2)#- /d 7J> 0
FORMAL REPORT
GERHTR-T35- -
UNITED STATES—GERMAN HIGH TEMPERATURE REACTOR RESEARCH EXCHANGE PROGRAM
Original report number 082-RW______Title Influence of the Method of Measurement on the Optical Aniaotropy Factor OPTAF of Pyro- r.a.rhan
Author(si K. Koizlik et al. Originating Installation Kemforschungsanlage Julich. fiesellschaft mlt beachrankter Haftung Date of original report issuanoe. July IQjk ______Reporting period covered______
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KERNFORSCHUNGSANLAGE JULICH Gesellschaft mit beachrankter Haftttng
Institute of Reactor Materials
Influence of the Method of Measurement on the Optical Anisotropy Factor OPTAF of Pyrocarbon by K. Koizlik, K. TSuber, H. Nickel and H. Wasmund*
Jul - 1082 - RW July 197%
* E. Leitz GmbH, Wetzlar
Printed as manuscript Oh the Influence of the Hethod of Bber den Einfluss des Hessverfahrens auf den
Measurement on the Optical Anisotropy Factor Wert des optischen Anisotropiefaktors OPTAF OPTAF of Pyrocarbon von Pyrokohlenstoff by von
K. Koizlik K. Koizlik K. TSuber K. TSuber H. Nickel . H- Nickel H. Wasmund ' H. Wasmund '
ABSTRACT KURZFASSUNG
This study describes the development and installation of an auto Die vor1iegende Arbeit beschreibt die Entwicklung und Installation matic microscope photometer for the measurement of the optical eines automatischen Mikroskopphotometers zur Hessung des optischen anisotropy factor OPTAF on the pyrocarbon coatings of fuel parti Anisotropiefaktors OPTAF an den Pyrokohl enstof fhiil Is chichten von cles . After a short representation of the physical basis of this Brennstoffteilchen. Nach einer kurzen Einfiihrung in die physika 1 i- procedure the new microscope photometer is introduced. First meas schen Grundlagen des MeBverfahrens wird das MeBgerSt vorgestelIt. urements for the adaptation of the new instrument of the so far Erste Messungen zur Anpassung des neuen an das bisher verwendete used microscope photometer are discussed. By these measurements GerSt werden diskutiert. Anhand dieser Messungen wird der Einflufi the influence of the instrument on the value of OPTAF in the case des Mikroskopphotometers bei bestimmten MeBablSufen auf den OPTAF of some special ways of measuring are explained and the appearance erklSrt und die Erscheinung einer scheinbaren Anisotropic begrundet. of an apparent anisotropy is interpreted.
+ ) + ^ Fa. £. Leitz GmbH» Wetzlar Fa. E. Leitz GmbH, Wetzlar Table of Contents
Page 1 Introduction
1. Introduction 2 The course of development of coated fuel particles for use in gas-cooled high temperature 2. Physical Principles of the Optical reactors (Kefs. 1 and 2), which is not yet Anisotropy Determination 5 complete even today, has necessitated all the time new and more accurate characterization 3* Microscope Photometer for Manual procedures. These procedures, which have Measurement 11 been devised in parallel with the continuous improvement in the production processes, Microscope Photometer for Automatic are aimed primarily at as complete as possible Measurement 17 a characterization of the coating material 4.1 Microscope Photometer 17 of the coated fuel particles, the pyrocarbon. Among the few, initially measurable material 4.2 Electronic Control 21 parameters particular importance was attached very early on to the anisotropy of the crystal 5* Calibration of the Automatic Measuring lographic orientation of the pyrocarbon in Microscope 24 regard to the use of the latter in the reactor 5.1 Apparent Orientation Anisotropy 26 (Ref. 3). In consequence of this the measurement of the orientation anisotropy became an 5.2 Apparent Orientation Anisotropy in essential part of the process of fuel particle Pyrocarbon Samples 31 characterization. 5*3 Influence of the Apparent Orientation Anisotropy on Measurement with theMPV XI 36 To permit investigations to be carried out 6. Summary 38 directly on the particle coatings, a new measuring technique had to be developed, since the classical X-ray method of structural determination was found at the time to be unsuitable, in particular as a result of the spherical symmetry of the orientation anisotropy of the particle coatings. Although an X-ray method (Ref. 4) is today available for this purpose, it is not as yet of far- reaching importance in view of the relatively complicated evaluation of the primary measurements.
It has long been known, on the other hand, that pyrocarbon possessed the property, like graphite, of exhibiting the phenomenon of bireflection (Refs. 5 and 6). This property
1 2 - is manifested in qualitative terms by the appearance on suitably ground and polished pyrocarbon surfaces, exposed to vertical illumination with linearly polarized light using crossed polarizers, of a so-called "Maltese cross" (Ref, 7)• Fig, 1 shows the Maltese cross in diagrammatic form and on a particle micrograph.
