Characterisation, Measurement & Analysis
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Applications

  • R&D 100 Award Winner: The SRS BGA244 Binary Gas Analyser

    The BGA244 Binary Gas Analyser, a winner of the 2016 R&D 100 award, can measure the ratio of two gases in a binary mixture or the purity of a single gas. It contains a comprehensive database of nearly 500 different gases allowing measurements on more than 50,000 different mixtures. Measurement accuracy depends on the gas species being measured, but is typically better than 0.1 percent.

    The BGA244 uses a colour touchscreen LCD for configuration and to display measurements. The product can also be controlled over USB, RS-232 and RS-422 computer interfaces. Multipurpose analogue inputs and outputs allow control and monitoring of external devices. Two user-defined relays are available for process control applications. The BGA244 operates at pressures from 5 to 150 psi with flow rates from 0 to 5000 sccm.

    With much improved accuracy, stability and resolution over thermal conductivity analysers, the BGA244 operates without lasers, filaments, chemical sensors, optical sources, separation columns, reference gases, or reagents. It runs virtually maintenance-free with a lower cost of ownership.

    Click here for full BGA244 product data. For further information, application support, demo or quotation requests, please contact us on 01582 764334 or click here to email.

    Each year for more than 50 years, R&D Magazine has honoured the 100 best innovations in research and development. For information on the 55th Annual R&D 100 Awards visit the R&D 100 Conference website.

  • NEW FS740 GPS Time and Frequency System

    From Stanford Research Systems (SRS), the recently launched FS740 Time and Frequency system is much more than just a 10MHz frequency reference.  With powerful capabilities and eye-catching performance it is a highly cost effective and multi-functional lab tool.

    The SRS FS740’s GPS disciplined 10MHz reference delivers caesium equivalent stability and phase noise, but for a fraction of the cost. A host of additional features include a 12-digit/s frequency counter, a DDS synthesised source with adjustable frequency and amplitude, built-in distribution amplifiers and event time tagging to UTC or GPS. Optional OCXO and Rubidium timebase clocks reduce phase noise to better than -130dBc/Hz.

    • GPS/DNSS Disciplined 10MHz reference
    • TCXO, OCXO or Rb Timebase (better than -130dBC/Hz phase noise)
    • Time Tagging to GPS and UTC
    • 12 digit/s Frequency Counter
    • Sine, square, triangle & IRIG-B source outputs
    • Built-in distribution amplifiers
    • Ethernet/RS-232 connectivity

     

    From SRS, a leader in innovative scientific instrumentation solutions, the FS740 is a powerful multifunctional tool for any lab with time/frequency reference and measurement needs.

    Click here for further information.

    To speak with a sales/applications engineer please call 01582 764334 or click here to email.

  • Scanning electron microscopy and pharmaceutical research topics

    Pharmaceutical research and questions linked to drug development and applications is a very diverse topic. Also the instrumentation involved in research and development is just as diverse. In this blog we will focus on the use of scanning electron microscopy (SEM) in three different research topics.

    Understanding nano-bio interfaces
    The diversity of technologies used within pharmaceutical research, with an emphasis on the importance of microscopy techniques, is summed up in the review of Jin et al.[1]. Nowadays, microscopic observations play a key role in nanotechnology research with applications in pharmaceutical sciences.

    Multi-scale observations are especially needed to understand nano-bio interfaces, where a wide range of phenomena are to be described. The benefit described in the SEM section is fewer artefacts, compared to atomic force microscopy, since no physical interaction with the sample is needed to gain images.

    In general, nano-bio interfaces can be understood as cell – nanotube or tissue – nanoparticle interaction and potential morphological changes occurring due to nanomaterials in cells and tissues. Observations of these interfaces enable future developments in pharmaceutical applications.

    Fig. 1: SEM image of tannic acid.
    Fig 2: SEM image of Prozac.

    Observing microvesicles
    Extracellular microvesicles (EMVs) are membranous nano-sized cellular organelles naturally released by cells in vitro and in living organisms. Microvesicles can be found in various human body fluids: blood plasma, urine, breast milk, and amniotic fluid.

    As they have been observed to carry functional proteins, RNA molecules and antigens, they can be understood as a novel way of cell-cell communication. Previous research work has shown that altered microvesicles gained from bovine milk containing mRNA and miRNA can be transferred to immune cells to potentially alter immune cell function.

    In the study described by Maburutse et al., various preparation techniques for microvesicles were compared [2]. The microvesicles were observed after preparation via SEM via secondary electron detection. To enable this observation to take place, the vesicles were fixed with paraformaldehyde, and air dried. The various preparation techniques resulted in a unique set of characteristics in the microvesicles.

     Optimising solid self-nano-emulsifying drug delivery

    As a final example, we can also consider drug delivery. Self-nano-emulsifying drug-delivery systems (SNEDDS) have emerged as effective delivery systems due to the development of enhanced bioavailability of lipophilic drugs. Dash et al. describe in a study an optimisation of solid self-nano-emulsifying drug delivery for enhanced solubility and dissolution [3].

    Involving scanning electron microscopy, it was concluded that there was no evidence of drug precipitation on the surface of solid SNEDDS. This will lead to a better future application, although investigations on animal/human models are needed.

