Quasi-switched inverter using space vector pulse width modulation with triangular comparison for photovoltaic applications

Este trabajo analiza un prototipo para un inversor elevador cuasi-conmutado (qSBI) alimentando una carga resistiva aislada desde una fuente de CC. Se propone el uso de una modulación de ancho de pulso de vectores espaciales (SPWM) con comparación triangular que genera un incremento en el factor de ganancia del qSBI, y se contrasta su desempeño con otro tipo de modulaciones de vectores espaciales, tales como las modulaciones discontinuas. Para verificar la validez de la extensión de rango de tensión en el convertidor qSBI, se desarrolló una plataforma de pruebas semi-personalizada. Esta plataforma utiliza una tarjeta DSP de punto flotante (Analog Devices ADSP-21369) para el procesamiento de las estrategias de control, y una tarjeta de interfaz que incluye un arreglo lógico programable (FPGA) de Xilinx (Spartan-3), que permite desarrollar la modulación sincronizada que el qSBI necesita. Los resultados experimentales demuestran mejoras en el desempeño del convertidor qSBI en cuanto al factor de ganancia, reducción del estrés de voltaje en el capacitor y los perfiles de corriente de entrada. Las estrategias discontinuas de modulación del vector espacial no presentan un buen desempeño cuando se compara con las modulaciones continuas SVPWM o SPWM, ya que los niveles de rizado en las corrientes tomadas del módulo PV son de aproximadamente el doble que en el caso de las técnicas de modulación continuas. Finalmente, el uso del convertidor qSBI como microinversor es puesto en evidencia por dos casos experimentales prácticos de un sistema fotovoltaico PV con un algoritmo de ajuste del máximo punto de potencia (MPPT).This work analyzes a prototype of a quasi-switched boost inverter (qSBI) feeding an isolated resistive load from a DC source. The use of spatial vector pulse width modulation (SPWM) with triangular comparison is proposed to increase the qSBI gain factor, and its performance is contrasted with other types of spatial vector modulations, such as discontinuous modulations. To verify the validity of the method for voltage range extension in the qSBI converter, a semi-customized test platform was developed. This platform uses a DSP floating point card (Analog Devices ADSP-21369) for processing and control strategies and an interface card that includes a programmable logic array (FPGA) from Xilinx (Spartan-3), which allows to develop the synchronized modulation qSBI needs. The experimental results show improvements in the performance of the qSBI converter in terms of gain factor, voltage reduction in the capacitor, and input current profiles. Discontinuous space vector modulation strategies do not perform well when compared to continuous SVPWM or SPWM modulations, because the ripple levels in the currents taken from the PV module are approximately twice as great as in continuous modulation techniques. Finally, the usefulness of a qSBI as PV microinverter is confirmed by two practical experimental cases of a PV photovoltaic system with a maximum power point adjustment algorithm (MPPT)

Distributed feedback lasers based on dichromated poly(vinyl alcohol) reusable surface-relief gratings

A simple, low-cost and versatile holographic method to produce reusable surface-relief gratings on dichromated poly(vinyl alcohol) (DCPVA) films, with periods Λ ≥ 270 nm and modulation depths up to 300 nm, is reported. DCPVA presents processing advantages with respect to other photoresists, such as dichromated gelatine, i.e. higher light sensitivity, simpler fabrication process and better batch to batch reproducibility. The successful use of these DCPVA relief gratings as distributed feedback (DFB) laser resonators is demonstrated. Second-order DFB devices emitting in the wavelength range 577-614 nm have been easily prepared by coating a dye-doped polymer film on top of the resonators.We thank the Spanish Government (MINECO) and the European Union (FEDER) for grant no. MAT2011-28167-C02

Quasi-chemical approximation for polyatomics: Statistical thermodynamics of adsorption

