In Situ High-Temperature Study Of Silver Behenate Reduction To Silver Metal Using Synchrotron Radiation

Transcription

1 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume In Situ High-Temperature Study Of Silver Behenate Reduction To Silver Metal Using Synchrotron Radiation Tom Blanton 1,3, Mark Lelental 1, Slawomir Zdzieszynski 2, Scott Misture 2 1 Eastman Kodak Company, Rochester, New York Alfred University, New York State College of Ceramics, Alfred, New York thomas.blanton@kodak.com ABSTRACT Silver behenate is utilized in commercially available photothermographic imaging elements. The silver metal (Ag ) image formation in these imaging elements is based on the heat-induced reduction of the silver behenate. The completed reduction process has historically been studied by conventional X-ray diffraction techniques. To enhance the understanding of the kinetics of silver metal formation, equipment and methods were developed allowing for in situ hightemperature monitoring of the AgBehenate Ag reduction process. To perform the necessary measurements a high-temperature sample stage and position sensitive detector were mounted on the X3B1 beam line powder diffractometer at the National Synchrotron Light Source, Brookhaven National Laboratories. The fraction of AgBehenate reduced and Ag developed was obtained from the synchrotron data and it was determined that the reaction rates for both processes followed a second order transformation. The activation energy was derived from the reaction rate data and was found to be 216 kj/mol for AgBehenate reduction and 156 kj/mol for Ag development. INTRODUCTION Medical imaging techniques such as CAT scans, MRI, and ultrasound utilize electronic means of image capture. However, there is still a need for hardcopy output of the image for further diagnostic evaluation, record keeping, and legal considerations. These hardcopy images are recorded using photothermographic films. Photothermographic films are comprised of silver halide, AgX (typically silver bromide, AgBr), for image capture, AgBehenate for image generation, binder, developer, toner, and additional chemicals necessary for optimizing the photographic performance, all coated onto a flexible polyester support. A digital image stored on a computer is transposed into an analog hardcopy by first scanning the photothermographic film with a tuned laser. The AgX captures the laser signal creating a latent image on the film. Unlike traditional wet chemistry techniques that require solutions of developer, stop bath, fixer and water rinse to convert the latent image into developed silver [1], photothermographic films utilize 15 seconds of thermal processing at 122 C to accomplish this conversion. The thermal process involves reduction of the AgBehenate to Ag and is made possible as a result of all of the required chemistry already present in the coated film medium [2]. Silver behenate is a crystalline long-chain silver carboxylate, CH 3 (CH 2 ) 20 COOAg that crystallizes as a dimer in a head-to-head configuration (Figure 1). The morphology of AgBehenate is typically plate-like, with the (00L) plane parallel to the plate surface [3]. When AgBehenate is dispersed in a binder and coated onto a polymeric support, these platelets will show preferential alignment parallel to the support resulting in the observation of a series of (00L) diffraction peaks in a diffraction pattern [4].

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume Figure 1. Head-to-head configuration of AgBehenate dimer,, - Ag atoms from respective molecules. Conventional X-ray diffraction (XRD) techniques have been utilized in the study of photothermographic films, typically in a pre- (starting amount of AgBehenate) and post- (final amount of AgBehenate and amount of developed Ag ) thermal processing mode. It is desirable to expand the understanding of the photothermographic process through the measurement of the kinetics of AgBehenate reduction and Ag development. The ability to study in-process AgBehenate reduction and Ag development by conventional laboratory source XRD is hampered by the speed at which the thermal process reaction occurs and the low coverage of silver in the silver containing phases in a photothermographic film (<250 µg/cm 2 ). Synchrotron radiation facilities lend themselves to the study of in-situ processes due to the availability of a high flux X-ray source. In this study, a high-temperature furnace and a position sensitive detector were installed on the X3B1 beam line at Brookhaven National Synchrotron Light Source. This experimental setup allowed for in-situ monitoring of the reduction of AgBehenate and development of Ag. The fraction of each phase transformed was then measured and activation energies calculated. EXPERIMENTAL Photothermographic Films All samples were prepared using commercially available photothermographic films (Eastman Kodak Company). The AgBehenate grains had a mean thickness of 0.1 µm and surface dimensions of µm. Silver halide grains were cubes with a mean edge length of 0.6 µm. The polymer support was 175 µm poly(ethylene terephthalate) (PET). Samples were exposed to conventional room fluorescent light before thermal processing. Data Collection X-ray diffraction measurements were performed at the National Synchrotron Light Source using beam line X3B1. A custom X-ray diffraction furnace built at Alfred University was attached to a Huber 424 θ-2θ goniometer. The diffraction furnace consists of Pt/Rh wire heating elements that create a 50 mm diameter spherical hot zone, capable of operating from room temperature to 1600 ºC. The temperature was controlled using a thermocouple mounted within 1 mm of the specimen with the power controlled using a Halder TC3 temperature controller. The photothermographic film specimen was mounted on a glass slide and the glass slide was mounted on an aluminum oxide plate with dimensions 10 by 100 mm. A smear of vacuum grease was utilized to ensure good thermal contact between the film and glass, and glass and aluminum oxide. To calibrate the temperature in this sample configuration, measurement of the phase transformation of AgI (β,γ-agi α-agi 147 C) was used. NIST SRM1976 was used to determine the exact wavelength used in the experiment, which was nm. Diffraction measurements were made with the incident angle fixed at 2 and an mbraun linear positionsensitive detector (PSD), which was fixed to cover an appropriate angular range to determine the