    Fig. 3: SEM image of pharmaceutical powder
    Fig. 4: SEM image of pharmaceutical powder.

    More on SEM in pharmaceutical research
    The three previous examples should provide a better understanding of the power and potential of SEM within pharmaceutical research. In short, the successful application of SEM fuels the development of more powerful, better-performing drugs.

    References
    1) Multi-Scale Observation of Biological Interactions of Nanocarriers: from Nano to Macro, Jin et al., Microsc Res Tech September 2010, 73 (9):813-823

    2) Evaluation and Characterization of Milk-derived Microvesicles Isolated from Bovine Colostrum, Maburutse et al., Korean J. Food Sci. An. 37 (5), 2017

    3) Design, optimization and evaluation of glipizide solid self-nanoemulsifying drug delivery for enhanced solubility and dissolution, Dash et al., Saudi Pharmaceutical Journal (2015)23, 528-540

    Topics: life sciences, R&D, pharmaceutical research

    About the author
    Dr. Jasmin Zahn is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. She is highly engaged in finding out more about the possibilities for Phenom-World products in various applications. In addition, Jasmin is active in sharing best practices with the outside world to encourage them to look outside their standard scope of use and to improve in their work.

  • SEM analysis of PVDF-HFP nanofibres for the fabrication of energy harvesters

    Nowadays, energy harvesting is seeing an increasing interest from the research community, a fact that is confirmed by the rising number of publications. Energy harvesting has a wide range of applications, ranging from portable electronics, such as wristbands, to implanted medical devices like pacemakers. In this field, researchers are focusing their attention on the development of new energy harvesters that satisfy strict requirements: they need to be light and small, but also cheap and highly portable. In this blog, we discuss the fabrication of energy harvesters made from PVDF-HFP nanofibers on PDMS and SF substrates. We investigate how these energy harvesters are characterised and what the role of SEM is in this study.

    Piezoelectric Energy Harvesting
    The increasing demand for innovative devices, such as embedded sensors in sportswear or smart watches, is drawing attention to energy harvesting. Energy harvesters have the capacity to convert external energy, which can be derived for instance from solar power or thermal energy, into electrical energy that can be used to power small electronic devices or wireless sensor nodes. Energy harvesters need to be small, light, inexpensive, portable, flexible and, in some cases, also biocompatible.

    One of the most common types of energy harvesters employs piezoelectric materials, which convert mechanical strain (for example human motion or acoustic noise) into electric current or voltage. A commonly-used piezoelectric material for energy harvesting applications is polyvinylidene fluoride (PVDF), which offers a good electro-mechanical coupling factor and is biocompatible, light and flexible.

    In a recent study, polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers were investigated as suitable candidates for energy harvesters (R. Najjar et al., Polymers 2017, 9, 479). The performance of the nanofibers was characterized in combination with two different substrates, namely polydimethylsiloxane (PDMS) and silk fibroins (SF).

    The first is a type of synthetic polymer, while the second is a natural protein that provides better biocompatibility and more favourable sustainability. The characterisation of the performance of energy harvesters includes the analysis of the morphology, the mechanical properties, and the mechanical-electrical measurements.

    Characterisation of PDMS and silk substrates through SEM analysis
    A scanning electron microscope was employed for the analysis of the morphology of PDMS and silk films. For this analysis, two types of silk fibroin films were investigated: pure silk fibroin and silk fibroin with 20% glycerol content.

    Because silk fibroins tend to become stiff and brittle over time, glycerol is added to the silk fibroin to make it more flexible. 20% is the optimum glycerol content for increasing the softness of the silk film, without the film disassembling in water.

    Fig 1. shows -SEM images of the surface of pure silk fibroin (A-C), silk fibroin with glycerol (D-F) and PDMS films (G-I), illustrating the surface microstructures and morphology. The cross-sections are shown in Figure 1, (J-L) for pure silk fibroin, (M_O) for silk fibroin with glycerol and (P-R) for PDMS films.

    All three materials show continuous and homogeneous structures without voids. The rough cross-sections indicate the tenacity fracture of the films that are related to strong mechanical properties.

    Fig. 1: SEM images showing the surface morphology of different types of pure silk fibroin (A-C), silk fibroin with 20% glycerol content (D-F) and PDMS films (G-I) plus SEM images showing the cross-sections of pure silk fibroin (J-L), silk fibroin with 20% glycerol content (M-O) and PDMS films (P-R).

    The study of the mechanical properties of the three types of film is also of utmost importance. Fig. 2 shows the stress-strain curves of the three materials. The PDMS (blue curve) is mostly elastic with a linear stress-strain curve until the fracture, showing a maximum stretch of more than 400% its total length, whereas the pure silk fibroin (pink curve) is stiffer and has lower yield point than PDMS.

    Fig 2: Stress-strain curve of PDMS and two types of silk fibroin films.

    The data from the measurements on silk fibroin prove that this material can survive larger forces and greater elongation, although it is stiffer than PDMS.

    SEM analysis of PVDF nanofibres

    The PDVF-HFP nanofibres were fabricated using the electrospinning process. Two different types of fibres were produced: random nanofibres and aligned and stretched nanofibres. Fig.3 shows SEM images of the two types of nanofibres (A and B).

