“Study of Electro-thermal Effects on PLA Materials Fed with AC Currents”

Given the rise in the emergence of new composite materials, their multifunctional properties, and possible applications in simple and complex structural components, there has been a need to unravel the characterization of these materials. The possibility of printing these conductive composite materials has opened a new area in the design of structural components which can conduct, transmit, and modulate electric signals with no limitation from complex geometry. Although several works have researched the behaviour of polymeric composites due to the immediate growth, however, the electrothermal behaviour of the material when subjected to varying AC applied voltage (Joule’s effect) has not been thoroughly researched. This study presents the characterization of the electrothermal behaviour of conductive composites of a polylactic acid matrix reinforced with conductive carbon black particles (CB-PLA). An understanding of this behaviour would contribute to the improved work in additive manufacturing of functional electro-mechanical conductive materials with potential application in energy systems, bioelectronics, etc. In this study, the electrothermal interplay is monitored under applied AC voltage, varying lengths, and filament printing orientations (longitudinal, oblique, and transverse). Each sample was printed using the fused deposition modeling technique such that each specimen has three different lengths (1L, 2L, 2.75L). To this end, deductions were made on properties that affect composite’s efficiency and life expectancy. The result of this study shows a great influence of printing orientation on material properties of 3D printed conductive composites of CB-PLA. The result also identifies the contribution of AC applied voltage to composites' stabilization time. This knowledge is important to provide experimental background for components' electrothermal interplay, estimate possible degradation and operating limits of composite structures when used in applications

Bio based carbon nano onion (CNO) and nanoengineered high-performance composites

In the face of mounting environmental concerns and the urgent need for greener technologies, sustainable carbon nanomaterials have emerged as a vital class of materials for a wide range of applications including structural composites, electronics, energy storage, and environmental remediation. Among these, carbon nano-onions (CNOs) have attracted growing attention due to their unique nested graphitic architecture, high surface area, electrical conductivity, and tunable surface chemistry. Despite their promising properties, the practical application of CNOs remains limited by complex, costly, and energy-intensive synthesis routes that often involve hazardous precursors, metal catalysts, or multi-step processing under extreme conditions. These limitations present significant barriers to large-scale, eco-friendly production of CNOs and hinder their integration into sustainable material systems. Therefore, developing a facile, scalable, and environmentally benign approach for CNO synthesis from renewable resources is of critical importance for advancing the field of green nanotechnology and enabling high-performance, and high-end applications. The first study, presents a transformative approach to the sustainable synthesis of carbon nano-onions (CNOs) by leveraging a rapid, solvent-free Joule heating method applied to lignin and biochar which is a renewable biomass residue. This method circumvents the conventional reliance on hazardous chemicals, metal catalysts, and high-vacuum systems, offering a scalable and energy-efficient pathway to produce oxygen-functionalized CNOs under ambient pressure. The resulting CNOs, with controlled particle sizes ranging from 33 to 36 nm and electrical conductivity between 3.73 and 3.95 S/m, exhibit excellent dispersibility in common solvents due to their surface oxygen functionalities. Importantly, both lignin- and biochar-derived CNOs demonstrated comparable structural and electrical characteristics, indicating the versatility of biomass feedstocks. When incorporated as a nanofiller in polylactic acid (PLA) at an optimized loading of 0.5%, CNOs significantly enhanced mechanical performance, increasing tensile strength and modulus by 43.7% and 128.4%, respectively, and improving impact strength by 60.4%. Additionally, CNO-modified PLA showed superior thermal stability, with increases of 6.3 °C in glass transition temperature and 13.7 °C in decomposition temperature, along with a 67.4% reduction in oxygen permeability and a 48.4% decrease in water vapor transmission. Compared to other carbon nanomaterials, the oxygenated CNOs exhibited superior reinforcement efficiency, affirming their potential as high-performance, green additives. With a low energy requirement of 15.6 MJ/kg for synthesis, this work pioneers a low-cost, eco-friendly route to engineer multifunctional CNOs for advanced polymer applications. In the second study, a fully biobased, high-performance nanocomposite was developed by integrating lignin-derived oxygen-functionalized carbon nano-onions (CNOs) into a polylactic acid (PLA) and wood flour matrix through a solvent-free, chemical-free melt extrusion process. This approach eliminated the need for any wood fiber pretreatment or the use of external coupling agents, significantly improving the sustainability and simplicity of the composite fabrication. The incorporation of a small amount of CNOs into the PLA/wood flour system led to simultaneous improvements in tensile strength, tensile modulus, impact resistance, and ductility—demonstrating that CNOs do not induce the typical strength–ductility tradeoff. Furthermore, CNO addition enhanced the thermal stability, flame retardancy, and reduced the water absorption of the resulting bio-composites. Microstructural analyses revealed a toughened fracture surface morphology characterized by matrix yielding and shear banding, suggesting effective energy dissipation during mechanical loading. The amphiphilic surface chemistry and turbostratic graphene structure of the CNOs facilitated their dual functionality as solid-state coupling agents and interfacial modifiers, enabling strong interactions with both hydrophilic wood fibers and the hydrophobic PLA matrix. The quasi-spherical geometry of CNOs contributed to a high interfacial surface area, promoting strong mechanical anchoring and molecular-level entanglement. Altogether, this study highlights the potential of CNOs as sustainable, multifunctional nanofillers for developing next-generation bio-composites with balanced mechanical and functional performance. In the final study, detailed in Chapter 4, biomass-derived carbon nano-onions (CNOs) were synthesized using a scalable Joule heating process and incorporated into epoxy/glass fiber composites to enhance mechanical performance. The morphology of the CNOs was controlled by tuning applied voltage and sample mass, with optimized conditions yielding well-defined, spherical graphitic shells. These CNOs were integrated into composites through three routes: epoxy matrix modification (Route I), fiber surface coating (Route II), and a hybrid combination (Route III). Mechanical testing revealed substantial improvements across all routes, with Route III showing the highest performance—tensile strength and modulus increased by 103% and 151%, respectively, while flexural strength improved by 39%. These enhancements were attributed to synergistic interactions between the oxygen-functionalized CNOs and the polymer matrix, including hydrogen bonding, π–π stacking, and covalent bonding, as well as mechanical interlocking. This study underscores the potential of biochar-derived CNOs as sustainable, high-performance nanofillers for advanced composite applications