The statistical thermodynamics of interacting polyatomic adsorbates (k-mers) on homogeneous surfaces was developed on a generalization in the spirit of the lattice-gas model and the quasi-chemical approximation (QCA). The new theoretical framework is obtained by combining (i) the exact analytical expression for the partition function of non-interacting linear k-mers adsorbed in one dimension and its extension to higher dimensions, and (ii) a generalization of the classical QCA in which the adsorbate can occupy more than one adsorption site. The coverage and temperature dependence of the Helmholtz free energy, chemical potential, configurational entropy, configurational energy, isosteric heat of adsorption and specific heat are given. The formalism reproduces the classical QCA for monomers, leads to the exact statistical thermodynamics of interacting k-mers adsorbed in one dimension, and provides a close approximation for two-dimensional systems accounting multisite occupancy. Comparisons with analytical data from Bragg-Williams approximation (BWA) and Monte Carlo simulations are performed in order to test the validity of the theoretical model. The resulting thermodynamic description is significantly better than the BWA and still mathematically handable.Fil: Dávila, M.. Universidad Nacional de San Luis; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; ArgentinaFil: Romá, Federico José. Universidad Nacional de San Luis; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis; ArgentinaFil: Riccardo, Jose Luis. Universidad Nacional de San Luis; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis; ArgentinaFil: Ramirez Pastor, Antonio Jose. Universidad Nacional de San Luis; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis; Argentin

Interaction of kinematic, kinetic, and energetic predictors of young swimmers’ speed

The aim of this study was to assess the interaction of kinematic, kinetic, and energetic variables as speed predictors in adolescent swimmers in the front-crawl stroke. Design: Ten boys (mean age [SD] = 16.4 [0.7] y) and 13 girls (mean age [SD] = 14.9 [0.9] y) were assessed. Methods: The swimming performance indicator was a 25-m sprint. A set of kinematic, kinetic (hydrodynamic and propulsion), and energetic variables was established as a key predictor of swimming performance. Multilevel software was used to model the maximum swimming speed. Results: The final model identified time (estimate = −0.008, P = .044), stroke frequency (estimate = 0.718, P < .001), active drag coefficient (estimate = −0.330, P = .004), lactate concentration (estimate = 0.019, P < .001), and critical speed (estimate = −0.150, P = .035) as significant predictors. Therefore, the interaction of kinematic, hydrodynamic, and energetic variables seems to be the main predictor of speed in adolescent swimmers. Conclusions: Coaches and practitioners should be aware that improvements in isolated variables may not translate into faster swimming speed. A multilevel evaluation may be required for a more effective assessment of the prediction of swimming speed based on several key variables rather than a single analysisThis work is supported by national funds (FCT–Portuguese Foundation for Science and Technology) under the project UIDB/DTP/04045/2020.info:eu-repo/semantics/publishedVersio

A high-temperature heat pump for compressed heat energy storage applications: Design, modeling, and performance

The current paper presents the design and performance of a high-temperature heat pump (HTHP) integrated in an innovative, sensible, and latent heat storage system. The HTHP has been designed to work between a heat source from 40 to 100 °C and a heat sink above 130 °C. An initial refrigerant analysis has revealed that R-1233zd(E) is the best candidate to meet the required performance and environmental considerations. The first part of this paper deals with the sizing and selection of the main components while discussing the challenges and working limits. A numerical model is also presented and validated. The second part of the paper is dedicated to develop parametric studies and performance maps under different operating conditions. The results show that the current HTHP, at a source temperature of 80 °C, consumes from 3.23 to 9.88 kW by varying the compressor’s speed from 500 to 1500 rpm. Heat production is achieved in the form of latent heat (7.40 to 21.59 kW) and sensible heat (from 6.35 to 17.94 kW). The heating coefficient of performance (COPHTHP) is around 4.This work has been partially funded by grant agreement No. 764042 (CHESTER project) of the European Union’s Horizon 2020 research and innovation program. The authors would like to express their deep gratitude to Prof. Dr. Jose Miguel Corberán Salvador for his perseverance, encouragement, and invaluable guidance during this work

Perylenediimide-based distributed feedback lasers with holographic relief gratings on dichromated gelatine

