This Lecture Is Part Of The Basic XRD Course.
Sample Preparation: IntroductionThis lecture is part of the Basic XRD Course.Basic XRD Course1
Sample Preparation: IntroductionBasic XRD Course2
Sample Preparation: IntroductionA measurement on a single sample should normally give the same result as anaverage measurement on a number of the same samples. Due to inadequatesample preparation, a significant variation in the results can occur. Therefore,sample preparation plays an important role.Note:It is nearly impossible to correct for sample preparation errors afterwards.Basic XRD Course3
Sample Preparation: IntroductionIn the next slides we will discuss: Sample displacement error Flat sample error Other positional errorsBasic XRD Course4
Sample Preparation: IntroductionThe peak shift effect due to specimen displacement is maximal for low anglesand zero at 180o 2θ.An indication of the specimen displacement effect: 2θ 0.06o at 2θ 30o for100 µm displacement.Basic XRD Course5
Sample Preparation: IntroductionThis solution requires:a. Fix sample (surface) at proper height in sample holder this is the responsibility of the userb. Position the sample holder properly in the instrument (in sample stage) included in design of instrumentA height positioning accuracy of 10 µm will be sufficient.Basic XRD Course6
Focusing Optics: Powder DiffractometersSample stages are used to allow the sample to be easily placed in the X-raybeam at the correct height and with the beam striking the middle of thesample.It is important when preparing and mounting the sample to ensure that it ispositioned at the right height. Any displacement away from this height willresult in a shift in the position of the peaks.If the sample is positioned so that it is too low then the peaks will beobserved at a lower angle than they should be.The peak shift is not the same for every peak. Low angle reflections areshifted more than high angle peaks.The change in peak position ( 2θ) is related to the sample height error (s),the instrument radius (R) and the diffraction angle θ by the formula:At a peak position of 30 º2θ a sample height error of 100 micron results in apeak shift of 0.06 º2θ.Basic XRD Course7
Sample Preparation: IntroductionThe peak shift effect due to the flat sample error is maximal at 90o 2θ andzero for 0 and 180o 2θ.An indication of the flat sample error effect: 2θ -0.02o at 90o 2θ.Basic XRD Course8
Sample Preparation: IntroductionNote:The radius of the focusing circle changes with the diffraction angle.To minimize the flat sample error one can reduce/minimize the irradiatedlength on the sample. This will cost intensity and hence a longermeasurement time will be needed.Basic XRD Course9
Sample Preparation: Introduction α1-2 α1 shift due to α2 contributionThe effect of peak shift due to overlap of α1 and α2 peak is maximal forintermediate angles.Solution: Apply profile fitting or α2-stripping. AD axial divergenceAt low diffraction angles, the peaks are asymmetric at the low angle side. Thepeak position shifts to lower angles.Solution: Use 0.02 rad Soller slits to minimize the axial divergence. T 100 cm-1 transparency effectAt low angles, the penetration/information depth is small and therefore thetransparency effect is also small. The same happens at high angles. The effectis maximal around 90o 2θ.Solution: For materials with a low linear absorption coefficient (between 1and 10 cm-1 for organic materials like polymers and pharmaceuticals), it isadvised to prepare a sample with a limited thickness (thin powder layer on azero background holder, between foils or capillary loading) instead of a bulkpowder sample.Note 1:The transparency effect is not the same as specimen displacement effect.Note 2:Some errors will more or less cancel each other out.Basic XRD Course10
Sample Preparation: IntroductionThe solutions of the previous slides are summarized here.As wetting agents, you can use for example ethanol, isopropanol, aceton orwater. Obviously, the wetting agent should not dissolve the sample.As binding agents, you can use for example Vaseline, grease or wax. Use onlya very small amount and the binding agent should not give any diffractionpeaks.In the next slides we will discuss intensity errors and solutions.Basic XRD Course11
Sample Preparation: IntroductionAnother (better) name for particle statistics is ‘crystal statistics’.The external particle size can be larger than the so-called crystallite size.Crystal statistics cause incorrect measured intensities. In case of severe crystalstatistics, the shape of the peaks will be affected as well. In this case, thepeaks shapes are not smooth but show a jagged (sawtooth-like) behavior.The peak position will also be affected in this case.This can cause problems to identify and/or quantify the proper phases presentin the sample.Basic XRD Course12
