Surface Oxide Removal Research Articles

Polarity controlled ScAlN multi-layer transduction structures grown by molecular beam epitaxy

We report on the molecular beam epitaxial growth and characterization of polarity-controlled single and multi-layer Scandium Aluminum Nitride (ScAlN) transduction structures grown directly on ScAlN templates deposited by physical vapor deposition (PVD) on Si(001) substrates. It is observed that direct epitaxial growth on PVD N-polar ScAlN leads to the flipping of polarity, resulting in metal (M)-polar ScAlN. By effectively removing the surface impurities, e.g., oxides, utilizing an in situ gallium (Ga)-assisted flushing technique, we show that high quality N-polar ScAlN epilayers can be achieved on PVD N-polar ScAlN templates. The polarity of ScAlN is confirmed by utilizing polarity-sensitive wet chemical etching and atomic-resolution scanning transmission electron microscopy. Through interface engineering, i.e., the controlled formation or removal of surface oxides, we have further demonstrated the ability to epitaxially grow an alternating tri-layer piezoelectric structure, consisting of N-polar, M-polar, and N-polar ScAlN layers. Such multi-layer, polarity-controlled ScAlN structures promise a manufacturable platform for the design and development of a broad range of acoustic and photonic devices.

Potential cycling boosts the electrochemical conversion of polyethylene terephthalate-derived alcohol into valuable chemicals

The electrocatalytic valorization of polyethylene terephthalate-derived ethylene glycol to valuable glycolic acid offers considerable economic and environmental benefits. However, conventional methods face scalability issues due to rapid activity decay of noble metal electrocatalysts. We demonstrate that a dynamic potential cycling approach, which alternates the electrode potential between oxidizing and reducing values, significantly mitigates surface deactivation of noble metals during electrochemical oxidation of ethylene glycol. This method enhances catalyst activity by 20 times compared to a constant-potential approach, maintaining this performance for up to 60 h with minimal deactivation. In situ Raman and X-ray absorption spectroscopy show that this effectiveness results from efficient removal of surface oxide during the reaction. The strategy is applicable to polyethylene terephthalate hydrolysates and various noble metals, such as palladium, gold, and platinum, with palladium showing a high conversion rate in recent studies. Our approach offers an efficient and durable method for electrochemical upcycling of biomass-derived compounds.

Electrical and optical properties of g-GaN/Al0.5Ga0.5N 2D/3D heterojunction under surface oxidation via first-principles

In this paper, the structural stability, charge transfer, band structure, density of states, absorption coefficient and reflectivity of g-GaN/Al0.5Ga0.5N heterojunction after surface oxidation are investigated. Our results reveal that formation energy of adsorptive oxidation process is negative and the oxidized surface can be formed spontaneously. The substitutional oxidation process is heat-absorbing and structure is unstable, which is not easy to occur. Band structure show that new energy levels are created due to charge transfer between O atom and heterojunction after surface oxidation. The electronic states of O-p orbitals hybridize with those of Ga-s, Ga-p, and Al-p orbitals, while O-s orbitals affect energy levels at low-energy end. Meanwhile, work function of heterojunction increases after oxidation, weakening electron emission properties of material surface. The results of optical properties illustrate that substitutional oxidation is detrimental to light absorption, while adsorption oxidation slightly enhances light absorption, especially when adsorption oxidation occurs in g-GaN layer, which is competitive for improvement of optical properties. This study can be useful for the removal of surface oxides from g-GaN/AlGaN heterojunction.

Low-loss α-tantalum coplanar waveguide resonators on silicon wafers: fabrication, characterization and surface modification

The performance of state-of-the-art superconducting quantum devices is currently limited by microwave dielectric loss at different interfaces. α-tantalum is a superconductor that has proven effective in reducing dielectric loss and improving device performance due to its thin low-loss oxide. Here, we demonstrate the fabrication of high-quality factor α-tantalum coplanar-waveguide resonators directly on pristine 300 mm silicon wafers over a variety of metal deposition conditions and perform a comprehensive material and electrical characterization study. Additionally, we apply a surface treatment based on hydrofluoric acid that allows us to modify different resonators surfaces, leading to a reduction in two-level system loss in the devices by a factor of three. This loss reduction can be entirely attributed to the removal of surface oxides. Our study indicates that large scale manufacturing of low-loss superconducting circuits should indeed be feasible and suggests a viable avenue to materials-driven advancements in superconducting circuit performance.