The Maltese cross becomes progressively clearer, the closer the alignment of the c-axes of the pyrocarbon crystallites parallel to the preferred direction, in other words the higher the value of the orientation anisotropy (Ref, 8) of the material. Although therefore there is a qualitative correlation between the intensity of the Maltese cross and the orientation anisotropy of pyrocarbon, no dear, quantitative relation between the Fig. 1 i Maltese cross on an equatorial section of a two can be established (Ref, 9)• coated particle, on the left in a photo graph and on the right in diagrammatic form However, the attempt to make use of miorophotometry (Refs, 10 and 11), which has been used for several decades, *.g., in mineralogy, as a key importance that it did a few years ago, new method of measurement of the orientation but is merely regarded as one, albeit important anisotropy proved successful. characteristic among other decisive material properties.
This method, which is also based on the bireflection phenomenon, permits a quantitative In the last two years, however, a microscope determination to be made of the orientation photometer with the corresponding equipment anisotropy of pyrocarbon with a high local has been developed for a largely automatic resolution. The use of microphotometry measurement of the orientation anisotropy, which represents a considerable improvement in this field began after 1965 and led to the development of various technically dif on the original laboratory instrument. The reason for this is that orientation anisotropy ferent instruments (Refs, 12-1%). The development has established itself as a characterization of the method may today be regarded as in parameter which is simple to measure but principle complete (Refs. 15 and 16). The nevertheless, of great statement value. improved knowledge of the material pyrocarbon, This means that the number of samples for its material parameters and its irradiation analysis is correspondingly high, in particular behavior in the reactor have meant that in the case of highly radioactive coated orientation anisotropy no longer has the fuel particles from irradiation experiments,
3 - % _ In. addition orientation anisotropy is gaining a function of the angle between the direction new importance in the study of the separation of polarization and the c-axis of the crystal : mechanism of pyrocarbon from the gas phase. if the direction of polarization and the This frequently involves in particular the c-axis are parallel, the reflection capacity need for systematic serial measurements, reaches its minimum value at 8 %, whereas which involve t&» use of large* numbers of if they are perpendicular, the maximum personnely unless carried out by means of value will be reached at 28 %. No variation an automated procedure. in the reflection capacity occurs in the case of reflection in the (a,c) and (b,c) planes, since the a and b direction of The present paper is a report on the new the graphite monocrystal are optically method of measurement. Comparative measurements identical. Fig. 2 shows in diagrammatic are described between the old, physically form the bireflection effect. simple instrument and the new photometer, which enable the correlation which was previously obtained (Ref. 1?) between X-ray measurements In the case of polycrystalline pyrocarbon, and optical determinations of the orientation which is believed to be built up only from anisotropy, to be applied to the new method small monocrystals of the graphite type, of measurement. The paper is concluded by the optical anisotropy is a function of a discussion of the results which have led the anisotropy of the distribution of the to a considerably improved understanding crystallographic c-axes of the individual of the optical determination of the orientation crystallites. The following function has anisotropy and which may perhaps enable already been derived for this relation a comparison to be made with the various (Ref. 20) : types of instruments which have in the meantime been introduced in other laboratories. OPTAF = A + *? fi + *? G • BAF , 2 .1? G + BAF ' 2. Physical Principles of the Optical Anisotropy Determination In this formula OPTAF denotes the optical anisotropy factor, BAF (Ref. 21) the - The physical principles underlying the optical e.g. measurable by X-ray analysis - crystal determination of orientation anisotropy lographic orientation factor and 7|s the have already been reported in detail (Refs. l8 and 19). In consequence only the most important ratio of the two coefficients of reflection of these need be repeated here. 28 % and 8 % of graphite monocrystals.
The basis of the process is the property If noncrystalline carbon is present in of graphite, and in consequence also Of the the reflecting surface of the pyrocarbon, pyrocarbon, of being optically uniaxial it has to be borne in mind that the latter and therefore hirefleeting. In the case is optically isotropic, so that equation of an ideal graphite monocrystal this means (1) is modified to equation (2) : that its reflection capacity with vertical incidence of linearly polarized light is
5 6 Fig. 3 shows in graph form the relation between the value of the aoncrystalline carbon fraction in the pyrocarbon and the quantity r) . •axis I fraction
•axis
60 - Amorphous
20 -
Fig, a : Diagrammatic representation, of bireflec tion on a graphite monocrystal (without Surface fraction of amorphous carbon as a taking into account a phase jump) function of the reflection ratio
+ ? PyC PyC ~ MF If the surface fraction of crystalline and n amorphous substance is designated tr, the 2 • ? PyC + (2) expression for t^p^c is constructed as follows :
r„ + €T ^PyC ^a^£es account the existence of & krist. (4) ^ PyC e ; noncrystalline carbon and is based on the value In this formula r denotes the reflection ^ PyC ~ ^ G = 8 - 3,5 *3) SIR capacity of the isotropically reflecting noncrystalline carbon and r„ and r^ the with an increasing noncrystalline carbon values for the reflection capacity of a fraction in relation to the value « 1. purely crystalline multlcrystal having a preferred orientation of the crystallites.
