Khoren Sahagian

Khoren Sahagian
Materials Scientist

Editorial February 2015

My colleague, Wally Hansen, and I have just released a new review paper that may be of interest to those of you in the aerospace industry.

As you know, aerospace has had decades of experience with metal bonding, sealing, and corrosion protection. However, many of these methods have not lent themselves well to automation, and they also generate environmental waste.

But new applications are emerging for atmospherically deposited nano coatings that can address these shortcomings. These nano coatings are formed by direct injection of select vapor chemistries into the plasma jet. Such molecular surface modifications yield long-term, environmentally stable interfaces for bonding.

Cleaning

Plasma surface treatment begins with a fine cleaning of the substrate to remove loosely bound organic contaminants (on metal oxide surfaces, the oxide layer is either removed or pre-conditioned for adhesion).

Atmospheric plasma jets provide a multi-faceted cleaning action. First, the charged species within the plasma neutralizes electrostatically bound particles. Next, a vortex of pressurized gas blows away unbound solids and oils. Finally, the substrate is bombarded by reactive gas plasma species.

Low molecule weight contaminants on the substrate surface are efficiently reduced into nascent compounds and removed from the material system. This final phase of cleaning takes place on the molecular scale.

Coating

After cleaning, a different plasma jet technology deposits a functional coating. The coating thickness is controlled by parameters such as distance to substrate and speed. Coatings have been developed that simultaneously provide adhesion promotion and corrosion protection of the bond line. Furthermore, some of these coatings act as reliable tie layers, effectively bridging contact between inorganic and organic systems. There is opportunity to replace liquid primers or otherwise laborious surface pre-treatments used in composite bonding.

So far, atmospherically deposited plasma coatings have demonstrated utility as a release layer, an adhesion promoter, or a corrosion barrier. In practice, the plasma-deposited nano coating combines with conventional protective top coats to amplify corrosion resistance and suppress ingress at damage sites.

The atmospherically deposited plasma coatings show substantial improvement in corrosion performance even at a thickness of less than 500 nm.

Conclusion

Atmospheric plasma cleaning and coating can offer significant advantages in surface preparation of metals and composite systems for bonding and sealing in the aerospace industry. The technology is scalable and amenable to automation as it enables high-throughput material processing.

Furthermore, some plasma implementations are able to combine multiple process steps into a single step while reducing waste and reducing operating costs – without compromising performance.

Recommend
12. February 2015   7:48 pm
Andy Stecher

Andy Stecher
Elgin, IL

I’m pleased and proud to let you know that Plasmatreat was awarded the “Würth Future Champion Award 2015,” which includes a €10,000 prize, at this year’s Summit Meeting for German World Market Leaders.

The award is presented each year by Adolf Würth GmbH & Co. KG, a specialist in the sale and distribution of assembly and fasting materials for professional use. It recognizes mid-sized German companies that demonstrate rapid and sustainable growth at an international level.

In presenting the award to Christian Buske – Plasmatreat GmbH founder, CEO, and managing partner – Joachim Kaltmaier of the Würth Group’s Central Management Board highlighted Plasmatreat’s accomplishments:

The company has discovered a niche market for atmospheric plasma surface treatment using plasma nozzles and has set global standards. Above-average, annual growth rates in double figures are testimony to their outstanding entrepreneurial spirit.

Christian is truly a pioneer in the world of atmospheric plasma treatment, developing technology that can be conveniently incorporated into existing production lines. Since 1995 he has built Plasmatreat to become the global leader in this technology. Many of you are already existing Openair customers, so you know how it works.

Targeted nozzles issue blasts of supercharged air that has been calibrated to a precise degree of ionization in order to effectively clean and modify the treated surfaces. The Openair process eliminates the need for wet chemistry processes and prepares the surfaces for optimal adhesion of subsequent coatings or adhesives.

Hearty congratulations from North America, Christian! I am proud and excited to be part of Plasmatreat’s international team, which is growing every day.

Recommend
1. February 2015   2:11 pm
Dr. K. L. Mittal, Dr. Robert H. Lacombe

Dr. K. L. Mittal, Dr. Robert H. Lacombe

The last issue of the  “SURFACES: THE INVISIBLE UNIVERSE” focused on viewing the chemistry of surfaces through the lens of X-ray Photoelectron Spectroscopy (XPS, aka ESCA).  In this issue we shift focus to the problem of surface modification and in particular polymer surface modification.  This topic is highly apropos in view of the upcoming Tenth International Symposium on Polymer Surface Modification: Relevance to Adhesion, to be held at the University of Maine, Orono, June 22-24 (2015).  This symposium will cover all aspects of polymer surface modification from mechanical roughening to laser modification.  The meeting will be concerned with the those areas where surface modification is a key technology which allows for the processing and manufacture of products which would otherwise be unobtainable.  This meeting will also delve into the realm of biopolymer materials with applications to forest products, medical implants and food processing.  All readers are cordially invited to join the symposium either to present a paper on their current work in this field or to simply attend and greatly expand their awareness of current developments.  Further details are available on the conference web site at:  www.mstconf.com/surfmod10.htm

