cotton fabric e-textiles prepared with aqueous carbon nanofiber inks

Cotton woven fabrics functionalized with aqueous inks made with carbon nanofibers (CNFs) and anionic surfactant are prepared via dip-coating followed by heat treatment, and their electronic properties are discussed. The e-textiles prepared with the inks made with the highest amount of CNFs (6.4 mg mL−1 ) show electrical conductivities (σ) of ∼35 S m−1 and a negative Seebeck (S) of −6 μV K−1 at 30 °C, which means that their majority carriers are electrons. The σ(T) of the e-textiles from 30 to 100 °C shows a negative temperature effect, interpreted as a thermally activated hopping mechanism across a random network of potential wells by means of the 3D variable range hopping (VRH) model. Likewise, their S(T) from 30 to 100 °C shows a negative temperature effect, conveniently depicted by the same model proposed for describing the negative Seebeck of doped multiwall carbon nanotube mats. From this model, it is deduced that the cause of the negative Seebeck in the e-textiles may arise from the contribution of the impurities found in the as-received CNFs, which cause sharply varying and localized states at approximately 0.085 eV above their Fermi energy level (EF). Moreover, the possibility of a slight n-doping from the cellulose fibers of the fabrics and the residuals of the anionic surfactant onto the most external CNF graphitic shells present in the e-textiles is also discussed with the help of the σ(T) and S(T) analysis.This research was funded by the project UID/CTM/00264/2021 of 2C2T under the COMPETE and FCT/MCTES (PIDDAC) cofinanced by FEDER through the PT2020 program. E.M. acknowledges financial support from ANID Anillo ACT/192023 and Fondecyt No 1190361

Thermoelectric properties of n-type poly (ether ether ketone)/carbon nanofiber melt-processed composites

The thermoelectric properties, at temperatures from 30 °C to 100 °C, of melt-processed poly(ether ether ketone) (PEEK) composites prepared with 10 wt.% of carbon nanofibers (CNFs) are discussed in this work. At 30 °C, the PEEK/CNF composites show an electrical conductivity (σ) of ~27 S m−1 and a Seebeck coefficient (S) of −3.4 μV K−1, which means that their majority charge carriers are electrons. The origin of this negative Seebeck is deduced because of the impurities present in the as-received CNFs, which may cause sharply varying and localized states at approximately 0.086 eV above the Fermi energy level (EF) of CNFs. Moreover, the lower S, in absolute value, found in PEEK/CNF composites, when compared with the S of as-received CNFs (−5.3 μV K−1), is attributed to a slight electron withdrawing from the external layers of CNFs by the PEEK matrix. At temperatures from 30 °C to 100 °C, the σ (T) of PEEK/CNF composites, in contrast to the σ (T) of as-received CNFs, shows a negative temperature effect, understood through the 3D variable-range hopping (VRH) model, as a thermally activated hopping mechanism across a random network of potential wells. Moreover, their nonlinear S (T) follows the same behavior reported before for polypropylene composites melt-processed with similar CNFs at the same interval of temperatures.A. J. Paleo gratefully acknowledges support from FCT-Foundation for Science and Technology by the project UID/CTM/00264/2021 of 2C2T under the COMPETE and FCT/MCTES (PIDDAC) cofinanced by FEDER through the PT2020 program and “plurianual” 2020–2023 Project UIDB/00264/2020. E. Muñoz acknowledges financial support from ANID Anillo ACT/192023 and Fondecyt No 1190361. M. Melle-Franco acknowledges support from the project IF/00894/2015 and within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC)

Thermoelectric properties of polypropylene carbon nanofiber melt-mixed composites: exploring the role of polymer on their Seebeck coefficient

The effect of polypropylene (PP) on the Seebeck coefficient (S) of carbon nanofibers (CNFs) in melt-extruded PP composites filled with up to 5 wt. % of CNFs was analyzed in this study. The as-received CNFs present an electrical conductivity of ~320 S m−1 and an interesting phenomenon of showing negative S-values of −5.5 μVK−1, with 10−2 µW/mK2 as the power factor (PF). In contrast, the PP/CNF composites with 5 wt. % of CNFs showed lower conductivities of ~50 S m−1, less negative S-values of −3.8 μVK−1, and a PF of 7 × 10−4 µW/mK2. In particular, the change in the Seebeck coefficient of the PP/CNF composites is explained by a slight electron donation from the outer layers of the CNFs to the PP molecules, which could reduce the S-values of the as-received CNFs. Our study indicates that even insulating polymers such as PP may have a quantifiable effect on the intrinsic Seebeck coefficient of carbon-based nanostructures, and this fact should also be taken into consideration to tailor conductive polymer composites with the desired thermoelectric (TE) properties.The authors affiliated with 2C2T acknowledge support from FCT-Foundation for Science and Technology within the scope of project UID/CTM/00264/2020. In addition, support through project IF/ A. J. Paleo et al. 00894/2015 and within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020 and access to the Navigator platform (LCA-UC) through the Advanced Computing Project CPCA/A2/2524/2020, financed by national funds through the Portuguese Foundation for Science and Technology I.P./ MCTES, is gratefully acknowledged

Comparative thermoelectric properties of polypropylene composites melt-processed using Pyrograf® III carbon nanofibers

The electrical conductivity (σ) and Seebeck coefficient (S) at temperatures from 40 ◦C to 100 ◦C of melt-processed polypropylene (PP) composites filled with 5 wt.% of industrial-grade carbon nanofibers (CNFs) is investigated. Transmission Electron Microscopy (TEM) of the two Pyrograf® III CNFs (PR 19 LHT XT and PR 24 LHT XT), used in the fabrication of the PP/CNF composites (PP/CNF 19 and PP/CNF 24), reveals that CNFs PR 24 LHT XT show smaller diameters than CNFs PR 19 LHT XT. In addition, this grade (PR 24 LHT XT) presents higher levels of graphitization as deduced by Raman spectroscopy. Despite these structural differences, both Pyrograf® III grades present similar σ (T) and S (T) dependencies, whereby the S shows negative values (n-type character). However, the σ (T) and S (T) of their derivative PP/CNF19 and PP/CNF24 composites are not analogous. In particular, the PP/CNF24 composite shows higher σ at the same content of CNFs. Thus, with an additionally slightly more negative S value, the PP/CNF24 composites present a higher power factor (PF) and figure of merit (zT) than PP/CNF19 composites at 40 ◦C. Moreover, while the σ (T) and S (T) of CNFs PR 19 LHT XT clearly drive the σ (T) and S (T) of its corresponding PP/CNF19 composite, the S (T) of CNFs PR 24 LHT XT does not drive the S (T) observed in their corresponding PP/CNF24 composite. Thus, it is inferred in PP/CNF24 composites an unexpected electron donation (n-type doping) from the PP to the CNFs PR 24 LHT XT, which could be activated when PP/CNF24 composites are subjected to that increase in temperature from 40 ◦C to 100 ◦C. All these findings are supported by theoretical modeling of σ (T) and S (T) with the ultimate aim of understanding the role of this particular type of commercial CNFs on the thermoelectrical properties of their PP/CNF composites.A.J.P. gratefully acknowledges support from FCT-Foundation for Science and Technology by the project UID/CTM/00264/2021 of 2C2T under the COMPETE and FCT/MCTES (PIDDAC) co-financed by FEDER through the PT2020 program and “plurianual” 2020–2023 Project UIDB/00264/2020. C.J.T. acknowledges the funding from FCT/PIDDAC through the Strategic Funds project reference UIDB/04650/2020-2023. E.M. acknowledges support from Project ANID PIA Anillo ACT/192023