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The correctness of this assumption is indicated of the method of measurement, here again also by recent OPTAF measurements on statically full details will not be given since the deposited pyrocarbon. These reveal for technical equipment has already been described the first time an anomaly in the optical (Ref. 30). Only a few of the more important anisotropy (Ref. 25) in that the a and fo aspects will be listed once again. directions of graphite-like crystals are no longer optically equivalent. This can at the present time only be interpreted The sample to be investigated is ground by means of the pyroaggregate hypothesis. in such a way that the section plane contains the possible preferred orientation. In the case of coated particles, for the However, no quantitative modification of characterization of which the process was the mathematical description of the OPTAF originally and mainly developed, the section measurement, applying these new results plane is located in the equatorial plane and concepts, has yet been made. There of the particles. The sample is secured is no doubt that the corrected interpretation to the rotating stage of the microscope of the OPTAF fine profiles in this sense photometer by means of a grinding stage can make in conjunction with cold oxidation and after an accurate adjustment is illuminated a further contribution to the understanding with linearly polarized light having a of pyrocarbon and to the so-called 2-component perpendicular incidence on the sample. model of this material (Refs. 26 and 27). The light reflected from the sample surface passes through the objective and measuring eyepiece and is measured by the photomultiplier 3. Microscope Photometer for Manual Measurement without any reflection by the reflector or prisms. The development of the process for the optical determination of the crystallographic orientation anisotropy at the KFA and its use in serial The simple geometry of the microscope photometer measurements, which have permitted the MPV I, coupled with the method of a fixed construction of a first model for the inter polarizer and a rotated sample, ensures pretation of structural variation processes that the very simple and in consequence in the pyrocarbon, have been carried out rough formula for the theoretical description with a production model of microscope photometer of the method of measurement exhibits very from Leitz/Wetzlar. In parallel with this good agreement with the experimental results a process has been developed in Austria without further corrections. From this which, although technically different, point of view there would have been no is based on the same physical principles need to develop a new instrument, especially (Ref. 28). An American method (Ref. 29), since the MPV I is still used at the present however, which has been developed for the time for new basic measurements in view same purpose, differs from the KFA technique of its simple geometry. There were, however, in the underlying physical principles also. two other decisive reasons for a further, technical development of the process for routine measurements. As in the case of the theoretical principles
11 12 The first reason, related to a fundamental axis of the rotating stage, must on the difficulty which led in the case of measure one hand run through the center of the ments with the MPV I to a by no means circular field of measurement, while on negligible error. The OPTAF determination the other hand it must coincide with the is carried out by adjusting initially the optical axis of the microscope photometer. direction of polarization of the light, This requirement can in practice not be which falls perpendicularly on the sample t met in spite of careful adjustment mainly perpendicular to the possible preferred due to the height of the instrument resulting orientation of the crystallites in the from its advantageous simple geometry. pyrocarbon - the intensity of the reflected A further factor is that with rectangular light is measured in this position. In fields of measurement, which can on accasion the case of spherical coated fuel particles be of advantage, this method of sample the preferred direction is that of the rotation cannot in principle be used. radii of the sphere. After the direction of polarization and the preferred orientation have been rotated by 90® in relation to If therefore it is necessary to eliminate each other, the direction of polarization the measurement errors caused by movement and the preferred orientation are parallel• of the field of measurement over the sample The intensity of the reflected light is surface as a result of rotation of the sample, measured in this position also. The ratio which become progressively greater the of the two intensities is the required optical smaller the field of measurement, and to anisotropy factor, which can then be converted permit the use of any desired measurement by means of the equation given above or field geometries, it is necessary to change by means of a correlation curve into the over to a method of measurement in which BAF which describes the anisotropy of the the sample is stationary and the direction crystal orientation. of the polarizer is rotated.
