Alexander Popp

Technical University Darmstadt, Darmstadt, Hesse, Germany

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Publications (13)

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    [Show abstract] [Hide abstract] ABSTRACT: Nanostructured materials lie at the heart of fundamental advances in efficient energy storage and/or conversion, in which surface processes and transport kinetics play determining roles. This review describes recent developments in the synthesis and characterization of composites which consist of lithium metal phosphates (LiMPO(4), M = Fe, Co, Ni, Mn) coated on nanostructured carbon architectures (unordered and ordered carbon nanotubes, amorphous carbon, carbon foams). The major goal of this review is to highlight new progress in using different three dimensional nanostructured carbon architectures as support for the phosphate based cathode materials (e.g.: LiFePO(4), LiCoPO(4)) of high electronic conductivity to develop lithium batteries with high energy density, high rate capability and excellent cycling stability resulting from their huge surface area and short distance for mass and charge transport.
    Full-text Article · Apr 2012 · Chemical Society Reviews
  • O Yilmazoglu · A Popp · D Pavlidis · [...] · G Battenberg
    [Show abstract] [Hide abstract] ABSTRACT: We report a simple method for the micro-nano integration of flexible, vertically aligned multiwalled CNT arrays sandwiched between a top and bottom carbon layer via a porous alumina (Al(2)O(3)) template approach. The electromechanical properties of the flexible CNT arrays have been investigated under mechanical stress conditions. First experiments show highly sensitive piezoresistive sensors with a resistance decrease of up to ∼35% and a spatial resolution of <1 mm. The results indicate that these CNT structures can be utilized for tactile sensing components. They also confirm the feasibility of accessing and utilizing nanoscopic CNT bundles via lithographic processing. The method involves room-temperature processing steps and standard microfabrication techniques.
    Article · Feb 2012 · Nanotechnology
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    [Show abstract] [Hide abstract] ABSTRACT: Hybrid materials composed of LiMPO4 (M = Fe or Co) with multiwalled carbon nanotubes (MCNTs) were synthesized by tethering lithium phosphoolevins on isolated stochastically disordered MWCNTs as well as on ordered 3D MWCNT arrays via solution based impregnation routes. Ordered 3D arrays of MWCNT monoliths comprising MWCNTs with nominal tube diameters of 60 and 200 nm were synthesized by a catalyst free, template based method, with porous aluminum oxide (PAOX) acting as a template. Consecutive selectiveetching processes gave free standing aligned 3D carbon nanotube (CNT) architectures that were used as supporting cathode structures for electroactive LiCoPO4. LiCoPO4 nanoparticle suspensions derived from an ethanol based organic phosphate source turned out to be superior for the tethering of LiCoPO4 nanoparticles onto the 3D aligned CNT arrays compared to an aqueous based tethering route, thus giving hybrid materials with better electrochemical battery performance compared to materials generated by the latter method. Li ion extraction within the ordered 3D CNT/LiCoPO4 composites seems to be a two-step process and the Li intercalation a one-step process, highlighting the enhanced kinetics of the Li insertion process in the 3D CNT/LixCoPO4 composite in comparison to a particulate mixture of LixCoPO4 that is typically present in a conventional carbon/LiCoPO4 cathode composite system.
    Full-text Article · Oct 2011 · Berichte der deutschen chemischen Gesellschaft
  • Oktay Yilmazoglu · Ravi Joshi · Alexander Popp · [...] · Jörg J. Schneider
    [Show abstract] [Hide abstract] ABSTRACT: Field emission from aligned carbon nanotube (CNT) blocks and bundles grown by two different chemical vapor deposition (CVD) methods were investigated. A single CNT block consists of about 1600 double-walled nanotubes per μm2 of area with an intertube distance in the range 15–20 nm, while a bundle consists of a few multiwalled CNTs, which contact each other in the tip region. Hydrogen/water assisted catalytic CVD on a silicon wafer resulted in ultrahigh aspect ratios for the CNT blocks with extremely high field enhancement coefficients >11 000, a turn-on field of 0.36 V/μm, and good field emission stability. The water and hydrogen concentration ratio were found to be a critical parameter for establishing high quality, ultralong (up to 2 mm) CNT growth. The second CVD growth method used was a noncatalytic template assisted technique for bundle generation. Special self-organized anodic alumina films controlled the synthesis of highly aligned carbon nanotube arrays with the desired tube geometry and adjustable intertube distances. The CNT bundle generation was adjusted by the CNT tube diameter, wall thickness, and length-controlled growth. Subsequent etching of the Al2O3 template resulted in substrateless flexible CNT bundles permitting simple gate electrode integration process. Field emission tests showed that the field enhancement factor, turn-on field, and field emission current density of the CNT cathode arrays could be optimized by adjusting the block or bundle distance. The authors obtained, for the block emitter, a smaller turn-on field and better stability. The block emitter with the better performance was additionally coated with CVD grown ZnO nanoparticles to obtain further improvement. The ZnO nanoparticle diameter was in the range o- - f 15–20 nm. First results showed partial clustering of the ZnO particles suggesting the need for further optimization of the deposition.
    Article · Mar 2011 · Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures
  • Jörg J. Schneider · Alexander Popp · Jörg Engstler
    Chapter · Jul 2010
