The Role of Development of the Rotating Anode X-ray Generator and

Waters Symposium: X-ray Diffraction of Powders and Thin Films

The Role of Development of the Rotating Anode X-ray Generator and the Use of Imaging Plates in Powder Diffractometer Instrumentation Jimpei Harada X-ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima-shi, Tokyo 196-8666, Japan; [email protected]

X-ray powder diffraction is a well-known technique that can provide diverse information about the structure of materials. It has been used mainly for identification of compounds by their diffraction pattern on the basis of comparison to existing databases such as the PDF (Powder Diffraction File) and ICSD (Inorganic Crystal Structure Database), and for characterization of industrial materials. It became an effective tool in analyzing the structure of materials on an atomic scale after the introduction of the Rietveld refinement (1) and has been used to determine electron charge density by the maximum entropy method (2). This paper sketches the growth of science and technology in X-ray powder diffraction. I will review the Rigaku Corporation’s role in this growth as a manufacturer specializing in X-ray analytical instrumentation. I will refer to (i) the automatic recording X-ray diffractometer Geigerflex, which was released in Japan in 1954 in response to customers’ demands; (ii) the development of rotating-anode X-ray generators of 6, 12, 18, 30, 60, and 90 kW (these generators greatly improved the quality of data by providing better signalto-noise ratios, thus permitting analysis of materials that diffract only weakly); (iii) the parallel X-ray beam technique, developed to measure surface stress, and a new synthetic graded multilayer monochromator with parabolic curvature; and (iv) a Debye–Scherrer diffractometer with cylindrical 2-D detector using an imaging plate.

Geigerflex Powder X-ray Diffractometer In 1952, just after its founding, Rigaku Corporation introduced a new X-ray generator, the “Model D”. It had a demountable tube and its maximum output power was 1.8 kW (60 kV × 30 mA) for a copper anode with a 1 × 10-mm2 focal spot. It is recorded in Crystallography in Japan (3) that in the same year North American Philips installed a Norelco X-ray diffractometer at the Faculty of Science, the University of Tokyo. The next year, Rigaku built a prototype X-ray diffractometer equipped with a demountable X-ray tube and a Geiger counter for X-ray detection. When it was completed as the Rigaku automatic recording X-ray diffractometer, Geigerflex, in 1954, a Philips sealed tube was introduced instead of the demountable X-ray tube. The Geigerflex was a horizontal goniometer with Bragg– Brentano (B–B) convergent optics (1946) (Fig. 1). This is the major difference between it and the vertical Norelco. Rigaku decided to manufacture a horizontal goniometer because its users considered it to be more convenient for installing heavy cooling or heating devices. Many users were constantly pushing the limits of the Geigerflex goniometer. Rigaku was requested to provide new instruments to address this issue. The versatile goniometers

Historical View of XRD in Japan Soon after its discovery by von Laue in 1912, the X-ray diffraction phenomenon was confirmed by Terada and Nishikawa, of the University of Tokyo, in crystalline materials and several textile products. In the 1920s, the first X-ray diffractometer built in Japan was fabricated at the Institute of Physical and Chemical Research. It was modeled after Bragg’s spectrometer, and employed an X-ray generator designed originally for medical use and an ionization chamber. To meet scientists’ demands, Rigaku Denki Seisakusho, the predecessor of Rigaku Corporation, was founded in 1923. Shimadzu and a few other manufacturing companies also participated in the development of instruments during this period. Several types of diffractometers and cameras could be found in many Japanese research laboratories. The X-ray generators used were either the Coolidge type or the demountable anode type. Rigaku Corporation was founded in 1951 and began to develop X-ray analytical instruments in responses to users’ requests. This led to the production of instruments not only for powder and single-crystal diffraction but also for X-ray radiography and X-ray fluorescence analysis.

Figure 1. Old Rigaku X-ray powder diffractometer, “Geigerflex” (1954).

JChemEd.chem.wisc.edu • Vol. 78 No. 5 May 2001 • Journal of Chemical Education

607


Figure 3. θ–θ goniometer with rotating anode X-ray generator, “TTR”.

