Dilution Gas Flow Research Articles

Mechanism and Effect of the Dilution Gas Flow Rate on Various Fe–Si/SiO2 Soft Magnetic Composites during Fluidised Bed Chemical Vapour Deposition

The effect of the dilution gas flow rate on inorganic oxide insulating layers can improve fluidised bed chemical vapour deposition (FBCVD) in Fe–Si/inorganic-oxide soft magnetic composites and obtain excellent magnetic properties. Herein, Fe–Si/SiO2 composite particles coated via FBCVD and deposited at a 125–350 mL/min Ar-dilution gas flow rate were prepared and sintered into soft magnetic composites. Results demonstrate that SiO2 deposited on the Fe–Si substrate particle surface changed from submicron SiO2 clusters (125 mL/min) to an incomplete SiO2 film, then to a complete SiO2 film, and finally to a porous SiO2 film as the Ar-dilution gas flow rate increased. SiO2 layers began to transform from the amorphous to the beta-cristobalite state with a hexagonal crystal structure between 1149.45 K and 1280.75 K. However, the SiO2 amorphous layers’ crystallisation did not affect the Fe–Si substrate particles’ crystal structure. With the increasing Ar-dilution gas flow rate, the saturation magnetisation of Fe–Si/SiO2 soft magnetic composites initially decreased and then increased. The electrical resistivity increased before 150 mL/min, followed by an increase between 150 and 250 mL/min and then decreased, whereas the total core loss exhibited the opposite trend. These results show that magnetic performance can be promoted by selecting a suitable dilution flow rate.

Optimization of plasma sampling depth and aerosol gas flow rates for single particle inductively coupled plasma mass spectrometry analysis

We performed experiments to assess the separate and also the combined effect of the sampling depth and the aerosol gas flow rates on the signal formation in single particle inductively coupled plasma mass spectrometry (spICP-MS) measurements by using dispersions containing Ag and Au NPs. It was found that the NP signal can significantly be improved by the optimization of the sampling depth. With respect to the „robust” setting, a signal improvement of nearly 100% could be achieved, which translates into a 25–30% improvement in size detection limits. It was also found that the shape of the spICP-MS signal histograms also change with the change of the plasma sampling depth. It was demonstrated that nanoparticle peak separation can also be significantly enhanced by using sampling depth optimization.The effect of the aerosol dilution gas flow, now standard in most ICP-MS instruments, on the spICP-MS signal formation was also studied for the first time in the literature, as this flow was hoped to make spICP-MS measurements more practical and faster via the on-line dilution of the aerosol generated from nano-dispersions. Our experimental results revealed that the dilution gas flow can only be used for a moderate aerosol dilution in spICP-MS measurements, if the gas flow going to the pneumatic nebulizer is proportionally lowered at the same time. This however was found to cause a significant worsening in the operation of the sample introduction system, which gives rise to a strong NP signal loss. Thus it was concluded that the use of the aerosol dilution gas flow, in its present form, can not be suggested for spICP-MS analysis.

Uncertainty analysis of the diffusion rate in the dynamic generation of volatile organic compound mixtures

The measurement of volatile organic compound (VOC) concentration changes, in the range of 10 ppt–1ppb, is essential in several applications. The World Meteorological Organization has identified a set of VOCs that are critical because of their long-term instability. The measurement of such changes demands generation of reference mixtures at a high accuracy and stability level. The dynamic preparation of gas mixtures based on diffusion is a suitable way of generating accurate reference mixtures, particularly of unstable VOCs. Diffusion rate and dilution gas flow rate uncertainties are the main contributors to the overall VOC concentration uncertainty. This paper presents a comprehensive overview of the uncertainty of the diffusion rate. Stress is laid on the advantages offered by a modified measurand equation. The validity of this analysis and the modified equation has been confirmed experimentally. Temperature variability and mass difference uncertainty make the main contributions to the uncertainty: a 0.01 °C variability results in a 0.2% uncertainty. The new measurand equation provides both a better indication of the systematic effects at a narrow temperature variability and a more realistic calculation of the uncertainty. The modified equation enhances calculation of uncertainty at the low diffusion rates needed to generate very low VOC concentration mixtures.

Study on the Absolute Density and Translational Temperature of Si Atoms in Very High Frequency Capacitively Coupled SiH4 Plasma with Ar, N2, and H2 Dilution Gases

The absolute densities and translational temperatures of Si atoms in very high frequency capacitively coupled SiH4 plasmas diluted with Ar, N2, and H2 gases were investigated by ultraviolet absorption spectroscopy with a ring dye laser and a hollow cathode lamp. It was found that the absolute density of Si atoms was of the order of 109–1010 cm-3 and the translational temperature of Si atoms ranged from 620 to 1130 K at a total pressure of 11 Pa, a dilution gas flow rate of 100 sccm, and a SiH4 flow rate of 0–15 sccm. The absolute densities and temperatures of Si atoms in plasma at an excitation frequency of 27 MHz were larger than those at 60 MHz under the conditions at the same electron density. Si atom heating was due to the energy of Si atoms released from the electron impact dissociation of SiHx (x=1–4). The translational temperatures of Si atoms in SiH4/Ar, SiH4/N2, and SiH4/H2 plasmas were evaluated to be 970, 1030, and 1130 K, respectively, at a frequency of 27 MHz, a SiH4 flow rate of 10 sccm, and a VHF power of 1500 W. The effect of Si atoms and SiH3 radicals on film deposition was investigated for SiH4/N2 in 27 MHz and 60 MHz plasmas. From the measurement using Fourier transform infrared absorption spectroscopy, the peak of the Si–H bond decreased and that of the N–H bond increased with increasing excitation frequency. Therefore, the film deposited at 60 MHz indicated a nitride-rich composition in comparison with that at 27 MHz. The contribution ratio of Si atoms to SiH3 radicals for film deposition in 27 MHz plasma was larger than that in 60 MHz plasma. These results are very important from the viewpoint of understanding neutral radical chemistries in the plasma and their related processing.

