Controllable Fabrication and Cathodoluminescence Performance of

Aug 8, 2008 - We have developed a unique technique, the facile electrical-field-assisted laser ablation in liquid (EFLAL) without any catalyst or orga...
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J. Phys. Chem. C 2008, 112, 13450–13456

Controllable Fabrication and Cathodoluminescence Performance of High-index Facets GeO2 Micro- and Nanocubes and Spindles upon Electrical-field-assisted Laser Ablation in Liquid P. Liu, C. X. Wang, X. Y. Chen, and G. W. Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, School of Physics Science & Engineering, Zhongshan UniVersity, Guangzhou 510275, P. R. China ReceiVed: March 23, 2008; ReVised Manuscript ReceiVed: June 26, 2008

We have developed a unique technique, the facile electrical-field-assisted laser ablation in liquid (EFLAL) without any catalyst or organic additives, to controllably fabricate the mass production of GeO2 micro- and nanoparticles with various shapes. By adjusting the applied electrical field, we synthesized the high-index facets GeO2 micro- and nanocubes and spindles, and propose the growth mechanisms of nanostructures upon EFLAL. On the basis of the cathodoluminescence measurements of dispersive GeO2 nanoparticles, we observed a shape-dependent red-shift of emission wavelength when the shape of GeO2 nanostructures transforms into spindle from cube, and then we established the physical model to address the anomalous red-shift of emission wavelength. Accordingly, we expecte EFLAL to be a general route to synthesize the micro- and nanostructures with metastable structures or metastable shapes. 1. Introduction Micro- and nanometer-scaled germanium dioxide materials have been well studied in the past decades because of their wide applications in biochemistry,1,2 catalytic chemistry,3 and the potential applications in nanoelectronics and optoelectronic nanodevices.4,5 For instance, one-dimensional GeO2 nanowires exhibit good optical properties and GeO2 nanocrystals are confirmed to be a blue-green luminescent material,6 which imply that GeO2 nanostructures can be considered to be an interest material for optoelectronic communications.7 Generally, the physical and chemical properties of nanoscaled materials strongly depend on their size and shape, which means that researchers can design the physical and chemical properties of nanostructural materials by controlling their size and shape. Therefore, a variety of GeO2 nanostructures have been synthesized in recent years.8-11 Also, the shape-controlling synthesis of GeO2 nanostructures is still of potential significance. In this contribution, we report that a unique technique, the facile electrical-field assisted laser ablation in liquid (EFLAL) without any catalyst or organic additives, has been developed for the controllable fabrication of micro- and nanostructures with metastable structures or metastable shapes. Using EFLAL, we synthesized the mass production of GeO2 micro- and nanocubes with high-index facets and a kind of GeO2 micro- and nanospindles. More importantly, we observed the shapedependent red-shift of emission wavelength when the shape of the synthesized nanostructures transforms into spindle from cube by the cathodoluminescence measurements of a single GeO2 nanoparticle. 2. Experimental Section A schematic illustration of the experimental setup of EFLAL is shown in Figure 1. The general introduction of laser ablation in liquid has been reported in many previous works.12-15 In this case, a single crystalline Ge target with 99.99% purity is * Corresponding author e-mail: [email protected].

