Tag Archives: SEM

Kuidas määrata metallide ja materjalide elemendilist koostist?

Metallide ja teiste materjalide koostisest sõltuvad otseselt nende omadused ja sellest tulenevalt rakendus erineval otstarbel. Elemendilise koostise määramiseks on aga mitmeid meetodeid, millest tulebki järgnevalt juttu.

telliuuring

WD-XRF ehk lainedispersiivne röntgenfluorestsents-spektroskoopia on meetod, kus katseobjekti kiiritatakse röntgenkiirgusega, mis ergastab proovi pinnakihis (mõned kuni kümned mikromeetrid) aatomeid. Nende aatomite relakseerumisel vabaneb karakteristlik röntgenkiirgus, mille detekteerimisel saadaksegi informatsiooni materjalis esinevate elementide ja nende sisalduse kohta. Laboratoorseteks uuringuteks kasutatav seade on suur ja kallis ning mõõtmine koos proovi ettevalmistusega võib aega võtta mitmeid tunde. See-eest on tegu ühe parima meetodiga elemendilise koostise määramiseks kuna see on täpne ja tundlik. Antud meetod ei võimalda siiski määrata väga kergeid elemente (nt vesinik) ja teatud elementidele vastavate piikide kattumisel saadakse mõnikord vale tulemus, mis vajab täiendavaid mõõtmisi teise meetodiga. Proovi ettevalmistusel on oluline, et katseobjekt oleks tasase pinnaga ja puhas.

ED-XRF ehk energiadispersiivne röntgenfluorestsents-spektroskoopia korral toimub samuti katseobjekti aatomite ergastamine röntgenkiirgusega ja tekkinud karakteristliku röntgenkiirguse mõõtmine. Seade on aga oluliselt lihtsama ja odavama ehitusega kui WD-XRF ja võib olla kas lauale pandav või lausa käeshoitav. Just käeshoitavaid püstoli kujuga ED-XRF analüsaatoreid kasutatakse tihti metallidega tegelevates ettevõtetes ja ka geoloogide poolt välitöödes kuna seade on kerge ja mõõtmine võtab aega vaid hetke. Tulemused ei ole muidugi eriti täpsed kuid alati seda polegi vaja ning täpsema analüüsi saab alati laborist tellida. Selline lihtsam püstoli kujuline ED-XRF seade on enamasti saadaval 20 000 – 40 000 euroga kuid selle müüja tuleb väga hoolikalt valida.

Kui teema pakub rohkem huvi, siis vaadake meie teadusvideot röntgenfluorestsents-spektroskoopiast, kus lisaks teooriale teeme ka ühe demonstratiivse mõõtmise;

 

SEM-EDX ehk energiadispersiivne röntgenmikroanalüüs skaneerivas elektronmikroskoobis. Selle meetodi puhul kiiritatakse katseobjekti fokuseeritud elektronkiirega, mis ergastab mõne mikromeetri sügavust pinnakihti. Aatomite relakseerumisel vabaneb karakteristlik röntgenkiirgus, mille detekteerimisel saab teada materjalis leiduvate elementide ja nende sisalduse kohta. SEM-EDX ei ole kaugeltki nii täpne ja tundlik kui WD-XRF ning ajakulu ühe objekti uurimiseks on võrreldav. SEM-EDX eelis WD-XRF ees on tema suur lokaalsus – kui WD-XRF korral kogutakse keskmistatud signaal mõne cm2 suuruselt alalt, siis SEM-EDX korral on signaali kogumiseks juba mitmeid võimalusi. Signaali saab koguda a) lokaalselt, vaid mõne ruutmikromeetri suuruselt alalt, huvipakkuvalt detaililt või b) keskmistatult nt 200 x 200 ruutmikromeeriselt alalt (ala suurust saab valida). Kolmas võimalus on elementide kaardistamine mikroskoopilisel skaalal huvipakkuval alal, kus lahutusvõime on ca 1 mikromeeter. Just viimane annab tihti täiendavat infot metallide või muude materjalide mikrostruktuuri kohta. See on oluline siis kui materjali keskmine elemendiline koostis on õige kuid valmistamisprotsessiga on midagi väga valesti läinud, mille tulemusena on materjalis tekkinud selline mikrostruktuur, mis halvendab tema mehaanilisi omadusi ja/või vastupidavust korrosioonile. Selliste uuringute raames on mõnikord vaja teha kvaliteetne ristlõige materjalist kas mehaaniliselt või fokuseeritud ioonkiirega ning uurida materjali sisemust, kaardistades seal elemente ja otsides seal mikromõrasid.

