One of the major national user facilities supported by ORNL is the High Temperature Materials Laboratory (HTML). Its two primary objectives are conducting research that will assist U.S. industry in meeting transportation challenges, and educating and training researchers from both academia and industry. The HTML comprises six user centers, which provide instruments for characterizing the microstructure, microchemistry, and physical and mechanical properties of materials over a wide range of temperatures.

The HTML’s new Aberration-Corrected Electron Microscope (ACEM), and several other ORNL microscopes also equipped with aberration correctors, allow for the first time the ability to record images of materials at sub-Ångström levels (an Ångström is one-ten millionth of a millimeter, about the diameter of an atom). These instruments all require a special environment in order to reach their design resolutions. Electromagnetic fields, mechanical vibration, noise, room temperature fluctuations, and barometric pressure changes can degrade microscope performance. The ACEM, for example, requires ambient electromagnetic fields significantly below 0.3 milligauss, a condition not found in the HTML or in any other laboratory at ORNL, which necessitated the AML project.

Project planning for the AML began in the fall of 2000, and construction was completed in May 2004. The 6,900-sf facility comprises an instrument building that houses four microscopes, and an adjacent (but mechanically separated) service building that houses HVAC equipment, instrument water chillers, and the main electrical distribution panels.

Close Proximity is Key for Design

When planners decided to construct the AML, they embarked on a learning experience to gather as much knowledge as possible about building a facility suitable for its imaging equipment. ORNL research staff member Dr. Lawrence Allard visited the Triebenberg Laboratory in Dresden, Germany, which was completed two years prior to the AML. This laboratory has a similar function to the AML, and had several design features that were incorporated into the AML’s design. He visited other labs such as the Juelich Research Center in Germany, home of the first prototype aberration-corrected instrument, and the Hitachi Advanced Research Lab in Hatoyama, Japan. He also visited Lawrence Berkeley Lab to see the addition on the National Center for Electron Microscopy, in which the NCEM “One-Ångström Microscope” is housed. The information gathered was used by ORNL’s engineering team and the architects to design the AML.

In placing the AML building adjacent to the HTML at a mere 110 feet, it is possible to take advantage of chilled water for HVAC, compressed air, and clean power supplied from the HTML. Clean power is supplied to the AML from a 75 kVA motor/generator (MG) set located in the HTML mechanical room. The MG set has uninterruptible power supply capability in the event of a power failure.

“There was a fine balance between getting the main electrical switchgear as close to HTML as possible but having the microscopes 70 to 80 feet away to provide an adequate buffer,” says Jeff Schantz, chief architect on the AML project when working with Lord Aeck and Sargent, the original architectural firm. “This location keeps the noisy generators, pumps, compressors, and the MG/UPS set totally isolated from the AML, allowing for optimum vibration, sound, air, and humidity stability for AML’s microscopes. This choice was the most practical approach for a low-cost, quiet site.”

Facility Designs, Specifications, and Requirements

There were four requirements for the facility design: The floor vibrations needed to remain below 1µm/sec (peak) on any axis at less than 30 Hertz. The airflow needed to be less than 5cm/sec across the column. Temperature stability was to remain at 0.2 degrees Celsius/hour or less. And finally, magnetic stray fields were to be less than 0.1mG rms at 60 Hertz.

Electromagnetic interference comes from both AC and DC sources. To address these issues, the AML was designed to minimize all potential environmental influences.

“The key factor in the facility design was starting with a clean site that required no remediation, and minimizing adverse environmental conditions by careful choice of construction materials and control of construction practices” says Allard.

For example, to address magnetic field issues, epoxy-coated reinforcement bars were used in separate concrete foundation slabs, to preclude the possibility for the re-bar net to conduct current to ground. The instrument building and room walls are constructed of concrete blocks, rather than metal studs, again eliminating the possibility for magnetic fields to be generated by current through the studs. Another critical point was the use of twisted pair electrical wiring.

“While this material is slightly more expensive than the standard parallel wiring that is used in most houses, it is critical in creating a field-free environment,” says Allard.

Finally, all metal air ducting, sprinkler piping, and water lines incorporated dielectric breaks to eliminate conductive paths for current flow.

Construction and Foundation Approach

The site and building are designed to allow for future expansion to twice its present size, accommodating up to four additional instruments. The foundation slabs and wall footings of the AML are on 8-foot engineered fill, separated into layers by geotextile fabric. An Exterior Insulation and Finish Systems (EIFS) construction was use for the building’s facing material. Floor slabs in the instrument rooms are one-foot-thick reinforced concrete, and cover the full floor area of the room. This slab-on-grade construction was deemed suitable because the site is inherently very quiet and provides exceptionally low vibration levels in the instrument rooms.

The AML provides a pair of rooms for each microscope—an instrument room and an associated “control room,” which contains the computers used to operate the microscope—and additional facilities for data analysis. This design effectively isolates an instrument from the disturbing influence of human operators during critical imaging, thereby minimizing noise and thermal fluctuations. A 4-foot square, quadruple-paned window is located between the instrument and control rooms.

A “building-in-building” design concept isolates the instrument rooms from the outside air and ensures that sound pressure would not affect the instruments themselves. The main instrument room section is constructed using this design philosophy, with the external shell built from 12-inch reinforced concrete masonry blocks filled with sand. The roof height is 26 feet in the instrument bay section.

“This approach reduces the pressure differential, outside noise, temperature variability, acoustic vibrations, and low-frequency vibrations produced by the wind moving over the building roof, with almost an airlock effect when entering the building,” says Schantz.

The laboratory also contains a small sample preparation room, an entry vestibule with associated airlock, and a building office. These rooms are contained within the service building, which is separated by a seismic joint from the instrument building.

Minimizing External Influences

Acoustic noise in the instrument rooms is minimized by the application of absorber/barrier blanket material on all four walls. The primary ceiling height in the instrument rooms is 18 feet. A 13-foot dropped ceiling, consisting of egg-crate grid and two layers of porous duct liner material provides additional noise-reduction. The dropped ceiling forms an air mixing plenum for the supply air, which enters through a pair of 12-inch diameter metal ducts having 50 percent pore area.

The control rooms have cloth-covered acoustic absorber panels on each wall to absorb noise from conversation and computer fans.

Variable frequency drives are used to provide tight temperature and airflow control. The design maximum airflow is 960cfm, which gives about 11 changes/hour, and less than 5cm/second vertical motion. However, experience has shown that 500cfm with about six changes/hour suffices to control the temperature to less than the design level, thus further minimizing air velocity in the room. To avoid the temperature variations caused by cycling air conditioning, the instrument rooms are relatively large (17 by 17 by 18 feet) and have only the minimum number of heat-producing sources. Air is returned from floor level through false walls on two opposing sides of the room.

Numerous Advantages Key to Lower Project Costs

After several proposal scenarios, a design-build scenario was selected at a cost of about $3 million for bricks and mortar. The breakdown of construction costs was $278,340 for foundations, $2,486,330 for the building itself, and $542,500 for support construction. The total project cost of $4.5 million includes all of the fees and management at $1,211,000.

It is important to note that the building went through a continuous design process. The construction phase was a continuous supervision process. The project benefited from several advantages including access to a plant from which to draw chilled water, a dedicated room for electrical power, access to 13 KVA electrical power, and a quiet site.

“We had numerous advantages,” says Schantz. “I think if you took all of those advantages away from the project, rather than being a $4 ½ million project, chances are good this would be a $6 million project, or more.”

By Lisa Brown