Figure 2. Top: Rotating anode assembly for the latest-model rotating-anode X-ray generator. Bottom: Sectional diagram of the rotating anode assembly.

produced as a result of such users’ demands are the diffractometers of the D/MAX-A, -B, and -C and 1000/2000 series, along with a variety of attachments for advanced methods. There are three options for the goniometer. One is a θ – θ goniometer system in which the sample surface is kept horizontal. This is suitable for the measurement of liquid samples. A second option is the vertical goniometer system with its easy sample mounting. This is the most popular configuration for powder diffraction. The third option is a horizontal system that permits safe mounting of heavy attachments such as an ultra-lowtemperature attachment or a high-pressure–high-temperature attachment. One can say, therefore, that Rigaku technological advancement has been driven by its customers. Rotating Anode X-ray Generator Several laboratories around the world were in great need of intense X-ray sources, even more than 50 years ago. Rigaku began to develop a rotating-anode X-ray generator in 1952, using the technology learned in manufacturing the demountable X-ray tube. Rigaku also used Taylor’s work (4 ) as a guide. After much trial and error, the first rotating-anode generator, called “Model U”, was completed in 1953. The anode was 608

100 mm in diameter and rotated at 760 rpm using a Wilsontype vacuum seal. The output power was 5 kW (50 kV × 100 mA) and the focal spot size was 1 × 10 mm2 for a copper anode. Continuous advancements have improved the performance and reliability of the generator. There were insatiable demands worldwide for Rigaku to increase the power of their X-ray generators even more. The first request was to build an X-ray generator of 30 kW (60 kV × 500 mA). Rotating an anode of 400-mm diameter at 3000 rpm allowed such a power level to be attained. This work was done in collaboration with Chikawa and Fujimoto (5) of the Science and Technical Research Laboratories of NHK in 1969. The development of a 90-kW X-ray generator, the most powerful in the world, was achieved in collaboration with a group at Nagoya University from 1975 to 1987 (6 ). The rotating anode was reduced to a diameter of 300 mm but rotated at 10,000 rpm. The challenge to attain higher-power X-ray generators and the collaboration with outside scientists brought about great changes in Rigaku’s technology. As a result, both the performance and the reliability of rotating-anode generators were greatly improved. These improvements were not restricted to high-power generators; they applied to standard generators as well. Turbo molecular vacuum pumps (6 ) and ferrofluidic vacuum seals (7) were introduced to these systems. Other improvements made the system even more reliable. Figure 2 shows a photograph of the latest-model rotating-anode generator and a cross section through the anode assembly. A direct-drive motor has replaced the motor, pulley, and belt used in earlier models. A mechanical seal has replaced a rubber seal in the cooling-water system. A further attempt to reduce weight and size made the latest model, the UltraX, portable enough to permit easy positioning not only on the generator tabletop but also on the θ arm of a θ – θ goniometer. The prototype θ – θ goniometer was built on a request from Nagoya University (8). It is now produced commercially as the TTR goniometer (Fig. 3). The maximum output power of the generator is 18 kW (60 kV × 300 mA). This machine has been very effective for the study of the structure of liquid surfaces and the characterization of thin films on substrate (9). Figure 4 shows the reflectivity curves from the free surface of a magnetic fluid at several temperatures; this is one of the more spectacular results obtained by this machine (10).

Journal of Chemical Education • Vol. 78 No. 5 May 2001 • JChemEd.chem.wisc.edu


a

b

Figure 4. Reflectivity curves from the free surface of a magnetic fluid at different temperatures, obtained by TTR (9). For clarity, each reflectivity is displaced vertically by 0.01. The Fresnel reflectivity from an ideal flat surface is shown by the solid curve. The crosses at 300 K indicate the reflectivity of silicone oil, an example of a noncolloidal liquid.