SEM-EDS ja WD-XRF praktilise rakenduse kohta saab vaadata meie teadusvideot meteoriidi uuringutest;

 

Millist meetodit siis ikkagi lõpuks valida? See lepitakse enamasti kokku uuringutele eelneva konsultatsiooni raames, kus selgitatakse välja milleks analüüsi reaalselt vaja on. Tihti on vaja selgitada välja metallisulami marki – näiteks kas tegu on sulamiga AISI 304 või AISI 316. Sellisel juhul piisab laboratoorsest WD-XRF uuringust. Mõnikord on aga tegu sulamiga, millest soovitakse valmistada kriitilisi detaile (nt autodele või lennukitele). Siis on otstarbekas viia läbi komplekt-uuring, mille raames viiakse läbi nii elementanalüüs WD-XRF meetodil, täiendav mikrostruktuuri uuring SEM-EDS meetodil, määratakse kõvadus (nt Rockwelliga) ja testitakse sulami vastupidavust korrosioonile sõltuvalt rakenduskeskkonnast.

Science Video Series: Under The Scanning Electron Microscope

We are currently making a science video series about scanning electron microscopy studies. In these videos we discuss the sample preparation and show how things actually look like in the microscopic scale.

Videos can be seen in our YouTube channel.

The scanning electron microscopy images made in these studies are uploaded to our gallery.

Horse Tooth Under the Scanning Electron Microscope

What does a horse tooth look like under a scanning electron microscope? In this episode we study a real horse tooth that was extracted from a horse due to dental problems.

Salt and Sugar Under the Scanning Electron Microscope

What do salt and sugar really look like in the microscopic scale? We try to answer this question by studying these compounds in unprecedented detail under a powerful scanning electron microscope.

Fly Under the Scanning Electron Microscope

What Does a fly look like under the scanning electron microscope? In this video we will explain how biological samples are prepared for scanning electron microscopy (SEM) studies. We will also take some images of the eye, leg, mouth and wing of the fly.

Scanning Electron Microscopy (SEM) Studies for Biometric OÜ

 

The company asked us to do elemental analysis for multiple dental implant components in order to confirm the quality of the metals. Due to the difficult three-dimensional shape of the substrates, the studies were carried out using a high resolution scanning electron microscope “Helios NanoLab 600” (FEI), equipped with an energy-dispersive X-ray spectrometry (EDX) analyzer INCA Energy 350 (Oxford Instruments). The samples were attached to the mushroom-shaped holders with a carbon tape. The studies showed that the metals used by Biometric OÜ are indeed high quality medical titanium. We also made high resolution images of the implants surface, which has been developed to be biocompatible, support osseointegration and have a good adhesion with the surrounding tissue.

biometric

Studied dental implant components (on the left) and the surface of an advanced dental implant (on the right).

Characteristic X-Ray Radiation

Characteristic X-Rays are generated when excited sample atoms undergo a relaxation process. For that the atoms need to be excited first and this can be done with high energy electromagnetic radiation (in XRF) or accelerated particles such as electrons (in SEM). The primary beam kicks out an inner shell electron and a vacant spot is left behind. As this state is unstable, a higher shell electron will soon move into this vacant spot and during this process energy is emitted in the form of X-Rays. This emitted radiation has a specific energy which depends on the binding energies of the two electrons that participated in this process. If this emitted ( characteristic ) x-ray radiation is detected then the composition of the material can be measured.

Vacuum Systems and Technologies

Vacuum can be understood as space from where matter (for example air) has been removed. It naturally exists in outer space but for certain applications, like materials characterization techniques, it needs to be achieved artificially. The desired level of vacuum is obtained with the help of a suitable vacuum pump. For example low vacuum (low quality vacuum with higher pressure) can be generated with a diffusion pump, scroll compressor pump, rotary vane pump, diaphragm pump or a sorption pump. High vacuum (high quality vacuum with very low pressures) however, can be obtained with high vacuum pumps such as the turbomolecular pump, ion pump, titanium sublimation pump and cryopump. The level of vacuum is measured with devices called vacuum gauges (vacuum meters) like the thermocouple gauge, pirani gauge, penning ionization gauge and the quadrupole mass spectrometer (analyzer). The working principle of vacuum pumps and vacuum gauges is explained with 3D animations in the video lecture above.