  • [Show abstract] [Hide abstract] ABSTRACT: A template assisted technique was used to synthesize carbon nanotubes (CNTs) in a highly ordered manner by a noncatalytic chemical vapor deposition technique. Special self-organized anodic alumina films control the syntheses of highly aligned carbon nanotube arrays with desired tube geometry, adjustable intertube distances, and unique CNT bundle generation, which are extremely important for CNT array optimization with high field enhancement factors and high current densities. These CNT arrays also meet industrial demands such as uniformity, low cost, and easy preparation. The CNT bundle generation was adjusted by the CNT tube-diameter, wall-thickness, and length-controlled growth. Field emission tests showed that the turn-on field of the CNT cathode arrays could be lowered by adjusting the intertube distance. A vacuum field emission triode with self-aligned gate using these CNT arrays as field emitter is also presented. The proposed CNT configuration allows the realization of cathode arrays on flexible substrates and is very promising for improving the characteristics of such devices. Moreover, micro/nanointegration opens the possibility for their mass production.
    Article · Apr 2010 · Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society
  • O. Yilmazoglu · R. Joshi · A. Popp · [...] · J. J. Schneider
    [Show abstract] [Hide abstract] ABSTRACT: Carbon nanotubes (CNTs) were grown by two different methods. The catalytic growth on a silicon wafer showed ultra high aspect ratios for the CNTs with extremely high field enhancement coefficients >25000 and a turn-on field of 0.36 V/μm, whereas the non-catalytic growth resulted in substrateless flexible CNT bundles with a simple gate electrode integration process. Results on an additional coating of the CNT emitter with ZnO with modified field-emission characteristics will be presented.
    Article · Jan 2010
  • Alexander Popp · Vassilios Siozios · Jörg J. Schneider
    [Show abstract] [Hide abstract] ABSTRACT: A novel approach is presented for the integration of highly aligned carbon nanotubes in ceramic alumina layers perpendicular to the tube axes. A template method is used to form carbon nanotubes by a noncatalytic chemical vapor deposition inside the pores of porous alumina before consequent thermal treatment of the resulting composite on the frontal surface. The thermal treatment leads to a well-defined layered phase transition accompanied by heterogeneous etching behavior of the alumina phases. Almost freestanding carbon nanotubes sandwiched in facial-oriented corundum layers are obtained after removal of the soluble phases. These ceramic layers drastically improve the handling and stability of highly aligned carbon nanotubes.
    Article · Dec 2009 · Journal of the American Ceramic Society
  • Alexander Popp · Jörg Engstler · Jörg J. Schneider
    [Show abstract] [Hide abstract] ABSTRACT: A monolithic carbon structure of macroscopic dimensions is obtained by non-catalytic chemical vapour deposition using templating porous alumina and removal of the alumina template. The carbon structure is mechanically stable, flexible and consists of freestanding, parallel aligned carbon nanotubes (CNTs) merging into a top and bottom carbon layer perpendicular to the tube axes. The nature of the carbonaceous material is further investigated by SEM, TEM, Raman spectroscopy and XRD. In a second templating step, the void space formerly occupied by the alumina is recaptured by metallic and ceramic materials resulting in CNT reinforced composites with well-defined tube interspaces, whereas the interior of the CNTs is still accessible.
    Article · Nov 2009 · Carbon
  • Alexander Popp · Oktay Yilmazoglu · Oktay Kaldirim · [...] · Dimitris Pavlidis
    [Show abstract] [Hide abstract] ABSTRACT: A novel approach towards fabricating free-standing and self-supporting highly aligned CNT arrays with integrated top and bottom contacts for the design and implementation of future CNT-based sensor devices is reported.
    Article · Jul 2009 · Chemical Communications
  • Alexander Popp · Jörg J Schneider
    [Show abstract] [Hide abstract] ABSTRACT: (Figure Presented) Processes on nanometer-scale dimensions are possible in a mechanically stable, flexible, and monolithic porous 3D carbon nanotube structure (see picture). The nanoscale chemical reactor exhibits an extremely high surface-to-volume ratio of up to 2×107 m2 m3, which depends on the tube diameter and tube separation. This exceeds values for current microreactors by at least two orders of magnitude.
    Article · Nov 2008 · Angewandte Chemie International Edition
  • Alexander Popp · Jörg J. Schneider
    [Show abstract] [Hide abstract] ABSTRACT: Mit einer mechanisch stabilen, flexiblen und porösen Funktionseinheit aus Kohlenstoffnanoröhren kann ein kontinuierlich betriebener nanostrukturierter Reaktor realisiert werden (siehe Bild). Je nach Röhrendurchmesser und -abstand erreicht der Reaktor extrem hohe Oberfläche-Volumen-Verhältnisse (bis 2×107 m2 m−3), was die Kennzahlen aktueller Mikroreaktoren um mindestens zwei Größenordnungen übertrifft.
    Article · Oct 2008 · Angewandte Chemie
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    [Show abstract] [Hide abstract] ABSTRACT: We have developed a chemical vapor deposition (CVD) process for the catalytic growth of carbon nanotubes (CNTs), anchored in a comose-type structure on top of porous alumina substrates. The mass-flow conditions of precursor and carrier gases and temperature distributions in the CVD reactor were studied by transient computational fluid dynamic simulation. Molecular-beam quadrupole mass spectroscopy (MB-QMS) has been used to analyze the gas phase during ferrocene CVD under reaction conditions (1073 K) in the boundary layer near the substrate. Field-emission (FE) properties of the nonaligned CNTs were measured for various coverages and pore diameters of the alumina. Samples with more dense CNT populations provided emitter-number densities up to 48,000 cm(-2) at an electric field of 6 V microm(-1). Samples with fewer but well-anchored CNTs in 22-nm pores yielded the highest current densities. Up to 83 mA cm(-2) at 7 V microm(-1) in dc mode and more than 200 mA cm(-2) at 11 V microm(-1) in pulsed diode operation have been achieved from a cathode size of 24 mm2.
    Full-text Article · Jul 2007 · Small