Computational Modeling of Black Phosphorus Terahertz Photoconductive Antennas using COMSOL Multiphysics with Experimental Comparison against a Commercial LT-GaAs Emitter

This thesis presents computational models of terahertz (THz) photoconductive antenna (PCA) emitter using COMSOL Multiphysics commercial package. A comparison of the computer simulated radiated THz signal against that of an experimentally measured signal of commercial reference LT-GaAs emitter is presented. The two-dimensional model (2D) aimed at calculating the photoconductivity of a black phosphorus (BP) PCA at two laser wavelengths of 780 nm and 1560 nm. The 2D model was applied to the BP PCA emitter and the LT-GaAs devices to compare their simulated performance in terms of the photocurrent and radiated THz signal pulse. The results showed better performance of the BP PCA compared with that of LT-GaAs emitter. The three-dimensional model (3D) improved the accuracy of the solution by eliminating some assumptions included in the 2D model of the BP PCA such as the application of the actual bowtie geometry of the electrodes and the inclusion of the distribution of the laser footprint in x- and y- directions. Furthermore, the 3D model investigated the temperature variation in the BP PCA emitter due to the Joule heating from the conduction of the current induced by the bias voltage and the laser heating produced by the electromagnetic power dissipation of the laser. However, the 3D model introduced computational challenges (i.e., solution time, CPU, and memory, RAM) because of the multi-scale nature of the BP configuration from nanoscale to microscale. The parallel version of the COMSOL package was executed on the supercomputer of XSEDE at Pittsburg and the AHPCC at the University of Arkansas to successfully overcome these challenges. This helped to simulate a large case of total number of unknown of 313, 252,784.00 that required 3,202.98 GB RAM and 25 h CPU time on XSEDE Bridges. In addition, the TeraAlign THz experimental system, purchased from TeraView, Cambridge, UK, was used to measure the THz signal radiation of the commercial LT-GaAs emitters, demonstrating good agreement in terms of the pulse width and shape