 SURFACES: LOW VOLUME BUT HIGH IMPACT ENTITIES

 An elementary calculation for just about any cube of a simple atomic or molecular material 1 centimeter on a side indicates that roughly one atom/molecule in ten million resides within ten angstroms of the outer surface.  Though relatively minuscule compared to the roughly 1023 atoms present in the sample, this surface layer determines many of the most important physical and technological properties of the material.  Common examples include optical properties such as reflection of light, all contact properties including friction, surface wetting and adhesion and many chemical phenomena including resistance to corrosion, biofouling and staining.  As important as these properties are, however, surface related phenomena received relatively scant attention during the early decades of the 20th century mainly due to the difficulty in performing careful experiments on such small amounts of material.  Well into mid century the solid state physics community was more focused on bulk properties of solids such as crystallographic structure, electronic band structure and bulk thermodynamic behavior.  What made all this work possible was the relative ease in preparing well characterized samples of uniform quality.  The influence of small amounts of contamination and crystallographic defects is small for bulk samples and more or less in proportion to the amount present. 

 In dealing with surfaces, however, we are faced with an entirely different situation.  The effect of small amounts of contamination and surface defects, rather than being more or less suppressed, can actually dominate the surface properties of the material under investigation.  Thus the basic problem of preparing well characterized surfaces for investigation is far more difficult than in the case of examining bulk properties.  Nonetheless, even though the sensitive properties of surfaces present daunting difficulties for those who want to perform careful investigations, these same properties present opportunities for the technologist who wants to alter surface behavior in order to create useful products and devices.  Nowhere is this opportunity more prevalent than for polymer materials which offer surfaces that can be modified by a number of physico-chemical methods to give specifically tailored results.  What makes this technology even more attractive is the fact that surface modification leaves all the bulk properties of the material intact so that one can, so to speak, have one’s cake and eat it too.  A typical example might be the case of a high modulus fiber being considered as a reinforcing agent in a particular matrix material.  One often finds that the fiber and matrix material are incompatible as obtained off the shelf.  However, appropriate surface treatment of the fiber can make it compatible with the matrix without sacrificing its reinforcing properties making possible the creation of a new composite material.

 MICRO-ORGANISMS AND THE FIBER-MATRIX INTERFACE

 One of the biologically oriented applications of surface modification technology is the removal of pathogens from food products such as berries, fruits and vegetables using atmospheric  plasma technology. Work in this area is currently underway and is planned to be presented at the symposium mentioned above.  However, on the flip side of this development, microorganisms, instead of being removed from a surface can also be added to improve adhesion as was discussed in a most interesting paper given at the first in the Polymer Surface Modification series.

 Since ancient times mankind has taken advantage of the bio-chemical synthesis skills of micro-organisms to create a number of desirable products.  Wine, beer and cheese derived from fermentation are the most recognized achievements of microbial industry.  However, to the best of my knowledge the work of Pisanova and Zhandarov is the first to put the microbes to work in modifying the surface properties of fibers for use in reinforced composites.[1]  These authors worked with the following fiber materials:

  1.    poly(caproamide) (Abbr. PCA)
  2.    poly(p- amidobenzimideazole) (Abbr. PABI)
  3.    poly(p-phenyleneterephthalamide) (Abbr.
  4.    PPTA)
  5.    poly(m-phenyleneisophthalamide) (Abbr. PPIA)
  6.    polyimde (Abbr. PI)

 The above fiber materials were considered as reinforcements for the following matrix materials:

  1.    polycarbonate   Abbr. PC
  2.    polysulfone   Abbr. PSF
  3.    polyethylene   Abbr. PE

 The fibers were exposed to the following micro-organisms:

  1.    Bacillus vulgaris (bacterium)
  2.    Bacillus cereus (bacterium)
  3.    Pseudomonas (bacterium)
  4.    Aspergillus (fungus)

 The authors knew that the above listed micro-organisms could attack and degrade polymer materials.  However, they reasoned that in the case of densely packed fibers the bacteria would be limited to performing their biochemical antics at the fiber surface leaving the bulk properties intact.  Thus each of the fibers was immersed in a nutrient medium containing one of the micro-organisms for up to two weeks to see what changes in the surface properties could be obtained.  The timing of exposure was important so as to limit the action of the organisms to just the fiber surface.  The mechanical properties of the fibers were measured beforehand by standard methods and the adhesion of the fiber to the matrix material was ascertained using the fiber pullout method. The results of their experiments were quite surprising in that in many cases they saw both improved mechanical properties of the fiber and improved adhesion of the fiber to the binding matrix material.  A sample of their results for the PCA/high density polyethylene (HDPE) system is given in Table I.  A cursory look at the data in Table I shows that the fiber mechanical properties are moderately improved by the micro-organism action with an increase in fiber modulus by as much as 18% and the strain at break by nearly 25%.  However, the bond strength of the fiber to the matrix in some cases could increase by nearly a factor of 2 as seen for the Bacillus vulgaris data.