One dimensional second-order distributed feedback (DFB) lasers consisting of polystyrene (PS) films doped with a perylenediimide laser dye, deposited over dichromated gelatine (DCG) photoresist layers with solvent resistant relief gratings recorded by holographic lithography, are reported. The advantage of using the grating on DCG in the final device is that the fabrication process is simplified and the grating pattern better preserved, since no etching methods to transfer the grating to another substrate are needed. A very simple model, proposed to explain the experimental waveguide properties, has allowed identifying the waveguide mode at which DFB emission appears, which was the key to optimize the device performance. In the frame of this model, the thickness of the PS and DCG films could be adjusted in order to minimize the laser threshold and to control its wavelength tuneability. The performance of these lasers is comparable to that of recently reported devices based on the same active material, but deposited over high-quality DFB gratings engraved on SiO2 by nanoimprint lithography.We thank support from the Spanish Government (MINECO) and the European Community (FEDER) through Grant Nos. MAT2008-06648-C02 and MAT-2011-28167-C02, as well as to the University of Alicante. M.G.R. was supported by a CSIC fellowship within the program JAE

Unraveling the Influence of the Electrolyte on the Polarization Resistance of Nanostructured La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes

Large variations in the polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes are reported in the literature, which are usually related to different preparation methods, sintering temperatures, and resulting microstructures. However, the influence of the electrolyte on the electrochemical activity and the rate-limiting steps of LSCF remains unclear. In this work, LSCF nanostructured electrodes with identical microstructure are prepared by spray-pyrolysis deposition onto different electrolytes: Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO), La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM), and Bi1.5Y0.5O3-δ (BYO). The ionic conductivity of the electrolyte has a great influence on the electrochemical performance of LSCF due to the improved oxide ion transport at the electrode/electrolyte interface, as well as the extended ionic conduction paths for the electrochemical reactions on the electrode surface. In this way, the polarization resistance of LSCF decreases as the ionic conductivity of the electrolyte increases in the following order: YSZ > LSGM > CGO > BYO, with values ranging from 0.21 Ω cm2 for YSZ to 0.058 Ω cm2 for BYO at 700 °C. In addition, we demonstrate by distribution of relaxation times and equivalent circuit models that the same rate-limiting steps for the ORR occur regardless of the electrolyte. Furthermore, the influence of the current collector material on the electrochemical performance of LSCF electrodes is also analyzedThis work was funded by PID2021-126009OB-I00 and PID2019-110249RB-I00 (Ministerio de Ciencia, Innovación y Universidades) and UMA18-FEDERJA-033 (Junta de Andalucia, Spain/FEDER). J.Z.G. thanks the Ministerio de Ciencia, Innovación y Universidades for his FPU grant (FPU17/02621). L.C.J. would like to thank Plan Andaluz de Investigación, Desarrollo e Innovación (PAIDI 2020) for the research support (DOC 01168)

Nanostructured composites as active layer to boost cathode performance in Solid Oxide Fuel Cells

Since the efficiency of Solid Oxide Fuel Cells (SOFCs) is largely limited by the high polarization resistance of the cathode, several strategies have been proposed to enhance the electrochemical activity of such electrodes. Among them, optimizing the electrode microstructure by using different preparation methods, such as infiltration and spray-pyrolysis deposition, have rendered excellent and durable electrochemical performance. In addition, the tailoring of the electrode/electrolyte interface by incorporating active layers have proven to be particularly useful to improve electrode properties. The present work proposes alternative active layers based on nanocomposites by combining the properties of the La0.8Sr0.2MnO3-d (LSM) cathode and different ionic conductors with fluorite-type structure. Different nanocomposite layers were prepared by spray-pyrolysis deposition at 450 ºC for 30 min on Zr0.8Y0.16O1.92 (YSZ) electrolyte, i.e. LSM-Ce0.9Gd0.1O1.95 (CGO) and LSM-Bi1.5Y0.5O3 (BYO). Thereafter, the LSM was screen-printed on the YSZ pellet and sintered at 1000 ºC. The nanocomposite active layers were studied by different structural and microstructural techniques, such as XRD, SEM-EDX and HRTEM. The electrochemical properties of active layers were also investigated by impedance spectroscopy at different dc-bias and distribution of relaxation times. Similarly, fuel cell tests were performed in a NiO-YSZ anode supported cell. The nanocomposite layers were dense with a thickness of approximately 700 nm. Specially LSM-CGO layers showed improved adherence to the electrolyte without the presence of cracks, delamination or undesired reaction. Cathodes with active layer showed Area Specific Resistance (ASR) associated with a lower charge transfer resistance and a fast oxide ion transport at the electrode/electrolyte interface.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Real-time monitoring of fenitrothion in water samples using a silicon nanophotonic biosensor

