Feed Gas Composition Research Articles

Cobalt@Silica Core‐Shell Catalysts for Hydrogenation of CO/CO2 Mixtures to Methane

AbstractCOx hydrogenation reactions for hydrocarbon synthesis, such as methane, are becoming more and more important in terms of the energy transition. The formation of the byproduct water leads to a hydrothermal environment, which necessitates stable catalyst materials under harsh reaction conditions. Therefore, novel nanostructured core‐shell catalysts are part of scientific discussion, since these materials offer an exceptional resistance against thermal sintering. Here we report on a core‐shell catalyst ‐ Co@mSiO2 ‐ for the hydrogenation of CO/CO2 mixtures towards methane. CO methanation experiments reveal a rapid temperature‐depended deactivation for temperatures above 350 °C caused by coking and possible blocking of the pores. In comparison to a Co/mSiO2 reference catalyst with the same Co particle size a significantly higher methane selectivity was found for CO2 hydrogenation, which we attribute to the confinement effect of the core‐shell structure and therefore a higher probability of CO readsorption. Finally, the simultaneous CO/CO2 co‐methanation experiments show a high flexibility of the catalyst materials on different gas feed compositions.

A Proton Conducting Solid Oxide Electrolysis Cell with Tri-Doping Electrolyte and Three-Phase Conducting Steam Electrode for Sustainable Hydrogen Production

Proton conducting solid oxide electrolysis cell (P-SOEC) is an emerging technology that enables sustainable hydrogen production at reduced operating temperatures (400~600 ○C). Economically competitive P-SOEC systems have distinct advantages over conventional oxygen-ion conducting counterpart (O-SOEC), but further technology development and widespread market acceptance will require continuous innovation of materials in order to improve cell performance, enhance system lifetime and reduce cost. In this work, a new type of tri-doping barium-zirconium-cerate electrolyte and a triple phase conducting steam electrode are explored as material candidates. The material synthesis, pellet sintering, and electrical conductivity are carried out to evaluate the feasibility as an electrolyte. For the triple phase conducting electrode, the conduction behavior and catalytic activity towards water splitting reaction are studied. In addition, these two materials are incorporated into P-SOEC button cells and the electrochemical performance is evaluated under different operating conditions (temperature, humidity, feed gas composition, etc). The long-term durability in high steam pressure is also carried out to examine the stability of the material system. Such an early-stage R&D showed feasibility of hydrogen production systems for field operations and the ability to cost effectively produce hydrogen at the scale needed to support military operations.

Mathematical Modeling of a Proton-Conducting Solid Oxide Electrolyzer Cell Using Computational Fluid Dynamics Simulation

The proton-conducting solid oxide electrolyzer cell (H-SOEC) is a novel electrolysis technology, which operates in the intermediate temperature range (500-700°C), as opposed to the high temperature (800-1000°C) operating range of oxygen ion conducting solid oxide electrolysis cells (O-SOECs), or the low temperature (70-80°C) operating range of proton-exchange membrane (PEM) electrolyzers [1]. Proton-conducting electrolyte has high ionic conductivity in this intermediate temperature range, which is not exhibited in O-SOECs; this means that there are fewer material challenges for the development of these devices because they do not need to withstand temperatures up to 1000°C [1,2]. Additionally, this temperature ranges yields the benefits of improved thermodynamics at increased temperature, meaning they require less electrical energy input as compared to PEM electrolyzers [1]. H-SOECs are a promising technology because they retain the thermodynamic benefits of operation at high temperature, without the material complications seen in O-SOECs. The purpose of this work is to model the 2D current density distribution of an H-SOEC at a applied electrolysis voltage. The model considers overpotential losses that occur due to kinetics of the electrochemical reaction, resistance in the cell components, and mass transport. The mathematical model uses electrochemical parameters fit with experimental data from [3] and is incorporated into the solving routine of computational fluid dynamics (CFD) software, Fluent. Figure 1 shows an example of the current density distribution in a planar H-SOEC cell that was obtained from the model simulation. Such parametric distribution is difficult to be measured, and the outcomes of this work would help the experimental development of this novel electrolyzer technology through optimization of operating conditions like temperature and gas feed compositions. The modeling approach also helps understand the thermal-flow management needs of this device as it is scaled up to the stack level. It will also provide a baseline understanding for more complex models that consider the internal transport phenomena in 3D, as only limited modeling papers on H-SOEC are available now.

