Time filter

Source Type

Esmeryan K.D.,Virginia Commonwealth University | Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Castano C.E.,Virginia Commonwealth University | Bressler A.H.,Virginia Commonwealth University | And 4 more authors.
Diamond and Related Materials | Year: 2017

Historically, the synthesis of diamond and graphite via combustion flames stands out as a simplified, scalable and inexpensive approach. Unfortunately, this method is not beneficial for industrial applications in coatings due to limitations related with the high flame and substrate temperatures. Here, we report novel findings about the formation mechanism of graphite-like and diamond-like supported nanostructures in low temperature laminar diffusion flames. Both materials are formed upon controllable combustion at atmospheric pressure of a cylindrical paper wick immersed in rapeseed oil. An accurate adjustment of the incident air flow and the amount of available fuel allow deposition of carbon soot or diamond-like carbon (DLC). The DLC formation is favorable in a narrow stoichiometric range at flame temperatures within ~ 210–260 °C and beyond this range the particles precipitate as soot. The comparative structural analysis using scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy, along with the full thermal and stoichiometric profiles for the chosen combustion conditions, suggest a kinetically driven graphite-to-diamond transformation rather than a thermodynamically induced phase transition. Our results reveal a new direction in the principles of graphite and diamond formation in flames that could be applied to surmount the existing shortcomings in flame synthesis. © 2017 Elsevier B.V.


Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Avramov I.D.,Georgi Nadjakov Institute of Solid State Physics | Radeva E.I.,Georgi Nadjakov Institute of Solid State Physics
Micromachines | Year: 2012

Temperature induced frequency shifts may compromise the sensor response of polymer coated acoustic wave gas-phase sensors operating in environments of variable temperature. To correct the sensor data with the temperature response of the sensor the latter must be known. This study presents and discusses temperature frequency characteristics (TFCs) of solid hexamethyldisiloxane (HMDSO) polymer coated sensor resonators using the Rayleigh surface acoustic wave (RSAW) mode on ST-cut quartz. Using a RF-plasma polymerization process, RSAW sensor resonators optimized for maximum gas sensitivity have been coated with chemosensitive HMDSO films at 4 different thicknesses: 50, 100, 150 and 250nm. Their TFCs have been measured over a (-100 to +110) °C temperature range and compared to the TFC of an uncoated device. An exponential 2,500ppm downshift of the resonant frequency and a 40K downshift of the sensor's turn-over temperature (TOT) are observed when the HMDSO thickness increases from 0 to 250nm. A partial temperature compensation effect caused by the film is also observed. A third order polynomial fit provides excellent agreement with the experimental TFC curve. The frequency downshift due to mass loading by the film, the TOT and the temperature coefficients are unambiguously related to each other. © 2012 by the authors.


Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Georgieva V.,Georgi Nadjakov Institute of Solid State Physics | Vergov L.,Georgi Nadjakov Institute of Solid State Physics | Lazarov J.,Georgi Nadjakov Institute of Solid State Physics
Bulgarian Chemical Communications | Year: 2015

The gas sensitivity of 16 MHz carbon soot coated quartz crystal microbalances (QCMs) towards various NO2 concentrations is investigated. The Scanning Electron Microscopy measurements reveal that the soot is deposited on the sensor surface irregularly, forming carbon nanospheres separated by nanoscale cavities. The inherent non-polar nature of the soot, along with its estimated root mean square roughness (Rrms) of 130 nm, transform the quartz surface into a superhydrophobic (SH) one, characterized with high static contact angle (∼153°) and low contact angle hysteresis (∼1°). Such surface configuration induces 15 Hz resonance frequency downshift of the SH QCM at low NO2 concentrations up to 500 ppm. Furthermore, the sensor demonstrates reversible gas sorption ability in the same gas range. Further gradual increase in the NO2 concentration up to 5000 ppm leads to an additional sensor response of 38 Hz; however, after dry air scavenging the sensor reaches its baseline with a slow rate of ∼0.33 Hz/min. In addition, upon ambient humidity increase of 45 %, the SH QCM shifts down its resonance with 45 Hz, whereas the 16 MHz WO3 coated counterpart with 212 Hz. These results clearly indicate the advantage of the soot coated QCM, which could be used for continuous in situ monitoring of air pollutants with negligible cross sensitivity to humidity of 1Hz/% RH. © 2015 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria.


