High fluxes available at modern neutron and synchrotron sources have opened up a wide variety of in situ and operando studies of real processes using scattering techniques. This has allowed the user community to follow chemistry in the beam, which often requires high temperatures, gas flow, etc. In this paper, we describe an integrated gas handling system for the general-purpose powder diffraction beamline Powgen at the Spallation Neutron Source. The Automated Gas Environment System (AGES) allows control of both gas flow and temperature (room temperature to 850 °C), while measuring the partial pressure of oxygen and following the effluent gas by mass spectrometry, concurrent with neutron powder diffraction, in order to follow the structural evolution of materials under these conditions. The versatility of AGES is illustrated by two examples of experiments conducted with the system. In solid oxide fuel cell electrode materials, oxygen transport pathways in double perovskites PrBaCo2O5+δ and NdBaCo2O5+δ were elucidated by neutron diffraction measurements under atmosphere with oxygen partial pressures (pO2) of 10−1 to 10−4 (achieved using mixtures of nitrogen and oxygen) and temperatures from 575 to 850 °C. In another example, the potential oxygen storage material La1−xSrxFeO3 was measured under alternating flows of 15% CH4 in N2 and air (20% O2 in N2) at temperatures from 135 to 835 °C. From the oxygen stoichiometry, the optimal composition for oxygen storage was determined.
I. INTRODUCTION AND DESIGN CRITERIA
In situ neutron diffraction has been a powerful tool for material characterization since the early days of neutron measurements. The high penetration depth of neutrons can simplify the design of furnaces, cryostats, and other sample environment equipment. While variable temperature measurements may be the most common, variable gas atmosphere measurements are vital for several key research areas, such as catalysis, fuel cells, and high temperature synthesis of materials. For example, the solid electrolytes in solid-oxide fuel cells are exposed to both oxidizing and reducing atmospheres, all while being held at elevated temperatures. Also, oxygen storage materials (OSMs) undergo significant changes in their oxygen stoichiometry when exposed to alternating oxidizing and reducing atmospheres. To enable these kinds of experiments, various scattering facilities, including both synchrotron1–3 and neutron sources,4,5 have designed flow-through sample cells and gas handling systems. For example, ISIS spallation neutron source designed and built a vanadium cell and furnace stick for use in catalysis experiments.4
For the Powgen neutron powder diffractometer (beamline 11A) at the Spallation Neutron Source (SNS) located at the Oak Ridge National Laboratory, the Automated Gas Environment System (AGES) has been developed as a fully-integrated system for controlled gas flow and monitoring during in situ neutron diffraction experiments. The design considerations required the controlled flow of various gases at rates up to 500 SCCM, including oxygen (O2), hydrogen/deuterium (H2/D2), carbon dioxide (CO2), and carbon monoxide (CO), to achieve a wide range of oxygen partial pressures (pO2 from 10−24 to 1 atm). The system needed to allow controlled mixing of different gases as well as heating of the sample. It was also critical to monitor the gas flow and have an independent measurement of the pO2 to correlate the structural parameters to other transport measurements carried out under these conditions. Finally, because SNS is a user facility, the system had to be easy to operate, even for those unfamiliar with it. Above all, due to the flammability of the gases and the need for high temperatures, the design required a high level of safety consideration.
AGES was originally installed at the beamline in October 2010 and has undergone numerous developments and refinements in the years since. An overview of AGES is shown in Fig. 1. Gas flows from cylinders housed in an external storage building to two gas mixing cabinets in the instrument hall. From the gas cabinets, the final gas stream flows through the sample in an insert housed inside a vacuum furnace, after which the gas is pushed out of the building via an exhaust blower. The available gases and their maximum flow rates are listed in Table I. Additionally, there is space to connect other non-hazardous gases as needed, currently pre-made mixtures of small amounts of O2 in N2. The gas stream may be humidified and is continuously monitored via an oxygen sensor and a universal gas analyzer (UGA). The system is typically used with a vacuum furnace, up to a maximum temperature of 850 °C. Since the original installation of AGES, a total of 18 experiments have taken advantage of the system, on topics ranging from oxygen storage materials to the in situ synthesis of battery materials.
|Gas type .||Category .||Maximum flow rate (SCCM) .|
|Carbon monoxide CO||Hazardous||100|
|Carbon dioxide CO2||Inert||100|
|Mix 4% H2 in He||Inerta||500|
|Air 21% O2 in N2||Oxidizer||500|
|Gas type .||Category .||Maximum flow rate (SCCM) .|
|Carbon monoxide CO||Hazardous||100|
|Carbon dioxide CO2||Inert||100|
|Mix 4% H2 in He||Inerta||500|
|Air 21% O2 in N2||Oxidizer||500|
This mix of 4% H2 is below the lower explosive limit (LEL).