    From these images the fibres diameter and orientation can be measured. In Fig. 3, the diameter distribution for random and aligned fibres is shown (graphs C and D respectively). In the first case, the diameter varies between 600nm to 1600nm, while for aligned fibres it ranges from 300nm to 700nm.

    The orientation distribution (shown in graphs E and F) shows that the random fibres have a larger range of orientation (from -50° to +50°), while the aligned fibres have orientation with one large peak around 0°.

    Fig 3: SEM images of traditionally prepared electrospun PVDF-HFP nanofibers (A) and stretched PVDF-HFP nanofibers (B). Diameter distribution histograms and orientation distribution of random nanofibers (C-E) and stretched nanofibers (D-F).

    Finally, the energy harvesting measurement was performed. Fig. 4 shows the voltage generated from PVDF-HFP random (A) and aligned (B) nanofibres on a PDMS substrate. The voltage generated from the stretched and aligned nanofibres is more than 12 times that of the electrospun random nanofibres.

    Fig. 4: Electrical output of OVDF-HFP nanofibres on PDMS substrates, for random nanofibres (A) and aligned nanofibres (B).

    The electro-mechanical characterisation was important in demonstrating that the aligned PVDF-HFP nanofibres have higher piezoresistivity and are therefore more suitable for energy harvesting applications. The SEM revealed to be a powerful instrument to analyse the morphology of the nanofibres and to measure the fibre diameter and orientation.

    From that, stretched nanofibres were shown to be better aligned with a more precise diameter control. They also outperformed the random nanofibres in the energy harvesting measurement by more than 10 times.

    Topics: materials science, fibres

    About the author
    Marijke Scotuzzi is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. Marijke has a keen interest in microscopy and is driven by the performance and the versatility of the Phenom SEM. She is dedicated to developing new applications and to improving the system capabilities, with the main focus on imaging techniques.

     

  • How SEM helps perform automated quality control on phosphate coatings

    We are surrounded by products that, for either decorative or functional purposes, are covered with coatings; from paintings and lacquers, to adhesive or protective coatings, optical, catalytic or insulating coatings. Of all these coatings, conversion phosphate coatings play an important role, especially in the automotive industry: they are used for corrosion resistance and lubricity. Since these coatings are used for critical parts, the coating process must undergo thorough quality checks. These checks consist of the analysis of the morphology of the coating as well as the percentage of coverage. In this blog, we describe and analyse how automated tools combined with SEMs can be helpful in quality checking phosphate coatings.

    Conversion phosphate coatings
    Coatings are not only used as a decorative feature, such as paint finishes or lacquers; most of the time have a functional purpose. Coatings can:

    • Serve as an adhesive,
    • Have optical, electrical or magnetic properties,
    • Be catalytic or light sensitive, such as those used to make photographic film.

    One of the biggest categories is that of protective coatings, ranging from insulation to waterproof and wear resistant to anti-corrosion. Coatings can be applied through chemical vapour deposition, physical vapour deposition, spraying, or chemical and electrochemical techniques, such as electroplating.

    Within this wide range of functionalities, materials and coating techniques, we focus our attention on conversion phosphate coatings. These are typically used in the automotive industry and serve as a protection layer on steel parts, preventing corrosion and providing lubricity. The main types of coatings are manganese, iron and zinc. The coating is applied by immersing the part in a bath containing a phosphoric acid, which causes the growth of a crystalline zinc, manganese or iron phosphate layer.

    Because of the critical use of the coated parts, the coating process must undergo thorough quality checks to ensure the performance of the coating. The quality check consists of the analysis of the coating morphology and the percentage of coverage. One way to carry out this analysis is by using a scanning electron microscope (SEM).

    How SEM helps perform quality control inspections on coating processes
    SEM is an ideal choice for quality checking of conversion coatings and for the analysis of the crystal morphology. Moreover, imaging with the BSE detector is the most suitable technique to analyse the coverage of the coated sample because of the difference in atomic number between the phosphate coating and the steel.

    Since steel is an alloy of iron, and therefore has a higher atomic number than the phosphate coating, it will appear brighter in the back scattered image. Fig.1 shows a BSE image overlaid with the coloured EDX map, where the yellow areas consist of iron and the light blue refers to zinc. The brighter areas of the BSE images overlay with the yellow, demonstrating the effectiveness of using these images for the measurement of coating coverage.

    Fig. 1: SEM image in backscatter mode of a steel sample covered with zinc phosphate coating overlaid with a coloured EDX map, showing the coated (yellow-iron) areas and the coating (light blue-zinc)

    Moreover, at the same time, BSE images also reveal the crystal structure of the coating, enabling the coating morphology to be analysed. Fig. 2 shows an SEM image of the crystal structure of zinc phosphate coatings.

    Fig. 2: SEM images in backscatter mode of asample covered with zinc phosphate coating, showing the different morphology of the crystal structure.

    Why automated quality control is key
    In quality control processes, it is ideal to have a system that enables the user to get fast results and avoid long downtime on the production line. In most cases, the analysis of coated samples is done in a laboratory that is located far from the production line, causing unwanted delays in instances of negative feedback. Having a fast and reliable way to check the quality of phosphate on site is important. Desktop SEMs are the ideal choice in this case, having a small footprint and being easy to use.