4 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume growth of silver metal and the decomposition of AgBehenate respectively. The Ag (111) line at 28.2 º2θ was used to determine the Ag growth rate, with the detector covering º2θ. The AgBehenate (004) line at 4.5 º2θ was used to track the AgBehenate reduction while the detector collected data from 3 to 14 º2θ. The PSD operation was fully computer-automated with the temperature controller, and data were binned into a multichannel analyzer using fixed count times with no delay between patterns. Data collection included isothermal measurements of Ag development and AgBehenate reduction at 106, 114, 122, 130, and 138 ºC, where 50 diffraction patterns were collected at each temperature after reaching the appropriate temperature. A fixed heating rate was used for all specimens, 120 ºC/min, and the temperature controller triggered the start of the XRD measurements immediately upon reaching the target temperature. In order to determine the amount of Agº formed after full conversion, each specimen was heated to 142 ºC after the sequential XRD measurements and a single diffraction pattern was collected. Data collection times ranged from 2 to 30 seconds per pattern, with shorter count times at higher temperatures where the reaction was observed to be more rapid. Data Analysis Data analysis was performed using Jade5 [5]. Several approaches for peak intensity determination were attempted, including profile fitting, automated peak height determination, and manual peak height determination. Except for the AgBehenate diffraction peaks early in the data collection, all diffraction peaks were weak and noisy. The most reproducible method for determining the fraction transformed vs. time was found to be manual peak height determination. Therefore each pattern was smoothed using a Savitsky-Golay filter and the background and peak heights were determined using a visual evaluation. All patterns for the Agº development were smoothed using a 51-point filter length, while the AgBehenate patterns were smoothed using a 13-point filter. Analysis of the kinetics was accomplished using the integral method, by attempting to fit the fraction transformed vs. time to the expected equations for first, second, third, and three-halves order transformations. After determining the appropriate order of the reaction, Arrhenius plots were used to determine the activation energy for AgBehenate reduction and Agº development. RESULTS AND DISCUSSION Examples of in-situ high-temperature XRD data for AgBehenate reduction and Ag development can be seen in Figures 2A and 2B respectively. Figure 2. Isothermal XRD patterns collected at 122 C, AgBehenate reduction Ag development.

5 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume In Figure 2A, several (00L) AgBehenate diffraction peaks are observed in the region of channels (~ θ). In the channel range, a portion of the diffraction pattern due to the PET support (broad peak) is also present. The loss of intensity of the AgBehenate diffraction peaks can be attributed to the reduction of AgBehenate during thermal processing. In Figure 2B, a peak in the region of channels (~ θ) is observed to grow with time. This peak is the (111) Ag peak, and is an indicator that silver development is occurring during thermal processing. The diffraction peak at ~200 channels is the (200) AgX peak, and an additional PET diffraction peak is present in the region of channels. Lower temperatures of thermal processing resulted in a slowing down of the reduction and growth processes, whereas thermal processing of the photothermographic film at elevated temperature caused an increase. The fraction of AgBehenate reduced and Ag developed as a function of time was determined for each isothermal dataset where the data collected for each sample at 142 C was defined as complete reduction of AgBehenate or maximum development of Ag (fraction transformed equals one). Figure 3 shows representative plots for data collected at 138 C for AgBehenate (Figure 3A) and Ag (Figure 3B). Fraction Transformed LB6 time vs LB6 FT Time (sec) Figure 3. Fraction transformed vs. time for AgBehenate reduction and Ag development based on isothermal data collected at 138 C. At 138 C, the reduction of AgBehenate and development of Ag both proceed at a fast rate. The scatter in the Ag data is a result of the weak intensity of the (111) silver metal peak. In contrast to the fast transformation times shown in Figures 3A and 3B, similar plots for data collected at lower temperatures have significantly slower times. For example, the time for 50% AgBehenate reduction at 138 C is ~18 seconds, at 122 C it is ~95 seconds, and ~600 seconds at 114 C. Several rate constant models were tested with the fraction transformed vs. time data. The best fit was obtained using second-order kinetics. kt = x/([a 0 ][A 0 -x]) (1) where: k = reaction rate x = fraction transformed Fraction Transformed ,, AB29 time vs AB29 Frac. Trans Time (sec)

