Dense and Optical Transparent Cdwo4 Films by Sol-Gel Processing for Scintillation Applications

Dense and optical transparent CdWO4 films by sol-gel processing for scintillation applications H.M. Shang Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98105 M. Bliss and S. Heald Pacific Northwest National Laboratory, Richland, Washington 99352 T.K. Sham and F. Heigl Department of Chemistry, University of Western Ontario, Ontario, N6A 5B7, Canada G.Z. Caoa) Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98105 (Received 5 October 2006; accepted 27 December 2006) In this paper, we report the first successful fabrication of dense and optically transparent cadmium tungstate (CWO) films by sol-gel processing and the study of their optical and x-ray scintillation properties. A new sol-gel processing method was developed using tungstic acid and cadmium nitrate as precursors and hydrogen peroxide as solvent; homogeneous and stable CWO sols were aged at room temperature and used for the preparation of CWO films. A rapid sintering process was investigated and found to be necessary to make dense and optically transparent nanocrystalline CWO films. CWO films were uniform, fully dense, and crack-free, with CWO as the only detectable crystalline phase, as determined by x-ray diffraction. The thickness, density, grain size, and crystallinity of CWO films are all found to be strongly dependent on the sintering conditions and in turn impact the optical and x-ray scintillation properties. Sol-gel-derived dense CWO films demonstrated intense photoluminescence and x-ray excited optical luminescence intensity. The relationships between sol-gel processing, nanostructures, and optical and x-ray scintillation properties are discussed in detail. I. INTRODUCTION Although CWO has been widely used, most studies Cadmium tungstate (CdWO , CWO) is a luminescent and applications of CWO were focused on single crystals 4 synthesized by the Czocharalski method at high tempera- material that absorbs the energy of high-energy radiation 6 such as x-rays, ␥-rays, and neutrons and then converts tures. Single-crystal CWO is difficult to grow at a high the energy to visible or near visible light within a short temperature since stoichiometric control is difficult to time of less than 10−8 s,1 making it a valuable scintilla- maintain due to incompatible vapor pressures of the con- tion material. Due to its high emission efficiency (∼40% stituents during crystal growth. The maintenance of that of sodium iodide),2 high radiation stability, little chemistry stoichiometry is critical because the scintilla- afterglow, and high density, CWO has been widely used tion properties of CWO are highly dependent on its in high-energy radiation imaging and detection, spec- chemical composition. Conventional ceramic processing trometry and radiometry of radionuclides in extra-low is difficult to apply to the fabrication because CWO de- composes at 1100 °C and highly volatile CdO prevents activities, and especially for computer tomography 6 (CT).3,4 Moreover, based on radiation capture reactions, densification at temperatures even far below 1100 °C. It CWO has been investigated for application in neutron is also difficult to use single-crystal CWO in fabricating detection.5 micro- and nanoscale devices that require films for high spatial resolution. Recent developments in film engineering have allowed more and more nanostructured poly- a) crystalline CWO to be made by various methods includ- Address all correspondence to this author. 7 8 e-mail: [email protected] ing sol-gel processing, pulsed laser deposition, spray DOI: 10.1557/JMR.2007.0215 pyrolysis,9 hydrothermal method,10 polymer-controlled J. Mater. Res., Vol. 22, No. 6, Jun 2007 © 2007 Materials Research Society 1527 H.M. Shang et al.: Dense and optical transparent CdWO4 films by sol-gel processing for scintillation applications crystallization,11 and liquid epitaxy growth.12 Of these particles and many large open-voids that result in inho- methods, sol-gel processing is particularly useful or the mogeneous morphology and low optical transparency. only choice for the formation of dense polycrystalline Multiple coatings did not improve the densification and complex oxide films that decompose at elevated tem- the optical transparency. Other modifications in sol-gel peratures or consist of highly volatile constituent ele- preparation also failed to achieve uniform dense CWO ments such as cadmium in CWO. Furthermore, the mod- films. The practical scintillation applications require erate sintering temperatures of sol-gel processing will thick, dense, and optically transparent films with desired also preserve nano-sized particles because grain growth stoichiometric composition and good crystallinity to cap- becomes predominant at high temperatures13; large ture as many of the incident high-energy particles as grains may cause unacceptable loss of luminescence by possible to generate the maximum number of photons; grain boundary scattering. good optical transparency is desired to permit the result- Previous