[EN] Due to the large quantities of pesticides extensively used and their impact on the environment and human health, a prompt and reliable sensing technique could constitute an excellent tool for in-situ monitoring. With this aim, we have applied a highly sensitive photonic biosensor based on a bimodal waveguide interferometer (BiMW) for the rapid, label-free, and speci¿c quanti¿cation of fenitrothion (FN) directly in tap water samples. After an optimization protocol, the biosensor achieved a limit of detection (LOD) of 0.29 ng mL¿¿1 (1.05 nM) and a half-maximal inhibitory concentration (IC50)of 1.71 ng mL¿¿1 (6.09 nM) using a competitive immunoassay and employing diluted tap water. Moreover, the biosensor was successfully employed to determine FN concentration in blind tap water samples obtaining excellent recovery percentages with a time-to-result of only 20 min without any sample pre-treatment. The features of the biosensor suggest its potential application for real time, fast and sensitive screening of FN in water samples as an analytical tool for the monitoring of the water quality.This work received financial support from DIONISOS Project (Retos Colaboracion RTC-2017-6222-5). The ICN2 is funded by the CERCA programme/Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, grant no. SEV-2017-0706)Ramirez-Priego, P.; Estévez, M.; Díaz-Luisravelo, HJ.; Manclus Ciscar, JJ.; Montoya, Á.; Lechuga, LM. (2021). Real-time monitoring of fenitrothion in water samples using a silicon nanophotonic biosensor. Analytica Chimica Acta. 1152:1-9. https://doi.org/10.1016/j.aca.2021.338276S191152Sánchez-Santed, F., Colomina, M. T., & Herrero Hernández, E. (2016). Organophosphate pesticide exposure and neurodegeneration. Cortex, 74, 417-426. doi:10.1016/j.cortex.2015.10.003Chough, S. H., Mulchandani, A., Mulchandani, P., Chen, W., Wang, J., & Rogers, K. R. (2002). Organophosphorus Hydrolase-Based Amperometric Sensor: Modulation of Sensitivity and Substrate Selectivity. Electroanalysis, 14(4), 273-276. doi:10.1002/1521-4109(200202)14:43.0.co;2-5Richardson, J. R., Fitsanakis, V., Westerink, R. H. S., & Kanthasamy, A. G. (2019). Neurotoxicity of pesticides. Acta Neuropathologica, 138(3), 343-362. doi:10.1007/s00401-019-02033-9Giordano, G., Afsharinejad, Z., Guizzetti, M., Vitalone, A., Kavanagh, T. J., & Costa, L. G. (2007). Organophosphorus insecticides chlorpyrifos and diazinon and oxidative stress in neuronal cells in a genetic model of glutathione deficiency. Toxicology and Applied Pharmacology, 219(2-3), 181-189. doi:10.1016/j.taap.2006.09.016Çakir, Ş., & Sarikaya, R. (2005). Genotoxicity testing of some organophosphate insecticides in the Drosophila wing spot test. Food and Chemical Toxicology, 43(3), 443-450. doi:10.1016/j.fct.2004.11.010Rahman, M. F., Mahboob, M., Danadevi, K., Saleha Banu, B., & Grover, P. (2002). Assessment of genotoxic effects of chloropyriphos and acephate by the comet assay in mice leucocytes. Mutation Research/Genetic Toxicology and Environmental Mutagenesis, 516(1-2), 139-147. doi:10.1016/s1383-5718(02)00033-5Yeh, S.-P., Sung, T.-G., Chang, C.-C., Cheng, W., & Kuo, C.-M. (2005). Effects of an organophosphorus insecticide, trichlorfon, on hematological parameters of the giant freshwater prawn, Macrobrachium rosenbergii (de Man). Aquaculture, 243(1-4), 383-392. doi:10.1016/j.aquaculture.2004.10.017Smith, A. G., & Gangolli, S. D. (2002). Organochlorine chemicals in seafood: occurrence and health concerns. Food and Chemical Toxicology, 40(6), 767-779. doi:10.1016/s0278-6915(02)00046-7Kumar, P., Kim, K.