Parallel Beam Technique The significance of parallel X-ray beam optics in powder diffraction has recently been seen in a new light as a result of several experiments using synchrotron radiation (SR) sources (11, 12). With an SR source the line profile obtained is symmetrical and free from the aberrations seen in conventional B–B convergent-beam optics. Moreover, the diffraction angle is insensitive to the sample position. This idea reminds us of the use of a simple Debye–Scherrer camera based on parallelbeam optics. For the identification of the composition of alloy samples, one is still encouraged to use a Debye–Scherrer camera to measure lattice parameters (13). Otsuki, Wakaomi, and Katayama had already pointed out the effectiveness of such parallel-beam optics in 1960 (14). In studies of surface stress, the systematic deviation of lattice parameters is measured by changing the penetration depth of the X-ray beam incident to the sample surface. Otsuki et al.

Figure 5. (a) Schematic experimental arrangement of parallel-beam optics used by Otsuki et al. (14) to measure stress. (b) Change of diffracted angle versus displacement of sample position.

proved that the diffraction angle is insensitive to the displacement of the sample position when parallel-beam optics is employed. Since no SR source was available at the time, they placed two sets of Soller slits on a diffractometer as shown in Figure 5a. One set was installed after the X-ray source to make the incident beam parallel and the other was placed in front of the detector to achieve sufficient resolution. This is exactly the same optics suggested by Parrish et al. (11). Figure 5b shows the result obtained by Otsuki et al. (14 ). We see that even a ±3-mm displacement of the sample from the correct position causes no serious error in the diffraction angle. This greatly simplified stress analysis measurements. A stress-measurement unit, the Strainflex, built at Rigaku in 1960, was based on this study. Equipment for micro-stress management, PSPC-MDG200, designed in 1984, uses the same idea. In this case a fine parallel beam is produced by a single glass capillary used in conjunction with a positionsensitive curved proportional counter (15).

Figure 6. A schematic experimental setup of optics in reflection mode.


609


New Optics for XRD

The Characterization of Thin Films For the study of thin films it is essential to use asymmetric diffraction conditions with parallel X-ray beam optics. The profile of a Bragg reflection is not seriously influenced, even in the 610

Figure 8. Diffraction profiles of the 533 reflection of a Si powder sample obtained by parallel-beam optics at various temperatures (17 ). The position of the peak moves toward the low-scatteringangle side owing to lattice expansion, whereas intensity decreases as temperature increases owing to the Debye–Waller factor.

4.83

13.16

4.82 4.81

Obs a

13.14

Calc a

13.12

Obs c 4.80

13.10

Calc c 13.08

4.79 13.06 4.78 13.04 4.77

13.02

4.76

Lattice Constant a / Å

The Measurement of Lattice Parameters One application of parallel-beam geometry is the study of the temperature dependence of lattice parameters, since the diffraction angles are less sensitive to sample position (14). Figure 8 shows the change of the Si (533) Bragg reflection with temperature. It is clearly seen that the peak position moves toward the low-angle side owing to lattice expansion. Intensity decreases because of the Debye–Waller effect as temperature increases. In Figure 9, the temperature dependence of lattice parameters a and c of Al2O3 powder measured by the parallel-beam method is compared with calculations based on thermal expansion coefficients up to 1500 K (19). When the data are normalized to room temperature, there is only a small disagreement between the observation and the calculation.

Figure 7. Comparison of diffraction profiles of Zeolite Na-LTA powder obtained using parallel-beam optics and Bragg–Brentano optics (18). The profile is symmetric for parallel-beam geometry, but asymmetric for the B–B method owing to inherent aberrations.