Esmeryan K.D.,Virginia Commonwealth University | Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Castano C.E.,Virginia Commonwealth University | Bressler A.H.,Virginia Commonwealth University | And 2 more authors.
RSC Advances | Year: 2016

A novel method for the flame synthesis of carbon nanoparticles with controllable fraction of amorphous, graphitic-like and diamond-like phases is reported. The structure of nanoparticles was tailored using a conical chimney with an adjustable air-inlet opening. The opening was used to manipulate the combustion of an inflamed wick soaked in rapeseed oil, establishing three distinct combustion regimes at fully-open, half-open and fully-closed opening. Each regime led to the formation of carbon coatings with diverse structure and chemical reactivity through a facile, single-step process. In particular, the fully-closed opening suppressed most of the inlet air, causing an increased fuel/oxygen ratio and decreased flame temperature. In turn, the nucleation rate of soot nanoparticles was enhanced, triggering the precipitation of some of them as diamond-like carbon (DLC). Surface characterization analyses using Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron spectroscopy confirmed this hypothesis, indicating a short-range ordered nanocrystalline structure and ∼80% sp3 bonds in the coatings deposited at fully-closed opening. Furthermore, three groups of 5 MHz Quartz Crystal Microbalances (QCMs) coated with soot and DLC, corresponding to each of the three combustion regimes, showed different frequency responses to aqueous ethanol and isopropanol solutions in the concentration range of 0-12.5 wt%. The DLC coated QCMs exhibited relatively constant frequency shift of ∼2250 Hz regardless of the chemical, while the response of soot coated counterparts was influenced by the quantity of heteroatoms in the film. Our method can be applied in chemical sensing for the development of piezoresonance liquid sensors with tunable sensitivity. © 2016 The Royal Society of Chemistry.


Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Radeva E.I.,Georgi Nadjakov Institute of Solid State Physics | Avramov I.D.,Georgi Nadjakov Institute of Solid State Physics
Journal of Physics D: Applied Physics | Year: 2015

A novel approach for the fabrication of durable superhydrophobic (SH) carbon soot coatings used in quartz crystal microbalance (QCM) based gas or liquid sensors is reported. The method uses modification of the carbon soot through polymerization of hexamethyldisiloxane (HMDSO) by means of glow discharge RF plasma. The surface characterization shows a fractal-like network of carbon nanoparticles with diameter of ~50 nm. These particles form islands and cavities in the nanometer range, between which the plasma polymerized hexamethyldisiloxane (PPHMDSO) embeds and binds to the carbon chains and QCM surface. Such modified surface structure retains the hydrophobic nature of the soot and enhances its robustness upon water droplet interactions. Moreover, it significantly reduces the insertion loss and dynamic resistance of the QCM compared to the commonly used carbon soot/epoxy resin approach. Furthermore, the PPHMDSO/carbon soot coating demonstrates durability and no aging after more than 40 probing cycles in water based liquid environments. In addition, the surface layer keeps its superhydrophobicity even upon thermal annealing up to 540 °C. These experiments reveal an opportunity for the development of soot based SH QCMs with improved electrical characteristics, as required for high-resolution gas or liquid measurements. © 2016 IOP Publishing Ltd.


Esmeryan K.D.,Virginia Commonwealth University | Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Bressler A.H.,Virginia Commonwealth University | Castano C.E.,Virginia Commonwealth University | And 2 more authors.
Applied Surface Science | Year: 2016

Although the superhydrophobic surfaces are preferable for passive anti-icing systems, as they provide water shedding before initiation of ice nucleation, their practical usage is still under debate. This is so, as the superhydrophobic materials are not necessarily icephobic and most of the synthesis techniques are characterized with low fabrication scalability. Here, we describe a rational strategy for the atmospheric icing prevention, based on chemically functionalized carbon soot, suitable for large-scale fabrication of superhydrophobic coatings that exhibit and retain icephobicity in harsh operational conditions. This is achieved through a secondary treatment with ethanol and aqueous fluorocarbon solution, which improves the coating's mechanical strength without altering its water repellency. Subsequent experimental analyses on the impact dynamics of icy water droplets on soot coated aluminum and steel sheets show that these surfaces remain icephobic in condensate environments and substrate temperatures down to −35 °C. Furthermore, the soot's icephobicity and non-wettability are retained in multiple icing/de-icing cycles and upon compressed air scavenging, spinning and water jetting with impact velocity of ∼25 m/s. Finally, on frosted soot surfaces, the droplets freeze in a spherical shape and are entirely detached by adding small amount of thermal energy, indicating lower ice adhesion compared to the uncoated metal substrates. © 2016 Elsevier B.V.


Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Avramov I.D.,Georgi Nadjakov Institute of Solid State Physics | Radeva E.I.,Georgi Nadjakov Institute of Solid State Physics
Sensors and Actuators, B: Chemical | Year: 2015

Experimental data on the temperature behavior of quartz crystal microbalances (QCMs) rigidly coated with solid polymer films of hexamethyldissiloxane (HMDSO) for gas and liquid-phase sensor applications are provided. 16 MHz AT-cut quartz resonators used as QCMs are coated at seven different HMDSO thicknesses in the 70-400 nm range by using a RF-plasma polymerization process. Their quasi cubic temperature frequency characteristics (TFCs) are measured over a temperature of -50°C to 90°C for each film thickness. A cubic spline approximation of the experimental data reveals a periodic oscillating behavior of the temperature coefficients of the TFC, while its inflection point oscillates between 24°C and 28°C in the above thickness range. Devices coated at 150, 260 and 380 nm, i.e. at a thickness period of 110-120 nm, were found to have identical thermal stability and TFC behavior. A simple practical method for correcting the sensor readings for thermally induced frequency shifts, based on an additional temperature measurement at each sensor reading, is suggested and verified experimentally. © 2015 Elsevier B.V.All rights reserved.


Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Yordanov T.A.,Georgi Nadjakov Institute of Solid State Physics | Vergov L.G.,Georgi Nadjakov Institute of Solid State Physics | Raicheva Z.G.,Georgi Nadjakov Institute of Solid State Physics | Radeva E.I.,Georgi Nadjakov Institute of Solid State Physics
IEEE Sensors Journal | Year: 2015

The organic vapor sensitivity of a quartz crystal microbalance (QCM) with an epoxy resin-carbon soot coating designed to be tolerant to humidity is reported. This is achieved using nonengulfed, but attached, hydrophobic carbon soot nanoparticles, coated in an irregular surface topography composed of islands and cavities. The root mean square roughness ( R rms ) of 130 nm, together with the hydrophobic soot, convert the epoxy surface to a superhydrophobic (SH) one with high static contact angle ( ∼151) and low contact angle hysteresis ( ∼1.1 ). The frequency shift of the SH QCM at 100% relative humidity is ∼7 × lower compared with an uncoated device, and thus indicating low water vapor adsorption due to superhydrophobicity. In addition, the SH QCM shows between three and six times higher gas sensitivity and lower detection limit compared with the conventional polymer coated QCMs. These results correlate well with the R rms and surface topography of the coating, which ensure enhanced sensing area. Furthermore, the sensor demonstrates reproducibility, reversibility, and fast response-recovery time ( ∼10 s) to ethanol, methanol, and isopropanol vapor. These experiments reveal that superhydrophobicity increases the organic vapor sorption at the expense of water vapor sorption, and thus allowing operation of the QCM gas sensors in an uncontrolled humidity environment. © 2001-2012 IEEE.


Esmeryan K.D.,Virginia Commonwealth University | Esmeryan K.D.,Georgi Nadjakov Institute of Solid State Physics | Castano C.E.,Virginia Commonwealth University | Bressler A.H.,Virginia Commonwealth University | And 2 more authors.
Applied Surface Science | Year: 2016

The fabrication of superhydrophobic coatings using a candle flame or rapeseed oil has become very attractive as a novel approach for synthesis of water repellent surfaces. Here, we report an improved, simplified and time-efficient method for the preparation of robust superhydrophobic carbon soot that does not require any additional stabilizers or chemical treatment. The soot's inherent stabilization is achieved using a specially-designed cone-shaped aluminum chimney, mounted over an ignited paper-based wick immersed in a rapeseed oil. Such configuration decreases the level of oxygen during the process of combustion; altering the ratio of chemical bonds in the soot. As a result, the fractal-like network of the carbon nanoparticles is converted into dense and fused carbon chains, rigidly coupled to the substrate surface. The modified carbon coating shows thermal sustainability and retains superhydrophobicity up to ∼300 °C. Furthermore, it demonstrates a low contact angle hysteresis of 0.7-1.2° accompanied by enhanced surface adhesion and mechanical durability under random water flows. In addition, the soot's deposition rate of ∼1.5 μm/s reduces the exposure time of the substrate to heat and consequently minimizes the thermal effects, allowing the creation of superhydrophobic coatings on materials with low thermal stability (e.g. wood or polyethylene). © 2016 Elsevier B.V. All rights reserved.


Dimitrov O.,Bulgarian Academy of Science | Nesheva D.,Georgi Nadjakov Institute of Solid State Physics | Levi Z.,Georgi Nadjakov Institute of Solid State Physics | Miloushev I.,Georgi Nadjakov Institute of Solid State Physics | And 3 more authors.
Journal of Physics: Conference Series | Year: 2012

Highly transparent, polycrystalline ZnO thin films were deposited by ultrasonic spray pyrolysis. The phase and chemical composition of the films were characterized by X-ray analysis (XRD) and X-ray photoelectron spectroscopy (XPS). According to the XRD analysis the films possess wurtzite structure with preferential (002) orientation. The morphology of the ZnO films was investigated by scanning electron microscopy (SEM). Optical transmittance higher than 80% is found in the visible region. The influence of both the substrate and postdeposition annealing temperatures on the dark conductivity and its activation energy were investigated. It has been found that the conductivity increases with increasing of the substrate and annealing temperatures while the activation energy decreases. These effects have been related to an increase of the density of oxygen vacancies. The sensing properties of the samples were tested in the presence of vapours of water, ammonia, ethanol and acetone. It has been revealed that the substrate temperature influences the gas sensitivity. The films deposited at 200°C have shown the highest sensitivity with the best results registered in the presence of ammonia vapours.

Loading Georgi Nadjakov Institute of Solid State Physics collaborators
Loading Georgi Nadjakov Institute of Solid State Physics collaborators