II. DESIGN DETAILS
A. Cylinder storage outbuilding
Starting at the source, the gas cylinders are housed in a detached outbuilding (Fig. 2). The outbuilding is separated into two isolated sections, one for non-hazardous gases (including oxidizers) and the other one for hazardous gases (including flammable gases). Both sides are continuously exhausted via fans and fan operation is remotely monitored. The N2, O2, and CO2 lines are provided with wall-mounted switchover regulators, in order to simplify switching cylinders. Gas lines from the outbuilding are all orbital welded stainless steel, up to the point where they enter the gas mixing cabinets, with no fittings, in order to reduce potential leak points inside the instrument hall.
B. Gas mixing cabinets
The incoming gases are mixed and the flow rates controlled are within two cabinets (Fig. 3), one for the hazardous gases and the other one for non-hazardous gases. The gas valves (Swagelok ultra-high purity diaphragm sealed) in the cabinets are pneumatic, being actuated by compressed air rather than electricity, in order to reduce the possibility of sparks. The valves are, with two exceptions discussed below, normally closed valves, meaning that in a de-energized state the valves are closed, making them fail-safe. The building compressed air line to each valve is controlled by a solenoid valve (SMC VCQ series). The solenoids are housed in the non-hazardous gas cabinet, with air lines running through feedthroughs into the hazardous gas cabinet for valves located there.
Flow rates are adjusted by thermal mass flow controllers (MFCs). In the non-hazardous gas cabinet, there are seven APEX (MC series) mass flow controllers (MFCs), one for each gas line. Most of the MFCs have a maximum flow rate of 500 SCCM. On the He/Ar and CO2 lines, the MFCs have a maximum flow rate of 100 SCCM, which allows better control at very small flow rates. In the hazardous gas cabinet, there are four MFCs (MKS 1479A series with an MKS 647C controller), two each with maximum flow rates of 100 and 500 SCCM. The piping in this cabinet is such that H2, CH4, and the H2/He mix can flow through MFCs with either 100 or 500 SCCM maximum flow rates, to gain the best combination of range and precision. The CO gas line may only flow through a 100 SCCM max MFC.
There are multiple safety features incorporated into the gas cabinets. On each hazardous gas line, a redundant pressure regulator provides an extra layer of safety in the event that the regulator on the gas cylinder fails. Vacuum pumps in each cabinet can be used to remove residual gas from the lines between gas changes, particularly when switching between oxidizing gases and flammable gases. The vacuum pump in the hazardous cabinet (Vacuubrand MZ 2C NT) is rated for hydrogen (ATEX: II 3G IIC T3 X internal). Each cabinet also includes a pressure indicator on the vacuum line to ensure that the pumps are operating correctly. Other safety features of the cabinets that relate more directly to the alarm system will be discussed below.
C. Furnace insert
The sample is held inside an insert placed in a vacuum furnace. This approach allows the gas-flow insert to be used with different furnaces without requiring alteration of the furnace itself. As shown in Fig. 4, the insert has a tube-in-tube design, where the gas flows down the inner tube, through the sample hanging at the bottom, and up and out the outer tube. The portions of the tubes in the hot zone of the furnace are made of quartz glass. Quartz is relatively stable in both oxidizing and hydrogen atmospheres at high temperatures. Additionally, quartz glass, being amorphous, does not add large peaks to the measured diffraction patterns. It does give wide amorphous bumps, but these background features are easier to subtract. Outside the hot zone, the quartz tubes are attached with metal-to-glass transitions (Technical Glass Products, Inc.) to 304 stainless steel tubes. These connect to the upper fixtures via quick connect fittings (Kurt J. Lesker Co.), which allow adjustment of the sample height. The gas inlet and outlet hoses, as well as a pressure relief valve hose, connect to the fixtures via VCO fittings. Additionally, there are two thermocouple feedthroughs to allow both primary and redundant Type K thermocouples for temperature control. The sample itself is held in a quartz glass basket, which has a quartz frit at the bottom and quartz wool on top of the sample to allow gas flow while still containing the sample. The basket hangs from the bottom of the inner tube via Macor pins. A Macor sleeve between the inner tube and the basket helps us to reduce the literal wiggle room, thereby improving the stability of the sample position.