 Also notable from the data in the table is the deleterious effect of overexposure to the microbial treatment as evidenced by the drop off in properties when the exposure time for Bacillus cereus was increased a factor of 4.

  

TABLE I:  Effect of micro-organism treatment on fiber mechanical properties for PCA and fiber/matrix bond strength for the PCA/HDPE system
TREATMENT (micro-organism) TENSILE STRENGTH (GPa) ELONGATION AT BREAK % BOND STRENGTH PCA/HDPE COMPOSITE (MPa)
No treatment 1.52 16 13.1
Bacillus vulgaris 1.74 17.8 21
Bacillus megaterium 1.74 18.2 17.6
Bacillus cereus 1.79 20.2 20
Bacillus cereus(8 week treatment) 1.23 15.5 19.9

 Though the precise details of the microbial action were not uncovered in these experiments a few things were made clear by microscopic examination.  In particular the first stage of the treatment  involved the filling of cavities, pores and microcracks on the fiber surface with the byproducts of the micro-organism’s metabolism which resulted in the healing of the defects and the subsequent improvement in mechanical properties.

In addition the surface chemistry of the fiber is significantly altered making it more compatible with the binding matrix and thus also improving the performance of the composite material.

 As a final word it is clear that this type of work is in its very early stages and that there is tremendous scope for expansion of the types of systems which can be looked at and the extent of improvement that can be achieved.  In addition, since the types of micro-organisms being used are commonly present in the environment this type of surface treatment could be considered more environmentally friendly than other methods which rely on harsh chemicals.

 Dr. Robert H. Lacombe, Chairman Materials Science and Technology CONFERENCES

3 Hammer Drive, Hopewell Junction, NY 12533-6124

Tel. 845-897-1654, FAX  212-656-1016 E-mail: rhlacombe@compuserve.com

[1] “Modification of Polyamide Fiber Surfaces by Micro-organisms”, Elena V. Pisanova and Serge F. Zhandarov, in Polymer Surface Modification: Relevance to Adhesion, Ed. K. L. Mittal (VSP, The Netherlands, 1995) p. 417

Recommend
22. January 2015   12:08 am
Andy Stecher

Andy Stecher
Elgin, IL

skiers

Photo courtesy Trysil via Flickr

I’m not much of a downhill skier myself – which is a good thing, as the terrain here in Chicagoland tends to be pretty flat – but I have many friends both here in the States and in Europe who revel in a day on the slopes. And I’ve got some exciting news for them.

Back in the day, a good coat of hand-applied wax was the only way you could hope to improve the performance of your skis. But now, as with so many things, Plasmatreat is helping to bring ski technology to a new level.

Plasma Nano-Tech at Envipark in Turin, Italy has been working to develop and file a patent application for the innovative “plasma ski,” the goal of which is to make skiers faster and more successful.

Davide Damosso, Director for Innovation and Development at Envipark, notes that the idea was to apply the maximum amount of absorbable wax to the running surfaces of racing skis – made from sintered UHMW-PE (ultra-high-molecular-weight polyethylene) – to improve sliding properties and wax retention. This was achieved using a targeted plasma treatment that modifies the functional characteristics of the surface coating.

“The combination of our Openair plasma technology and PlasmaPlus atmospheric nano-coating process offered the perfect conditions for this project,” says Giovanni Zambon, head of Plasmatreat’s Italian subsidiary, who was responsible for supplying the plasma systems and providing Envipark with technical support during the test phase.

After nine months and 40 laboratory tests, the results have been published – and they are very impressive! Thanks to the microfine plasma cleaning, high level of activation, and plasma coating, which was developed specifically for this purpose and applied with the aid of the PlasmaPlus system, there was a sixfold increase in wax absorption compared with the conventional (but otherwise identical) wax impregnation method.

We are, needless to say, very excited about this – and so is Simone Origone, the world champion speed skier who set a new world record of 252.454 km/hour last March in the French Alps.

“In our discipline we are constantly looking for opportunities to improve our performance,” Origone says. “This new process is extremely interesting. If it transpires that I will be able to ski even faster on snow with this technology, it will prove invaluable to me and the skiing world as a whole.”

Interesting stuff, yes? I am continually amazed by the ingenuity of Plasmatreat’s R&D team and the new, exciting applications for our technology. Perhaps I will see you on the slopes one of these days with your new pair of speedy plasma skis (I will be warm and cozy in the lodge, cheering you on).

Recommend
Category: AUTOMOTIVE
8. January 2015   12:17 am
Andy Stecher

Andy Stecher
Elgin, IL

Photo courtesy Nathan Laurell via Flickr

Photo courtesy Nathan Laurell via Flickr

Have you heard the story (likely apocryphal) about Picasso sketching in the park?