Sample Preparation: IntroductionThe graph shows the relation between the relative error in the measuredintensity and the crystallite size for a constant irradiated volume.Warning:The above figure indicates the effect only.Note:The relative error in the measured intensity is a statistical expected averageerror. The real occurring error is not predictable and varies from step to step.It can be zero, much larger or much smaller and it can be positive or negative.Hence, any correction afterwards is not possible.Basic XRD Course13
Sample Preparation: IntroductionMeasured on standard NIST SRM 640. This is a relatively coarse grained siliconstandard powder.NumberIntensity pread14.1%0.5%Basic XRD Course14
Sample Preparation: IntroductionNote:The relative error in the measured intensity is a statistically expected averageerror. The real occurring error is not predictable and varies from step to step.It can be zero, much larger or much smaller and it can be positive or negative.Hence, any correction afterwards is not possible.Basic XRD Course15
Sample Preparation: IntroductionTwo Phi-scans have been performed on a Si test sample using the 28.44 o2θ(111) peak.One scan is performed with a ½o divergence slit (irradiated length 11.4 mm)and the other scan with 1/16o divergence slit (irradiated length 1.4 mm).The sample mask (illuminated width) was 20 mm for both scans.The scan with the smaller irradiated area (blue) shows much larger statisticalvariation. This is because a smaller (average) number of particles contributeto the diffraction process.Note 1:The two scans in the graph are scaled with respect to each other for bettercomparison.Note 2:In a Phi-scan, the intensity is measured for a slowly rotating sample as afunction of the rotation angle (phi) around the normal of the sample surface.The rotation cycle repeats after one complete revolution of the sample.Basic XRD Course16
Sample Preparation: IntroductionVarious Phi-scans have been performed on a Si test sample using the 28.44o2θ (111) peak.The divergence slit varied from 1o (irradiated length 22.7 mm) to 1/32o(irradiated length 0.7 mm). The sample mask (illuminated width) was 20mm for all scans.Further reductions of the irradiated area have been achieved with a smallerbeam mask (illuminated width): 10 and 5 mm combined with the smallestdivergence slit (1/32o).The average and standard deviation of the measured intensity has beencalculated. The graph shows the errors on a relative basis.The counting statistical error for these scans is indicated as well. In allsituations, this error is much smaller than the crystal statistical error. Ifnecessary, the counting statistical error can be reduced by counting longer(i.e. collecting more counts).The crystal statistical error will not change for experiments with longercounting time!Note 1:In a Phi-scan, the intensity is measured for a slowly rotating sample as afunction of the rotation angle (phi) around the normal of the sample surface.The rotation cycle repeats after one complete revolution of the sample.Note 2:A larger divergence slit to increase the illuminated area will reduce theresolution.Basic XRD Course17
Sample Preparation: IntroductionThe penetration depth can be influenced by choosing another X-raywavelength (for example Cr or Co). Depending on the material, thedifference in the penetration depth can be 3 to 5 times (for example for Fesamples).For good particle statistics (see later), a large number of crystallites shouldcontribute to the diffraction process. A way to achieve this is to reduce theaverage particle/crystallite size by grinding (if possible). One shouldinvestigate different grinding procedures (method, time, cooling, et cetera)for each sample type. There can be very different for example for mineralsand organic materials (e.g. pharmaceuticals).Basic XRD Course18
Sample Preparation: IntroductionInstrumental Increasing the irradiated volume by increasing the incidence divergenceand/or a larger beam mask is only possible if the sample size allows it. Forpowder samples, a larger sample can be made only if enough powder isavailable. Sample oscillation (linear movement) is usually done for stress or texturemeasurements. Sample rocking (angular movement) is implicitly done when you use line(1D) detectors. Sample rocking can be simulated by adding repeated scans each with adifferent omega-offset (usually up to 2 degrees) also called “wobble” scan.Basic XRD Course19
Sample Preparation: IntroductionAn example of a reactive phase is ‘free lime’ in cement clinker material. Theamount of free lime influences the quality of the cement clinker. Due toreaction with moisture (and CO2) in the air, free lime disappears within a fewdays. Hence, the proper amount of this phase can only be obtained when youmeasure freshly prepared samples made from correctly stored material.Free lime Water Portlandite: CaO H2O Ca(OH)2Portlandite Carbondioxide Calcite Water: Ca(OH)2 CO2 CaCO3 H2ONote 1:The water from the second reaction will be re-used in the first reaction.Other example: Smectites (clay mineral) show a tremendous peak shift inrelation to humidity.Note 2:If the composition of the sample changes during a long measurement, thelow and high angle regions do not represent the same average composition.A good average pattern for the whole angle range can be obtained bymaking multiple short scans (in time) and summing up the results afterwards.Basic XRD Course20