Data Sets Formation on the Physical Properties of Oxide Scale Components for Theoretical Assessment of Efficiency Parameters of Laser Cleaning of Carbon Steels and Related Processes

There is a need in machine-building industries nowadays to automate technologies, in particular, laser ones, to remove surface oxide layers – mill scale, rust – from steel products/pieces in order to improve the energy effectiveness of processing. Herewith, a theoretical assessment method for the intensity of heating of the oxide layer and the phase transition in it can be used to optimize laser cleaning (LC) of the steel surface. To realize this, it is possible to use some calculation and modeling procedures that require, as a first step, the data collection and verification on the temperature-dependent properties of iron-containing condensed phases, as possible components contained, in particular, in scale, which is typically widespread into various metal products. In this regard, the formation of database for characteristics of oxide scale components by the way of selection of information on thermophysical (including optical) properties of the components mentioned and of steel base, which are required for a reliable calculation of the thermal efficiency parameters of the technology for laser cleaning of carbon steels, as well as such actively developed related technologies as laser cutting, drilling, coating remelting, etc., was chosen as the task of our research. An analytical overview of published experimental data made it possible to systematize information on a number of transport and other physical properties of iron-containing components at ambient pressure, including thermal conductivity (k) and diffusivity (a), density ρ, irradiation absorptance and integral emissivity in the temperature range from T ≈ 298 K to the melting temperatures of oxide and metal phases and above them. At the same time, a preliminary thermochemical estimation shows (on the calculated data) the existence of such thermodynamically stable forms of the condensed phase in the heating spot of scale layers during its LC at the melting point and above it, as Fe3O4, FeO, and Fe, which is consistent with known experimental data. Comparison of the values of a calculated by us (using the published values of k, ρ and molar heat capacity and using extrapolation in the high-temperature region) for the types of scale components under consideration with a set of experimental values of this parameter in current literature revealed the presence of differences for both oxide and metal phases. These new values make it possible to fill in a gap in the temperature range T = 1600–1800 K that existed in the data on the thermal diffusivity. The value of a = (0.83–0.92)·10–6 m2/s was also calculated for liquid iron oxide for the T ≈ 1800 K, which was not measured experimentally, that, obviously, prevented modeling of not only laser surface processing, melting and cleaning of steels, but also calculations in the field of metallurgical and other technologies, which are characterized by the presence of iron oxide melts during heating.

Enhancement of device performance in β-Ga2O3 Schottky barrier diodes with tetramethylammonium hydroxide treatment

Schottky barrier diodes were fabricated on tetramethylammonium hydroxide (TMAH)-treated β-Ga2O3 surfaces with Ni/Au as Schottky metal contacts. TMAH wet surface treatment efficiently reduced the roughness of the β-Ga2O3 surface. An X-ray photoelectron emission analysis of the TMAH-treated β-Ga2O3 surface chemical binding features indicates the effective removal of unwanted surface oxides and contaminants formed unintentionally by chemisorption during exposure to ambient conditions. The TMAH-treated Au/Ni/β-Ga2O3 Schottky diode displayed improved rectification behavior with an increment of barrier height by 0.30 eV compared to the untreated diode. The breakdown voltages (Vbr) for the untreated and TMAH-treated Au/Ni/β-Ga2O3 Schottky barrier diode are 298 and 358 V, respectively. The decrease in the turn-on resistance (Ron) in the Au/Ni/β-Ga2O3 Schottky barrier diode by TMAH treatment can be ascribed to the improved surface morphology. The improved electrical behavior of the Au/Ni/β-Ga2O3 Schottky barrier diode formed on the TMAH-treated β-Ga2O3 surface can be accredited to the oxygen vacancies reduction, that deceased the carrier concentration in the β-Ga2O3 epilayer and to the improved the surface morphology. These results demonstrate the effectiveness of TMAH wet surface treatment on β-Ga2O3 and its potential as a simple method for fabricating β-Ga2O3-based devices with improved performance.

