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In this study we have successfully performed NR measurements of deuterated polystyrene (d-PS) thin films in CO2 at P < 700 bar (Fig. 2). The results show an anomalously large swelling maximum (AV/V » 25 %) which occurs only at the supercritical fluid/gas phase boundary. Atomic force microscopy confirmed the swelling is uniform throughout the films and does not produce large voids. Such a swelling maximum at the supercritical fluid/gas phase boundary has never been seen in bulk PS-CO, mixtures. In addition, an isobaric experiment at 79 bar showed that the same swelling behavior did not appear at the phase boundary between the liquid and supercritical fluid CO2 phases.
At this point we can only speculate that the anomalous swelling behavior may be related to the higher order nature of the gas/supercritical fluid transition that is accompanied by larger density fluctuations than the first-order liquid/supercritical fluid transition. Experimentally these fluctuations are clearly seen by the sharp increase in background scattering from the CO, at the transition boundary. Adsorption of CO2 into the viscous polymer film may suppress these fluctuations thereby lowering the energy of the sys
tem. This effect was further investigated by observing the swelling of the d-PS film as a function of temperature at P = 79 bar (Fig. 3.) Recently we have expanded our research by measuring chain mobility in bilayer polymer films of d-PS and hydrogenated PS (h-PS). We have used secondary ion mass spectrometry and NR to measure the diffusion coefficient as a function of pressure, temperature and molecular weight. This work established that swelling at the phase boundary is accompanied by an increase in interdiffusion between the polymer layers and a large decrease in the PS glass transition temperature. These results show that scCO, can potentially be used to process multi-component thin polymer films that may otherwise not be miscible. Potential applications are in the areas of graded index of refraction waveguides and lower dielectric constant films for use in microelectronic devices.
 M.A. McHugh and V.J. Krukonis, Supercritical Fluid Extraction, 2nd Ed., Butterworth-Heinemann, 1993.
COLD NEUTRON DEPTH PROFILING OF LITHIUM BATTERY MATERIALS
s part of a continuing collaboration between NIST and Tufts
University, we have employed cold neutron depth profiling (NDP) to measure elemental concentrations versus depth for several lithium ion battery materials. One goal is to rationally design the thermo-mechanical properties of amorphous lithium phosphorous oxynitride (lipon, a solid ionic conductor), particularly the thermal stress (thermal expansion coefficient). A part of this study is to relate the resulting thermal stress to starting composition and temperature of evaporant, and of the composition and pressure of the background gas during deposition. We are also studying LiCoO, films (a material that can be used as an electrode in batteries) to determine if the ion beam assisted deposition process used has the capability of controlling not only the degree of crystallinity and orientation of crystallites, but also of the Li/Co ratio.
The NIST cold neutron depth profiling instrument and technique have been described previously . The technique of neutron depth profiling (NDP) permits the determination of depth profiles in thin films up to a few micrometers for several light elements. The most readily analyzed elements are lithium, nitrogen, and boron. We have previously reported measurement of lithium mobility in electrochromic devices . The lithium depth profiles are based on the measurement of the energy of alpha particles and/or tritons from the 'Li(n,a)3H reaction. Nitrogen depth profiles are based on the measurement of the energy of protons from the 14N(n,p)11C reaction. Samples are placed in a beam of cold neutrons, and the emerging particles are intercepted by surface barrier detectors that measure their number and energy. Comparing the emission intensity with that of a known standard leads to quantitative determination of the lithium and nitrogen concentrations. Moreover, the emitted charged particles lose energy as they exit the film; this energy loss provides a direct measurement of the depth of the originating lithium nucleus. A great advantage of the NDP technique is that it is non-destructive, which allows repeated observations of the concentrations under different conditions. When combined with other techniques, e.g., activation analysis, ratios to other constituents can be determined.
Figure 1 gives an example of profiling results using the NDP technique. Shown here are profiles for two lipon samples manufactured under different conditions. The elemental concentrations in atoms/cm3 are presented as a function of depth. Because the alpha particle loses energy at a greater rate than the proton, the resolution for the lithium profile is better than that for nitrogen. One observes that the sample Y245 has a much more uniform distribution of lithium than sample Y239. By integrating the areas under the curves, one obtains the total concentration of lithium and nitrogen, respectively, and therefore the ratio of the two in the sample. Table I gives measured lithium/nitrogen ratios thus obtained and the corresponding thicknesses of four lipon samples.