Since the rotation of the direction of The second reason for the introduction of polarization and the preferred orientation a new microscope photometer emerged during by 90® only has to be made in relation the first systematic study of the coatings to each other, it is in principle Immaterial of coated fuel particles. whether the polarizer is rotated with a fixed sample or the sample rotated with a fixed polarizer. Since in the MPV X In the initial years of use of the method the polarizer cannot be reproduceably rotated of measurement, the anisotropy measurements without considerable labor, the sample were carried out with a circular field is rotated for execution of the measurements. of measurement having a diameter of 15-20 p.m, This in fact is the heart of the problem. less often 10 pm. This meant that the It is immediately obvious that the precision OPTAF represented a mean value of a relatively of the field of measurement on the sample large surface area. With layer thicknesses must not vary as a result of the rotation. of a few 10 pm, such as are normal with This means that the axis of rotation, round coated fuel particles, only 2 to 3 measurements which the sample is rotated, i.e., the in a radial direction could be carried
13 14 out with such large fields of measurement. This resulted in two of the presentations of the orientation anisotropy value most commonly used even today : in the first place the indication of the moan anisotropy factor for a complete particle batch, possibly supplemented by the maximum and minimum values found and also the standard deviation, and secondly the plotting in graph form of the so-called layer thickness/anisotropy curve. Fig. 5*
This curve relates the layer thickness at azxy point on a coated fuel particle to the mean value of the orientation anisotropy, formed from individual measurements on a number of particles of the same layer thickness. The layer thickness/anisotropy curve is particularly important in non- spherical particle batches•
It was discovered relatively soon, however, that pyrocarbon layers frequently exhibit marked property gradients. Even where no Fig. 5 : Layer thickness/ana.sotropy curve of a substantial gradients are present, we find particle variety. As is normal, the on occasion substantial the local variations coating has higher anisotropy values in the property values to which reference at points of small layer thickness than has been made above. Fig. 6 shows such at points of large layer thickness• a particle coating (Ref* 31)» examined both by the optical method of anisotropy examination and also by cold oxidation time with x 450 magnification about 4 pm and determination of the density. Since and with x 1200 magnification about 1*5 pm* these variations are of importance both for the irradiation behavior and. also for fundamental investigations on the deposition Two properties of the MPV I render these mechanism, for some time now it has been measurements extremely difficult. In the customary in addition to the above measure first place the very small dimensions of ment s to determine the anisotropy values the field of measurement necessitate an as a function of the distance of the site extremely careful adjustment and continual of measurement from the particle kernel. readjustment of the microscope• The reason In this case the minimum possible field for this, the rotation of the sample with of measurement is used, at the present a fixed polarizer, has already been discussed above• In addition for a series of measurements
15 16 along a single radius with a mean layer it is no longer necessary to rotate the thickness of for example 50 pm 10 to 20 individual measurements are necessary. sample during the measurement. This eliminates
The above two conditions make this type the possibility of error discussed above, of anisotropy measurement extremely laborious. namely that of the field of measurement "wandering" over the sample surface during rotation as a result of a not completely accurate adjustment of the photometer. It was an obvious solution, therefore, to introduce a method of measurement for routine or series measurements of the orientation anisotropy, which retained the advantages For this purpose two polarizers are arranged of the optical procedure described above in a two-way movement carriage in such a but eliminated where possible the disadvantages. way that by means of an electromagnet either one or other polarizer can be brought into the ray path of the illumination apparatus. 4. Microscope Photometer for Automatic Measurement The two polarizers are positioned with their directions of polarization mutually perpendicular For the further development of the process and perpendicular to the axis of the light for the optical determination of the orientation bundle. The polarizer carriage is shown anisotropy use was made of the production in diagrammatic form in Fig. 7. model of microscope photometer MPV IX from Leitz/Wetzlar. Nevertheless it had to be substantially modified in order to perform the required task. These modifications were planned jointly with Leitz and carried out by Leitz, so that an instrument Polarizer I Polarizer II is available today capable of meeting all requirements. Lifting
4.1 Microscope Photometer Position 0 Position 90 As stated above, the basic instrument of the new microscope photometer was the MPV IX Control (Ref. 32) supplied by Leitz/Wetzlar. In addition to a number of production improvements, this microscope photometer had quite decisive advantages over the previous instrument which rendered it particularly suitable Fig. 7 : Diagrammatic representation of the for use as a routine characterization polarizer two-way carriage in the new instrument. microscope photometer
A variation of the direction of polarization However, it is just this very advantageous means that in the new OPTAF measuring instrument displacement of the direction of polarization
17 - 18 - instead of the sample that gives rise to a fundamentally new problem in the execution of the OPTAF measurements. On its path from the light source to the polarizer, the light is deflected by mirrors. As a result of the property of reflecting surfaces of reflecting light which is not perpendicularly incident at different intensities, depending on the position of the direction of oscillation of the E-vector of the light in relation to the plane of incidence, the light is partially polarized even before reaching the polarizer. This phenomenon is generally known in optics as polarization by reflection (Kef. 33), Fig. 8. UJ tU
This phenomenon leads in conjunction with the polarizers to the fact that in the case of a rotation of the polarizer instead of a rotation of the sample an apparent _8 : Occurrence of polarized light by anisotropy occurs, which immediately makes reflection at an angle other than 90* an isotropic metal reflector appear anisotropic. (Ref. 34) These conditions are examined in greater detail below. orientation anisotropy in statically deposited pyrocarbon, since according to initial An addition to the MPV II facilitates in results it appears certain that the orientation a second important respect the series OPTAF anisotropy and its systematic study can measurements and the determination of OPTAF provide importeint information on the deposition fine profiles. This consists of the re mechanism of pyrocarbon. placement of the originally manually adjustable cross-stage, which was secured to the microscope rotating stage and which carried in turn Although the scanning stage available at the magnetic section stage, by a scanning the present time has a relatively large stage. The scanning stage is displaced minimal step width of 5 pm in relation in the x and y directions by electric motors. to a fine scanning stage, the corresponding It is possible with this stage, which is overall transport path of the stage is operated by a central control unit, to 10 mm, a factor which renders it particularly record not only automatic line-scans of suitable for the investigation of coated the orientation anisotropy, but also surface fuel particles and statically deposited scans. It is this latter possibility which pyrocarbon. may in the near future become important in the examination of variations in the In regard to the remaining components the
19 20 microscope photometer is in principle a a suitable control of the course of the
production model. Fig. 9 shows a photograph measurement. This control unit, the design of the modified MPV XI with the corresponding of which was worked out in conjunction i supply and control unit. with the firm Leitz, has been developed and constructed to the final stage of readiness by Leitz.