Sample Preparation: IntroductionGlass capillaries can be used to shield the sample powder from the air (toavoid reaction with oxygen or moisture).The sample surface can also be covered with Mylar foil to shield the samplepowder from the air (for lightly reactive samples).Highly reactive samples need to be prepared in a glass capillary inside aglove-box under a neutral atmosphere (like Nitrogen). The glass capillarymust be sealed at both ends.Note:In a so-called humidity chamber, you can also study the reaction of thesample with moisture in relation with a controlled humidity and/ortemperature. This is especially interesting for pharmaceuticals. Clay mineralsare also studied in humidity chambers (usually at room temperature).Basic XRD Course21
Sample Preparation: IntroductionBasic XRD Course22
Sample Preparation: IntroductionThe options of powder sample preparation are presented here.As wetting agents (in case of small amounts of sample), you can use forexample ethanol, isopropanol, aceton or water. Obviously, the wetting agentshould not dissolve the sample.As binding agents, you can use for example Vaseline, grease or wax. Use onlya very small amount and the binding agent should not give any diffractionpeaks.Basic XRD Course23
Sample Preparation: IntroductionThis solution requires:a. Fix sample (surface) at proper height in sample holder this is the responsibility of the userb. Position the sample holder properly in the instrument (in sample stage) included in design of reflection-transmission spinnerA height positioning accuracy of 10 µm will be sufficient.Basic XRD Course24
Sample Preparation: IntroductionSample preparation Grinding powder sample should be done with care! Too much might resultin too small particles or lattice damage and this will result in broadenedpeaks. Hence, do some testing in order to find out how much grinding isallowed. Start with manual grinding using mortar and pestle (the so called“grinding bowl”).Certain materials might change their crystal structure due to the forcesapplied during grinding. Usually, the kind of change is from Alpha to Betaform and vice-versa. Especially for pharmaceuticals, this side effect is notwanted.Basic XRD Course25
Sample Preparation: IntroductionNon-spherical particles are: flakes, needles, cubes, Basic XRD Course26
Sample Preparation: IntroductionThe random oriented sample is prepared by dusting onto a greased surface(of a silicon single crystal substrate).For the preferred oriented sample, the sample surface is pressed (flattened)with a glass plate. The flat mica particles are pushed against the substratesurface. Hence, preferred orientation is introduced when one touches thesample surface.The next sheets will show the differences in the measured scans.Basic XRD Course27
Sample Preparation: IntroductionThe sample is prepared by dusting onto a greased surface (of a silicon singlecrystal substrate).The sample shows some preferred orientation. The 00l reflections showrelatively more intensity than the other reflections. However, the otherreflections are still measurable. Let us assume that this example is close torandom orientation.Basic XRD Course28
Sample Preparation: IntroductionThe sample is initially prepared by dusting onto a greased surface (of a Siliconsingle crystal substrate).After that, the sample surface is pressed (flattened) with a glass plate. Theflat mica particles are pushed against the substrate surface. Hence, thepreferred orientation is much stronger.The intensities of the 00l reflections have been increased (more particles with00l orientation). The intensities of the other intensities have been decreased.Some are not measurable anymore. This is a sample with a strong preferredorientation.The next slide shows zoomed parts of the two patterns.Basic XRD Course29
Sample Preparation: IntroductionThe dusted sample has a random orientation.The pressed sample has a preferred orientation (reflections of type hk0disappear).Basic XRD Course30
Sample Preparation: IntroductionOther solutions instead of randomizing:1. Deliberately create a sample with extreme (and known) preferredorientation.2. Create samples with a known but constant preferred orientation.For back loading: prepare the sample against a rough reference surface. Forexample, emery paper (fine sand-paper) covered with thin Mylar foil (againststicking) can be used as reference surface. Also a special tool with aroughened surface can be used.Sometimes the quality of the measurement can be improved by dispersing asmall amount of the powder on the sample surface (last step of the samplepreparation). You can also brush very carefully.Amorphous fillers could be anything from glass particles to instant coffee!Basic XRD Course31