used as the starting material and is fixed on the bottom of a rectangular quartz chamber with dimensions 6.0 × 3.8 × 7.0 cm3. The liquid is the deionized water. Attentively, a dc electrical field with adjusting voltage, which is brought by two quadrate parallel-electrodes with a distance of 1.6 cm, is applied above the target. An illustration of the applied electrical field is shown in Figure 1b, and the electric field intensity can be deduced to be about 9.06 × 102 ∼ 2.0 × 103 V/m according to the geometry of electrodes. Then, a second harmonic is produced by a Q-switched YAG laser with a wavelength of 532 nm, pulse width of 10 ns, repeating frequency of 5 Hz, and energy of 150 mJ for the ablation of the target. The whole system is maintained at room temperature and is placed inside an ultrasonic vibrator with an oscillate frequency of 70 kHz. After the laser interacted with the target for about 180 min, the gray-white powders were collected from the liquid and desiccated at 60 °C in an oven before the further measurements. Scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectrometer, and cathodoluminescence (CL) spectroscopy carried out by a Gatan MonoCL3 system attached field emission scanning electron microscopy (FESEM) were employed to characterize the morphology, structure, and luminescence of the assynthesized samples. 3. Results and Discussion Figure 2, panels a and b, shows two typical SEM images of micro- and nanocubes and spindles. Clearly, we can see that homogeneous micro- and nanocubes and spindles are synthesized using EFLAL. In detail, these cubes exhibit smooth surfaces and a perfect three-dimensional geometry, and the edge lengths of cubes are in the range of 200∼500 nm, as shown in Figure 2a. A high magnification SEM image shows that all corners and edges of the micro- and nanocubes are rather sharp but are not slightly truncated as that of metal cubes in Figure 2c.16 Additionally, we can see another kind of the synthesized nanostructure, as shown in Figure 2, panels b and d, that is, spindle-like micro- and nanoparticles, in which we can see that

10.1021/jp802529r CCC: $40.75  2008 American Chemical Society Published on Web 08/08/2008

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Figure 1. Illustration of the electrical-field-assisted pulsed-laser ablation in liquid (a) and a detailed depiction of the reactivity field instance (b).

the sizes of most micro- and nanospindles are in the range of 200∼400 nm. Note that these micro- and nanocubes and spindles are synthesized under the applied electrical fields, in which the electrical fields are set to 14.5 and 32 V for cubes and spindlelike syntheses in our cases, respectively. Two corresponding XRD patterns are obtained and are shown in Figure 2, panels e and f, respectively, in which the only highest main peak is assigned to be from the {1011} planes of the hexagonal GeO2 (R-phase of quartz-like structure) (JCPDS Card File No. 431016), whereas no Ge peaks are observed. Additionally, other peaks show the good agreement with that of the hexagonal GeO2. Therefore, these as-synthesized micro- and nanoparticles are GeO2. Interestingly, the strong peak of the {1011} planes indicates that these as-synthesized micro- and nanoparticles have high index facets, which leads to the good uniformity of cubes and spindles. Moreover, it can be noticed that, besides the peak of the {1011} planes, there are some unknown peaks that appear in Figure 2e compared with that of Figure 2f. We attribute them to the differences of the crystallographic orientations and shapes of two kinds of particles. To compare to the case of the EFLAL that utilized the external electrical field, we carry out the same LAL experiment without the external electrical field. One pure Ge target is ablated by pulsed-laser in the conventional LAL condition without any external electrical field, and the results are shown in Figure 3. Clearly, we can see that almost all the products are sphericallike particles, and the size of most particles is in the range of 300-400 nm. Interestingly, these results are consistent with many previous reports.17,18 Therefore, the application of an electrical field upon laser ablation of a Ge target plays the important role in the formation of the high-index facets microand nanocubes and spindle-like particles in our experiment. Moreover, the EDS data show that these spherical-like clusters are mainly composed of pure germanium element. This result further indicates that the extent of the water decomposition upon LAL is very limited without any electrical field. In fact, the water decomposition upon LAL provides the oxygen sources of GeO2 in our case. Thus, the application of the electrical field is an indispensable factor in significantly affecting the formation and shape evolution of the GeO2 micro- and nanocubes and spindles. Figure 4, panels a and b, shows the TEM bright-field images of single a cube and spindle, and these results exhibit a high symmetry of the cubic morphology and the spindle. The corresponding selected area electron diffraction (SAED) patterns are shown in Figure 4, panels c and d, in which the 4-fold symmetry of the SAED patterns confirms that the cube and