To obtain information on other isotopes that are not measurable by NDP, a combination of techniques is employed. In the following example the ratio of lithium to cobalt in two thin film LiCoO2 samples is determined. The lithium concentration is determined by NDP, as described above; and the cobalt concentration is determined by instrumental neutron activation analysis (INAA). Figure 2 gives lithium depth distributions from NDP measurement for two thin films of LiCoO2. The integral under the curves gives the total amount of lithium in the film. After the depth distributions
were obtained, the samples were encapsulated in polyethylene "rabbits" for irradiation in the core of the NIST reactor. The total cobalt concentration was then determined by INAA in which the 60Co gamma decay intensity was measured and compared with a standard. Table II gives the lithium and cobalt atom area density obtained from NDP and INAA respectively, as well as the lithium/ cobalt atom ratios for these and other samples. The INAA technique does not provide any depth information, so that the ratio values listed in the table are for the average over the entire depth.
To summarize, depth profiles of two different lithium ion battery materials have been measured. For the lipon sample, profiles were obtained for lithium and nitrogen as well as the total quantity of each of these elements in the film. To date, an insufficient number of samples have been measured to obtain a good correlation with the physical properties of the films. We are also investigating the possibilities of measuring Li/P ratios of both starting materials and resulting films by combining NDP with RNAA for phosphorous.
For the lithium cobalt oxide sample, the NDP technique was combined with INAA to determine the ratio of lithium to cobalt in the samples. Although further work is needed to better quantify the relative evaporation rates of lithium and cobalt, it has been demonstrated that the measured Li/Co ratio varies in direct proportion to the relative evaporation rates of lithium and cobalt, as anticipated. Furthermore, the results indicate that the Li/Co ratio can be controllably varied from being less than one to greater than one.
 R. G. Downing, G. P. Lamaze, J. K. Langland, and S. T. Hwang, NIST Journal of Research. 98, 109 (1993).
 G. P. Lamaze, H. H. Chen-Mayer, A. Gerouki, and R. B. Goldner, Surf. Interface Anal. 29, 637 (2000).
SERVING THE SCIENTIFIC AND TECHNOLOGICAL COMMUNITIES
The role of the NCNR as a national user facility has expanded significantly over the past year, as the final few instruments envisioned in the original cold neutron project have become operational. The Disk-Chopper Spectrometer, the Filter-Analyzer Neutron Spectrometer, the High Flux Backscattering Spectrometer, and the Neutron Spin-Echo spectrometer now permit U.S. scientists to carry out neutron spectroscopy with greatly enhanced resolution and sensitivity. In addition the new thermal perfect-crystal diffractometer small angle neutron scattering instrument (USANS) has been commissioned this year, expanding the length scale available by this technique to 104 nm. User experiments show a steadily increasing diversification in subject area and technique, enabled by the new instruments. We anticipate that the trend will continue over the next few proposal cycles. (See the highlights on USANS, FANS, and DCS in this issue. The 1999 NCNR report featured a highlight on the NSE.)
User participation over the past 14 years shows continuing growth (see Fig. 1). The NCNR currently accommodates more than half of all neutron users in the United States. It has assumed greater importance to the neutron user community this year with
the announcement of the permanent shutdown of the High Flux Beam Reactor at Brookhaven National Laboratory. As the Spallation Neutron Source is being built at Oak Ridge, the NCNR continues to be the Nation's premier facility for providing neutrons to the U.S. research community.