Toward modeling of performance of hydrogen selective mixed matrix membrane using artificial neural network

ABSTRACTRecently hydrogen gas introduced as valuable gas which could be recovered from various streams and membrane science is one of the applicable and economically attractive way for the separation of hydrogen. In this present contribution, artificial neural network modeling was used to model the gas permeability of hydrogen at various operational condition and different composition of feed. Also, multi-layer perceptron (MLP) used as optimization algorithm for improving the accuracy of developed ANN model. Feed pressure, gas composition and weight percent of nano filler was used as input parameter and hydrogen selectivity is the output of the model. Modeling results show that ANN modeling shows best performance among the all models reported in the open literature. Moreover, statistical parameters resulted from ANN modeling compared with other artificial intelligence-based models. Coefficient of determination, root-mean-square error, and average absolute relative deviation of ANN modeling is 0.9984, 0.01184, and 4.85, respectively, which shows the accuracy and robustness of the developed model for the prediction of hydrogen selectivity.

Biogas separation using a membrane gas separator: Focus on CO2 upgrading without CH4 loss

The CO2 in biogas could be utilized as a resource for industrial or agricultural application, even though it was treated as impurity in many previous studies for biogas upgrading. In this study, biogas was upgraded in terms of recovering highly concentrated CO2 without incurring CH4 loss. Tests under various conditions were conducted to enhance CO2 concentration and recovery efficiency. First, the effects of temperature and pressure on CO2 upgrading were investigated. 40°C and 7bar being selected as the optimal conditions that were then applied for further experiments. Separation tests under varying CO2 and CH4 compositions in a synthetic biogas were performed. The CO2 concentration in the permeate gas was upgraded to a maximum of 70.1%, 77.3% and 83.1% at feed gas compositions of 30%, 35%, and 40% CO2, respectively. Next, when applying a triple-stage separation configuration, a simultaneous improvement in both the CO2 concentration and CH4 loss were observed. These results indicate that an increase in CO2 concentration caused a reduction of CH4 loss. Finally, at biogas plant, CO2 was concentrated at 95.6% with 0.57% CH4 loss, which could be utilized in area to want the carbon source such as industrial and agricultural field.

Dynamic modeling of syngas fermentation in a continuous stirred‐tank reactor: Multi‐response parameter estimation and process optimization

Syngas fermentation is one of the bets for the future sustainable biobased economies due to its potential as an intermediate step in the conversion of waste carbon to ethanol fuel and other chemicals. Integrated with gasification and suitable downstream processing, it may constitute an efficient and competitive route for the valorization of various waste materials, especially if systems engineering principles are employed targeting process optimization. In this study, a dynamic multi‐response model is presented for syngas fermentation with acetogenic bacteria in a continuous stirred‐tank reactor, accounting for gas–liquid mass transfer, substrate (CO, H2) uptake, biomass growth and death, acetic acid reassimilation, and product selectivity. The unknown parameters were estimated from literature data using the maximum likelihood principle with a multi‐response nonlinear modeling framework and metaheuristic optimization, and model adequacy was verified with statistical analysis via generation of confidence intervals as well as parameter significance tests. The model was then used to study the effects of process conditions (gas composition, dilution rate, gas flow rates, and cell recycle) as well as the sensitivity of kinetic parameters, and multiobjective genetic algorithm was used to maximize ethanol productivity and CO conversion. It was observed that these two objectives were clearly conflicting when CO‐rich gas was used, but increasing the content of H2 favored higher productivities while maintaining 100% CO conversion. The maximum productivity predicted with full conversion was 2 g·L−1·hr−1 with a feed gas composition of 54% CO and 46% H2 and a dilution rate of 0.06 hr−1 with roughly 90% of cell recycle.