-H., & Deep, A. (2015). Recent advancements in sensing techniques based on functional materials for organophosphate pesticides. Biosensors and Bioelectronics, 70, 469-481. doi:10.1016/j.bios.2015.03.066Schellin, M., Hauser, B., & Popp, P. (2004). Determination of organophosphorus pesticides using membrane-assisted solvent extraction combined with large volume injection–gas chromatography–mass spectrometric detection. Journal of Chromatography A, 1040(2), 251-258. doi:10.1016/j.chroma.2004.04.006Sánchez, M. E., Méndez, R., Gómez, X., & Martín‐Villacorta, J. (2003). Determination of Diazinon and Fenitrothion in Environmental Water and Soil Samples by HPLC. Journal of Liquid Chromatography & Related Technologies, 26(3), 483-497. doi:10.1081/jlc-120017184Sherma, J. (1993). Pesticides. Analytical Chemistry, 65(12), 40-54. doi:10.1021/ac00060a004Grigoryan, H., Li, B., Xue, W., Grigoryan, M., Schopfer, L. M., & Lockridge, O. (2009). Mass spectral characterization of organophosphate-labeled lysine in peptides. Analytical Biochemistry, 394(1), 92-100. doi:10.1016/j.ab.2009.07.008Thompson, C. M., Prins, J. M., & George, K. M. (2010). Mass Spectrometric Analyses of Organophosphate Insecticide Oxon Protein Adducts. Environmental Health Perspectives, 118(1), 11-19. doi:10.1289/ehp.0900824Wang, J., Chatrathi, M. P., Mulchandani, A., & Chen, W. (2001). Capillary Electrophoresis Microchips for Separation and Detection of Organophosphate Nerve Agents. Analytical Chemistry, 73(8), 1804-1808. doi:10.1021/ac001424eWatanabe, E., Kanzaki, Y., Tokumoto, H., Hoshino, R., Kubo, H., & Nakazawa, H. (2001). Enzyme-Linked Immunosorbent Assay Based on a Polyclonal Antibody for the Detection of the Insecticide Fenitrothion. Evaluation of Antiserum and Application to the Analysis of Water Samples. Journal of Agricultural and Food Chemistry, 50(1), 53-58. doi:10.1021/jf0108359Hua, X., Yang, J., Wang, L., Fang, Q., Zhang, G., & Liu, F. (2012). Development of an Enzyme Linked Immunosorbent Assay and an Immunochromatographic Assay for Detection of Organophosphorus Pesticides in Different Agricultural Products. PLoS ONE, 7(12), e53099. doi:10.1371/journal.pone.0053099Liu, G., & Lin, Y. (2005). Electrochemical Sensor for Organophosphate Pesticides and Nerve Agents Using Zirconia Nanoparticles as Selective Sorbents. Analytical Chemistry, 77(18), 5894-5901. doi:10.1021/ac050791tMane, P. C., Shinde, M. D., Varma, S., Chaudhari, B. P., Fatehmulla, A., Shahabuddin, M., … Chaudhari, R. D. (2020). Highly sensitive label-free bio-interfacial colorimetric sensor based on silk fibroin-gold nanocomposite for facile detection of chlorpyrifos pesticide. Scientific Reports, 10(1). doi:10.1038/s41598-020-61130-yEnsafi, A. A., Rezaloo, F., & Rezaei, B. (2017). Electrochemical Determination of Fenitrothion Organophosphorus Pesticide Using Polyzincon Modified-glassy Carbon Electrode. Electroanalysis, 29(12), 2839-2846. doi:10.1002/elan.201700406Qi, P., Wang, J., Wang, X., Wang, X., Wang, Z., Xu, H., … Wang, X. (2018). Sensitive determination of fenitrothion in water samples based on an electrochemical sensor layered reduced graphene oxide, molybdenum sulfide (MoS2)-Au and zirconia films. Electrochimica Acta, 292, 667-675. doi:10.1016/j.electacta.2018.09.187Kant, R. (2019). Surface plasmon resonance based fiber–optic nanosensor for the pesticide fenitrothion utilizing Ta2O5 nanostructures sequestered onto a reduced graphene oxide matrix. Microchimica Acta, 187(1). doi:10.1007/s00604-019-4002-8Zinoviev, K. E., Gonzalez-Guerrero, A. B., Dominguez, C., & Lechuga, L. M. (2011). Integrated Bimodal Waveguide Interferometric Biosensor for Label-Free Analysis. Journal of Lightwave Technology, 29(13), 1926-1930. doi:10.1109/jlt.2011.2150734Fernández Gavela, A., Grajales García, D., Ramirez, J., & Lechuga, L. (2016). Last Advances in Silicon-Based Optical Biosensors. Sensors, 16(3), 285. doi:10.3390/s16030285Maldonado, J., Estévez, M.