    However, this may be not sufficient. An automated tool for quality checking that does not require a dedicated and experienced user gives more advantages in terms of time saving and the reliability of the results. The Phenom Programming Interface (PPI) is the right platform to implement an automated tool for quality checks.

    Fig. 3 shows the process flow of the automated tool designed for the quality checking of the coating coverage. It enables the user to scan a large area of the sample by collecting a set of BSEs images at low magnification and saving them in a selected folder.

    These images are then automatically analysed by applying a threshold on the grey level, effectively separating the coating (appears darker) and the uncovered steel (appears brighter). The average coverage percentage and the statistics of the measurement are then saved in a report that the user can print out later. It is also possible to load a set of images, select the correct threshold, do the analysis, and generate the report.

    Fig. 3: Process flow of the automated tool for the quality check of the phosphate coatings coverage.
    Topics: materials science, scanning electron microscope automation

     

    About the author
    Marijke Scotuzzi is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. Marijke has a keen interest in microscopy and is driven by the performance and the versatility of the Phenom SEM. She is dedicated to developing new applications and to improving the system capabilities, with the main focus on imaging techniques.

  • How scanning electron microscopy fuels biomedical research

    Biomedical research is a wide field. It describes an area of science devoted to the study of the processes of life, the prevention and treatment of diseases, and the genetic and environmental factors related to diseases and health. And since the field is so diverse, its range of investigation equipment is too. Scanning electron microscopy (SEM) is one of those types of equipment, and is used to describe tissue or organ structures to gain insights into possible alterations and diseases. To show the variety of topics explored with a SEM — demonstrating its power and vast range of applications — this blog will introduce you to three scientific studies.

    Revealing variations of the human cochlea
    The human cochlea appears to have variations, as proven with images by Rask-Andersen et al.[1]. These SEM images reveal the anatomical variations to raise awareness of them and their impact on cochlear implantation.

    The researchers point out that studies about the fine structure of the human cochlea may provide a better understanding of the intracochlear tissue interacting with an electrode during insertion and electric stimulation.

    To enable an observation with SEM, the cochlea was, after decalcification, divided mid-modiolarly using a razor blade. The organ of Corti with stereocilia tufts, as well as various hair cells and further structures, could be shown nicely after the preparation. In addition, possible trauma sites for cochlear implantation were analysed in further detail.

    Figure 1: (a) Human inner hair cell stereocilia - low-frequency region. (b) Outer hair cell stereocilia - high-frequency region. (c) Outer hair cell stereocilia - low-frequency region. Reprint from Rask-Andersen et al.

    Investigating the possibility of suppressing post-operative intimal hyperplasia
    Endothelial injury is considered to be the first step towards post-operative hyperplasia; an increase in the growth of organic tissue. Hyperplasia is seen to be the most common cause of vein graft occlusion.

    In their SEM study, Yamamurra et al. describe the possibility of suppressing post-operative intimal hyperplasia.[2] To be able to observe the impact of a free radical scavenger, edaravone, vein grafts were prefixed in glutaraldehyde and postfixed in osmium tetroxide. After critical point drying and sputter coating, the vein crafts were imaged and analyzed.

    The research team showed that the endothelial cells in unoperated veins had a cobblestone-like appearance. Comparing this to samples after a 1-hour bypass, the endothelial cells in the saline group showed a comb-like structure and the adherence of monocytes, while in the edaravone group the cobblestone-like structure remained and far fewer monocytes were observed. The team therefore concluded that edaravone might not only suppress hyperplasia, but also atherosclerosis.

    Figure 2: The fragile structure of the fenestrated bony columns is presented in (d,e). Reprint from Rask-Andersen et al.

    Reviewing ECM with SEM
    As a third and final example, we highlight the review on the ocular corneal extracellular matrix (ECM) by Quantock et al.[3] Electron optical imaging has already contributed to the research of ocular ECM for more than 60 years, and has found its place in describing fine structural anatomy and tissue changes in pathological conditions since the 1970s.

    While SEM has initially been employed in ocular matrix research along with studies of cell and lamellar organization, the development of FIB-SEM (focused ion beam) has taken it to a new level. As a result, it is now possible to image three-dimensional micro-anatomy.

    With these three examples we hope to have offered you a glimpse into how SEM is an invaluable research tool in biomedical topics, covering a vast range of applications, and fueling numerous future developments.

    References

    1) Human Cochlea: Anatomical Characteristics and Their Relevance for Cochlear Implantation, Rask-Andersen et al., The Anatomical Report 295: 1791-1811 (2012).

    2) Supression of postoperative intimal hyperplasia of vein grafts with edaravone in rat models – a scanning electron microscope study, Yamamurra et al, Int J Angiol Vol 16 No. 4 Winter 2007.

    3)  From Nano to Macro: Studying the Hierarchical Structure of The Corneal Extracellular Matrix, Quantock et al., Exp Eye Res 2015 April, 133: 81-99.

    Topics: scanning electron microscope, life sciences

     

    About the author
    Dr. Jasmin Zahn is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. She is highly engaged in finding out more about the possibilities for Phenom-World products in various applications. In addition, Jasmin is active in sharing best practices with the outside world to encourage them to look outside their standard scope of use and to improve in their work.