A passer-by recognized him and begged him to draw her portrait, which he graciously did. After enthusing about how wonderful it was, the woman asked Picasso how much he owed her for the drawing.

“Five thousand francs, Madame,” the artist replied.

“Five thousand francs?! That’s an outrage! It took you all of five minutes,” she sputtered.

“No, Madame,” Picasso replied coolly. “It took me my entire life.”

Believe it or not, there are parallels to be drawn here between the great Spanish/French artist and Plasmatreat’s plasma technology.

We here at Plasmatreat are particularly excited this New Year’s because we are celebrating 20 years in business in 2015. Our first atmospheric plasma jet sale for the automotive industry took place in Germany in 1995. Since then, both the science and our service have been evolving—and improving—to the point where we are the leader in our industry.

That type of mastery doesn’t come overnight—as Picasso noted, it takes years and years. A lifetime. We take pride in making it look easy, but behind the scenes we are continuing to hone our craft every single day.

It also means, as you may have surmised, that we are not generally the cheapest plasma technology option available. However, when you look at value per dollar spent, we are confident that we are the very best there is (and our ever-growing list of testimonials lets us know that we’re on the right track).

If you are already a customer of ours, thank you for 20 great years…and counting. We look forward to many more.

And if you’re not already a customer, but you’re thinking about it, give us a call. We’d love to talk to you and explain what makes us stand head and shoulders above the competition.

Two decades of experience in the automotive industry have brought us to a level of expertise that is simply unparalleled (even if our portrait sketches still look more like the work of a two-year-old than the next Guernica!).

Recommend
Category: AUTOMOTIVE
18. December 2014   2:24 pm
Andy Stecher

Andy Stecher
Elgin, IL

I recently returned from Detroit, discussing projects with OEMs and tier suppliers. This really is an exciting time in automotive: CAFE standards, lightweighting, reduction of carbon emissions, and increased occupational safety requirements call for new materials selections and advanced production methods.

We very much appreciate all of our wonderful automotive customers and partners, and we are always looking for ways to serve you better. Along those lines, I wanted to talk to you a bit about what makes servicing our automotive clients so special for us.

Perhaps you, like me, occasionally receive communications from other companies claiming plasma knowledge in the automotive business. While their technologies may appear somewhat similar to ours, nothing could be further from the truth! Plasmatreat is proud to be the only company that offers:

  • ISO 9001, CE, and UL/CSA certifications
  • A proven track record of successful value contribution in the automotive industry for 20 years
  • Fully integrated, automated automotive plant solutions supported by a global service team
  • In-the-field personal technical engineering support plus 3 different laboratories in North America for individual testing
  • Robust R&D services that continually expand our growing list of industry solutions
  • Award-winning industrial product design technology
  • What we believe to be the most diverse private-sector atmospheric and low pressure plasma equipment suite and surface chemistry offerings in the world
  • Exceptional product reliability and customer service. (As one satisfied customer just told me, “I am extremely impressed by Plasmatreat’s level of service. The extra effort with the in-plant training and support is above and beyond expectations.” We are pleased to have many other similar comments in our files!)

 

In short, we here at Plasmatreat take great pride having established the “gold standard” of our industry; we call this Plasmersion!

We hope to have many opportunities in 2015 and beyond to demonstrate Plasmersion to you first-hand. If you ever have any questions about our value proposition, please do not hesitate to get in touch – we’d be delighted to speak with you.

You may also wish to check out our featured case study article in the January 2015 issue of Engine Technology International magazine, which discusses why Plasmatreat’s technology is both earth-friendly and highly effective for various engine manufacturing processes. Click here to read it.

Best wishes for a wonderful holiday season and a productive and prosperous 2015!

Recommend
11. December 2014   8:37 pm
Dr. K. L. Mittal, Dr. Robert H. Lacombe

Dr. K. L. Mittal, Dr. Robert H. Lacombe

  X-RAY PHOTOELECTRON SPECTROSCOPY – XPS

 In last months issue of the INVISIBLE UNIVERSE essay series we established two fundamental reasons why surfaces remain essentially invisible to us even though they are the most common entity with which we interact every waking moment.  The first reason was the fact that our eyes only detect about 2% of the radiation that any given surface can beam at us making us rely on special experimental technologies such as XPS in order to discern otherwise invisible surface structures.  The second reason is that surfaces are subject to the laws of quantum mechanics which determines not only the details of surface structure but also governs the interactions of all forms of radiation with the atomic and molecular entities of which all surfaces are composed.

 Last month’s discussion laid out the fundamental precepts of quantum theory and we now apply them to understand the workings of the XPS experiment.

 As pointed out in last months discussion the essence of the XPS experiment is when an X-Ray ( a high energy photon) impinges on a surface causing an electron ( called a photo electron due to being ejected by a photon) to be ejected from the surface.  The laws of quantum mechanics insure that the ejected electron will come off at a very specific energy which uniquely identifies the type of atom from which it was ejected.   This fact is what makes the XPS experiment so useful in probing the chemistry of any given surface.  To understand this better we need to understand a little more about how quantum mechanics determines the atomic and molecular structure of all matter.