Adsorption isotherms of CO2 – CH4 binary mixture using IAST for optimized ECBM recovery from sub-bituminous coals of Jharia coalfield: an experimental and modeling approach

ABSTRACTPrediction of adsorption equilibria of methane (CH4)/carbon dioxide (CO2) on coal at various feed gas composition is essential for the commercial development of primary and enhanced coalbed methane (ECBM) recovery by CO2 sequestration from coal reservoirs. In this article, an attempt is made to determine the adsorption equilibria of CH4 and CO2 mixture over a wide range of pressure and free gas compositions with the help of developed computational approach using the Ideal Adsorption Solution theory (IAST) for multicomponent adsorption. The advantage of the model is that it can predict the adsorption equilibrium of the mixture from the pure gas equilibrium data. Considering the complex adsorption during gas transportation and efficient gas injection modes at constant pressure or constant composition, the two different computational approaches were developed to solve the iterative calculation of IAST, i.e. constant mole fraction with variable pressure (P = 0 to 1000 psi) and constant pressure with mole fraction as a variable (y = 0 to 1). Individual adsorption isotherm of CH4 and CO2 for the two different sub-bituminous coal samples was determined up to a pressures of 1250 psi. Pure gas isotherms reveal that volume wise CO2 has 2.00 to 2.63 times more adsorption capacity than CH4 for the samples respectively. Computational model developed using ideal solution theory can predict the adsorbed gas volume at any pressure on the experimentally studied samples collected from Indian CBM fields. The developed computational code using MATLAB© software was validated using the binary adsorption data for the two coal samples from global CBM fields published in the literatures, with calculated error lying within the permissible limit.

Methanol Synthesis Using CO₂ and H₂ on Nano Silver-Ceria Zirconia Catalysts: Influence of Preparation Method.

Silver catalysts supported on ceria-zirconia (CZ) mechanically mixed oxide were synthesized by wet impregnation and chelating methods. Nominal loadings of 5 wt.% of Ag was deposited on the CZ support. These catalysts were tested for the CO₂ hydrogenation reaction to methanol with feed gas composition of CO₂-H₂ = 3:1 at 250 °C, 20 bar total pressure and GHSV of 1800 h-1. The calcined and reduced catalysts were characterized using XRD, BET, TPR, SEM-EDS, XPS and FTIR-DRIFTs techniques. Finely deposited silver crystallites sized in the range of 20-50 nm were observed through SEM and HR-TEM analysis. TPR and XRD studies demonstrated the presence of Ag₂O and metallic silver (Ag0) on CZ support. About 10% of CO formation was observed on chelating catalyst (5Ag/CZ CHE). However, only, 5% CO was observed on impregnated (5Ag/CZ IMP) catalyst. The greater CO formation was associated with ease reduction of Ag₂O to metallic silver in 5Ag/CZ CHE catalyst. Further, 70% of methanol selectivity was observed on 5Ag/CZ IMP due to the presence of Ag₂O on CZ. FTIR-DRIFTs results revealed the methanol formation via formate intermediates and CO formation via RWGS reaction on the studied catalysts.

Microstructure and mechanical properties of CVD TiN/TiBN multilayer coatings

Application of a multilayered coating architecture based on two alternating hard materials provides a strategy to enhance the mechanical properties of hard coatings for the metal cutting industry. Within this work, the mechanical behavior of CVD TiN/TiBN multilayer coatings was correlated with their microstructure and compared to the respective single-layers. Multilayers with different bilayer periods (1400, 800, 300 and 200 nm) were prepared in an industrial-scale thermal CVD plant by alternate variation of the feed gas composition. Complementary X-ray diffraction, Raman spectroscopy and transmission electron microscopy investigations confirmed a dominating face-centered cubic TiN structure, accompanied by hexagonal TiB2 and amorphous TiB in the B containing coatings. The addition of B and a decreasing bilayer period resulted in a decreasing grain size and increasing hardness. Micromechanical bending tests revealed an increase of strength and fracture toughness with addition of B and increasing layer number. While the highest strength was observed in a TiBN single-layered coating, the multilayered TiN/TiBN with a bilayer period of 200 nm was the hardest and at the same time toughest of the investigated coatings.