  • Food science research: how scanning electron microscopy is used

    In food sciences, researchers face many different and challenging microscopy tasks: from particle and fibre analysis, to food preservation, food microbiology and food pathogens. Many different microscopy techniques are used within food science. Food science research and scanning electron microscopy (SEM) have been strongly related for many years. SEM can be used in various studies from herbs to fruits, and from engineered food to natural components. This blog should offer you some insights into how SEM is used and what benefits and challenges go along with its use.

    SEM analysis on the effect of drying procedures
    As a first example, let’s take a look at the analysis of the effect of drying procedures on the physiochemical properties and antioxidant activities from China’s major cultivated shellfish [1]. The Pacific oyster, Crassostrea gigas, is known to be a precious food and medicine resource. The morphology of the polysaccharides of Crassostrea gigas were analysed to see if different drying methods might result in a changed surface topography and structure of the polysaccharides.

    Due to the use of scanning electron microscopy, Hu et al. could show that spray-drying created smaller and more uniform polysaccharide particles, while freeze-drying and rotary evaporation drying created a more oval and smooth surface topography. A smooth surface topography can indicate a higher bioactive and antioxidant activity. Based on the results that they found, spray drying is the recommended method for producing polysaccharides.

    Figure 1: Example of baking yeast imaged with SEM

    The investigation of banana starch
    On a similar topic, starch from major banana (matooke) was investigated in more detail. Starch is one of the polymers of glucose, which serves in higher plants as the main energy reserve. Starch is an inexpensive, abundant, biodegradable and renewable material that is available from a variety of plants, e.g. wheat, maize or potato.

    In the study by Ssonko and Muranga, the major banana was used as a source of starch [2]. The starch samples were placed on stubs and, after Pt-Pd coating, analysed inside a scanning electron microscope. The shapes observed differed between elongated, spheroid and oval grains.

    The results confirm observations by other researches, who had already reported that banana starch consisted of very different shapes. Only starch from Fougamou, a cooking banana, showed a triangular and pear-shaped structure. Overall, the results of the study provide useful information about the physicochemical and functional properties of starches, enhancing their application in the food industry as a substitute for commercially-available starches.

    The investigation of plants and essential oils derived from them
    Last but not least, an example of essential oils there was an interesting study by Kotronia et al [3]. Oregano is well known to be an aromatic plant, which commonly grows wild in countries close to the Mediterranean Sea. And although the Oregano onites plant and the essential oils derived from it are well known and have been used for centuries, it is only recently that the oils have attracted more research in various fields of applications.

    The oils have been shown to be antimicrobial, antioxidant and anti-inflammatory, resulting from the phenolic content - mainly from carvacrol. Carvacol is even suggested to have significant potential in preventing neurodegenerative disorders.

    In the study, scanning electron microscopy was used to observe final dried β-cyclodextrin-oregano essential oils inclusion complexes. To gain statistical data, 100 particles were imaged, and their average particle size was determined. Kotronia et al were able to observe that the inclusion complexes did not show a special morphology, but had prisms, parallel and smooth sides.

    Additionally, they observed agglomerates of different sizes, with larger particles possibly attracting smaller ones. The average size that could be determined was 2.2µm. Looking to the future, diverse applications for encapsulated essential oils are foreseen, including food, cosmetics and the agricultural sectors.

    Figure 2: Imaging of rosemary with SEM

    Speaking of plants, and the oils derived from them,imaged by a scanning electron microscope; take a look at the leaf tissue in Figure 3 below. We recently found out that some leaves have these star-shaped patterns on their surface. And their size is just 100 µm, or a tenth of a millimetre!

    Figure 3: SEM image of a plant’s star-shaped leaf tissue

    References

    1. Effect of drying procedures on the physiochemical properties and antioxidant activities of polysaccharides from Crassostrea gigas – Hu et al., PLoS ONE, 2017
    2. Partial characterization of starches from major banana (matooke) cultivars grown in Uganda - Ssonko and Muranga, Food Sci Nutr 2017 (5)
    3. Encapsulation of Oregano Essential Oil in β-cyclodextrin: synthesis and characterization of the inclusion complexes – Kotronia et al. bioengineering 2017 (4)

    Topics: life sciences

    About the author
    Dr. Jasmin Zahn is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. She is highly engaged in finding out more about the possibilities for Phenom-World products in various applications. In addition, Jasmin is active in sharing best practices with the outside world to encourage them to look outside their standard scope of use and to improve in their work.

  • Emission stability in SEM thermionic electron sources: CeB6, LaB6 and W filaments

    Typically, desktop scanning electron microscopes (SEM) make use of thermionic sources, from which electrons are emitted when warming up the SEM filament. Although the working principles are the same, different thermionic sources show a different performance. Phenom SEMs are equipped with a CeB6 source because of its higher brightness and longer lifetime. A parameter that plays a crucial role is the emission current stability. How is the CeB6 source performing in terms of stability? What are the engineering smarts that enable the Phenom source to maximize a CeB6 source's potential? This blog answers these questions.