 The story basically begins with a series of  experiments carried out by Hans Geiger and Ernst Marsden1 over the time period from 1908 to 1913 where they were bombarding gold foils with alpha particles and detecting how the particles were scattered.  Up to that time no one really had any idea as to what to expect.  The mechanics of materials at the time assumed all matter to be a sort of homogenous continuum and on the basis of that assumption the best guess was that the particles would pass straight through or be somehow absorbed by the material.  What they found instead was that a fraction of the particles were scattered through large angles up to 180 degrees. Ernst Rutherford the Director of the Cavendish Laboratory commented on these remarkable results as follows:

 It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.

 This remarkable insight led to the notion that atomic matter was something like a miniature solar system where electrons orbit a central nucleus much as earth and the planets orbit the sun.  This all seemed quite plausible for a while since it was known that positive and negative particles attracted each other by an inverse square power law much as the sun attracts each of the planets.  This notion quickly fell apart, however, since the then well known laws of electrodynamics unambiguously predicted that such a system would be unstable.  Classical electrodynamics clearly predicted that the orbiting electrons would quickly radiate away their orbital energy and fall into the central nucleus.

 The conundrum of how atomic matter manages to exist was solved only when physicists attacked the problem using the principles of quantum theory.  In particular Schrödingers equation could be solved exactly for hydrogen, the simplest atom of all, and the resulting solution provided a remarkable template for working out the atomic structure of the rest of the periodic table and in fact provided the theoretical foundation of why the periodic table exists.  The picture that emerged is that a typical atom consists of a small ultra dense positively charged nucleus surrounded by a cloud of electrons where each electron occupies what is called a quantum eigenstate with a very sharply defined quantized energy level.

 At this point we have to wonder what on earth is a quantum eigenstate?  To proceed further we need to pass through the looking glass into the nether world of atomic matter as describe quantum theory which is the most consistent and accurate description that we have.  Ostensibly, from the work of Rutherford and his colleagues, the typical atom consists of negatively charged electrons somehow circulating around a positively charged center being held in a tightly contained cluster by the inverse square Coulomb interaction.  The first thing to note is that the notion that the electrons circle around the nucleus in a manner similar to the way the planets circle around the Sun is completely out the window.  The Heisenberg Uncertainty Principle in particular dictates that we cannot know, even in principle, where inside the atom a particular electron might be at any given time.  All we can know is where the electron tends to spend most of its time, i.e. the probability of finding the electron at any given point at any given time instant.  This comes about because quantum theory dictates that the state of any given electron is prescribed by what is known as a wave function which must be a solution of Schrödingers equation which leads us into the realm of some fairly abstract mathematics dealing with the solutions of differential equations.  Sorting through the details of solving Schrödingers equation would lead us rather far into the hinterlands of differential equation theory but we don’t have to make that journey to appreciate the final result.  The basic results that emerge from laboring through the details are as follows:

Lacombe1

Figure: Approximate energy level diagram for atomic matter

 1) The time independent solutions of Schrödingers equation which describe the atom at equilibrium are solutions to what is commonly known as an eigenvalue problem.

 2) The typical eigenvalue problem states that the differential equation under consideration can have solutions only if certain key parameters have very specific values which arise out of the general eigenvalue solution procedure.

 3) For the case of a simple atom such as hydrogen the key parameters are the spatial coordinates of the orbiting electron or what are commonly called its degrees of freedom.  In Cartesian coordinates these are the (x,y,z) position values.  For atoms it is more convenient to use spherical coordinates so instead of  (x,y,z) we use (ρ,θ,φ) i.e. a radial coordinate plus two angular coordinates.

 4) Thus, for the hydrogen atom the state of the electron is described by 3 eigenvalues one for each independent coordinate.  For the radial coordinate ρ the eigenvalue is called n where n can be any integer 1,2,3 …4. For the angular coordinate θ the quantum number is designated by the integer l which must be less than or equal to n – 1 or in symbols l # n – 1.  For the φ coordinate the eigenvalue number is designated by m which is subject to the constraint -l # m # +l

 5) There is yet one more phantom degree of freedom the electron can have and that is its spin which is designated by the symbol σ and can take on only the values ±1/2.  The electron spin is a wholly unexpected and mysterious degree of freedom that has to exist since it accounts for the magnetic moment of the electron.

 Figure: Approximate energy level diagram for atomic matter

6) The final piece of the puzzle which completes the quantum

description is the Pauli exclusion principle which simply states that no two electrons within an atom can have exactly the same quantum numbers.  Thus each electron must be described by different values of the numbers n,l,m and s.