Lattice Constant a / Å

General Aspects A graded parabolic multilayer optic, often referred to as a Goebel mirror (private communication with H. Goebel, 1987), has come onto the market as a result of techniques developed by Osmic, Inc., which enable a fairly efficient parallel X-ray beam to be easily produced in the laboratory. In principle, the X-ray intensity obtainable from it depends on the length of the mirror, which determines the acceptance angle of X-rays from the source, and also the brightness of the X-ray source. To create the brightest parallel beam possible in the home laboratory, we examined the combination of an 18-kW rotating-anode X-ray generator with an Osmic GO-13B mirror. Figure 6 shows the schematic setup of the optics installed on a D/MAX 2000 powder X-ray diffractometer in reflection mode (16 ). With a line focus source of 0.05 × 10 mm2, a parallel beam of 1 × 10 mm2 was obtained with horizontal divergence of 0.05°. Two Soller slits were set, one after the parabolic multilayer mirror and the other in front of the detector, to reduce vertical divergence of the beam. Otherwise the diffraction profile becomes asymmetric owing to the wellknown umbrella (or shadow) effect observed in powder diffraction profiles. A parallel slit analyzer must be set between the sample and the detector (17) (Fig. 6), as discussed by Otsuki et al. for a laboratory X-ray source (14) and by Parrish et al. for an SR source (11). Its divergence must be the same order of magnitude as that of the incident beam, 0.05°. This angle is regulated by the geometric structure of the parabolic multilayer mirror. Because of the use of two sets of Soller slits, the complete profiles of the diffracted beams are very symmetrical compared to those obtained by the convergent-beam method, as shown in Figure 7 (18). Two diffraction patterns for zeolite Na-LTA are compared. One was obtained by B–B optics and the other by the parallel-beam optics under discussion. The intensity was reduced by 1⁄3. For this examination, therefore, it is concluded that a good-quality parallel beam is obtainable by using the new optics, but it is recommended that optics with a rotatinganode X-ray generator be used to compensate for the intensity reduction.

13.00

4.75

12.98 200

400

600

800

1000

1200

1400

1600

Temperature / K

Figure 9. Temperature dependence of two lattice parameters of an Al2O3 powder sample measured by a parallel-beam method (17 ). The observed results are compared with calculations based on published thermal-expansion coefficient data. All data are normalized to room temperature.

extreme case of the asymmetric diffraction condition using grazing-incidence geometry, because of the use of a parallel beam. A diffractometer system for thin-film analysis was designed by Kobayashi and Hirashima on the basis of this idea (20) and built by Rigaku in 1984, although the Goebel mirror was not available at the time of its release.


Waters Symposium: X-ray Diffraction of Powders and Thin Films X-ray source: Cu Target 0.05 × 10 mm; 50 kV 300 mA Beam conditioning slit 0.1 × 10 mm Parabolic multilayer

Incident slit 1 × 10 mm

CuKα1 Channel-cut monochromator Ge(111)

Figure 10. One possible arrangement of parallel-beam optics. To select only Kα1 radiation from Cu radiation, the Ge (011) asymmetric-channelcut monochromator and an ordinary channel-cut monochromator are set antiparallel after a parabolic multilayer. One can get a very thin parallel beam by this optical configuration.

ω

Sample

2θ Attenuator

Double slits 0.2 × 10 mm

X-ray detector

X-ray optics appropriate for the study of the surfaces of nearly perfect crystals differ from those used to study thin films. There are various degrees of imperfection in thin crystalline films, which range from nearly amorphous thin films to perfect epitaxial thin crystalline films. Even thin films require different optics depending on their degree of imperfection. A system in which a Ge (011) asymmetric-channel-cut monochromator is set after a parabolic multilayer is shown in Figure 10. Using this optic we can select only K α radiation from Cu radiation and can get a very thin parallel beam. This is extremely powerful for the characterization of nearly perfect epitaxial thin crystalline films. To measure reflectivity of thin amorphous SiO2 film, however, a single multilayer optic is sufficient if it is combined with a multilayer analyzer. Figure 11 shows reflectivity measured from 1, total reflection, down to the order of 10᎑8. Note that such a wide range can only be obtained by using these optics together with a 15-kW rotatinganode X-ray generator; otherwise, we could reach only the order of 10᎑7 (16, 17 ). The in-plane as well as out-of-plane structure of thin films can be investigated if the counter arm is set so as to rotate around the axis perpendicular to the thin-film sample. This is the principle of the design of the ATX-G goniometer with parallel-beam optics recently developed at Rigaku. Using this diffractometer, Omote et al. clearly observed the Co (002) in-plane Bragg reflection from a polycrystalline magnetic hard disk, and it has a highly preferred orientation in the plane of the disk (21). This is the first observation of in-plane diffraction from a polycrystalline thin film using a homelaboratory X-ray source and not using an SR source. The new parallel X-ray beam has interesting applications to the study of interfacial roughness and to the structural study of thin films (22–24 ). 1