GENERATION AND CALIBRATION OF STANDARD GAS MIXTURES OR VOLATILE FATTY ACIDS USING PERMEATION TUBES AND SOLID-PHASE MICROEXTRACTION

Volatile fatty acids (VFAs) are major components of odors associated with agricultural operations and livestockhousing, solid waste processing and disposal, industrial and municipal wastewater collection, and treatment systems.Emission estimation and assessment of odor control methods depend on reliable air sampling and analysis methods for VFAs.The objective of this research was to develop and test a method for continuous and reliable generation of standard gas mixturesfor VFAs based on permeation tubes. Standard gas mixtures for acetic, propionic, isobutyric, butyric, isovaleric, valeric, andhexanoic acids were generated with permeation tubes and monitored for a 100-day period. The gravimetric loss of VFAs fromeach tube was measured periodically and used to calculate the emission rate for each permeation tube. Emission rates wereas high as 2,011 ng/(min cm) for acetic acid to as low as 49 ng/(min cm) for isovaleric and hexanoic acids. The emission ratewas combined with the dilution flow rate to calculate standard concentrations. Five different concentrations for each VFAwere obtained by adjusting the dilution flow rate. Gas concentrations were monitored with DVB/Carboxen/PDMS 50/30 .mSPME fibers using triplicate 1 min extractions. Maximum standard gas concentrations ranged from 21.9 ppmv for acetic acidto 0.22 ppmv for hexanoic acid. Minimum standard gas concentrations ranged from 3.0 ppmv for acetic acid to 0.03 ppmvfor hexanoic acid. The relative standard deviations (RSDs) for all VFA concentrations ranged from .1.6% to .7.8%. Rapid(1 min) SPME extractions were sufficient to preconcentrate significant amounts of VFAs for separation on a GC-FID withoutderivatization. The SPME technology proved to be very useful for monitoring standard gas mixtures. Dilution gas flow ratedid not affect the emission rates from permeation tubes. In contrast, low acid levels affected permeation rates of the acidsfrom the tubes. The methodology described in this article could be used to generate and test standard gas mixtures for otherodorous gases.

Electronic transport in low-temperature silicon nitride

Perpendicular current transport through thin silicon nitride films deposited at 100 ° C by radio frequency chemical vapor deposition (RF) is measured between patterned square contacts with side lengths between 5 and 200 μm . Hydrogen dilution, silane-to-ammonia ratio, and total gas flow were varied to achieve control of film properties. The dependence of the current on the applied field and measurement temperature are correlated to structural parameters such as the index of refraction, etching rate in buffered hydrofluoric acid and infrared vibrational band strengths. Using the appropriate deposition parameters, it is possible to prepare, at 100 ° C , silicon nitride dielectric films with electronic properties compatible with use as gate dielectrics of thin-film transistors.

Continuous dynamic-gravimetric preparation of calibration gas mixtures for air pollution measurements

The preparation of calibration gas mixtures for air pollution measurements by the dynamic-gravimetric method was investigated using sulphur dioxide in nitrogen as a model. The target mole fraction was 200×10–9 mol/mol, with the option of also getting smaller mole fractions. Thermal mass flow meters calibrated with reference mass flows were used to measure the dilution gas flow (nitrogen). The relative standard uncertainty of the dilution gas flows between 10 mg/s (approx. 500 ml/min) and 40 mg/s (approx. 2000 ml/min) was 0.15%. The mass flow of the target component measured as the permeation rate was determined via the quasi-continuous observation of the loss in the permeation tube mass during the measuring time. A magnetic coupling system and an adapted microbalance were used for this purpose. The results presented show permeation rates measured over the lifetime of a tubular permeation source. The measurement cycles took between 3 days and 7 h at least. The relative standard uncertainty of the mixture composition did not exceed 2%. First comparisons with gas mixtures prepared by the static-gravimetric method show compatibility. The applicability of the system is not restricted to the SO2/N2 mixture. It can also be used for preparing other gas mixtures in this field of application.

Predominant influences on the bulk properties and surface morphology in hydrogenated amorphous silicon carbide films

Abstract The influence of H2 dilution ratio, gas flow and substrate temperature on the bulk and surface properties of hydrogenated amorphous silicon carbide films is discussed. Through infrared and electron spin resonance studies, it is revealed that microstructural heterogeneity can be reduced by controlling surface reactions. On the other hand, atomic force microscopy observations reveal for the first time that surface morphology is dominated by growth rate under supply-limited conditions. This implies that improvement in the bulk properties does not always lead to films with smoother final surfaces. Both surface reaction control and growth rate control are found to be crucial for obtaining films with excellent bulk and surface properties.