Impact of Char Properties and Reaction Parameters on Naphthalene Conversion in a Macro-TGA Fixed Char Bed Reactor

Catalytic tar removal is one of the main challenges restricting the successful commercialization of biomass gasification. Hot gas cleaning using a heterogeneous catalyst is one of the methods used to remove tar. In order to economically remove tar, an efficient low-cost catalyst should be applied. Biomass char has the potential to be such a catalyst. In this work, the reactor parameters that affect the conversion of a model tar component “naphthalene” were investigated employing an in situ thermogravimetric analysis of a fixed bed of biomass char. The following reactor and catalyst parameters were investigated: bed temperature (750 to 900 °C), gas residence time in the char bed (0.4 to 2.4 s), char particle size (500 to 1700 μm), feed naphthalene concentration, feed gas composition (CO, CO2, H2O, H2, CH4, naphthalene, and N2), char properties, and char precursor. It was found that the biomass char has a high activity for naphthalene conversion. However, the catalytic performance of the biomass char was affected by the gasification reactions that consumed its carbon, and the coke deposition that reduced its activity. Furthermore, high ash and iron contents enhanced char activity. The results of this work will be used in the design of a process that uses biomass char as an auto-generated catalyst in the gasification process.

Catalytic DBD plasma approach for methane partial oxidation to methanol under ambient conditions

Abstract CuO/γ-Al2O3 promoted partial oxidation of methane to methanol was carried out in a non-thermal plasma DBD reactor operating under ambient conditions. The catalysts were synthesized by wet impregnation method, characterized systematically and tested for a single step methane partial oxidation to methanol under ambient conditions. Typical results highlighted the complementary effect of plasma and catalyst, where integration of catalyst in plasma zone improved both conversion of methane and selectivity to methanol. The influence of specific input energy and feed gas composition was studied and the intermediate species formed in plasma catalytic reaction was monitored by an optical emission spectroscopy. The best CH3OH selectivity of ˜37% was achieved over 5 CA catalyst against the ˜27% CH3OH selectivity with plasma system. The improved performance of the hybridized system was asserted due to high charge deposition and modified gas phase chemistry due to catalyst integration.

Study of MoO3-γAl2O3 catalysts behavior in selective catalytic reduction of SO2 toxic gas to sulfur with CH4.

In the present study, a detailed investigation was carried out on MoO3 alumina-supported catalysts behavior in selective catalytic reduction of SO2 to sulfur with CH4. At first, four different molybdenum catalysts with weight rates of 0, 5, 10, and 15 were impregnated on γ-alumina to be characterized using XRD, SEM, BET, BJH, and N2 adsorption. Then, to find the most active catalyst, temperature dependency test was performed on all of the prepared catalysts and the result representing Al2O3-Mo10 as the best catalyst. In next step, the effects of feed gas composition, space velocity, and long-term activity, as an important industrial factor, were tested on Al2O3-Mo10. It was revealed instantaneously from the beginning, MoO3 specie started to convert mainly into MoS2 and MoO2, and a minor part into Mo2C, which is terminated after 750min achieving a stable condition. Thereafter, SO2 conversion and sulfur selectivity increased from 85.8 to 89.4% and 99.4 to 99.7%, respectively. XRD graph of the used catalyst and TPO thermogravimetric/mass-spectra proved possible happening of the proposed mechanism in long-term activity. At the end, mean activation energy was determined based on Arrhenius model in temperature range of 550 to 800°C, with a value of 0.33eV for Al2O3-Mo10.

CASE STUDY: ECONOMIC AND TECHNICAL ANALYSIS OF RETROGRADE GAS PROCESSING ON OFFSHORE PLATFORMS

Recent discoveries of retrograde gas fields in Brazilian deepwaters are more distant from the shore, and away from available infrastructure for processing gas. In this scenario, processing the gas offshore becomes an important challenge, equipping these fields to become viable producers. The present study aims to evaluate the economic motivation for this concept by comparing two scenarios of retrograde gas production. The scenarios consider processing the gas to sales based on: 1. Production offshore and gas export to a new built onshore processing plant, and 2. Production and treatment of the gas offshore. The second scenario considered three hydrocarbon dew point treatments: Joule-Thomson, Turboexpansion, and Refrigeration. The economic assessment used the software QUE$TOR®. Following the assessment, process simulations used ASPEN HYSYS® to perform the technical evaluation. The economic results were promising despite connected strongly to gas price. The simulation results indicated that the use of dew point treatments are viable but depend on feed gas composition.

Stability of amine-functionalized CO2 adsorbents: a multifaceted puzzle.