The block diagram in Fig. 10 shows the connection of the various control units and the flowsheet in Fig. 11 shows the logical course of a measurement in the case of the pointwise scanning of a surface (surface scan).
Apart from the peripheral drive mechanisms for the switching prism, the polarizer two- way carriage and the servomotors of the scanning stage, the "control center" is formed by the electronic control unit which interrelates chronologically and logically the various individual stages in a measurement or a surface scan. No details of the actual electronics can be given here. Only one characteristic of the control unit will nevertheless be mentioned, which permits without the use of a process control computer a largely flexible configuration of the sequence of individual measurements : the external programming by a crossbar distributor. Fig. 12.
Fig. 9 : Microscope photometer MPV XI with structural modifications, supply and The circuit diagram of the crossbar distributor. control unit for automatic OPTAF Fig. 13a, shows first of all the inter measurements on pyrocarbon connection of the individual functions for the measurement of an individual measuring point. In this diagram the 4.2 Electronic Control function "Magnet aus" (magnet off) denotes that the polarizer two-way carriage is Apart from the modifications to the microscope in the rest position. In this position photometer itself, the most important pre during the function "Messen" (measuring) requisite for a largely automated determination the intensity of the light reflected from of the optical anisotropy factor OPTAF is the sample is measured. An important point
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n-n 1 a a Cb o o a c a 3 <■+ g* 3 Qi (T> tr c (W Abb, 10: Blockschaltbild der verschiedenen Einheiten des fur automatische MPV II OPTAF-Messung umgebauten MPV II brought into the ray path. On conclusion of the measurement of the light intensity with this polarizer setting the scanning stage moves to the next measuring position in the x direction and completes the course of the function. The two intensity values issued must now be divided by each other and, as will be shown below, corrected. This concludes the determination of the optical anisotropy at this single measurement point on the pyrocarbon sample.
The circuit of a surface scan is shown in the next circuit diagram. Fig. 13b. It is possible in this case to preselect any desired number of steps in the x direction (cycle II) and also any desired number of steps in the y direction, i.e., the number of lines of the surface scan (cycle I). The step dimension can also be freely selected as a complete multiple of the smallest step dimension of 5 jum, and in fact for the x and y directions independently.