Several other options are available to users. Before the gas stream enters the insert, it may be flowed over water to be humidified. The temperature of the water, and therefore the degree of humidification, is controlled with a Thermocube 200/300/400 chiller. Additionally, the gas stream may be sampled, either before or after the sample, by a universal gas analyzer (Stanford Research Systems UGA100) to determine the atomic composition. Finally, an oxygen sensor (Imtech 122-10) monitors the gas exiting the insert, using a YSZ sensor to determine the partial pressure of oxygen (pO2). Such systems are commonly used in solid-oxide fuel cell research. Our model can measure values ranging from 10−24 (typical value for H2) to 1 (O2).
D. Control system
The brain of AGES is a Programmable Logic Controller (PLC), which controls the valves, MFCs, pressure indicators, and vacuum pumps. The PLC (Compact Logix L3) is interconnected through a Local Area Network (LAN) to Linux-based Experimental Physics and Industrial Control System (EPICS) software.6 The PLC is programmed using ladder logic and communicates over the TCP/IP network to an Input Output Controller (IOC) using the ETHERIP EPICS module. The PLC contains most of the logic to handle user-level control, as discussed below. It also interfaces with the vacuum pumps, pneumatic valves, and oxygen and pressure sensors. The PLC has three types of modules. The analog input module is connected to the pressure sensors and the oxygen sensor. These pressure sensors provide a linear voltage output that corresponds to the pressure and the oxygen sensor provides an analog voltage, which is fed into the Nernst equation to give the partial pressure of oxygen. The PLC also has two digital output modules, which are used to actuate the valves. The last module contains relays, which are used to control power to the hazardous and non-hazardous cabinet vacuum pumps.
The EPICS IOC also controls the MFCs. In the hazardous gas cabinet, the software communicates to the MKS 647C controller using an ethernet to RS-232 serial converter. It is capable of setting correction factors and flow setpoints, and of reading back the setpoints, actual flow rates, and gas correction factors. In the non-hazardous gas cabinet, the software also uses an ethernet to RS-232 serial converter. It communicates to an RS-232 Alicat serial hub and, using string commands, can set the flow setpoint and correction factor and read back the flow setpoint, actual flow rate, absolute pressure, and correction factor. The EPICS IOC communicates to the MFCs using TCP/IP. The IOC then transforms the setpoints and reads back into the channel access (CA) protocol. Once all the variables are in CA, they can be set or read at the beamline, monitored away from the beamline, archived into a database, and scripted.
The graphical user interface was designed using Control System Studio7 (CS-Studio, Fig. 5). The states of the pneumatic valves, pressures, and flows are archived in a database and can be plotted in CS-Studio to facilitate debugging and experiment analysis. AGES has two operational modes. Administrator mode allows full control of individual valves, MFCs, and pumps and is password-protected to only allow access to beamline staff. User mode is automated to allow gas flow by simply selecting the desired gas or gases and setting the flow setpoint for each gas. The software automatically opens the appropriate valves for each gas. Upper and lower flow limits may be set for each gas so that notification will be sent if the flow rate falls outside the desired range. Users may also choose whether or not to humidify the gas and may control the final valve leading to the sample area. Finally, there are automated subroutines to remove residual gas from the AGES piping that automatically open and close the proper valves and start vacuum pumps in the proper sequence. In addition to the GUI controls, the gas flow rates may be controlled through a scripting interface, which allows the automatic collection of data under multiple different gas flow settings.
Given the dangers inherent with some of the gases, safety was a primary concern in the design of AGES. During the design, preference was given to engineered controls over administrative controls, as the former are much more effective in preventing accidents. The maximum allowable flow rate for each hazardous gas was calculated based on the worst-case scenario of a disconnected hose allowing an entire cylinder to empty into the instrument hall, taking into consideration the building’s air-exchange rate, in order to stay below regulatory exposure limits. These calculations were used to guide the selection of MFC maximum flow rates, as well as sizing of flow-restricting orifices (diameters of 0.004–0.007 in.) installed in each gas line in the outbuilding. Excessive pressure on the upstream end of an orifice, for example, due to a through-failure of the regulator, can dislodge the orifice, allowing higher flow than anticipated. Therefore, pressure relief valves (125 psi) were installed immediately downstream of each regulator. Another pressure relief valve (5 psi) was installed on the sample space to protect the insert from over-pressure.