Formation and Microwave Losses of Hydrides in Superconducting Niobium Thin Films Resulting from Fluoride Chemical Processing

AbstractSuperconducting niobium (Nb) thin films have recently attracted significant attention due to their utility for quantum information technologies. In the processing of Nb thin films, fluoride‐based chemical etchants are commonly used to remove surface oxides that are known to affect superconducting quantum devices adversely. However, these same etchants can also introduce hydrogen to form Nb hydrides, potentially negatively impacting microwave loss performance. Here, comprehensive materials characterization of Nb hydrides formed in Nb thin films as a function of fluoride chemical treatments is presented. In particular, secondary‐ion mass spectrometry, X‐ray scattering, and transmission electron microscopy reveal the spatial distribution and phase transformation of Nb hydrides. The rate of hydride formation is determined by the fluoride solution acidity and the etch rate of Nb2O5, which acts as a diffusion barrier for hydrogen into Nb. The resulting Nb hydrides are detrimental to Nb superconducting properties and lead to increased power‐independent microwave loss in coplanar waveguide resonators. However, Nb hydrides do not correlate with two‐level system loss or device aging mechanisms. Overall, this work provides insight into the formation of Nb hydrides and their role in microwave loss, thus guiding ongoing efforts to maximize coherence time in superconducting quantum devices.

Magnesium and magnesium alloy dissolution by high intensity focused ultrasound: erosion/cavitation vs. Wave propagation

The dissolution of metals, influenced by mechanical and chemical factors, plays a crucial role in various applications. Ultrasonic irradiation has been explored for its ability to enhance dissolution rates and modify surface characteristics. In this study, we investigate the dissolution of magnesium (Mg) and magnesium alloys under high-intensity focused ultrasound (HIFU) conditions with frequency sweeping (wobbling). Our findings reveal distinct effects of cavitation and acoustic streaming on the dissolution process. For pure magnesium, ultrasonic treatment significantly increases dissolution rates compared to silent conditions. Negative frequency sweeps result in the highest dissolution rates, linked to increased cavitation activity, while positive sweeps reduce dissolution rates but maintain acoustic streaming effects. The removal of surface oxides is accelerated in all sonication conditions. Macro- and micro-roughness patterns on the surface correspond to the wobbling frequency range, with wavelengths matching the average ultrasonic frequency. However, dissolution is not uniform across the sample, and preferential attack occurs at the focal point during negative frequency sweeps. In contrast, magnesium alloys exhibit lower dissolution rates than pure Mg. The alloy's mechanical properties make it less susceptible to cavitation erosion but more sensitive to acoustic streaming-induced dissolution. Grain boundaries are preferentially attacked, revealing differences between ductile pure Mg and the harder, more cavitation-resistant, alloy. This study highlights the complex interplay between cavitation and acoustic streaming in the dissolution of magnesium and its alloys under HIFU conditions, shedding light on the limits and potential applications of this technique, particularly in microstructure analysis.

Dicarboxylic Acid‐Assisted Surface Oxide Removal and Passivation of Indium Antimonide Colloidal Quantum Dots for Short‐Wave Infrared Photodetectors

AbstractHeavy‐metal‐free III–V colloidal quantum dots (CQDs) are promising materials for solution‐processed short‐wave infrared (SWIR) photodetectors. Recent progress in the synthesis of indium antimonide (InSb) CQDs with sizes smaller than the Bohr exciton radius enables quantum‐size effect tuning of the band gap. However, it has been challenging to achieve uniform InSb CQDs with band gaps below 0.9 eV, as well as to control the surface chemistry of these large‐diameter CQDs. This has, to date, limited the development of InSb CQD photodetectors that are sensitive to 1400 nm light. Here we adopt solvent engineering to facilitate a diffusion‐limited growth regime, leading to uniform CQDs with a band gap of 0.89 eV. We then develop a CQD surface reconstruction strategy that employs a dicarboxylic acid to selectively remove the native In/Sb oxides, and enables a carboxylate‐halide co‐passivation with the subsequent halide ligand exchange. We find that this strategy reduces trap density by half compared to controls, and enables electronic coupling among CQDs. Photodetectors made using the tailored CQDs achieve an external quantum efficiency of 25 % at 1400 nm, the highest among III–V CQD photodetectors in this spectral region.

Dicarboxylic Acid-Assisted Surface Oxide Removal and Passivation of Indium Antimonide Colloidal Quantum Dots for Short-Wave Infrared Photodetectors.