 The above half dozen results that arise out of solving Schrödingers equation for the hydrogen atom form the basis of the energy level diagram depicted in figure (1).  Thus the energy of the electron which depends on all the degrees of freedom, but primarily on n, must also be quantized and therefore must be depicted by sharply defined discreet levels as shown in the figure.

 This is really a rather wondrous result as it lets us piece together atomic structure and the periodic table from the simple hydrogen atom up to very complex multi electron structures.  Lets cobble together the first few atoms to see how it works:

 1) HYDROGEN: With only one electron which occupies the lowest energy level traditionally designated by the label s which stands for angular momentum l=0.  The x axis labels in figure (1) correspond to the θ quantum number l and again by tradition is given the labels s, p, d, f … corresponding to the l values 0, 1, 2, 3, … respectively.  So for the hydrogen electron in its lowest energy state corresponding to n = 1 it can only have l = 0 according to rule 5 above and thus m must also be 0 according to the same rule.  The spin quantum number σ can be either ±1/2. Thus from the point of view of quantum theory the hydrogen electron is described by the following quadruplet of magic numbers (n,l,m,σ) = (1,0,0,1/2) where in the absence of any external magnetic field the spin quantum number σ could be either + or – ½.

 2) HELIUM: As the process continues to more complex atoms with more electrons we simply fill the levels shown in figure (1) one by one always filling the lowest energy levels first so that at equilibrium the atom has the lowest possible energy.  Helium has a nucleus with 2 protons and thus accommodates two electrons to achieve electrical neutrality and the lowest possible energy configuration has both electrons in the n = 1 quantum level with l = m = 0.  Now the Pauli exclusion principle comes into effect and dictates that one electron will have spin value +1/2 and the other -1/2.  By rule 6 above the n = 1 quantum level is now filled with one electron in the (1,0,0,+½) state and the other in the (1,0,0,-½) state.   Once an n level has the maximum number of electrons allowed by the above rules it is said to be filled and atoms with a filled n level tend to be very stable.  Thus helium is at a filled level and is predicted to be very stable as is well attested by experiment.

Lacombe2

Figure: XPS spectrum of a clean silicon wafer

 LITHIUM: Adding one more electron brings us to the element lithium with three electrons.  The first two electrons go into the n = 1 state with l = 0 and σ = ±1/2. Since the n = 1 state is now filled the third electron has no choice but to move into the n = 2 level with l = 0 and σ = ½ which is the 2s level shown in figure(1).

 HIGHER ATOMS: The process continues with the next element beryllium having 4 electrons completely filling the 1s and 2s levels.  The next atoms consisting of boron through neon must move into the next higher angular momentum state with l = 1 and designated as the 2p level in figure(1).  Since l = 1 now allows the m quantum number to take on the values -1, 0, +1 this level can hold up to 6 more electrons since each m level can accommodate 2 electrons by Pauli’s exclusion principle.  So the next 6

electrons go into the 2p level in the diagram and account for boron through neon.  At neon the 2p level is completely filled and neon is thus predicted to be a chemically stable molecule as is also born out in experiment.  The process continues progressively filling the higher energy levels shown in figure(1) and terminates finally at uranium with 92 electrons and thus 92 protons in the nucleus.  At this level of electric charge the electromagnetic forces in the nucleus reach a par with the normally much stronger nuclear forces and the nucleus itself becomes unstable and prone to fission.  Thus uranium is the largest atom found naturally in the environment though even larger atoms have been produced in large accelerators they have very short lifetimes.

 The above picture immediately implies that the electronic configurations of the different atoms provide unique tags for identifying each one since each configuration comes with a unique binding energy which is simply the energy required to remove an electron from a specific energy level into the void.  Consider for example a fluorine atom sitting on a surface.  The 1s electrons in fluorine have a binding energy of 17.4 ev (electron  volts) so if we see electrons at this energy being emitted from the surface we know that fluorine must be present. This is the basic concept underlying the XPS experiment.  The XPS spectrometer basically bombards the surface with X-rays which have sufficient energy to blast electrons bound to the various surface atoms out of their energy levels and then sorts the emitted electrons by their unique binding energies thus revealing which atoms are present at the surface.  Also the XPS experiment samples only the very top layers of the sample since once an electron has been emitted from its energy level it cannot travel very far through the surrounding material since its electric charge causes it to interact strongly with all the surrounding atoms which can capture or deflect it back into the substrate.  A free electron can typically go no more that about 100 angstroms through the material before being captured or deflected so in effect the electons which are ejected come from the top 100 angstroms of the surface effectively making XPS a surface sensitive technique. 