Fig. 12 : Crossbar distributor and various functions of the central control The first measuring point and the orientation electronics of the sample on the scanning stage must of course be set manually. The calculation of the OPTAF and also of the BAF from is that it is possible by means of the the determined OPTAF can be carried out time setting to select a sufficiently long on-line by means of a process control duration of measurement (delay), so that computer or in the case of data output any vibrations occurring in the microscope on punched tape in any desired computer. photometer due to the switching magnets In the case of individual measurements of the polarizer carriage or the servomotors and in special investigations the automatic of the scanning stage have time to die away. equipment can of course be disconnected The measuring signal can be issued either and the measurement carried out manually. in analog or digital form. After the lapse of the timing interval set, e.g., 2 sec, the control unit switches to the function 5* Calibration of the Automatic Measuring "Magnet ein" (magnet on), which causes Microscope a sufficient displacement of the two-way carriage for the second polarizer to be To test the new equipment and to establish whether the new OPTAF values could be
23 - 24 converted by suitable corrections into 5.1 Apparent Orientation Anisotropy the old anisotropy factors, samples of different materials were examined loth As described in a previous section, the with the old and also with the new instruments. light emitted in unpolarized form from This was intended primarily to clarify the light source is deflected by mirrors the question of the effect of the pre in each case by 90° on its path to the polarization of the light referred to polarizer. This means that the angle above and its influence on the orientation of incidence of the light onto the mirror anisotropy. The aim of these measurements is %5®. As a result of this reflection was to relate the new OPTAF measuring the light is partially polarized. In instrument to the old measuring procedure quantitative terms the polarization due as the "standard" procedure. Fig. 14 to reflection is given by the Fresnel shows the two measuring instruments with formulae which for substances with a high the corresponding supply, control and indicator absorption, i.e., for the metals used instruments• No process control computer as a mirror coating, have the following is at the present time connected to them. form :
2 2 (n1-n..ocs y ) +(n„. 6 .cosf) Rp = ---- L-J.------_ (n^+r^.cos y ) +(n2-ff • cos y)
(ly-i^.cos y )2+(n2. S’ )2 (n2+nrcos y )2+(n2. S )2
R and R denote the coefficients of reflection P » of the metal surface for the case that the polarizer direction is parallel or perpendicular to the plane of incidence. n and n denote the refraction indices of the media in front of and behind the reflecting boundary surface, Y denotes the angle of incidence of the light and
Rp 7 Rs - 0,803. _ 50 % J2 " 40 % = 1,25
A single reflection at 45® at one such mirror will thus cause about 20 % pre This intensity difference of the light polarization of the light. Very similar falling on a sample produces an apparent values are obtained for the mean optical orientation anisotropy even in the case constants of high grade steel. of crystallographic isotropic material.
In MPV I the light of the measuring illumination On this phenomenon is superimposed a second in front of the polarizer is deflected effect. The theory of optical anisotropy by 90® by a single mirror, Fig. 15. The assumes the perpendicular incidence of mirror is positioned vertically, so that the light on the sample surface. This the light is prepolarized in a vertical condition is by no means met in the two direction. With a fixed polarizer and microscope photometers used, as is shown rotation of the sample the prepolarization by Fig. 16. has no effect on the measurement result, irrespective of whether the preferred 27 General illumination Corresponding measurements on MPV I with Source of Mirror polished metals as samples yielded the measurement illumina values in Table 1 : tion Illumination two-way car riage OPT A F 3 Polarizer Material Polarizer fix Sample fixed ed Sample Polarizer rotated rotated Aluminium 1,00 1,43 Opaque illuminator Copper 1,00 1,45 Steel 1,00 1,45 Table 1 EiJL>~Al Ray path in the MPV I from the source of In spite of the highly simplifying assumptions measurement illumination to the opaque in the theoretical considerations on the illuminator apparent anisotropy phenomenon, the agreement between estimate and measurement is surprisingly good. The reason why the measured values are lower than the calculated values is that the assumption of the incidence of light on the sample at an angle of 45® is too highly simplified. This results in the second component of the apparent anisotropy of 0,733 1,25 0,588 being too large. This brought out very clearly in a measurement on an aluminium mirror with general illumination instead of the measurement illumination. Since the general illumination follows a rectilinear path in the illumination two-way carriage, Fig. 16 : Diagrammatic representation of the in the case of rotation of the polarizer reflection of linearly polarized light only this second component will contribute on the sample surface to the apparent anisotropy. As is to be expected, the measurement recorded in this case is not the 29 30 OPTAF (apparent calculated) 1.25 OPTAF (actual) =1.44 but an With a fixed sample and rotation of the polarizer, on the other hand, the recorded OPTAF (apparent, measured) = 1.15. measurement