Heavy-metal-free III-V colloidal quantum dots (CQDs) are promising materials for solution-processed short-wave infrared (SWIR) photodetectors. Recent progress in the synthesis of indium antimonide (InSb) CQDs with sizes smaller than the Bohr exciton radius enables quantum-size effect tuning of the band gap. However, it has been challenging to achieve uniform InSb CQDs with band gaps below 0.9 eV, as well as to control the surface chemistry of these large-diameter CQDs. This has, to date, limited the development of InSb CQD photodetectors that are sensitive to 1400 nm light. Here we adopt solvent engineering to facilitate a diffusion-limited growth regime, leading to uniform CQDs with a band gap of 0.89 eV. We then develop a CQD surface reconstruction strategy that employs a dicarboxylic acid to selectively remove the native In/Sb oxides, and enables a carboxylate-halide co-passivation with the subsequent halide ligand exchange. We find that this strategy reduces trap density by half compared to controls, and enables electronic coupling among CQDs. Photodetectors made using the tailored CQDs achieve an external quantum efficiency of 25 % at 1400 nm, the highest among III-V CQD photodetectors in this spectral region.

Characterization of laser-induced surface structures of aluminum in an oxygen-free atmosphere

Brazing of materials with high oxygen affinity, such as aluminum alloys, requires the removal of surface oxides. This is commonly achieved using chemical fluxes. The approach described here utilizes IR-pulsed laser radiation at a pulse duration of 45 ns to remove the oxide layer. To prevent reoxidation, an oxygen-free atmosphere is used by introducing monosilane (SiH4) into the process chamber, which binds the oxygen molecules. The laser-induced surface structuring that occurs during oxide removal of aluminum alloy EN AW-6082 was investigated. Pulse energies between 60 – 300 µJ and different pulse overlaps between 0 – 90 % were analyzed for topography and roughness based on the process atmosphere. The results suggest that the roughness increases significantly only at 90 % overlap. Additionally, laser structuring with high energy and overlap in the presence of O2 results in a porous surface layer, while in the absence of O2, it leads to pillar formation with higher roughness.

Surface Oxide Removal in Preparation for Controlled Liquid Metal Embrittlement

Surface Oxide Removal in Preparation for Controlled Liquid Metal Embrittlement

Nanoporous Cu-Sn Intermetallic Material for Interconnects in Electronic Packaging

Nanostructured and nanoparticulate Cu-based materialshave emerged as viable replacements of conventional solder materials for realizing high-performing and reliable interconnection in today's 2.5D/3D electronic packaging. Unlike the traditional interconnects, those materials offer better compliance during the overall packaging routines. Owing to their highly developed surface area resulting of their uniform interconnected porosity, said nanomaterials also enable joint formation by sintering by thermal compression realized at high pressure and at much lower temperatures compared to bulk counterparts. Recently, sintering of nanoporous (np) Cu was demonstrated under high pressure and temperature after initial pre-treatment in acid for surface oxide removal. Separately, other studies have been focused on the attachment of Cu pillars to pads by using nanoparticulate Cu pastes as sintering material. The ultimate goal is to develop materials that facilitate a highly effective and durable chip-to-substrate interconnection.In this report, we discuss the development and use of nanoporous uniform and continuous CuSn films for low-temperature joint formation. The novelty of this work is realized through the introduction of tin (Sn) to work along with the np-Cu structure for ensuring lower sintering temperatures with a goal of producing packaging interconnects based on CuSn intermetallic compound (IMC). The addition of Sn to the np-Cu system is performed using a bottom-up, all-electrochemical synthetic approach (see Figure 1). The method is founded on the initial synthesis of thin films of np-Cu by dealloying of Cu-Zn alloys, followed by a conformal coating with a thin layer of Sn. The main goal of the proposed approach is to form a np-CuSn composite that upon sintering at high temperature and pressure, transitions into a densely compressed Cu3Sn IMC bonding material. Realized accordingly as interconnect layer, said material ensures good stability at high current densities (greater than 1×105 A ⋅ cm2 ) and low electrical resistivity (equal or less than 8.5 µΩ ⋅ cm).The synthetic details and then the applicability of the Cu-Sn nanomaterials obtained using the approach for synthesis of a low-temperature joint formation material are also discussed. The Sn-coating process was administered by galvanic pulse plating, which was optimized to provide a conformal Sn coating and thus, to retain the initial np-Cu overall porous structure. Another optimization effort was made for ensuring a Sn atomic fraction that allows for the formation of Cu6Sn5 IMC. After the optimization process for determining the necessary plating conditions for reaching mostly Cu6Sn5 composition ratio, the material was subjected to sintering at temperatures in the range from 300ºC to 200oC, and at a pressure of 20 MPa. We found that the joints formed were heavily densified and consisted mainly of Cu3Sn IMC. The report presents the new approach as a facile and cost-effective method for synthesizing nanostructured Cu-Sn films as viable candidates for a new-generation of interconnect materials together with the demonstration of assembly processes that optimize robustness and reliability. The discussion also shows the proposed approach’s applicability by realization of low-temperature sintering of nanostructured CuSn films in pre-designed fine pitch assemblies. Figure 1: Schematic representation demonstrating key steps of the approach for synthesis of np-CuSn composites. Figure 1