Carbonaceous materials for their use as aircraft lightning strike protection

The main motivation behind this work, was to substitute the current technology used as lightning strike protection in the aircraft industry. This protection is composed of metallic meshes of foils, normally bronze, copper, and in some exceptional cases, for example in some fairings, aluminum in which cases, Glass Fiber Reinforced Polymer (GFRP) material will be added to avoid corrosion that direct contact between the Carbon Fiber (from the structural Carbon Fiber Reinforced Polymer material (CFRP)) and the Al might cause [1]. The bronze mesh adapts better to parts with complex geometries and is cheaper than cooper materials, however, its electrical conductivity is lower than the ones exhibited by copper meshes or foils. For those areas that need, not only Lightning Strike Protection (LSP), but also electromagnetic shielding, copper mesh or foils will be used such as Expanded Copper Foils (ECF), which is an epoxy pre-impregnated expanded copper foil that allows automated placement on the CFRP part.Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de MadridPresidente: Mauricio Terrones - Secretario: Francisco Javier Velasco Lopez - Vocal: José Sánchez Góme

Multifunctional Carbon Based Felt

Pyrolýza se běžně používá jako strategie pro zpracování odpadních textilií, přičemž přeměna textilního odpadu s vysokým obsahem uhlíku na uhlíkaté materiály je potřebná pro přípravu ekonomicky výhodných produktů s vyšší užitnou hodnotou ("upcycling") a zároveň zmírňuje dopad textilních odpadů na životní prostředí. Uhlíkové plsti jsou široce používána pro svoji relativně nízkou hmotnost a vysokou elektrickou vodivosti díky vnitřní 3 D vodivé síti. Přímé použití odpadních textilních plstí jako prekurzoru k výrobě uhlíkových plsti je však zkoumáno jen omezeně. Cílem této dizertační práce je optimalizace karbonizace odpadních plstí na bázi acrylic za kontrolovaných podmínek pro výrobu uhlíkových plstí a realizaci jejích multifunkčních aplikací. Byl zkoumán vliv různých metod zatížení odpadních plstí na bázi akrylových vláken s PTFE zátěrem na proces smršťování při pyrolýze. Byly hodnoceny mechanické vlastnosti, elektrické vlastnosti a tepelné vlastnosti finální uhlíkové plsti. Výsledky naznačují, že aplikace okrajového zatížení na vzorky během fáze karbonizace pomáhá snížit rychlost smrštění konečného produktu, což umožňuje uhlíkové plsti získat pružnost a vytvořit dobře strukturovanou vodivou síť. Pro studium vlivu zátěru PTFE na pyrolýzu plstí na bázi akrylových vláken byly plsti potaženy různými koncentracemi PTFE a následně podrobeny pyrolýze. Zkoumáním morfologie, mechanických vlastností a elektrických vlastností vzorků potažených PTFE bylo zjištěno, že vyšší koncentrace povlaku měly pozitivní dopad na mechanické a elektrické vlastnosti výsledné uhlíkové plsti. Vysoké koncentrace povlaku však vedly také ke ztrátě pružnosti uhlíkové plsti, což by mohlo vážně omezit jejich použitelnost. Zkoumáním morfologie a chování uhlíkových plstí připravených při různých teplotách karbonizace v režimu zatížení okraje bylo zjištěno, že zvýšení teploty karbonizace podpořilo růst krystalinity ve vláknech a tvorbu uspořádané grafitické struktury. Bylo dosaženo vytvoření husté, vysoce vodivé sítě s vysokou porozitou. Výsledky stínění elektromagnetického záření (EMI) prokázaly, že výsledná uhlíková plsť dosáhla vysoké účinnosti stínění EMI 55 dB a specifické účinnosti stínění 2676,9 dBcm?g1, čímž překonala mnoho uhlíkových kompozit. Kromě toho uhlíková plsť vykazovala vynikající účinnost ohmického ohřevu a vysoké rychlosti ohřevu. Strukturální stabilita byla zkoumána pomocí speciálně navrženého experimentu. Výsledky ukázaly, že si uhlíková plsť udržela stabilitu vnitřních vodivých drah po více cyklech ohybu. Bylo také zkoumáno využití připravených uhlíkových plstí pro tvorbu filtračních vrstev použitelných v respirátorech a filtračních maskách (filtrace vzduchu). Vynikající elektrická vodivost uhlíkové plsti umožnila její použití nejen jako respirační filtrační vrstvu, ale také pro vysokoteplotní elektrickou dezinfekci kontaminantů a mikrobů/virů. Konstrukce masky a vhodná konfigurace elektrod umožnila řízený odporový ohřev, zajišťující spolehlivost vysokoteplotní dezinfekce uhlíkové plsti. Účinnost