H. Vargas

Ammonia detection by using quantum-cascade laser photoacoustic spectroscopy

A pulsed quantum-cascade distributed-feedback laser, temperature tunable from -41 degrees C to +31.6 degrees C, and a resonant differential photoacoustic detector are used to measure trace-gas concentrations to as low as 66 parts per 10(9) by volume (ppbv) ammonia at a low laser power of 2 mW. Good agreement between the experimental spectrum and the simulated HITRAN spectrum of NH3 is found in the spectral range between 1046 and 1052 cm(-1). A detection limit of 30 ppbv ammonia at a signal-to-noise ratio of 1 was obtained with the quantum-cascade laser (QCL) photoacoustic (PA) setup. Concentration changes of approximately 50 ppbv were detectable with this compact and versatile QCL-based PA detection system. The performance of the PA detector, characterized by the product of the incident laser power and the minimum detectable absorption coefficient, was 4.7 x 10-9 W cm(-1).

Photoacoustic thermal characterization of kaolinite clays

The usefulness of photoacoustic techniques in characterizing the thermal properties of kaolinite clays, which are a very abundant mineral in some regions of Brazil, was investigated. Different behavioural patterns are observed below and above a heating temperature of 950 °C and can be attributed to the change from metakaolinite to an amorphous three-dimensional mineral which crystallises to mullite and crystobalite above 1200 °C. The implications of the results in the manufacture of clay bricks are briefly considered. This is important in tropical regions where a knowledge of the thermal properties of materials used in the building construction industry is highly desirable.

Application of laser photoacoustic spectroscopy for the analysis of gas samples emitted by diesel engines

Abstract A homemade CO 2 laser photoacoustic spectrometer has been constructed for monitoring gas emissions from several sources. Numerous air pollutant gases are emitted by the exhaust of industries, vehicles, power plants, etc. In Brazil, there has been an increasing demand for power generators, driven by diesel oil and other fossil fuels, because of the lack of rain and consequent decreased availability of hydroelectric energy, leading to the rationing of energy. It is known that the combustion of diesel oil in power generators is the main source of many contaminant gases such as nitrogen oxides, SO 2 , CO and hydrocarbons. In this work, we performed a thorough analysis, using photoacoustic technology, of gas samples collected in a closed room where a power generator was turned on for 60 and 30 min to saturate the ambient air. Some gases e.g. SO 2 and NO 2 were detected in the ppm range.

Measurement of the thermal properties of liquids using a thermal wave interferometer

In this paper we discuss the use of an alternative photothermal technique for measurements of thermal properties of liquids. The proposed technique is based upon the concept of thermal wave interferometry. The liquid sample is confined between two thin pyroelectric detectors. One of these detectors acts as a modulated absorber of light while the other is used for sensing the temperature fluctuations transmitted through the liquid layer. The good agreement between the values of the thermal properties we got with the present technique and those reported in the literature demonstrates the capability of the technique for full characterization of the thermal properties of liquids.

Optical and thermal properties of liquids measured by means of an open photoacoustic cell

A double-purpose open photoacoustic cell, suitable for optical and thermal characterization of liquid samples is described. An experimental method for the determination of very low concentrations of contaminants in liquid substances is proposed. The proposed technique was used to determine the concentration of Chromium (VI) in water via the measurement of its optical absorption coefficient. To test the suitability of the proposed technique for experimental determination of thermal properties of liquids, studies on ethanol were performed. The agreement between experimentally obtained thermal properties and values reported in the literature is good.

On the use of the thermal wave resonator cavity sensor for monitoring hydrocarbon vapors

A gas sensing device based on a thermal wave resonator cavity is outlined. It is experimentally tested by monitoring the presence of several hydrocarbon vapors in air via the measurement of the thermal diffusivity. It is also shown that its time-dependent response may be used to follow the vapor diffusion. It is shown that its characteristic response time is linearly correlated to the thermal diffusivity value of the mixture. The steps toward the development of a practical sensing device are further discussed.

Application of the thermal wave resonator to the measurement of the thermal diffusivity of gas mixtures

A gas analyzer device based on thermal wave interference in a cavity is presented. The thermal diffusivity of CO2:air mixtures as a function of the relative concentration is measured. It is demonstrated that different concentrations of CO2 in air can be detected with accuracy using the described experimental device. The results presented here open the possibility to perform routine measurements of thermal diffusivity of binary gas mixtures and using this parameter to monitor the relative gas concentration.

Characterization of the thermal properties of gases using a thermal wave interferometer

A method suitable for the thermal characterization of gases is described. It is based on thermal wave interference (TWI) in a cavity. A TWI device was constructed and the thermal diffusivity of different binary gas mixtures as a function of their relative concentration was measured. From these values, using a logarithm mixing model for the thermal conductivity of a two-phase gas system, the thermal properties of the corresponding pure gases were calculated. The data obtained for several test gases show good agreement with values reported in the literature.

Photoacoustic Monitoring of Internal Plastification in Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Copolymers: Measurements of Thermal Parameters

Propriedades termofisicas de copolimeros poli(3-hidroxibutirato-co-3-hidroxivalerato) poli(3HB-co-3HV) foram investigadas com o objetivo de entender a influencia das unidades monomericas de 3-hidroxivalerato (3HV) incorporadas durante a copolimerizacao aleatoria. Os resultados mostram forte evidencia de que uma plastificacao interna e introduzida no copolimero pelas unidades de 3HV. Foi observado que a condutividade termica do copolimero aumentou linearmente com a concentracao de 3HV. Por outro lado, a difusividade termica foi muito sensivel as mudancas de composicao do copolimero, apresentou um aumento brusco inicial e atingiu um plato de saturacao. Os graficos de amplitude contra frequencia indicam a predominância de um mecanismo termoelastico. Neste trabalho e apresentando uma nova montagem fotoacustica para as medidas de efusividade termica.

Diesel engines gas emissions monitored by photoacoustic spectroscopy

In Brazil, there has been an increasing demand for power generators, driven by diesel oil and other fossil fuels, because of the lack of rain and consequent decreased availability of hydroelectric energy, leading to the rationing of energy. It is known that the combustion of diesel oil in power generators is the main source of many contaminant gases such as nitrogen oxides, SO2, CO, and hydrocarbons. In this work, we carried out a thorough analysis, using the photoacoustic technique of gas samples collected in a closed room where a power generator was turned on for 60 min to saturate the ambient. Some substances such as SO2 and NO2 were detected in the parts per million range.