Transport kinetics of protium and deuterium in titanium: Experiments and modeling

We conducted absorption experiments on Ti using hydrogen isotopes, protium and deuterium, to gain insights into the kinetics of hydrogen atoms, particularly across the Ti surface. Rather than relying on various surface pretreatments, we identified the initial temperature of Ti in vacuum as the determining factor in achieving a high absorption rate and content. Specifically, we found that the initial Ti temperatures of approximately 900 and 980 °C are optimal for protium and deuterium, respectively, likely due to sufficient removal of the native surface oxide on Ti. The dependence of the absorption rate and amount of hydrogen isotopes on the temperature during absorption was partially explained by the phase transformation and exothermicity of the hydrogenation reaction. Moreover, this dependence strongly indicated the existence of a tunneling transport process. Additionally, we developed a kinetic model for hydrogen isotope transport in Ti to conduct numerical simulations. The absorption curves, calculated using the developed model, closely matched a series of experimental curves obtained at different temperatures. These curves were from an identical set of equations and kinetic parameters for both protium and deuterium. Our numerical kinetic model has the potential to serve as a valuable simulation tool for various applications, such as the design of high-performance hydrogen storage systems and maintenance strategies against hydrogen embrittlement of structural materials.

Oxide-Free SiC-SiC Direct Wafer Bonding and Its Characterization

In this study, the feasibility of oxide-free room temperature wafer bonding process was demonstrated for 4H-SiC wafers with in situ surface oxide removal. The investigations covered three areas: incoming metrology of the original wafer, characterization of activated single wafer and analysis of bonded wafer pairs. The focus was on compositional, chemical, mechanical and morphological analysis of the surfaces and of the bonded interfaces. Incoming wafers were inspected whether they fulfill the requirements of wafer bonding, and activated wafers were characterized to measure the surface modifications. The quality of the bonded wafers and the bonding energy were verified using scanning acoustic microscopy measurements as well as the Maszara blade test. Furthermore, cross-section transmission electron microscopy was used to investigate the amorphous layer at the bonding interface. The work reported is a demonstration of the capability of different characterization methods regarding SiC-SiC wafer bonding.

Significant effect of thin oxide layer on characteristics of p-InGaN/GaN nonalloyed ohmic contacts

In an interconnected high-vacuum illustration system, the surfaces of p-InGaN/GaN heterostructures grown with integrated metalorganic chemical vapor deposition were treated in O2 gas for different times at room temperature and then transferred to another chamber for fabricating ohmic contacts via sputtering Pd/Pt/Au multi-layers. X-ray photoelectron spectroscopy measurements unveil that the proportions of Ga–O of the treated samples significantly increase compared with that of the as-grown sample, indicating that a thin layer of GaOx forms on the p-InGaN/GaN surface after exposing to oxygen or air atmospheres. Meanwhile, the samples exposed to O2 or air were found to have much higher specific contact resistance, i.e., higher by two orders of magnitude than that of the as-grown sample. The specific contact resistance of the as-grown sample was derived as 9.3 × 10−5 Ω cm2 using the circular transmission line measurement. Furthermore, the Schottky barrier height of the samples was determined from the measured I–V curves with the thermionic field emission model and was revealed to be closely related to surface treatments. Despite the fact that the insulating layer of GaOx was as thin as 1.2–1.4 Å for the samples exposed to oxygen at room temperature, it can act as an extra barrier layer causing significant increase in the specific contact resistance via blocking the tunneling of carriers. Therefore, the effective removal of p-InGaN surface oxide plays a vital role in preparing good ohmic contacts.