 

Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy

The scanning electron microscope is a powerful tool used by many scientists for studying and developing materials. This device uses electrons instead of photons and that allows the operator to visualize even nano-scaled surface details. Therefore it is very useful for also studying corrosion processes and creating efficient thin coatings which have a thickness of less than a micrometer.

In the video above you will see the working principle of this microscope and how it is operated.

The electrons are generated in the electron gun on top of the microscope. There are different source types but one of the oldest used is a tungsten filament that is heated over 2400 degrees celsius by passing a current through it. At this temperature electrons emit from the filament and are pulled down the column with the help of an anode. The operator selects the voltage that accelerates the electrons. It can be up to 30 kV in conventional scanning electron microscopes. The accelerated electrons are scattered and need to be focused – this is done with the help of multiple focusing lenses. These lenses also tune the amount of electrons that reach the sample. For example by broadening the electron beam, the electrons are absorbed by the column and dont reach the sample. In order to scan across the sample row by row, the electron beam needs to be moved in such a manner. This is achieved with the help of scanning coils that are placed below the focusing lenses. The final focusing of the beam is done with objective lenses located below the scanning coils. In order to get an image of the substrate the surface is scanned row by row with the electron beam. In each spot signals like secondary electrons, backscattered electrons or characteristic X-Rays are generated. By detecting the signal emitted from each scanned spot, an image of the surface is generated. So for example by detecting secondary electrons a secondary electron image is generated.

Secondary electrons are generated when the primary beam electrons kick out electrons from the substrate atoms. In that process primary beam electrons lose energy as it is transfered to the secondary electrons. These secondary electrons generaly have low energy and escape only from near the surface, giving a good image of the substrates topography. The electrons are collected with a special detector that has a positively charged collector  for pulling the negatively charged electrons towards it.

Backscattered electrons are primary beam electrons that are scattered back in a similar direction as they interact with the substrate atoms nucleous. In that process they dont lose much energy and can be emitted even from deep layers of the substrate. Therefore they carry the bulk information of the sample and generated images are not completely topographic. The electron yield strongly depends on the atomic number of the substrate. For example regions on substrate with higher atomic number appear brighter on the image as more electrons are backscattered in these regions.

Characteristic X-Rays are generated when sample atoms exited by primary beam electrons are undergoing a relaxation process where the inner shell vacant spot is filled with an outer shell electron. The emitted radiations energy depends strictly on the atoms number and on the electrons binding energies that are involved in this process. Therefore this signal can be used to detect different elements and do quantitative element analysis.

For a real SEM study the sample first needs to be prepared – it has to be dry, conductive and with a suitable size. Non-conductive samples can be coated with a thin (couple nanometers) layer of metal (Au, Pt..), which allows SEM studies as they conduct away the heat and negative charge caused by electrons. The sample is moved into the microscope through an airlock or the main chamber door and placed on a stage with a special holder. The stage can move in any desired direction (x,y,z, rotate, tilt). The sample is then moved under the electron column to a suitable working distance. After focusing and other adjustments the first electron image can be taken.

Element microanalysis can be done in several ways but first an electron image of the sample is obtained. The most common way to do element analysis is selecting a characteristic spot on the sample and then bombard it with the narrow (couple nm wide) electron beam. As a result X-Rays are emitted from the couple micrometer wide interaction volume. This gives information from a very localized area but in some cases an average composition of the material is needed. For that a larger area (lets say 50 x 50 microns) is bombarded by electrons and the X-Rays emitted from that area are detected. It is also possible to map the distribution of elements on a larger area by scanning across the surface with the electrons row by row and collect X-Rays from each scanned point. Based on the detected X-Rays an image is created that shows the distribution of certain elements.

In order to see what is inside a material a focused ion beam (FIB) is used to make  a cross-section or a thin lamella. The cross-sections or lamella are then studied with electrons at a suitable angle by tilting it with the help of the stage.

Scanning electron microscopes generally work in a high vacuum in order to prevent surface contamination and electron or x-ray interactions with the gas in the chamber which would affect the quality of the image. However there are also specially designed environmental scanning electron microscopes (ESEM-s), that work in low vacuum. The ESEM can therefore even be used to study living cells or bacteria.