spindle are single crystals. For the cube, the square facet is undoubtedly indexed to be the (101j1) planes. Further, two highresolution TEM (HRTEM) images of one facet of the cube and one edge of the spindle are recorded as shown in Figure 4, panels e and f. Interestingly, the dominant two-dimensional lattice fringes as shown in Figure 4e are measured to be 0.235 and 0.438 nm, respectively, which are close to the (112j0) and (101j0) lattice spacing of the R-phase quartz-like structure. Additionally, in Figure 4f, the lattice planes with a spacing of 0.433 nm is measured, which is undoubtedly assigned to be the (011j0) planes of the hexagonal GeO2 crystallite. Therefore, all the TEM analyses assuredly confirm that the synthesized cubes and spindles are both single crystals. A striking property of GeO2 nanostructures is that they can emit stable and bright blue-green light.19-21 Considering that CL spectroscopy is a powerful methods for obtaining information on electron-photon interaction and the transitions of the valence band in crystals,22 we employ CL spectroscopy to characterize the luminescence of the as-synthesized samples. The SEM images of two cubes and two spindles are shown in Figure 5, panels a-b, and the corresponding clear emissions images observed at room temperature are displayed in the insets. In detail, a broad CL peak with the luminescence center located at about 427 nm is observed in Figure 5c, which indicates that a stable and bright blue light emits from the nanocubes. Further, a Lorentzian deconvolution of the solid line spectrum is calculated, and then the result shows that the shoulder corresponds to an emission band at 508 nm, and the blue part has two components, one at 440 nm and another one at 398 nm. The 508 nm component with emission energy of 2.44 eV is in agreement with the previous studies of GeO2 luminescence,7 which seems to be the characterized luminescence peak of GeO2 nanostructures. For the components at 398 and 440 nm, they are similar to that of GeO2 nanocrystals,23,24 which are suggested to be from the emissions induced by defects in the matrixnanocrystal interface.20 However, the emission from nanospindles definitely exhibits a different behavior from that of nanocubes, as shown in Figure 5d. It is well-known that there is always only one luminescence peak in the CL spectrum of GeO2 nanostructures, as described in many previous reports.7,20 Nevertheless, we can clearly see two CL emission peaks in our CL spectrum. Using the analysis of a most-fitted peak deconvolution result, we obtain that the luminescence spectrum consist mainly of two peaks with center frequencies at 425 and 510 nm, respectively. Compared with our analysis above, the low peak at 425 nm is in agreement with the main peak that is shown in Figure 5c, which is the CL

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Figure 2. SEM images of the synthesized micro- and nanocubes and spindles. Low-magnification SEM images of cubes (a) and spindles (b). High-magnification SEM images of a cube (c) and several spindles (d). Corresponding XRD patterns of the hexagonal GeO2 cubes (e) and spindles (f).

luminescence with three complex blue-green bands. Meanwhile, the long wavelength component of 510 nm is considered to be an emission from GeO2 nanowires.7 Accordingly, we can have a clear and general insight into the basic physical mechanisms involved in the luminescenceband-shift of GeO2 nanostructures with different shapes by combining with the previous investigations and our work. For

the three-dimensional confinement GeO2 nanoparticles such as spheres and cubes, they mainly emit the short wavelength luminescence, that is, the blue light luminescence, which is attributed to the interaction between oxygen vacancies and germanium-oxygen vacancies centers.19 In detail, the electron acceptors would be formed by (VGe, VO)× (the germanium-oxygen vacancies center), and the donors would be formed by Vo¨ (the

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Figure 3. Low-magnification SEM image of the synthesized spherical-like products upon conventional LAL method (a), and a corresponding high-magnification SEM image of the Ge nanoparticles (b).

oxygen donor). After excitation of the acceptor, a hole on the acceptor and the electron on the donor would be created according to the following equation.

(VGe, VO)× +Vo¨ + 3e f (VGe, VO)″ + Vo¨

(1)

Then, the blue emission occurs via an unprompted reverse reaction according to the following formal equation.19

(VGe, VO)″ + Vo¨ f (VGe, VO)×+VO× + 2hν

(2)