The potential for leaks inside the gas mixing cabinets is ameliorated by a blower (Amtek Rotron) that continuously, actively evacuates the space inside the cabinets to keep them under a slight negative pressure. A back-up blower on an Uninterruptable Power Supply (UPS) automatically starts if the primary blower stops. A differential pressure sensor in the hazardous cabinet monitors if the required level of negative pressure is maintained. The capacity of the blowers was sized to ensure that in the event of a worst-case failure, the dilution factor would be sufficient to keep the flammable gas concentrations below 25% of their lower explosive limits (LELs). The limiting condition involves simultaneous failures of the regulators and the MFC on the hydrogen line allowing maximum flow of H2, in which case a blower flow of 2.86 × 105 SCCM is required. Additionally, two sets of gas sensors (Mine Safety Appliances MSA Ultima XE) monitor both the exhaust from the gas cabinets and the area around the insert (Fig. 6). Each set contains one CO sensor, set to an alarm limit of 15 ppm, and one combustible sensor, set to an alarm limit of 25% of the LEL.
Hydrogen is flammable with a wide explosive range in air, ranging from a lower explosive limit of 4% to an upper explosive limit of 75%. Therefore, to prevent mixing with O2, the H2 line contains an extra valve. This valve is normally open and is actuated from the same solenoid and the same air hose as the normally closed valve on the O2 line. In this way, only one of these two valves can ever be open at a given time, preventing the simultaneous flow of O2 and H2. In addition, there are procedures for cleaning out the gas lines when switching between hazardous and oxidizing gases.
The safety systems are controlled through a configuration-controlled PLC, separate from the gas flow control PLC, and is monitored at all times while in operation by the shift Instrument Hall Coordinator. An alarm is sent to them if any problem occurs with the gas cabinet differential pressure sensor, exhaust blowers on the main system and the gas outbuilding, or the gas leak detection sensors. In the event of a loss of negative pressure on the gas cabinets, a local white strobe is activated and the shut-off valves in the gas outbuilding are activated, stopping all hazardous gas flow into the instrument hall. In the event of a detected gas leak, a local red strobe and an audible alarm are activated, the shut-off valves are activated, and all power is cut to the gas mixing cabinets, which are on a dedicated circuit, closing the valves.
III. SCIENCE ENABLED BY AGES
The ability to simultaneously control the gas atmosphere and the temperature of a sample enables unique research opportunities in fields with significant societal impact such as energy storage materials, oxidation-reduction chemistry, catalysis, and in situ monitoring of materials synthesis. High temperature measurements in neutron facilities have traditionally been carried out in vacuum furnaces, which provide a slightly reducing atmosphere. In cases where samples change stoichiometry in such an atmosphere or require a certain pO2 level for the process of interest, it is critical to be able to control the gas environment. This was the primary science motivation for building AGES. In Sec. III A, we provide two examples of work done using this system.
A. Solid oxide fuel cells
Solid Oxide Fuel Cells (SOFCs) are devices capable of generating power cleanly through the direct oxidation of a variety of hydrogen rich fuels. While there are commercially available cells, primary research has been concentrated on finding materials that can operate at significantly reduced temperatures to lower cost and increase the lifetime of these devices. The majority of the materials of interest for both electrodes and electrolyte in SOFCs are crystalline mixed metal oxides often from the perovskite, spinel, and Ruddlesen-Popper phases. The general mechanism for oxygen transport is attributed to the large off-stoichiometry often present in these oxide compounds. Neutrons are ideally suited to study this phenomenon due to their sensitivity to oxygen in the presence of heavier metal elements. Two user groups led by Professor Steven McIntosh at Lehigh University8–10 and Professor Arumugam Manthiram from The University of Texas at Austin11 have carried out multiple experiments to study various candidate electrode materials for intermediate temperature SOFCs.