-C., Fernández-Gavela, A., González-López, J. J., González-Guerrero, A. B., & Lechuga, L. M. (2020). Label-free detection of nosocomial bacteria using a nanophotonic interferometric biosensor. The Analyst, 145(2), 497-506. doi:10.1039/c9an01485cHuertas, C. S., Fariña, D., & Lechuga, L. M. (2016). Direct and Label-Free Quantification of Micro-RNA-181a at Attomolar Level in Complex Media Using a Nanophotonic Biosensor. ACS Sensors, 1(6), 748-756. doi:10.1021/acssensors.6b00162Maldonado, J., González-Guerrero, A. B., Domínguez, C., & Lechuga, L. M. (2016). Label-free bimodal waveguide immunosensor for rapid diagnosis of bacterial infections in cirrhotic patients. Biosensors and Bioelectronics, 85, 310-316. doi:10.1016/j.bios.2016.04.095González-Guerrero, A. B., Maldonado, J., Dante, S., Grajales, D., & Lechuga, L. M. (2016). Direct and label-free detection of the human growth hormone in urine by an ultrasensitive bimodal waveguide biosensor. Journal of Biophotonics, 10(1), 61-67. doi:10.1002/jbio.201600154Chocarro-Ruiz, B., Herranz, S., Fernández Gavela, A., Sanchís, J., Farré, M., Marco, M. P., & Lechuga, L. M. (2018). Interferometric nanoimmunosensor for label-free and real-time monitoring of Irgarol 1051 in seawater. Biosensors and Bioelectronics, 117, 47-52. doi:10.1016/j.bios.2018.05.044Chocarro-Ruiz, B., Pérez-Carvajal, J., Avci, C., Calvo-Lozano, O., Alonso, M. I., Maspoch, D., & Lechuga, L. M. (2018). A CO2optical sensor based on self-assembled metal–organic framework nanoparticles. Journal of Materials Chemistry A, 6(27), 13171-13177. doi:10.1039/c8ta02767fManclús, J. J., Primo, J., & Montoya, A. (1996). Development of Enzyme-Linked Immunosorbent Assays for the Insecticide Chlorpyrifos. 1. Monoclonal Antibody Production and Immunoassay Design. Journal of Agricultural and Food Chemistry, 44(12), 4052-4062. doi:10.1021/jf960144qDante, S., Duval, D., Fariña, D., González-Guerrero, A. B., & Lechuga, L. M. (2015). Linear readout of integrated interferometric biosensors using a periodic wavelength modulation. Laser & Photonics Reviews, 9(2), 248-255. doi:10.1002/lpor.20140021

Critical behavior of the frustrated antiferromagnetic six-state clock model on a triangular lattice

We study the anti-ferromagnetic six-state clock model with nearest neighbor interactions on a triangular lattice with extensive Monte-Carlo simulations. We find clear indications of two phase transitions at two different temperatures: Below $T_I$ a chirality order sets in and by a thorough finite size scaling analysis of the specific heat and the chirality correlation length we show that this transition is in the Ising universality class (with a non-vanishing chirality order parameter below $T_I$). At $T_{KT}(<T_I)$ the spin-spin correlation length as well as the spin susceptibility diverges according to a Kosterlitz-Thouless (KT) form and spin correlations decay algebraically below $T_{KT}$. We compare our results to recent x-ray diffraction experiments on the orientational ordering of CF$_3$Br monolayers physisorbed on graphite. We argue that the six-state clock model describes the universal feature of the phase transition in the experimental system and that the orientational ordering belongs to the KT universality class.Comment: 8 pages, 9 figure