Thus, the luminescence process could be divided into two steps. At the first step, the electron in the donor band is captured by a hole in an acceptor to form a trapped exciton. Then, the trapped exciton radiatively recombines and emits the blue photon. However, a long wavelength emission starts to appear when GeO2 nanostructures exhibit one-dimensional shapes such as nanowires. In other words, the main luminescence center transforms into a green emission band,7,19,20 which is attributed to the effect of oxygen-deficient centers. Interestingly, it can be found in our work that the shape of nanospindles seems an intermediate phase of the shape evolution from nanoparticles to nanowires. Thus, the competition between the emission mechanisms of nanoparticles and nanowires naturally leads to two different emissions simultaneously appearing in the CL spectrum of nanospindles, as shown in Figure 5d. Accordingly, the illustrative summary of this emission-shift of GeO2 nanostructures is illustrated in Figure 5e. Now we turn to the position to discuss the growth mechanisms of GeO2 nanostructures upon EFLAL. In general, the crystal shape is determined by the crystallographic planes. When crystals form under the equilibrium conditions, their crystalline habits are determined by the surface energies.25 The fastest growing plane always occurs in the direction that is perpendicular to the face with the highest surface energy, which leads to the weakening of the high-energy surfaces while the lowenergy surfaces are enhanced. Thus, this plane evolution causes the final shape of crystals. The general growth mechanisms of nanostructures upon laser ablation in liquid have been addressed in many groups’ previous publications.12-15 Therefore, our interest lies specifically in the influence of the applied electrical field on the shape-controlling in synthesis of nanostructures upon EFLAL. It is well-known that laser ablation in liquid is a very fast and far from equilibrium process, in which many metastable

structures and metastable shapes formed in the initial, intermediate, and last synthesis process can be reserved in final products.15 The initial process of laser ablation at a liquid-solid interface is an interaction between laser and Ge target. First, Ge species having large initial kinetic energy will form a dense region in the vicinity of the solid-liquid interface, which is called the laser-induced plasma plume. Because the plasma is strongly confined in liquid, a shockwave will be created at supersonic velocity in front, which would induce an extra pressure in the plasma plume.26 The extra pressure is usually called the laserinduced pressure. Then, the laser-induced pressure induces a temperature increasing in the plasma plume, and the temperature is estimated to be above 4000 K.27 Therefore, the plasma plume formed upon laser ablation in liquid is in the high-temperature, high-density, and high-pressures (HTHDHP) state. On the other hand, the deionized water would first be electrolyzed into H and O ions under the applied electrical field in our case. It is noticed that these pieces with metastable phase exist in the plasma plume, and the ultrasonic dispersive action is used in our reactions. Although there may be some relatively large Ge clusters ejected from the pulsed-laser ablation that then survive provisionally, the ultrasonic wave can facilely make these unstable clusters distribute dispersedly in the liquid environment. Then, the plenteous active O element (O ion and even molecular O) from the water decomposition will easily combine with the Ge components in the plasma plume to form the initial GeO2 micro- and nanostructure nuclei. Moreover, the violent oscillation wave could destroy and disperse the precipitated layer that forms on the electrodes due to the ultrasonic vibrator, which inhibits depositions on the electrodes. Thus, this result makes the ions double charge layer have a weak effect on the formation of these micro- and nanoparticles. In addition, the quenching time of the laser-induced plasma plume is very short. Although there would be some charges appearing in the plasma plume, their influence on the formation of nanocrystals is undoubtedly an instantaneous one. Thus, this behavior naturally brings very small perturbation in the relatively strong applied electrical field. On the other hand, the internal ionic-electrical field could affect the instantly initial nucleation in the plasma plume. Therefore, this influence seems to be ignored in the sustaining growth of GeO2 micro- and nanostructure particles according to the discussion above. Moreover, a possible contribution of the

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Figure 4. TEM images of an individual GeO2 nanocube (a) and a dispersive GeO2 spindle (b). Corresponding SAED patterns of the cube (c) and spindle (d). HRTEM images of an edge of the GeO2 nanocube (e) and one side of the spindle (f).

laser influence is examined in our case. Our results show that many GeO2 micro- and nanoparicles are repeatedly grown by incorporating the nearby GeO2 pieces during the one-by-one pulsed-laser irradiation, which is similar to that suggested in previous references.18 Accordingly, the intensity of laser ablation brings an obvious influence on the growth rate of the products. Then, we find that there is hardly any effect on the characteristics of products when the energy of laser is adjusted finitely.