    Working principle of thermionic sources in scanning electron microscopes
    In an earlier blog on electron sources, we discussed the performance of CeB6 and tungsten sources extensively. CeB6 and tungsten are both thermionic sources with a filament called a cathode, from which electrons are emitted. The emission starts when the electrons are provided with enough energy to cross the potential barrier, given by the work function of the cathode material, which can be either tungsten or CeB6.

    The energy is provided by heating up the cathode, which in turn is done by letting current flow through it. A Wehnelt electrode that is negatively charged with respect to the cathode pushes the unwanted electrons back into the filament, effectively determining the size of the emitting area.

    Below the cathode and the Wehnelt electrode, an anode provides a strong electric field, or a strong lens that makes the electron beam converge into a crossover between the Wehnelt and the anode. Figure 1 shows the schematics of the CeB6 source, consisting of a filament, a Wehnelt electrode and an anode. The filament is at high potential, as well as the Wehnelt, whereas the anode is grounded. The circuitry positioned in between the filament and the anode measures the emission current.

    Fig.1: Schematics of a thermionic source, consisting of a CeB6crystal (the filament), a Wehnelt electrode and the anode. In red, the trajectories of the electrons that are pushed back in the filament thanks to the Wehnelt voltage and the trajectories of the emitted electrons, forming the primary beam.

    Comparison between thermionic electron sources: CeB6, LaB6 and W

    CeB6 is not the only cathode for thermionic sources, LaB6 and tungsten are also used.

    Tungsten cathodes are hair-pin filaments that are bent to reduce the size of the emitting surface. They are typically warmed up to a temperature of 2500-3000 K to achieve high current density, being the work function of tungsten 4.5 eV. At 2800 K, a practical value of current density is 3 A/cm2.

    The lifetime of tungsten cathodes, which can vary between 40 and 200 hours, is limited by the evaporation of the cathode material, resulting in the wire breaking when it becomes too thin. To prevent too much oxidation, a vacuum of 10-3 Pa is kept at the source.

    Hexaboride crystals (CeB6 and LaB6) cathodes are rods with a flat tip, and are typically heated up to 1400-2000 K, as the work function is lower than the tungsten (2.7 eV for LaB6 and 2.5 eV for CeB6). A low work function and low temperatures yield a higher current density than tungsten cathodes, in the range of 20-50 A/cm2.

    Typically, hexaboride cathodes are 10 times brighter than tungsten cathodes, meaning they provide higher beam current in a smaller spot size at the sample. Also, the lifetime of hexaboride cathodes is higher, typically 10 times that of tungsten cathodes.

    However, hexaboride cathodes need a vacuum of better than 10-4 Pa to prevent oxidation. The performance of hexaboride cathodes strongly depends on vacuum and temperature. Studies suggest that CeB6 cathodes are less likely to be affected by carbon contamination than LaB6 cathodes. Also, CeB6 cathodes have a lower evaporation rate at a working temperature of 1800 K compared to LaB6. Therefore, the shape of a CeB6 cathode tip lasts longer.

    The following table summarises the physical properties of the three thermionic sources:

    Emission current stability in the Phenom CeB6 electron source
    The stability of the emission current is a key requirement for thermionic sources. During the operation of the microscope, the emission current is kept stable by adjusting the Wehnelt voltage in a constant control loop. The emission current is measured in the source, by a circuitry between the filament and the anode, as shown in Fig. 1. The Wehnelt voltage is then adjusted according to the read out of the emission.

    It is of utmost importance that the current at the sample is kept constant, for given settings. An automated function measures the sample current as a function of the emission current. The emission current is adjusted by varying the voltage on the Wehnelt, thereby regulating the amount of electrons pushed back into the filament, for a constant filament temperature. The current at the sample can be measured indirectly from the signal of an image taken with the BSD detector on a reference material.

    The optimal case, shown in blue in Figure 2, is when the current at the sample has a maximum at 62 µA. If the peak is before or after this value, it means that the temperature of the filament is either too low or too high and needs to be adjusted. The automated function sets the new temperature of the filament and measures the current at the sample for different emission currents again, until the peak falls at the ideal emission current of 62 µA.

    Fig.2: Current at the sample (I spot) as function of the emission current for different filament temperatures.

    Once the temperature is adjusted through the automated function, the voltage on the Wehnelt is set for an emission current of 40 µA, thus on the left of the peak of the blue curve. An emission of 40 µA is chosen as the optimum between resolution and beam current, which translates into image quality.

    The current stability of thermionic sources is typically better than 1% RMS. Measurements on sample current stability of a CeB6 source in a Phenom microscope show that the fluctuation of the beam current is about 0.3 % in the first 5 hours from switching on the source and is 0.2 % from 5 hours up to 15 hours after switching on the source, as represented in Figure. 3. The drop in the first hour is measured to be approximately 10% and is caused by the stabilization of the temperature in the source unit.

    Moreover, the vacuum stability at the source does not affect the emission current of the CeB6 source in a relevant way.

    Fig. 3: Representation of the measured current at the sample over a period of 15 hours.

    There’s much more to discover in Phenom desktop SEMs
    So, the CeB6 source drives the highly reliable and durable character of a Phenom desktop SEM, but it’s hardly the sole contributor. If you take a closer look at Phenom scanning electron microscopes, you will realise that they have a multitude of interesting specifications worth investigating further, like their light and electron optical magnification, resolution, and digital zoom.