This review focuses on important stability issues facing amine-functionalized CO2 adsorbents, including amine-grafted and amine-impregnated silicas, zeolites, metal-organic frameworks and carbons. During the past couple of decades, major advances were achieved in understanding and improving the performance of such materials, particularly in terms of CO2 adsorptive properties such as adsorption capacity, selectivity and kinetics. Nonetheless, to pave the way toward commercialization of adsorption-based CO2 capture technologies, in addition to other attributes, adsorbent materials should be stable over many thousands of adsorption-desorption cycles. Adsorbent stability, which is of utmost importance as it determines adsorbent lifetime and operational costs of CO2 capture, is a multifaceted issue involving thermal, hydrothermal, and chemical stability. Here we discuss the impact of the adsorbent physical and chemical properties, the feed gas composition and characteristics, and the adsorption-desorption operational parameters on the long-term stability of amine-functionalized CO2 adsorbents. We also review important insights associated with the underlying deactivation pathways of the adsorbents upon exposure to high temperature, oxygen, dry CO2, sulfur-containing compounds, nitrogen oxides, oxygen and steam. Finally, specific recommendations are provided to address outstanding stability issues.

High-Temperature Co-Electrolysis: A Versatile Method to Sustainably Produce Tailored Syngas Compositions

High-temperature co-electrolysis of carbon dioxide and steam is a promising method to produce ‘white’ syngas by making use of renewable energy and carbon dioxide as sustainable feedstock. The technological key advantage is the possibility to tailor syngas compositions over a broad range. This paper presents a systematic investigation of the syngas tailoring process by establishing relationships between feed gas compositions and flow rates to the syngas ratio. A linear dependence between the H2O:CO2 ratio in the feed gas and the H2:CO ratio in the output gas was observed. Furthermore, the syngas ratio remains mostly invariant upon variations in electrochemical potential and fluctuating gas utilizations/flow rates during operation of a co-electrolysis cell. Most importantly, the co-electrolysis performance was demonstrated to operate at high current densities of up to 3.2 A·cm−2 over a broad range of feed gas compositions with faradaic efficiencies of nearly 100%. The possibility to operate co-electrolysis under transient load conditions renders this method particularly suitable in future scenarios of intermittent availability of renewables. The results described here illustrate the versatility of co-electrolysis, which can produce all relevant syngas compositions in a single-step process at constantly high performance.

Effect of Oxygen Partial Pressure on Solid Oxide Electrolysis Cells

High-temperature (700-900 degrees C) steam electrolysis based on solid oxide electrolysis cells (SOECs) is valuable as an efficient and clean path for large-scale hydrogen production with nearly zero carbon emissions, compared with the traditional paths of steam methane reforming or coal gasification. The operation parameters, in particular the feeding gas composition and pressure, significantly affect the performance of the electrolysis cell. In this study, a computational fluid dynamics model of an SOEC is built to predict the electrochemical performance of the cell with different sweep gases on the oxygen electrode. Sweep gases with different oxygen partial pressures between 1.01 x 10(3) and 1.0 x 10(5) Pa are fed to the oxygen electrode of the cell, and the influence of the oxygen partial pressure on the chemical equilibrium and kinetic reactions of the SOECs is analyzed. It is shown that the rate of increase of the reversible potential is inversely proportional to the oxygen partial pressure. Regarding the overpotentials caused by the ohmic, activation, and concentration polarization, the results vary with the reversible potential. The Ohmic overpotential is constant under different operating conditions. The activation and concentration overpotentials at the hydrogen electrode are also steady over the entire oxygen partial pressure range. The oxygen partial pressure has the largest effect on the activation and concentration overpotentials on the oxygen electrode side, both of which decrease sharply with increasing oxygen partial pressure. Owing to the combined effects of the reversible potential and polarization overpotentials, the total electrolysis voltage is nonlinear. At low current density, the electrolysis cell shows better performance at low oxygen partial pressure, whereas the performance improves with increasing oxygen partial pressure at high current density. Thus, at low current density, the best sweep gas should be an oxygen-deficient gas such as nitrogen, CO2, or steam. Steam is the most promising because it is easy to separate the steam from the by-product oxygen in the tail gas, provided that the oxygen electrode is humidity-tolerant. However, at high current density, it is best to use pure oxygen as the sweep gas to reduce the electric energy consumption in the steam electrolysis process. The effects of the oxygen partial pressure on the power density and coefficient of performance of the SOEC are also discussed. At low current density, the electrical power demand is constant, and the efficiency decreases with growing oxygen partial pressure, whereas at high current density, the electrical power demand drops, and the efficiency increases.