Preparation and Combustion Mechanism of Boron-Based High-Energy Fuels

Due to the characteristics of high energy density and a high calorific value, boron has become a high-energy fuel and shows great potential to be a high-performance candidate for propellants. However, the wide applications of boron are still limited by the characteristics of easy oxidization, ignition difficulty, a long combustion duration, and combustion products that readily adhere to the surface and inhibit full combustion. Therefore, how to overcome the shortcomings and improve the combustion efficiencies of boron-based fuels have become the highlights in exploring novel high-performance energetic materials. In this paper, the prevalent preparation methods and the corresponding combustion mechanisms of boron-based energetic materials are briefly summarized. The results showed that the boron-based energetic materials can be prepared by surface coating, mechanical milling, and ultrasonic mixing methods. At the same time, the corresponding ignition delay and combustion efficiency were also analyzed according to different combustion tests. The results showed that the boron-based composites with different additives had different combustion characteristics. The combustion of boron-based energetic materials can be optimized by removing surface oxide layers, providing extra heat, inhibiting the formation of or the rapid removal of the combustion intermediates, and increasing the diffusion rate of oxygen. With the improvement of the combustion efficiency of boron-based energetic materials, boron-based high-energy fuels will become more and more widely adopted in the future.

Microbe Regulates the Mineral Photochemical Activity and Organic Matter Compositions in Water

Photochemical reactions that widely occur in aquatic environments play important roles in carbon fate (e.g., carbon conversion and storage from organic matter) in ecosystems. Aquatic microbes and natural minerals further regulate carbon fate, but the processes and mechanisms remain largely unknown. Herein, the interaction between Escherichia coli and pyrite and its influence on the fate of carbon in water were investigated at the microscopic scale and molecular level. The results showed that saccharides and phenolic compounds in microbial extracellular polymeric substances helped remove pyrite surface oxides via electron transfer. After the removal of surface oxides on pyrite, the photochemical properties under visible-light irradiation were significantly decreased, such as reactive oxygen species and electron transfer capacity. Unlike the well-accepted theory of minerals protecting organic matter in the soil, the organic matter adsorbed on minerals preferred degradation due to the enhanced photochemical reactions in water. In contrast, the minerals transformed by microbes suppressed the decomposition of organic matter due to the passivation of the chemical structure and activity. These results highlight the significance of mineral chemical activity on organic matter regulated by microbes and provide insights into organic matter conversion in water.

Surface preparation of metal films by gas cluster ion beams using organic acid vapor for Cu–Cu bonding

Surface-activated bonding (SAB) of Cu by gas cluster ion beam (GCIB) irradiation with acetic acid vapor was studied. GCIB irradiation realizes surface smoothing and surface reaction enhancement without severe damage. Therefore, it is promising for SAB. In this study, acetic acid vapor was introduced during Ar-GCIB irradiation to assist the removal of surface oxides on the Cu surface. XPS results showed that Cu(OH)2 was effectively removed by reaction with adsorbed acetic acid, and there was no residue by acetic acid adsorption. In addition, surface roughness decreased by Ar-GCIB irradiation with acetic acid because of the preferential removal of protrusion. Preliminary bonding experiments showed an increase of Cu–Cu bond strength by Ar-GCIB irradiation with acetic acid vapor.

Surface preparation of metal films by gas cluster ion beams using organic acid vapor for Cu-Cu bonding

Abstract Surface activated bonding of Cu by gas cluster ion beam (GCIB) irradiation with acetic acid vapor was studied. GCIB irradiation realizes surface smoothing and surface reaction enhancement without severe damage. Therefore, it is promising for surface-activated bonding (SAB). In this study, acetic acid vapor was introduced during Ar-GCIB irradiation to assist removal of surface oxides on Cu surface. XPS results showed that Cu(OH)2 was effectively removed by reaction with adsorbed acetic acid, and there was no residue by acetic acid adsorption. In addition, surface roughness decreased by Ar-GCIB irradiation with acetic acid because of the preferential removal of protrusion. Preliminary bonding experiments showed increase of Cu-Cu bond strength by Ar-GCIB irradiation with acetic acid vapor.