is In consequence the value of the apparent OPTAF (rotation of polarizer) = 2.39 anisotropy occurring with the measurement illumination in the case of rotation of On the basis of theoretical considerations the polarizer and a fixed sample the measured value should be : OPTAF (rotation of polarizer, calculated) OPTAF (apparent, overall) = 1.25 x 1.15 = 1.45 1.44 x 1.45 = 2.09. is in excellent agreement with the measurements The difference between the calculated and in question. If in place of the Berek measured values need only be taken into prism in the opaque reflector a mirror account here by an additional correction is used, the apparent anisotropy has to factor : increase once again by a factor of 1.25 : 2.39 1.14. OPTAF (apparent, opaque reflector) = 1.45 x 2.09 1.25 » 1.8i. The apparent anisotropy in pyrocarbon thus becomes The corresponding measurement with an aluminium reflector gives in good agreement OPTAF (apparent, pyrocarbon) = 1.66 with the above a value of 1,70. by comparison with the value in metals 5.2 Apparent Orientation Anisotropy in Pyrocarbon OPTAF (apparent, metals) = 1.45. Samples More important than this variation in The discussed phenomenon of apparent orientation the value of the apparent anisotropy is anisotropy in the MPV I naturally affects a fundamental difficulty in the anisotropy also the measurements on pyrocarbon samples, measurement of pyrocarbon samples with when operating with a fixed sample and rotation of the polarizer due to the fact rotation of the polarizer. The value that the anisotropy value depends on the of 1.45 for the apparent OPTAF has in this position of the preferred direction in case to be taken into account, since the the field of measurement. measurements are always carried out with the Berek prism. This is confirmed, for With a fixed polarizer the OPTAF can be example, by the anisotropy determination defined in simple fashion as follows : on graphite rods coated with pyrocarfcon Reflected intensity with a polarizer under static conditions. Measurement OPTAF - direction preferred direction with a fixed polarizer yields the actual Reflected intensity with a polarizer optical anisotropy of direction " preferred direction 31 32 This definition is still only conditionally if it allows little light to pass, a measured valid in measurements with fixed samples. value is obtained which, after division If in fact the preferred orientation of by the correction factor described above the pyrocarbon and the field of measurement K = 1.5i gives the true OPTAF. If on is such that with parallel positioning the other hand the preferred orientation of the preferred orientation and the polarizer is directed in such a way that with the direction the polarizer is perpendicular preferred orientation and polarizer direction to the direction of prepolarization, i«e., perpendicular to each other, i.e., in the position of amplified reflection of the pyrocarbon, the polarizer is set on low Polarizer ' Preferred transmission, the anisotropy will be in parallel orientation part eliminated. A too low OPTAF is then apparently recorded. Since the influence of the anisotropy depends in this case Light of Strong on the OPTAF itself, a second correction Prepolarization high intensity reflection factor is obtained, which is no longer constant but increases with a rise in in, the OPTAF• These conditions are shown in diagrammatic form in Fig. 17* Light of Weak low intensity reflection This effect may result in an anisotropic 'Polarizer ’Preferred sample appearing apparently isotropic, perpendicular orientation so that under certain circumstances it may even have an apparent anisotropy of less than 1. Polarizer Preferred OPTAF, r otation of OPTAF, ro tation of polarize r without polarizer with influenc e of influence of Sample OPTAF ani sotropy anisotr ^ ppy 10 Measured Correction Measured Correction value Ki 1/Ki value K2 1/K2 - i ! 1 1.014 1.520 1.49 0.67 1.520 1.49 0.67 1,00 2 1. 171 1.815 1.55 0.64 1.312 1.12 0.89 1.38 3 1.202 1.828 1.52 0.66 1.257 1.05 0.95 1.45 4 1.538 2.308 1.50 0.66 1.000 0.65 1.54 2.31 5 1.440 2.174 1.51 0.66 1.088 0.76 1.32 1.99 Fig. 17 : Influence of the direction of the prefer red orientation of the pyrocarbon sample Table 2 in the field of measurement on the measured OPTAF value 33 correction factor anisotropy polarizer Fig. value on has of measurements polarizer. diagram. the accordance of anisotropy The conversion confirmed the rotation the measurements a case 18 with high aniaotropy. sample : that with rotationof of ation conversion Dependence The with direction, this is rotation transmission, of in of Table . the found influence the is the the The the influence perpendicular of expectation, orientation anisotropy carried preferred OPTAF 2 case the polarizer correction This to of contains of of where 35 polarizer be the of the the of value is Fig. out of very rotation the the the sample OPTAF orientation correction shown the a on constant to polarizer 17 in factor much orientation sample correction to few upper the orientation the into the values in the is, of corresponding MPV dependent OPTAF Fig. for case OPTAF on in for the factors I with the 18. factor OPTAF values value rot for Sample Al Steel Copper which found the i.e. to always direction values factor care out of the correction the The considerations Table anisotropy by in illumination light in The described The 5.3 be the OPTAF very two this MPV MPV following in agreement on two conditions Influence_of_the has in passing always to able 3 has practice orientation is MPV mirrors 1.45 Measurement 1.45 1.43 the II, II, respect instruments similar