    References

    • Introduction to charged particle optics, P. Kruit, Delft University of Technology
    • Scanning electron microscopy, Physics of image formation and microanalysis, L. Reimer, Springer
    • Cathodes for Electron Microscopes, CeBix and LaB6 Filaments Standard Tungsten Loop Filaments, Electron Microscopy Sciences

    Topics: scanning electron microscope, sem filament

    About the author
    Marijke Scotuzzi is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. Marijke has a keen interest in microscopy and is driven by the performance and the versatility of the Phenom SEM. She is dedicated to developing new applications and to improving the system capabilities, with the main focus on imaging techniques.

     

     

  • SEM and TEM: what's the difference?

    Electron microscopes have emerged as a powerful tool for the characterisation of a wide range of materials. Their versatility and extremely high spatial resolution render them a very valuable tool for many applications. The two main types of electron microscopes are the Transmission Electron Microscope (TEM) and the Scanning Electron Microscope (SEM). In this blog we briefly describe their similarities and differences.

    Working Principle of Scanning Electron Microscopes and Transmission Electron Microscopes
    Let's start with the similarities. For both techniques, electrons are used in order to acquire images of samples. Their main components are the same;

    • An electron source;
    • A series of electromagnetic and electrostatic lenses to control the shape and trajectory of the electron beam;
    • Electron apertures.

    All of these components live inside a chamber which is under high vacuum.

    Now over to the differences. SEMs use a specific set of coils to scan the beam in a raster-like pattern and collect the scattered electrons (read more about the different type of electrons detected in a SEM).

    The transmission electron microscopy (TEM) principle, as the name suggests, is to use the transmitted electrons; the electrons which are passing through the sample before they are collected. As a result, TEM offers invaluable information on the inner structure of the sample, such as crystal structure, morphology and stress state information, while SEM provides information on the sample’s surface and its composition.

    Moreover, one of the most pronounced differences between the two methods is the optimal spatial resolution that they can achieve; SEM resolution is limited to ~0.5 nm, while with the recent development in aberration-corrected TEMs, images with spatial resolution of even less than 50 pm have been reported.

    Which electron microscopy technique is best for your analysis?
    This all depends on what type of analysis you want to perform. For example, if you want to get information on the surface of your sample, like roughness or contamination detection, then you should choose a SEM. On the other hand, if you would like to know what the crystal structure of your sample is, or if you want to look for possible structural defects or impurities, then using a TEM is the only way to do so.

    SEMs provide a 3D image of the surface of the sample whereas TEM images are 2D projections of the sample, which in some cases makes the interpretation of the results more difficult for the operator.

    Due to the requirement for transmitted electrons, TEM samples must be very thin, generally below 150 nm, and in cases that high-resolution imaging is required, even below 30 nm, whereas for SEM imaging there is no such specific requirement.

    This reveals one more major difference between the two techniques; sample preparation. SEM samples require little or no effort for sample preparation and can be directly imaged by mounting them on an aluminum stub.

    In contrast, TEM sample preparation is a quite complex and tedious procedure that only trained and experienced users can follow successfully. The samples need to be very thin, as flat as possible, and the preparation technique should not induce any artefacts (such as precipitates or amorphisation) to the sample. Many methods have been developed, including electro polishing, mechanical polishing and focused ion beam milling. Dedicated grids and holders are used to mount the TEM samples.

    SEM vs TEM: differences in operation
    The two EM systems also differ in the way they are operated. SEMs usually use acceleration voltages up to 30 kV, while TEM users can set it in the range of 60 – 300kV.

    The magnifications that TEMs offer are also much higher compared to SEMs: TEM users can magnify their samples by more than 50 million times, while for the SEM this is limited up to 1-2 million times.

    However, the maximum Field of View (FOV) that SEMs can achieve is far larger than TEMs, which users can only use to image a very small part of their sample. Similarly, the depth of field of SEM systems are much higher than in TEM systems.

    Figure 1: Electron microscopy images of silicon. a) SEM image with SED offers information on the morphology of the surface, while b) TEM image reveals structural information about the inner sample.

    In addition, the way images are created are different in the two systems. In SEMs, samples are positioned at the bottom of the electron column and the scattered electrons (back-scattered or secondary) are captured by electron detectors. Photomultipliers are then used to convert this signal into a voltage signal, which is amplified and gives rise to the image on a PC screen.

    In a TEM microscope, the sample is located in the middle of the column. The transmitted electrons pass through it, and through a series of lenses below the sample (intermediate and projector lenses). An image is directly shown on a fluorescent screen or via a charge-coupled device (CCD) camera, onto a PC screen.

    Table I: Summary of the main differences between a SEM and a TEM

    Generally, TEMs are more complex to operate. TEM users require intensive training before being able to operate them. Special procedures need to be performed before every use, with several steps included that ensure that the electron beam is perfectly aligned. In Table I, you can see a summary of the main differences between a SEM and a TEM.

    Combining SEM and TEM technology
    There is one more electron microscopy technique to mention, which is a combination of TEM and SEM, namely Scanning Transmission Electron Microscopy (STEM). It can be applied to both systems, but its full capabilities are revealed when applied to a TEM tool. Most modern TEMs can be switched to “STEM mode” and the user only needs to alter their alignment procedure. In STEM mode, the beam is finely focused and scans the sample area (as SEM does), while the image is generated by the transmitted electrons (like in TEM).