filtrace a výsledky antibakteriálních testů ukázaly, že uhlíková plsť dosáhla více než 90 % účinnosti filtrace pro inhalovatelné částice a účinně inhibovala výskyt mikrobů. Flexibilní uhlíkové plsti mají obecně nižší výrobní náklady a vykazují dobrou chemickou a strukturální stabilitu. Výsledky funkčních testů ukázaly, že připravené uhlíkové plsti mají významný potenciál pro aplikace v oděvních ohřívačích, flexibilním stínění EMI, respiračních filtrech a dalších typech funkčních materálů.Pyrolysis has emerged as a strategy for processing waste textiles, with the conversion of high-carbon-content textile waste into carbonaceous materials being beneficial for recovering its economic value while mitigating the environmental impact of textile waste. Carbon felt is widely used due to its lightweight nature and internal 3D conductive network. However, limited research exists on directly using waste textile felts as a precursor to produce carbon felt. The aim of this thesis is to carbonize acrylic-based waste felts under controlled conditions to produce carbon felt and enable its multifunctional applications. To achieve the conversion of acrylic-based felts into flexible carbon felts with excellent performance, this study aims to investigate the impact of different loading tension methods and PTFE coatings during the pyrolysis process on the shrinkage rate, mechanical properties, electrical properties, and thermal properties of the resulting carbon felt. The results indicate that applying edge load to the samples during the carbonization stage helps to reduce the shrinkage rate of the final product, allowing the carbon felt to gain flexibility and form a well-structured conductive network. To study the impact of PTFE coating on the pyrolysis of acrylic -based felts, acrylic -based felts were coated with different concentrations of PTFE and subsequently subjected to pyrolysis. By examining the morphology, mechanical properties, and electrical properties of PTFE-coated samples, we found that higher coating concentrations had a greater impact on the performance of the resulting carbon felt. Although high coating concentrations increased the material's modulus and electrical conductivity, they also led to a loss of flexibility in the carbon felt, which could severely limit its application scope. By characterizing the morphology and structure of carbon felts prepared at different carbonization temperatures under an edge loading mode, it was found that increasing the carbonization temperature promoted higher crystallinity within the fibers and the formation of an ordered graphite structure. The formation of a dense, highly conductive network and high porosity was achieved. EMI shielding results demonstrated that the resulting carbon felt achieved a high EMI shielding effectiveness of 55 dB and a specific shielding effectiveness of 2676.9 dBcm?g1, surpassing many carbon composites. Additionally, the carbon felt exhibited excellent heating efficiency and high heating rates in resistive heating tests. Structural stability was investigated through a custom-designed experiment. The results showed that even under heating conditions, the carbon felt could maintain internal conductive pathway stability through multiple bending cycles. This work also investigated the feasibility of converting acrylic -based filter felts into carbon felts for use in respiratory filtration layers. The excellent electrical conductivity of carbon felt allows it to be used not only as a respiratory filtration layer but also for high-temperature electrical disinfection. The design of the mask body and the corresponding electrode configuration enabled controlled resistive heating performance, ensuring the reliability of high-temperature disinfection of the carbon felt. Filtration efficiency and antibacterial testing results showed that the carbon felt achieved over 90% filtration efficiency for inhalable particles and effectively inhibited microbial growth due to its antibacterial properties. Flexible carbon felt offers lower manufacturing costs and exhibits good chemical and structural stability. Functional testing results indicate that it demonstrates significant potential for applications in wearable heaters, flexible EMI shielding, respiratory filters, and other related fields