As is well-known, there are two stable phases of GeO2 under normal pressure and temperature: the tetragonal phase (rutile structure) and the hexagonal R-phase (R-phase quartz-like structure).28 In general, GeO2 structure will smoothly convert to a regular hexagonal β-phase (β-phase quartz-like structure) when the temperature rises above 1049 °C, and then the unstable β-phase will transform to the stable R-phase upon quenching.10 Therefore, the products in our case are the R-phase hexagonal GeO2 micro- and nanocubes and spindle-like particles that finally

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Figure 5. (a) SEM image of two single cubes, and the inset is the corresponding CL image. (b) SEM image of two spindles and the inset of the corresponding CL image. The corresponding CL spectrum of cubes (c) and spindles (d). (e) An illustration of the luminescence-shift of GeO2 nanostructures with various shapes.

formed after the reactions in the high-temperature and highpressures plasma region. Accordingly, the formation of these GeO2 micro- and nanocubes and spindles involves three steps. First, Ge particles undergo a conformation change from solid phase to vapor phase in the HTHDHP plasma plume induced by the pulsed-laser. Meanwhile, the active O element, which is electrolyzed from the deionized water, will encircle the plasma. Second, Ge pieces in vapor phase continuously react with the O element and then nucleate and grow into GeO2

particles. Note that the growing GeO2 particles are in the β-phase because the vapor phase cannot transform directly to the R-phase at the HTHDHP state. Finally, with the temperature and pressure gradually decreasing during plasma quenching, the β-phase GeO2 particles will change into the R-quartz-like structure and form the final shape. Furthermore, as a result of the liquid confinement, the growth time (the plasma quenching time) of the synthesized particles is very short. Thus, the size of the grown crystals is usually in the micro- and nanometer scale.

13456 J. Phys. Chem. C, Vol. 112, No. 35, 2008 Considering the equilibrium of a small crystal with its ambient phase, there should be a minimum surface energy to determine the equilibrium shape. From the standard JCPDS card No. 431016 (the R-quartz-like irregular hexagonal structure) of GeO2, we know that the {1011} crystal facets of the R-phase GeO2 polycrystallline planes have the minimal surface energy. Thus, it is suggested that the applied electrical field plays the importance role in the growth of the {1011} planes with low surface energy. In other words, the applied electrical field could stabilize and enhance the growth of the {1011} planes. Therefore, the stability of the {1011} crystallographic planes becomes so strong that the crystal growth perpendicular to these planes would be hindered, due to the effect of electrical field. Accordingly, the six {1011} planes of the hexagonal GeO2, which are perpendicular or parallel to each other, would easily induce the formation of a cubic morphology as shown in Figure 2a. However, the {1011} plane growth would be elongated in the certain direction such as the [0001] crystallographic orientation of the hexagonal structure when the applied electrical field becomes strong, which would induce the spindle-like shape formation. Importantly, these deductions are confirmed by the analysis of the XRD pattern shown in Figure 2f and the TEM analysis shown in Figure 3d of the as-synthesized micro- and nanospindles. For example, compared with the XRD pattern of cubes, we only see the [0001] peak in the XRD pattern of spindles. Thus, the strong electrical field not only enhances the {1011} planes growth but also induces the preferential [0001] crystallographic orientation. 4. Conclusion We have developed a novel methodological approach, the electrical field assisted laser ablation in liquid (EFLAL), to controllably synthesize micro- and nanostructures with metastable shapes. Using EFLAL, we not only synthesized GeO2 micro- and nanocubes and spindles but also addressed the influence of the applied electrical field on the shape-controlling of nanostructures upon laser ablation in liquid. Moreover, we have observed the red-shift of emission wavelength with the shape evolution from cube to spindle in the CL spectra of the synthesized nanostructures and have proposed a general physical explanation for the luminescence behaviors of GeO2 nanostructure with various shapes. Our investigations have showed that two advantages of EFLAL over the general LAL techniques for the micro- and nanostructures synthesis are as follows. (i) The fabrication technique is not only chemically “simple and clean” but also is synthetically controllable and designable, which has the simple starting materials without any catalyst or organic additives and the adjustablity of the applied electrical