6 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume t = time A 0 = original concentration of reactant In Figure 4, plots for determination of rate constants are shown for isothermal data collected at 114 C. As with the fraction transformed data of Figure 3, the Ag development data show more scatter due to weak diffraction peak intensity. LB15, Micro, 114C 4 AB21, Micro, 114C 1 LB15 time vs LB15 x/(1-x) x column 2 vs y column 2 3 AB21 time vs AB21 x/(1-x) x column 4 vs y column 4 x/(1-x) x/(1-x) time (sec) Time (sec) Figure 4. Rate constant plots for AgBehenate reduction and Ag development based on isothermal data collected at 114 C. The slope of the linear best-fit line is the rate constant, k, shown in equation 1. Taking the slope for the curves in Figures 4A and 4B, along with similar plots for the additional isothermal data collected in this study, one can generate an Arrhenius plot for AgBehenate reduction (Figure 5A) and Ag development (Figure 5B) Ag Behenate, Micro Arrhenius results second order: y = x, R2 = 0.91 Ea = 216 kj/mol = 51.5 kcal/mol 0-2 Micro Ag Growth, Second Order Kinetics y = x Ea = 156 kj/mol = 37.3 kcal/mol -3 ln k -4-5 ln k -4-6 Recip T vs ln k 3 pt. x column 5 vs y column /T (K) vs ln k x column 4 vs y column /T (K) /T (K) Figure 5. Arrhenius activation energy plots for AgBehenate reduction and Ag development. Rate constants generally are dependent on temperature, and increase with increasing temperature [6]. The Arrhenius equation can be used in many cases to determine the activation energy of a reaction:

7 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume k = Ae -Ea/RT (2) can be rearranged to give: ln k = ln A - E a /RT (3) where: k = reaction rate Ea = activation energy R = gas constant T = temperature A = preexponential factor A plot of ln k vs. 1/T will give a straight line if the Arrhenius equation is obeyed, with slope -E a /R and intercept A. Based on the slopes derived from Figures 5A and 5B, the activation energy for AgBehenate reduction is 216kJ/mol and Ag development is 156 kj/mol. Based on equation 2, the lower activation energy for Ag development is an indication that the formation of silver is a faster process than the reduction of AgBehenate under the conditions utilized in this study. This result suggests that the minimum energy required to break the appropriate AgBehenate covalent bonds required to free up Ag atoms, is greater than the energy required to generate Ag metallic bonds resulting in the formation of the developed Ag image observed in a processed photothermographic film. SUMMARY Photothermographic films can be utilized as hardcopy media for many digital medical diagnostic applications. XRD is ideally suited to study the silver containing phases in photothermographic films due to the differences in crystal structure of the principle components AgBehenate, Ag, and AgX. In situ high-temperature synchrotron radiation experiments were determined to be well suited for studying the reduction of AgBehenate and Ag development. The ability to study the reaction kinetics of photothermographic films will allow for improved materials design and improved product features. ACKNOWLEDGEMENTS The SUNY X3 beam line at NSLS is supported by the Division of Basic Energy Sciences of the U.S. Department of Energy (DE-FG02-86ER45231). Financial support was provided by Eastman Kodak Company as well as the New York State Center for Advanced Technology at Alfred University. REFERENCES [1] Carrol, B.H., Higgins, G.C., James, T.H., Introduction to Photographic Theory, The Silver Halide Process, John Wiley & Sons, New York, 1980, 1-3. [2] Sahyun, M.R.V., J. Imaging Sci. Technol., 1998, 42(1), [3] Huang, T.C., Toraya, H., Blanton, T.N., Wu, Y., J. Appl. Cryst., 1993, 26, [4] Blanton, T.N., Barnes, C.L., Lelental, M., J. Appl. Cryst., 2000, 33, [5] Materials Data Incorporated, 1224 Concannon Blvd., Livermore, California [6] Levine, I.N., Physical Chemistry, McGraw-Hill Inc., New York, 1978,