Carbon-based Functional structures from Pyrolysis of Kevlar Fabric Wastes

Tato disertační práce se zabývá využitím vlákenných aromatických polyamidových (Kevlarových) odpadů pro vývoj mikroporézních a elektricky vodivých materiálů na bázi aktivovaného uhlíku. Je použita metoda řízené, jednostupňové karbonizace. Odpadní Kevlarové textilie byl získány z regionálního výrobního závodu v České republice. Aktivované uhlíkové struktury byly připraveny pyrolýzou za různých podmínek, tj. několika typů inertní atmosféry, optimalizovaného časově teplotního režimu tepelného namáhání a finální teploty karbonizace (v rozmezí 500 °C až 1200 °C) tak, aby vznikly porézní a elektricky vodivé struktury. Byla zkoumána tepelná degradace Kevlaru, a složení těkavých produktů jeho pyrolýzy. Geometrické, fyzikální, morfologické, elektrické a termoelektrické vlastnosti připravených aktivovaných uhlíkových struktur byly studovány s ohledem na různé tepelné režimy a různé typy inertní atmosféry. Schopnost stínění proti elektromagnetickému rušení (EMI) ve vysokofrekvenčních oblastech (tj. na 2,45 GHz) a nízkofrekvenčních oblastech (tj. pod 1,5 GHz) byla zkoumána pomocí metody vlnovodu a metody koaxiálního přenosu. Dále bylo studováno chování ohmického ohřevu aktivovaných uhlíkových struktur jako funkce použitého elektrického výkonu a času. Progresivní změny koncentrací plynných produktů tepelného rozkladu Kevlaru v závislosti na teplotě pyrolýzy a jejich rozdíly v těkavosti jsou vyhodnoceny pomocí algoritmu pro separaci směsných spekter získaných UV spektroskopií.This dissertation is dealing with the use of fibrous aromatic polyamide (Kevlar) waste for the development of microporous and electrically conductive materials based on activated carbon. A method of controlled, one-stage carbonation is used. The waste Kevlar fabric was obtained from a regional manufacturing plant in the Czech Republic. Activated carbon structures were prepared by pyrolysis under different conditions, i.e. several types of inert atmosphere, optimized time-temperature mode of heat stress, and final carbonization temperature (in the range of 500 °C to 1200 °C) so as to create porous and electrically conductive structures. The thermal degradation of Kevlar and the composition of volatile products of its pyrolysis were investigated. The geometric, physical, morphological, electrical, and thermoelectric properties of the prepared activated carbon structures were studied with respect to different thermal modes and different types of inert atmospheres. The electromagnetic interference (EMI) shielding capability in the high frequency (i.e. at 2.45 GHz) and low frequency (i.e. below 1.5 GHz) regions was investigated using the waveguide method and the coaxial line transmission method. Furthermore, the ohmic heating behavior of activated carbon structures was studied as a function of the applied electric power and time. Progressive changes in concentrations of gaseous products of thermal decomposition of Kevlar depending on the pyrolysis temperature and their differences in volatility were evaluated using an algorithm for the separation of mixed spectra obtained by UV spectroscopy

The Effect of Templated Graphitization on Electromechanical Properties of Carbon Nanofiber

Theory predicts that carbon nanofibers (CNFs), processed via carbonizing polymeric nanofibers, as down-sized version of carbon fibers (CFs) should be significantly stronger than CFs, due to size-dependent defects in CFs such as skin-core radial inhomogeneity. To close the gap between the predictions and experimentally achieved strength of CNFs, the processing-microstructure-properties relationship in CNFs was studied. The CNFs in my study were fabricated by thermal stabilization and carbonization of electrospun polyacrylonitrile (PAN) nanofibers which contain CNTs inclusions. In this research the formation of graphite-like structures (turbostratic domains) within CNFs was promoted by adding CNTs to the precursor in a process known as templated graphitization, in which the presence of CNTs can facilitate the arrangement of carbon atoms, obtained as a result of the carbonization of PAN, into a graphite-like structure (sp² carbon bonds) similar to what exists in CNTs. It is further demonstrated that the templating effect of CNTs is more pronounced when PAN chains are aligned with each other and with CNTs, as was achieved in this research by hot-drawing the precursors. The existence of CNTs effectively promotes the formation of highly ordered polymer interphase. The study on the microstructure and mechanical properties of CNFs confirms that the modification of the precursor microstructure, such as enhanced chain alignment, can be maintained during carbonization, and indeed leads to enhanced graphitic alignment. Based on the MEMS-based nano-mechanical tension tests on CNFs, the combined effect of precursor hot-drawing and graphitic templating effect resulted in CNFs with tensile strength and modulus of 6.9 GPa and 250 GPa, respectively, which are the largest values reported up to date for this type of material. Moreover, the multifunctional properties of electrospun CNFs, including piezoresisitivty and electrical conductivity, were restudied both experimentally and via continuum models, demonstrating the strong correlation between microstructure modification and properties improvement. In summary, a clear strategy for developing low-cost high performance CNF/CNTs hybrid nanofibers was obtained based on new understanding of the load bearing mechanism within CNF. These nanofibers can be used as the multifunctional building blocks for a host of applications, including aerospace and automotive industry