Figure(2) shows an XPS spectrum of a nominally clean silicon wafer which as manufactured consists of 100% pure single crystal silicon.  The XPS spectrometer, however, reveals a surprisingly different picture of the surface of the wafer.  Those in the microelectronics industry know that an initially clean silicon surface will react fairly rapidly with ambient oxygen to form a layer of silicon dioxide SiO2.  After a day or so sitting out in a clean room the initially pure silicon surface gathers a layer of SiO2 up to about 1000 angstrons thick which is way too thin to be detected by the human eye but is readily revealed in the XPS spectrum in figure(2).  The large central peak in the figure comes from electrons emitted from the oxygen 1s level in the SiO2.  The two small peaks to the far right come from the silicon 2s and 2p levels respectively.  This is not all, however, as the peak just to the right of the central oxygen 1s peak comes from the carbon 1s level and reveals that a contamination layer of a carbon containing molecule is also present.  This carbon contaminant most likely comes from some hydrocarbon or another which are present in nearly every ambient atmosphere.  Finally, the very tiny peak to the left of the central oxygen line reveals that a minute amount of fluorine is present as this peak represents electrons being emitted from the fluorine 1s level.  It is quite unusual for fluorine to be present on a clean silicon wafer and the contaminant level revealed in figure(2) entered through a rather surreptitious mechanism and also caused some rather unexpected problems with the adhesion behavior of this surface.

 Figure(2) represents a clear example of the invisible nature of surfaces by revealing three properties of a supposedly clean silicon wafer that would have been entirely invisible to the naked eye.

 1Geiger, Hans; Marsden, Ernest. “The Laws of Deflexion of α Particles through Large Angles”. Philosophical Magazine. Series 6 25 (148): 604-623(1913)

Recommend
5. December 2014   7:59 pm
Andy Stecher

Andy Stecher
Elgin, IL

PIA_Header720x320

We’re pleased to let you know that Plasmatreat has been selected to present at a brand-new, groundbreaking conference hosted by Plastics News. The conference, Plastics In Automotive: Building Tomorrow’s Car, will take place in Detroit, MI, from January 13-14, 2015.

As more automakers turn to new material choices to reduce the weight of their vehicles and to maximize fuel efficiency, innovations in plastics are driving the development of new vehicles that will spur further growth. Meanwhile, such issues as CAFE standards and the use of composite structures provide grounds for further discussion.

The conference will be exploring many of these important topics. I am delighted to be speaking, on behalf of Plasmatreat, on “Automotive Surfaces and the Prospects for Plasma Coatings.” Plasmatreat offers enabling technology for automotive lightweighting.

The conference will coincide with Industry Preview at the North American International Auto Show and include numerous presentations and panels featuring leading OEMS, Tier One suppliers and other experts on the development of automotive plastics. Attendees will also receive a ticket to the world’s largest auto show, where they can witness the latest in innovation and new vehicles.

As an added bonus, attendees have an exclusive opportunity to register to attend the Automotive News World Congress networking dinners, featuring keynote addresses from Mary Barra, CEO of General Motors, and Sergio Marchionne, Chairman & CEO of Chrysler Group LLC and CEO of Fiat S.p.A.

We’d love to see you there! Click here for more information about the conference.

Recommend
Khoren Sahagian

Khoren Sahagian
Materials Scientist

Editorial July 2014

Plasma treatments are a permanent and covalent substrate modification.  However many references note diminishing effects of plasma treatments with time.  One generalized conclusion is that the plasma modification is a temporary effect.  This conclusion is not inherently accurate or applicable to all plasma and material systems.  In truth there are many factors that govern the success and longevity of a plasma modification.  Research in plasma lacks harmonization in equipment, setup/configuration, and material selection.  These are key variables in a plasma modification.  Results from one method may not necessarily translate well to another experimental setup or class of material.  For this reason some engineering reviews of gas plasma do more to confound than to elucidate the scientific dialogue within industry.

 

Equipment design is of particular relevance in plasma industry.  This includes but is not limited to the electrode configuration, matching, RF frequency, and equipment geometry.  Many apparatus used in academia boast custom fabricated equipment or custom modification to existing tools.  Their equipment exemplifies engineering capabilities.  In my opinion the effectiveness of the equipment to a material system is specific and rarely generalizable to all materials or apparatus.

 

Plasma chemistry and substrate material should be matched correctly.  Some polymer systems may be either resistant or sensitive to specific plasma chemistry.  It is not enough to report gas, pressure, and power.   A complete characterization should understand the plasma stoichiometry and a hypothesis of the surface interaction.  Furthermore it must be accepted that many polymer systems are mobile, may swell with gas or moisture, or may undergo relaxation mechanisms.  Therefore be careful to consider pairing a material system with appropriate plasma source and plasma chemistry.

Recommend
7. July 2014   2:26 pm
Dr. K. L. Mittal, Dr. Robert H. Lacombe

Dr. K. L. Mittal, Dr. Robert H. Lacombe

We continue the INVISIBLE UNIVERSE blog after a delay due to our heavy involvement in the recently completed 9th INTERNATIONAL SYMPOSIUM ON CONTACT ANGLE, WETTABILITY AND ADHESION held at Lehigh University June 16-18, 2014.