has been well proven since its invention in 1916. We can see the distribution of crystalline orientation from the variation of the intensity distribution along each Debye–Scherrer ring recorded on an X-ray film. In other words, 2-D Debye– Scherrer rings provide more information than 1-D profiles about the structure of crystallites that comprise the sample. From the 2-D data, the secondary structure of materials (distribution of grain size, crystalline orientation, strain, and so on) can be deduced. However, the Debye–Scherrer camera has been left as it was for a long time because of the lack of automation, whereas standard XRD systems have become very popular as a result of computer automation. It is now possible to replace X-ray film with an imaging plate (IP) or a CCD detector and to extract 2-D data with the aid of high-performance computers (25). A single-crystal X-ray diffractometer for small-molecule structure analysis with a cylindrical IP detector of size 466 × 256 mm2 and camera length 127.4 mm, the Rigaku R-AXIS Rapid, was introduced in 1997. This diffractometer can also be used to collect Debye– Scherrer patterns. A version of the R-AXIS Rapid specifically for general X-ray diffraction, the Rigaku D/Max Rapid (Fig. 12), was introduced in 1999 (26 ). It is designed for flat samples

Debye–Scherrer Diffractometer with Imaging Plate The well-known Debye–Scherrer camera for powder diffraction, based in principle on a parallel X-ray beam, has been utilized for making accurate measurements of lattice parameters. It can provide a 2-D diffraction pattern for a sample, which an ordinary XRD system cannot do. Its usefulness

Figure 11. Two reflectivity curves obtained by using the optics shown for two amorphous SiO2 thin films on a Si substrate of different thicknesses (16 ). Reflectivity from 1 down to the order of 10᎑8 is clearly observed.


611


Figure 12. D/MAX-Rapid, the latest model of the Debye–Scherrer diffractometer.

Figure 13. Typical diffraction pattern obtained by D/MAX-Rapid for a quartz powder sample of medium grain size.

to be set with a 45° tilt to the direct beam, so that one can collect wide-angle diffraction data (204° and 90° in 2θ in the horizontal and vertical directions, respectively) for a selected area of the sample. The 2-D Debye–Scherrer diffraction pattern data obtained by the new diffractometer for a sample of SiO2 powder is shown in Figure 13. The intensity is not smooth for a sample with coarse grain size along a Debye– Scherrer ring owing to an insufficient number of crystallites in the area exposed to the incident beam. However, if the intensity is integrated along the line it is possible to identify its structure on the basis of the ICDD database. This is one of the advantages of this diffractometer. There are several other advantages as well (26 ).