    When working in STEM mode the users can take advantage of the capabilities of both techniques; they can look at the inner structure of samples with very high resolving power (even higher than TEM resolution), but also use other signals like X-rays and electron energy loss. These signals can be used in spectroscopic techniques; the energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS).

    Of course, EDX is also a common practice in SEM systems and is used to identify the chemical composition of samples by detecting the characteristic X-rays that are emitted from the materials when they are bombarded with electrons.

    EELS can only be realised in a TEM system working in STEM mode and enables the investigation of the atomic and chemical composition, the electronic properties as well as local thickness measurements of materials.

    Choosing between SEM and TEM
    From everything we have mentioned, it is clear that there is no “better” technique; it all depends on the type of analysis that you require. TEM is the choice when you want to get information from the inner structure, while SEM is preferred when surface information is required. Of course, major decision factors are the big price difference between the two systems, as well as the ease of use. TEMs may enable much more resolving power and versatility to the user, but they are much more expensive and larger than SEMs and require more effort in order to acquire and interpret results.

    About the author
    Antonis Nanakoudis is Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. Antonis is extremely motivated by the capabilities of the Phenom SEM on various applications and is constantly looking to explore the opportunities that it offers for innovative characterisation methods.

  • SEM and fibre analysis for filtration systems quality control

    The production of filters and membranes undergoes several quality control steps to ensure that the properties of the product are up to specification. Different tools can be used for such analysis, but only one can provide the best results. Find out in this blog how Scanning Electron Microscopes (SEM) can be used to investigate imperfections in metallic filters.

    Different purpose – different filter
    Whether we are aware of it or not, we use many filters on a daily basis – directly or indirectly. For example, tap water is filtered several times before undergoing chemical treatment, prior to becoming drinking water. This is to remove all kind of particles, from big rocks to small sand grains. In extreme cases, the filters can become so thick that they can catch bacteria.

    Air conditioners also employ filtering systems. They are mostly used to trap dust and small particles, but combined with activated carbons, they can catch odours and purify the air.

    Also, some chemical reactions catalysts are typically shaped as metallic meshes with micropores to let the chemicals flow while, at the same time, maximizing the exposed surface and minimizing the pressure required for the fluid to flow in the reactors.

    Whatever their application, all filtration systems typically require cleaning or replacement, to make sure that they will operate correctly over time. The cleaning operation, combined with normal usage, will eventually determine the end of the life cycle of a filter, which will require a replacement.

    Comparison between BSD (image 1a) and SED (image 1b) image of a metallic mesh. In BSD images the color contrast highlights differences in the atomic weight of the analyzed material, making contamination detection easier. SED signal enhances the sample topography and the surface structure of the sample.

    Deeper inspections with SEM

    The easiest way to investigate a filter is obviously to observe it. Optical microscopes can, in many cases, provide plenty of information on larger filters, but in other cases the resolution that they provide is not sufficient for in-depth inspection.

    Scanning electron microscopes, therefore, prove to be the best analysis technique to inspect any kind of filter thanks to the higher magnification and better resolution. SEM show not just the size of the pores in-between the fibers, but also the surface of the fibers themselves, their roughness and much more. So exactly what kind of data can be obtained?

    One tool for multiple analysis

    An electron microscope is an imaging tool that enables the user to perform a highly detailed visual inspection of the imaged samples. However, modern image processing techniques have turned these devices into automated analysis tools.

    A clear example of this is the automated system to measure the diameter of fibers displayed in the image below. Here, software can reconstruct the direction of the fiber and measure it — an operation that would require tremendous efforts and a huge amount of time from an operator.

    In the case of multi-layer materials or coated fibres, the SEM can be used to determine that the size of each layer has the right size, the distribution of the material on the substrate is homogeneous and the roughness is within specifications.

    Filtration efficiency and product lifetime, as well as resistance to washing processes, can largely benefit from the improvements derived from knowing this information.

    Beyond imaging: chemical composition analysis with EDS

    Most electron microscopes are equipped with an EDS detector. This detector collects the EM radiation emitted by the sample to identify the nature of the element it is composed of.

    This analysis can be used on the fibre’s material to determine, for example, how a different composition can enhance the lifetime of the filter. Even more interesting information can be obtained when analysis is performed on the collected particles, providing precious hints on their nature.

    Image processing for measurement automation

    Image processing technologies have improved dramatically over the last few years and software can now do tedious operations that would steal very precious time from an operator. Applying these techniques to fibre analysis has created a powerful tool to completely characterise not just the fibres diameter and their orientation, but also the dimensions of the holes that are formed in between.

    An example of automatic detection of fibers (image 2a) and results displayed in a diameter size distribution histogram (image 2b).
    An example of automatic detection of pores (image 3a) and results displayed in an area size distribution histogram (image 3b).

    About the author

    Luigi Raspolini is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. Luigi is constantly looking for new approaches to materials characterisation, surface roughness measurements and composition analysis. He is passionate about improving user experiences and demonstrating the best way to image every kind of sample.

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