Members of the double perovskite family have been studied for application in cathodes for SOFCs. These layered perovskites have the general formula AA′B2O5+δ and often show tetragonal distortion at high temperatures. The A/A′ layering along the c-axis leads to localization of the oxygen vacancies within the layer which can be directly observed by carrying out in situ neutron powder diffraction as shown by the McIntosh group.9,10 They examined PrBaCo2O5+δ and NdBaCo2O5+δ from 575 to 850 °C and pO2 values of 10−1 to 10−4 (achieved using mixtures of nitrogen and oxygen) atmosphere. Similar results were observed for both compounds where oxygen vacancies are localized in the Ba plane. Fractional occupancies for the three distinct oxygen sites could be determined using Rietveld analysis. A total oxygen stoichiometry value for Nd compound ranged from 5.11 to 5.51 as a function of temperature and pO2 value. Analysis of atomic displacement parameters indicated a curved oxygen transport pathway, with oxygen hopping from a vacancy rich oxygen site in the Nd-O layer to the nearest neighbor O-site in the Co-O layer (as shown in Fig. 7).
B. Oxygen storage materials
Oxygen storage materials (OSMs) can alternatively hold and release large amounts of oxygen as they convert between different phases and crystal structures. They are often used in chemical looping reactions, including methane reformation for the production of hydrogen and efficient capture of CO2. Professor Efrain Rodriguez and his collaborators at the University of Maryland have used AGES to study La1−xSrxFeO3 as a potential oxygen storage material.12
Improving our knowledge of the evolution of structure and composition under reaction conditions is crucial for future design of new OSMs. A major factor that determines the performance of these materials is the transport of oxygen between the bulk and the surface of the OSM. The chemical looping reaction generally takes a few minutes to cycle and as such a very fast diffraction technique such as synchrotron x-ray diffraction (SXRD) had been used to follow the kinetics and structural phase transitions. Very high resolution is also needed to distinguish the subtle differences of the structure going from pseudocubic (below 200 °C) to cubic perovskite (235 °C–335 °C) and eventually transitioning to the brownmillerite structure (535 °C–635 °C). The reduced material finally reverts to the cubic perovskite structure above 735 °C. While kinetics and structure can be easily followed using SXRD, it is still difficult to determine accurate oxygen stoichiometry due to the presence of much heavier Sr and Fe in the sample. Therefore, neutron powder diffraction (NPD) measurements were carried out using AGES, which enabled alternating flows of 15% CH4 in N2 and air (20% O2 in N2) over the sample while measuring neutron diffraction patterns at a variety of temperatures from 135 to 835 °C for four different sample compositions (x = 0, 0.5, 0.67 and 1). They were able to track changes in the lattice parameters and observed significantly higher unit cell volumes when under 15% CH4 than at the same temperatures under air (Fig. 8). They also investigated changes in the oxygen stoichiometry in order to determine the oxygen storage capacity of each sample and were able to determine the optimal composition for application as an oxygen storage material. From this study, the authors were able to conclude that this series of compounds has an envelope of oxygen storage capacity over a given temperature range under which oxygen can easily and reversibly be inserted and removed.
In the recent past, AGES has also been used to look at structural changes in battery materials and synthesis of candidates for electrodes and electrolytes for Li battery materials (unpublished work). This showcases the versatility of the types of measurements that this system has enabled at the beamline.
In summary, an Automated Gas Environment System (AGES) was developed for the Powgen powder diffractometer (BL-11A) at the Spallation Neutron Source. AGES allows the controlled flow and mixing of the following gases: nitrogen (N2, a maximum flow rate of 500 SCCM), oxygen (O2, 500 SCCM), helium (He, 100 SCCM), argon (Ar, 100 SCCM), carbon dioxide (CO2, 100 SCCM), carbon monoxide (CO, 100 SCCM), hydrogen/deuterium (H2/D2, 500 SCCM), methane (CH4, 500 SCCM), and pre-made mixtures of 4% H2 in He (500 SCCM) and air (500 SCCM). Users may control the gas flow rates through an EPICS-based GUI as well as through scripting, for ease of data collection. The system may be placed within a furnace, allowing measurement at temperatures up to 850 °C. A chiller incorporated into the system allows controlled humidification of the gas stream. A universal gas analyzer and an oxygen sensor allow monitoring of the input or exhaust gases. AGES has enabled the in situ neutron powder diffraction of a variety of technologically important materials, including materials for solid oxide fuel cells and oxygen storage materials.
The authors wish to thank the groups of Steven McIntosh and Efrain Rodriguez, whose research provided excellent examples of the application of AGES, as well as ORNL engineers Bruce Hill, Randy Summers, Lorelei Jacobs, and Donald Montierth, who contributed to the development of AGES. Additionally, we would like to thank Zhonghe Bi and J. H. Kim. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.