Highly Selective, Iron‐Driven CO2 Methanation

AbstractCO2 methanation has gained traction for its potential in renewable energy storage, though the high cost of renewable hydrogen production and costly metals used in methanation catalyst synthesis remain a significant barrier to implementation. Herein we present a Ru−Fe@NCNT catalyst, consisting of ruthenium and iron nanoparticles on nitrogen‐doped carbon nanotubes, as a highly selective, hydrogen efficient, iron‐driven alternative to typical nickel and ruthenium catalysts used for CO and CO2 methanation. Ru−Fe@NCNT offer competitive CO2 conversion and methane selectivity, and a reduction of up to 80 % in ruthenium loading versus similar literature and commercial catalysts. It is proposed that this desirable CO2 methanation performance, using an atypical optimal feed gas composition where PH2/PCO2=3, is the result of effective cooperation between the iron‐catalysed reverse water gas shift and methane‐selective Fischer‐Tropsch, and ruthenium‐catalysed CO methanation reactions.

Enhanced dry reforming toward hydrogen production over Ni/CeO2[sbnd]SiO2 via different catalyst synthesis routes

Dry reforming of methane to produce hydrogen-rich syngas is widely studied, where the productivity of the reaction is greatly dependent on the activity and stability of the catalyst. In this investigation, a binary catalyst support consisting of a SiO2CeO2 combination was developed to produce an efficient catalyst support with high resistance to carbon deposition during the reaction. Four different preparation methods (deposition-precipitation, DP; impregnation, IMP; sol-gel, SG; and ball milling, BM) were used to synthesize SiO2CeO2 supports, and 5 wt % nickel was added to the support via wet impregnation. The catalyst performance was recorded in the order: Ni/CS-DP ˃ Ni/CS-IMP ˃ Ni/CS-BM ˃ Ni/CS-SG at 800 °C for 10 h reaction with a CH4/CO2 feed-gas composition ratio of 1:1. Ni/CS-DP was seen to be the most promising catalyst, exhibiting H2 and CO selectivities of ∼95% and ∼73%, respectively, with a H2/CO ratio of >1:1, and maintaining 97% CH4 conversion up to 10 h of reaction with very low coke deposition. The superior catalytic performance of Ni/CS-DP was achieved as a result of the synergistic effects of high basicity and low reducibility, which was achieved via direct alteration during the preparation of the support.

Hydrogen isotopes separation validation of frontal displacement chromatography for various compositions of feed gas and tritium extraction simulation for TBM

Based on the previous study in frontal displacement chromatography (FDC) packed with Pd-Al2O3, three groups of separation tests were carried out to verify the separation performance of the constituted FDC device for various compositions of feed gas and to validate the application probability of FDC in the Tritium Extraction System (TES) of ITER and China Fusion Engineering Test Reactor (CFETR). The separations were conducted by the FDC procedure with characteristics of the feed gas one-time flushing though the column and then reasonable separation performance had been obtained. The results indicate that the FDC process could be applied to deal with the desorbed gas mixtures from TES and/or further to extract and thereafter enrich the breeding tritium in ITER or CFETR, which would take the advantages of system compactness and efficiency over the present route of TES. Comparing to other related displacement chromatography procedures, the FDC process could be applied in tritium pre-enrichment for the mixtures of low tritium concentrations, which is highlighted by the outstanding merit of operation simplicity.

Development of surface barrier detectors of low leakage current employing argon cold plasma assisted silicon surface cleaning prior to detector fabrication

The performance of silicon surface barrier detectors was improved by atmospheric pressure argon cold plasma assisted surface cleaning compared to routine cleaning using organic solvents, prior to edge protection using epoxy, then followed by metallization. The silicon wafers' surfaces were exposed to plasma etching, after lapping and chemical etching. The plasma cleaning conditions were optimized with respect to feed gas composition, flow rates and exposure time. Various Au/n-Si surface barrier detectors fabricated thereof were characterized by measuring forward and reverse Current-voltage (I-V) characteristics and alpha energy spectra obtained on exposure to α-particles. The variation of alpha energy resolution with applied reverse bias voltage for each detector was also studied. The results reveal that the leakage current of the new detectors treated with cold plasma formed by a mixture of Argon and Air (90% Argon and 10% Air) was substantially lower than that of surface barrier detectors fabricated following conventional methods.