be to , (apparent) of very in is to the factor by X best x be in in applies the to Section use correction always direction. particular of a from . which same taken good. can occurring only factor the in fashion suited sample the This K the be anisotropy g that with the direction OPTAF . polarizer only with - located measurements with to expected Apparent 5.1, applied constant All increases 36 This of source - for in ensure x in with the factor 1.81 1.81 1.79 with differ routine - a For also other 1.25 1.25, are ( the apparent the - pyrocarbon means fixed MPV horizontally the this and, itself directly and is of Orientation the found correction field that the MPV - has relevant Table the in SI measured MPV measurement deflected measurements sample, are that MPV case in influence principle II ) been of the apparent to on OPTAF II. order carried the 3 I, with to ; , reoccur measurement preferred the : MPV , 1.73 1.73 1.79 Anisotropy (apparent) IX , OPTAF (true) = OPTAF (MPV II, measured) x 0.64 Sample Kj/K (MPV I) K /K (MPV II Table 4 below contains individual measurements which confirm the low degree of scattering 1 1.00 1.00 of the correction factor values. 2 1.38 1.38 3 1.45 1.45 We shall not discuss further here why the correction factor on the MPV XI is not 4 2.31 2.25 5 1,99 as in the case of the metal samples higher 1.83 than the value for MPV I by a factor of 1.25, but only by a factor of 1.05. The important point is that the optical conditions have Table 5 an equivalent influence on the OPTAF measure ments in the MPV I and MPV II. This is reflected inter alia by the fact the values of the In all OPTAF measurements in the future, Kf/Kg ratio are identical in the MPV I and therefore, the value obtained on the MPV II will be multiplied by 1/K^ = 0.64 and converted MPV II instruments, i.e. , that in the case of pyrocarbon both and also are displaced in this way into the true optical anisotropy factor. A further important point appears by the same factor in the MPV II, Table 5. to be that l/K^ in the case of very low values of the orientation anisotropy, i.e., with OPTAF 2£L 1.01, has to be raised to a value of l/K^ = 0.70 for reasons of error Sample OPTAF (MPV I) OPTAF (MPV II) Correction l/Ki factor statistics, in order to avoid too frequent recording of OPTAF values which are apparently 1 1.004 1.576 1.570 0.637 <;i.oo. 2 1.038 1.621 1.563 0 s640 3 1.041 1.647 1.583 0.632 6. Summary 4 1.065 1.672 1.570 0.637 After irradiation experiments had demonstrated 5 1.138 1.754 1.542 0.649 the importance of the material property 6 1.177 1.863 1.583 0.632 "orientation anisotropy" of the pyrocarbon coatings of coated fuel particles for the 7 1.488 2.320 1.561 0.641 irradiation behavior of the particles, an optical method of measurement was developed in the Institute of Reactor Materials of Table 4 the Julich Nuclear Research Center for determination of the orientation anisotropy. This method, which enables the anisotropy of the crystallographic orientation to be 37 - 38 - measured with a high local resolution by means of a microscope photometeri has been On the basis of the first routine measurements directly correlated with the structure of with the MPV XX carried out at the Institute the pyrocarbon both empirically and of Reactor Materials, the correction instructions theoretically. on p. 38 of the KFA Report No, Jul-1082-HW should in the future be handled as follows ; The optical method of measurement of the OPTAF (MPV II, measured) ^ 1,563 — X o, 64 orientation anisotropy has in the meantime 1,563 > " > 1,540 X 0,65 proved its importance for the characterization tt of pyrocarbon in numerous systematic studies. 1,560 3? 1,515 X 0,66 H It is today an essential characterization 1,515 > 3<- 1,495 —^ X 0,67 procedure in pre- and postirradiation studies t! l,47o X o, 68 of particle irradiation experiments. It 1,495 > > (! has in addition contributed extremely important 1,47o » 1,45o — X c , 69 new knowledge in the fundamental study of 1,4 5o > " X 0,7o the deposition mechanism of pyrocarbon and on the structure of this material. There is therefore no question of this method of the determination of the orientation anisotropy disappearing from the category of important characterization procedures for pyrocarbon. With the introduction and calibration of a new and automatic microscope photometer, the Institute of Reactor Materials now has available two efficient instruments which are extremely well suited both for individual f.SfO and serial studies. The most important result of the studies described here, apart from the adaptation of the new OPTAF measuring instrument to the old instrument, the fact that with the information now available on the parameters, which form the basis of the method of measurement itself, the technically different OPTAF methods of measurement can now be mutually adapted by computer calculations. 39 - 40 o tH CD CO CO C" e- uD k X cO tn X X LO d- V X X X CO X CO OJ X X k CN H K K jt X j ft) E vorw. ruckw. 0 ’ vorw. ruckw. <0 > riickw. zu C •H c •H s CO W < D D i-H W CJ 1,40 J------— w OO £ £ u >b JO CD to Tisch Tisch Tisch Gittermonochromator Gittermonochromator h Magnet 0 50 100 150 jjm O E- VerschluB < ac U Q u lu o X x; :c a* 3 > ^5 X >3 Abb. 6: Lokale Schwankungen von Materialeigenschaften in einer Pyrokohlen- Abb. 13: Prograramablauf am Kreuzschienenverteiler stoffhullschicht beschichteter Brennstoffteilchen als Funktion des 13 a: fur die Messung eines Einzelpunktes Abstandes des MeBortes vom inneren Schichtrand 13 b: fur die Messung eines Flachenscans ■Literatur ' - 22. ) H. 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