THE NCNR USER PROGRAM
Researchers may obtain use of NCNR neutron beam instruments in several ways, the most direct being through the formal proposal system. Approximately twice a year, a Call for Proposals is issued. After a thorough review process by external referees and by the NCNR Program Advisory Committee (PAC), approved proposals are allocated beam time. The PAC is a panel of distinguished scientists with expertise across a broad range of neutron methods and scientific disciplines. It is the body primarily responsible for proposal review and recommending user policies for the NCNR, working closely with the Center's Director and staff. Its current membership includes Sanat Kumar (Penn State University, chair), Robert M. Briber (University of Maryland), Michael K. Crawford (DuPont), Dieter K. Schneider (Brookhaven National Laboratory), Thomas P. Russell (University of Massachusetts), Sunil K. Sinha (Argonne National Laboratory), Laurence Passell (Brookhaven National Laboratory), and Gabrielle G. Long (NIST).
At the recent PAC meeting in May 2000, the PAC considered 71 proposals for SANS and reflectometry, in addition to 43 for inelastic neutron scattering. Although we expect that both categories will see increased user demand in future proposal rounds, the latter area is likely to see more growth, since the new inelastic scattering spectrometers will offer capabilities that in aggregate have not been available previously at U.S. neutron facilities.
THE CENTER FOR HIGH RESOLUTION
Several NCNR instruments are supported by the National Science Foundation (NSF) through the Center for High Resolution Neutron Scattering (CHRNS), a very important component of the user program. The instruments include a 30 m SANS machine, the SPINS triple-axis spectrometer, and USANS. Approximately 40 % of the instrument time allocated by the PAC goes to experiments carried out on CHRNS instruments. In the near future, another SANS diffractometer, the 8 m machine on neutron guide NG-1, which is presently used primarily for NIST programmatic research, will be
upgraded to a more powerful 9 m instrument with a new detector, and made available to users. The NSF is currently reviewing a proposal to further expand the scope of CHRNS, so that it will encompass several of the newer cold neutron spectrometers. Including these instruments in the CHRNS project will provide maximal technical support and accessibility to the user community.
SIXTH ANNUAL SUMMER SCHOOL
The NCNR held its annual Summer School on Neutron Scattering on June 5-9, 2000. The course this year focused on the complementary techniques of SANS and neutron reflectometry (NR) and was attended by a group of 32 graduate students and postdoctoral fellows, predominantly from university chemical engineering and materials science departments. By devoting an entire week to just two techniques, it was possible to cover both theoretical and practical aspects, as well as applications, in some depth. Sixteen NCNR staff members led the participants through lectures, demonstrations and hands-on experiments at the NCNR's two 30 m SANS instruments and two reflectometers. Included for the first time in this year's course were demonstrations of newly developed computational tools for planning and simulating SANS and NR experiments now accessible through the NCNR Web site.
The final day of the course consisted of parallel lecture sessions in the morning on applications of the two techniques drawn from recent research in polymer science, complex fluids, magnetism, and structural biology. The course closed with a session in which representatives from each team presented their experimental results to the whole class and staff, which prompted several lively discussions. Comments received throughout the week and on the course evaluation forms indicated that the course was successful in enabling the attendees to assess the applicability of neutron scattering to their own research interests. As in the past, this summer school was jointly sponsored with the National Science Foundation, which provided financial assistance to many of the university participants.
FIGURE 2: Participants gain hands-on experience in SANS measurements at the 6th annual summer school on neutron scattering, June 5-9, 2000
Direct collaborations on specific experiments remain a common way for users to pursue their ideas using NCNR facilities, accounting for approximately half of the number of instrument-days. The thermalneutron triple-axis spectrometers are mainly scheduled in this way. Most of the time reserved for NIST on these and all other NCNR instruments is devoted to experiments that are collaborations with non-NIST users. Collaborative research involving external users and NIST scientists often produces results that could be not obtained otherwise.
Another mode of access to the NCNR is through Participating Research Teams (PRTs). In this case, groups of researchers from various institutions join forces to build and operate an instrument. Typically, 50% to 75 % of the time on the instrument is then reserved for the PRT, and the remaining time is allocated to general user proposals. For example, a PRT involving ExxonMobil, the University of Minnesota, and NIST cooperates on the NG-7 30 m SANS instrument. Similar arrangements involving other PRTS apply for the horizontal-sample reflectometer, the high-resolution powder diffractometer, the filter-analyzer spectrometer, and the neutron spinecho spectrometer.
Photography by L. A. Shuman