Sample Preparation: IntroductionThese solutions have in common that only a relatively small amount ofpowder is used for preparing the XRD sample.Note:Another way of preparing a randomized powder sample with a small amountof powder is filling a glass capillary. However, this requires a different opticalset-up.Basic XRD Course32
Sample Preparation: IntroductionBe aware of the particle (crystal) statistics problem! Apply spinning!Note 1:If the sample surface is also small (micro-diffraction), it will be difficult toavoid crystal statistics.Note 2:The single crystal substrate is also called zero-background holder.Basic XRD Course33
Sample Preparation: IntroductionClay minerals and paste-like samples can be measured on a porous ceramicfilter plate. The material is deposited on the plate using the suction method[ref]. When designing the suction device, consideration should be given tothe dimensions (diameter range of the disk is between 31.8 mm and 32.2 mm,with thicknesses between 3.3 mm and 3.7 mm).The porous ceramic filter plate is made out of porous aluminium oxide, meanpore size 3.5 µm.[Ref]: E.G. Kinter, S. Diamond, (1956) “A new method for preparation andtreatment of oriented-aggregate specimens of soil clays for X-ray diffractionanalysis”, Soil Sci. 81, 111-120.Basic XRD Course34
Sample Preparation: IntroductionEnvironmental analysisXRD is playing a big role in this application area since it is used not only foranalysis of filter samples taken from mining and refractory production sites,but also for research on soil additives for reclamation, quantitative analysisof asbestos in building demolition waste and identification of deposits foundin automotive and boiler components.A sample of respirable dust is collected on a membrane filter using arespirable dust sampler. The filter is then placed directly into the samplebeam of an X-ray diffractometer. The mass of crystalline silica on the filter isdetermined from X-ray diffraction response, calibrated against filters loadedwith known amounts of standard quartz or cristobalite. Since the volume ofair sampled is known, the concentration of airborne crystalline silica is readilycalculated.Basic XRD Course35
Sample Preparation: IntroductionBasic XRD Course36
Sample Preparation: IntroductionSolid objects can be fixed in the sample holder for non-standard sample sizesand shapes using modeling wax or plasticine.The diameter of the cavity is 44 mm, the maximum sample height is 6.5 mm.Flat samples can be first fixed on a glass slide using photo glue. In this case itis easy to remove the sample after the measurement is completed. Alsopositioning the sample on a flat glass slide helps avoiding tilt of the sampleand creates a good contrast to the sample stage table.Note:Extra care is needed for proper sample mounting for stress and texturemeasurements. In case of stress measurements, the sample height is evenmore critical. In case of texture measurements, the sample height is lesscritical. For both applications, avoid that a sample tilt is introduced withsample mounting.Basic XRD Course37
Sample Preparation: IntroductionThin films or thin solid objects as well as fiber samples can be easily fixed inframe-like sample holders such as rectangular powder holders or for examplea film strip holder.Basic XRD Course38
Sample Preparation: IntroductionPosition the sample holder properly in the instrument (in sample stage) isincluded in design of both reflection-transmission spinner and stage for flatsamples.Basic XRD Course39
Sample Preparation: IntroductionIn case of using stages with no reference surface placing of the sample at aproper height can be done in following ways:a. Alignment with a height sensor, which is controlled from Data Collectorsoftware. This device is used in combination with the stages, which haveprogrammable Z axis (cradles, modular stage with programmable Zadjustment).b. Adjustment of the sample height using dial gauge. This option is usedwhen the Z axis of the stage is controlled manually (multi-purpose samplestage, modular stage with manual Z-adjustment).In the last case to fix the sample at the proper height is the responsibility ofthe user.A height positioning accuracy of 10 µm is sufficient.Basic XRD Course40
Sample Preparation: Introduction α1-2 α1shift due to α2contribution Basic XRD Course 10 The effect of peak shift due to overlap of α1and α2peak is maximal for intermediate angles. Solution : Apply profile fitting or α2-stripping. AD axial divergence At low diffraction angles, the peaks are asymmetric at the low angle side. The
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