tion Inaugural Conference; Singapore, Nov 13–16, 1992; 15Q-11. Tajiri, K.; Yamada, N.; Orihara, H.; Takahashi, I.; Terauchi, H.; Harada, J.; Ishibashi, Y. J. Phys. Soc. Jpn. 1995, 64, 3157. Takahashi, I.; Ueda, K.; Tsukahara, Y.; Ichimiya, A.; Harada, J. J. Phys. 1998, 10, 4489. Parrish, W.; Hart, M.; Erickson, C. G.; Masciochi, S.; Huang, T. C. Adv. X-ray Anal. 1998, 29, 243. Barnea, Z.; Clapp, R.; Creagh, D. C.; Sabine, T. M.; Stevenson, A. W.; White, J. W.; Wilkins, S. W.; Harada, J.; Hashizume, H.; Kashihara, Y.; Sakata, M.; Osumi, K.; Zemb, T. Rev. Sci. Instrum. 1989, 60, 2537. Debye, P.; Scherrer, P. Physik. Z. 1916, 17, 277. Otsuki, N.; Wakaomi, I.; Katayama, T. In Proceedings of the 3rd International Conference on Nondestructive Testing; Tokyo and Osaka, March 1960; Dan-Pacific Press: Tokyo, 1961; pp 733–736. Nakazawa, H. X-ray Divergence-Angle Limitter; Japanese Patent 93-27840, June 1984. Omote, K.; Fujinawa, G. Adv. X-ray Chem. Anal. Jpn. 1999, 30, 165. Fujinawa, G.; Toraya, H.; Staudenmann, J. L. J. Appl. Crystallogr. 1999, 32, 1145. Fujinawa, G.; Sasaki, A. Adv. X-ray Chem. Anal. Jpn. 2000, 31, 11. Mitsunaga, T.; Saigo, M.; Fujinawa, G. Adv. X-ray Chem. Anal. Jpn. in press. Kobayashi, Y.; Hirashima, O. Rigaku-denki J. 1985, 16, 15; in Japanese. Omote, K. X-ray Spectrom. 1999, 28, 440. Hirano, T.; Matsuo, R.; Tomiyama, K.; Yazawa, I.; Wada, H.; Otaki, M.; Omote, K. In Proceedings of the 19th Annual Symposium on Photomask Technology; Monterey, CA, Sep 15–17, 1999; SPIE: Bellingham, WA, 1999; Vol. 3873, p 562. Ulyanenkov, A.; Takase, A.; Kuribayashi, M.; Ishida, K.; Ohtake, A.; Arai, K.; Harada, T.; Yasuda, T.; Yao, T.; Tomita, H.; Komiya, S. J. Appl. Phys. 1999, 85, 1520. Ulyanenkov, A.; Matsuo, R.; Omote, K.; Inaba, K.; Harada, J.; Ishino, M.; Nishi, M.; Yoday, O. J. Appl. Phys. 2000, 87, 7255. Amemiya, Y. Methods Enzymol. 1997, 276, 233–243. Dosho, A.; Ortega, R. Rigaku J. 1999, 16, 51.

9. 10. 11. 12.

13. 14.

Acknowledgments I thank K. Omote of the X-ray Research Laboratory and G. Fujinawa of the Application Laboratory of Rigaku Corporation for providing their valuable data and J. Ferrara of Rigaku/MSC, The Woodlands, for his kind critical reading of the manuscript. I also thank R. Ortega of Rigaku/MSC and A. Dosho of Rigaku Corporation for their helpful comments on the Debye–Scherrer diffractometer.

15. 16. 17. 18.

Literature Cited 1. Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. 2. Wilkins, S. W.; Varghese, J. N.; Lehman, M. S. Acta Crystallogr. A 1983, 39, 573. Sakata, M.; Sato, M. Acta Crystallogr. A 1990, 46, 263. 3. Kubo, K.; Kato, S. In Crystallography in Japan; Sakurai, T., Ed.; Crystallographic Society of Japan: Tokyo, 1988; Chapter 7. 4. Taylor, A. J. Sci. Instrum. 1949, 26, 225. 5. Fujimoto, I.; Chikawa, J. J. Crystallogr. Soc. Jpn. 1970, 12, 249; in Japanese. 6. Kato, N. In Proceedings of X-ray Diffraction Study; High Power X-ray Laboratory, Nagoya University, Ed.; Nagoya University: Nagoya, Japan, 1979; Vol. 1, p 3; in Japanese. 7. Harada, J. J. Crystallogr. Soc. Jpn. 1977, 19, 36; in Japanese. 8. Harada, J.; Takahashi, I.; Shimura, T.; Miyazaki, T.; Yamada, K.; Masuda, K. Presented at Asian Crystallographic Associa-

612

19. 20. 21. 22.

23.

24. 25. 26.


The Role of Development of the Rotating Anode X-ray Generator and

Recommend Documents