Liu et al. field. (ii) The products with various metastable shapes or structures can be homogeneously synthesized in ambient conditions without extreme temperature and pressure. Acknowledgment. NSFC (50525206 and U0734004) and the Ministry of Education (106126) supported this work. References and Notes (1) Shea, R.; Chopin, T. J. Appl. Phycol. 2007, 19, 27. (2) Lin, C. H.; Chen, S. S.; Lin, Y. C.; Lee, Y. S.; Chen, T. J. Neurotoxicology 2006, 27, 1052. (3) Jing, C. B.; Hou, J. X.; Zhang, Y. H. J. Am. Ceram. Soc. 2007, 90, 3646. (4) Cumberland, B. A.; Popov, S. V.; Taylor, J. R.; Medvedkov, O. I.; Vasiliev, S. A.; Dianov, E. M Opt. Lett. 2007, 32, 1848. (5) Lange, T.; Njoroge, W.; Weis, H.; Beckers, M.; Wuttig, M. Thin Solid Films 2000, 365, 82. (6) Wu, X. C.; Song, W. H.; Zhao, B.; Sun, Y. P.; Du, J. J. Chem. Phys. Lett. 2001, 349, 210. (7) Hidalgo, P.; Mendez, B.; Piqueras, J Nanotechnology 2007, 18, 155203. (8) Hu, J. Q.; Li., Q.; Meng, X. M.; Lee, C. S.; Lee, S. T. AdV. Mater. 2002, 14, 1396. (9) Bai, Z. G.; Yu, D. P.; Zhang, H; Ding, Z. Y.; Wang, Y. P.; Gai, X. Z.; Hang, Q. L.; Xiong, G. C.; Feng, S. Q Chem. Phys. Lett. 1999, 303, 311. (10) Tang, Y. H.; Zhang, Y. F.; Wang, N.; Bello, I.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 1999, 74, 3824. (11) Chen, X.; Cai, Q.; Zhang, J.; Chen, Z.; Wang, W.; Wu, Z. Y.; Wu, Z. H. Mater. Lett. 2007, 61, 535. (12) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B 2000, 104, 9111. (13) Compagnini, G.; Scalisi, A. A.; Puglisi, O. J. Appl. Phys. 2003, 94, 7874. (14) Kazalevich, P. V.; Simakin, A. V.; Voronov, V. V.; Shafeev, G. A. Appl. Surf. Sci. 2006, 252, 4373. (15) Yang, G. W. Prog. Mater. Sci. 2007, 52, 648. (16) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (17) Sylvestre, J. P.; Poulin, S.; Kabashin, A. V.; Sacher, E.; Meunier, M.; Luong, J. H. T. J. Phys. Chem. B 2004, 108, 16864. (18) Ishikawa, Y.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. Appl. Phys. Lett. 2007, 91, 161110. (19) Wu, X. C.; Song, W. H.; Zhao, B.; Sun, Y. P.; Du, J. J. Chem. Phys. Lett. 2001, 349, 210. (20) Fitting, H. J.; Barfels, T.; Trukhin, A. N.; Schmidt, B. J. Noncryst. Solids 2001, 279, 51. (21) Nogales, E.; Montone, A.; Cardellini, F.; Mendez, B.; Piqueras, J. Semicond. Sci. Technol. 2002, 17, 1267. (22) Yang, Y. H.; Chen, X. Y.; Feng, Y.; Yang, G. W Nano Lett. 2007, 7, 3879. (23) Hidalgo, P.; Mendez, B.; Piqueras, J. Nanotechnology 2005, 16, 2521. (24) Zacharias, M.; Fauchet, P. M. Appl. Phys. Lett. 1997, 71, 380. (25) Mullin, J. W. Crystallization; Butterworths, London, 1971. (26) Berthe, L.; Fabbro, R.; Peyer, P.; Bartnicki, E. J. Appl. Phys. 1999, 85, 7552. (27) Saito, K.; Takatani, K.; Sakka, T.; Ogata, Y. H Appl. Surf. Sci. 2002, 197-198, 56. (28) Sarver, J. F.; Hunvmel, F. A. J. Am. Ceram. Soc. 1960, 43, 336.

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