sol-gel-derived CWO films have demon- ant luminescent photons to come out of the scintillation strated photoluminescence and the capability to detect films so that the photons can be detected by a charge- x-rays, ␥-rays, and neutron emissions.14 However, CWO coupled device (CCD) or photomultiplier tube (PMT) films that are simultaneously transparent, thick, and and thus achieve high scintillation efficiency. dense have never been achieved despite various modifi- In this paper, we report our recent study on the syn- cations and improvement reported in the literature. A thesis of dense and optically transparent CWO films by a common problem in sol-gel processing is the control of combination of a new sol-gel processing and rapid sin- hydrolysis and condensation reactions and the prevention tering, which effectively prevented the precipitation of an of precipitation or phase separation, particularly in com- undesired parasitic phase during the sol preparation and plex oxide systems. For CWO sols, the stabilization of the loss of volatile CdO during sintering at elevated tem- tungsten precursors is particularly difficult because most perature. A systematic study of the relationship between tungsten precursors are highly moisture sensitive. Many sintering conditions, microstructure, crystallinity, and different tungsten precursors have been investigated to optical and x-ray scintillation properties of the sol-gel- fabricate tungsten-related films by sol-gel processing, in- derived CWO films is presented and discussed. cluding tungsten alkoxide,15 sodium tungstate,16 tungsten chloride,7 and tungsten oxychloride.17 Tungsten alkoxide is arguably the standard sol-gel precursor for tung- II. EXPERIMENTAL sten systems, but it is extremely moisture sensitive and Cadmium tungstate films were made by a new sol-gel difficult to deal with at ambient conditions. Sodium tung- processing with tungstic acid (H2WO4, 99%, Aldrich, state can be used but will introduce sodium into the sol as St. Louis, MO) and cadmium nitrate as [Cd(NO3)2·4H2O, an impurity that is hard to remove from the film. The 99%, Aldrich] precursors and hydrogen peroxide (H2O2, other frequently used tungsten precursors were those pre- 30%, Aldrich) as solvent. First, 2.62 g tungstic acid was cursors with chlorine, such as WCl6 and WOCl4, because dissolved in 35 ml hydrogen peroxide aqueous solu- they were more stable and less expensive.7,18 When these tion. The mixture was stirred for2hat60°C to obtain precursors react with alcohol, such as iso-propanol, the a clear solution of tungstic acid. Then a stochio- following reaction is expected to occur19: metrically equivalent amount of cadmium nitrate was added into above solution. This mixture was stirred at + ͑ ͒ → ͓ ͑ ͒ ͔ WOCl4 4 CH3 2CHOH WO OCH CH3 2 xCl4−x room temperature for 30 min and then divided equiva- + xHCl . (1) lently into two beakers. In the first beaker, ethylene glycol (EG, CH2OHCH2OH, Aldrich) and glycerin [Gly, The byproduct of this reaction is hydrogen chloride, CH2OHCH(OH)CH2OH, Aldrich] were added with a ס which decreases the pH value of sol, leading to a long volume ratio of H2O2:EG 1:3. In the other beaker, gelation time, and leaves chloride as an impurity in CWO ethylene glycol and glycerin were added with a volume ס film after sintering. Hydrogen chloride in the sol will also ratio of H2O2:EG:Gly 1:0.9:0.1. The two sols were lead to the formation of crystalline cadmium chloride stirred continuously for another 1 h and aged at room upon the addition of cadmium nitrate during the sol temperature for 20 days prior to making films. preparation, resulting in the formation of milky and rela- CWO films were made by the dip-casting method on tively unstable sols. These crystalline particles were eas- glass substrates. Forty microliters CWO sol from the first ily observed by microscope in CWO sol and film without beaker were dropped on a 1 × 1 in. glass substrate, which any thermal annealing; the results were confirmed by was heated on a 60 °C hot plate. The film was dried at x-ray diffraction (XRD).17 The presence of cadmium room temperature for 30 min. Then, 40 ␮l CWO sol from chloride crystals in the CWO film would effectively the second beaker was dropped on top of it when it was prevent the full densification by sintering at elevated heated at 60 °C. Five films were made by exactly the temperatures; typically, the films will consist of large same procedure and were dried at 100 °C for 1 h. Those 1528 J. Mater. Res., Vol. 22, No. 6, Jun 2007 H.M. Shang et al.: Dense and optical transparent CdWO4 films by sol-gel processing for scintillation applications films were then put directly into 500 °C furnace and kept introduced to a system consisting of more than one metal for different periods of time to investigate the sintering element.22,23 Such cross catalytic effects often lead to effects on the microstructure, crystallinity, and optical homogeneous condensation between the same element and scintillation properties of the resultant CWO films.

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