FABRICATION, CHARACTERIZATION AND APPLICATIONS OF HIGHLY CONDUCTIVE WET-SPUN PEDOT:PSS FIBERS

Smart electronic textiles cross conventional uses to include functionalities such as light emission, health monitoring, climate control, sensing, storage and conversion of energy, etc. New fibers and yarns that are electrically conductive and mechanically robust are needed as fundamental building blocks for these next generation textiles. Conjugated polymers are promising candidates in the field of electronic textiles because they are made of earth-abundant, inexpensive elements, have good mechanical properties and flexibility, and can be processed using low-cost large-scale solution processing methods. Currently, the main method to fabricate electrically conductive fibers or yarns from conjugated polymers is the deposition of the conducting polymer onto an inert fiber support by using different techniques. However, the volume occupied by the electrically active coating is generally very small relative to the volume of insulating fiber acting as support. Therefore, when considering the total volume, the bulk electrical conductivity of these coated textiles is usually small, often lower than 10 S/cm, which limits their applications. An interesting alternate approach would be to fabricate fibers directly from the electrically conductive material avoiding the need for an inert-fiber support. Therefore, in this work, a wet-spinning process for the fabrication of PEDOT:PSS fibers with high electrical conductivity and robust mechanical properties is described. The process includes a coagulating step, a drawing step in a dimethyl sulfoxide bath and two drying steps. The effect that drawing the fibers in the DMSO bath has on the electrical, thermoelectric and mechanical properties of the fibers is studied and correlated to the changes observed in the fibers’ structure. In general, the fibers with the highest state of preferential orientation of crystal planes are also the most conductive and stiffest. In order to further improve the electrical properties of the fibers, substituting the DMSO drawing step by a sulfuric acid drawing step in the fabrication process is investigated. The sulfuric acid drawn fibers have higher electrical conductivities and better mechanical properties than the DMSO drawn fibers. In fact, electrical conductivities as high as 4039 S/cm and break stresses around 550 MPa are obtained which, to the best of our knowledge, are the highest reported for a PEDOT:PSS fiber. The mechanism by which sulfuric acid enhances the electrical and mechanical properties of the fibers is also investigated. It is found that the sulfuric acid treatment is very efficient removing PSS from the fibers while also promoting substitution of PSS by sulfates as counterions. The removal of PSS and substitution of counterions leads to a reorganization of the crystal structure of the fibers that is more favorable for charge transport. The last part of this work focuses on the application of the fibers. The mechanical properties of the fibers are compared to traditional textile fibers. Additionally, the time stability of the electrical conductivity of the fibers is also studied. Moreover, the maximum current carrying capacity or ampacity of the fibers is investigated together with some Joule heating-based applications such as thermochromic textiles. A thermoelectric textile device is also demonstrated using the fibers as the p-type legs. Finally, electrochemical applications of the fibers are discussed and demonstrated

Artificial Muscles

Course material for "Artificial Muscles" e-course

Ultrafast carrier dynamics in terahertz photoconductors and photomixers: beyond short-carrier-lifetime semiconductors

Efficient terahertz generation and detection are a key prerequisite for high performance terahertz systems. Major advancements in realizing efficient terahertz emitters and detectors were enabled through photonics-driven semiconductor devices, thanks to the extremely wide bandwidth available at optical frequencies. Through the efficient generation and ultrafast transport of charge carriers within a photo-absorbing semiconductor material, terahertz frequency components are created from the mixing products of the optical frequency components that drive the terahertz device – a process usually referred to as photomixing. The created terahertz frequency components, which are in the physical form of oscillating carrier concentrations, can feed a terahertz antenna and get radiated in case of a terahertz emitter, or mix with an incoming terahertz wave to down-convert to DC or to a low frequency photocurrent in case of a terahertz detector. Realizing terahertz photoconductors typically relies on short-carrier-lifetime semiconductors as the photo-absorbing material, where photocarriers are quickly trapped within one picosecond or less after generation, leading to ultrafast carrier dynamics that facilitates high-frequency device operation. However, while enabling broadband operation, a sub-picosecond lifetime of the photocarriers results in a substantial loss of photoconductive gain and optical responsivity. In addition, growth of short-carrier-lifetime semiconductors in many cases relies on the use of rare elements and non-standard processes with limited accessibility. Therefore, there is a strong motivation to explore and develop alternative techniques for realizing terahertz photomixers that do not rely on these defect-introduced short-carrier-lifetime semiconductors. This review will provide an overview of several promising approaches to realize terahertz emitters and detectors without short-carrier-lifetime semiconductors. These novel approaches utilize p-i-n diode junctions, plasmonic nanostructures, ultrafast spintronics, and low-dimensional materials to offer ultrafast carrier response. These innovative directions have great potentials for extending the applicability and accessibility of the terahertz spectrum for a wide range of applications