Speaking of CONTACT ANGLE phenomena this would be a good time to review this topic as the contact angle method is by far and away the most popular surface analysis method in use today. This technique is of special interest to anyone doing surface modification either by plasma or any other method as it is the simplest and least expensive method for analyzing the impact of any surface treatment whatever. It is fortunate that our office has received a recently published volume on this topic which we will review shortly but first a rudimentary introduction to the concept of contact angle behavior of droplets for the sake of those who may be new to this topic.

Classic definition of the equilibrium contact angle of a drop of liquid on a surface as the balance of three surface tensions.

Fig.(1). Classic definition of the equilibrium contact angle of a drop of liquid on a surface as the balance of three surface tensions.

When a drop of some liquid is placed on a surface it will typically bead up and form a sessile drop as exhibited in Fig.(1). The angle that the edge of the drop makes with the underlying solid is determined by the balance of three surface tensions or surface energies if you prefer. The concept of surface energy has been covered in some detail in the February 2014 issue of this blog so it will not be reviewed here only to point out that surface tensions as measured in Nt/m are dimensionally the same as surface energies (J/m2) which are readily derived from surface tensions by multiplying the numerator and denominator of the units by m (meter) giving Nt-m/m2 or J(joules)/m2.

And now to our book review:

Wetting of Real Surfaces, By Edward Yu. Bormashenko, (Walter de Gruyter GmbH, Berlin/Boston, 2013)

In this rather compact monograph Prof. Bormashenko provides a rather comprehensive account of the theoretical developments in the field of contact angle and wettability of surfaces.

Interestingly, Prof. Bormashenko points out that the field of contact angle and wettability remained rather a backwater endeavor in the field of modern physics from the time of Thomas Young’s pioneering work up to roughly the 1990′s despite the fact that scientific heavyweights such as Einstein, Schrödinger and Bohr devoted a significant portion of their research activity to this topic. Much of this stems from the fact that surfaces presented a rather messy and intractable research topic due to the difficulty in obtaining well defined surfaces free of contamination and other defects. Indeed the eminent theoretical physicist Wolfgang Pauli remarked that “God created matter but the Devil created surfaces”. Thus the solid state physics literature up to about the early 1980′s tended to be dominated by topics such as superconductivity, electronic band structure, phase transitions, semiconductors and similar topics dealing primarily with the bulk behavior of solids.

This all started to change significantly by about the 1980′s being led in large part by the microelectronics industry which was fabricating multilevel thin film structures which were becoming more and more dominated by interfacial surfaces between metals, insulators and semiconductors.  Even by the early 1970′s it was becoming apparent that in order to fabricate devices with higher and higher circuit densities it was critical to understand the nature of the interactions between the various material components at their contact surfaces. This need was supported by advances in microscopy starting with electron microscopy and evolving further to electron tunneling microscopy and finally to the now ubiquitous atomic force microscopy. On top of this a number of surface analysis techniques emerged nearly too numerous to mention the most popular of which being X-ray Photoelectron Spectroscopy (XPS also called ESCA Electron Spectroscopy for Chemical Analysis).

The need for understanding surface properties was of course not limited to the microelectronics industry. The entire coatings industry needed to understand the wetting properties of various paints and inks and the biotechnology industry dealing with medical implants needed to understand how the surfaces of their devices would interact in the in vivo environment. The contact angle technique thus started to emerge as a low cost and highly sensitive method for exploring the wetting behavior of surfaces.

A critical juncture of sorts was achieved with the work of Barthlott and Neinhuis in 1997[1] who first studied the extreme hydrophobicity of the lotus leaf and its effect in removing all manner detritus from the leaf’s surface. This work lead to a literal explosion of work on the superhydrophobic effect and a variety of applications to self cleaning surfaces and other highly innovative technologies.

Getting back to Prof. Bormashenko’s volume a brief look at the table of contents reveals a rather wide range of topics:

  1. What is surface tension
  2. Wetting of ideal surfaces
  3. Contact angle hysteresis
  4. Dynamics of wetting
  5. Wetting of rough and chemically heterogeneous surfaces: the Wenzel and Cassie models
  6. Superhydrophobicity, superhydrophilicity and the rose petal effect
  7. Wetting transitions on rough surfaces
  8. Electrowetting and wetting in the presence of external fields
  9. Nonstick droplets

There is clearly not enough space here to cover all of the above topics in any detail so we will focus on chapter 6 dealing with super hydrophobic and hydrophilic phenomena which, as Prof. Bormashenko points out, are among the currently most actively researched topics in the contact angle field.

Schematic illustrating the difference between a truly superhydrophobic surface and one exhibiting only a high contact angle

Fig.(2). Schematic illustrating the difference between a truly superhydrophobic surface and one exhibiting only a high contact angle

Interestingly, the author points out that exhibiting a high contact angle is not sufficient to define a state of superhydrophobicity as might casually be assumed. In addition the contacting water drop must also exhibit low contact angle hysterisis.  That is the advancing and receding contact angles must be approximately the same. This property is required for the so called “lotus leaf” effect where water drops not only form with a high contact angle but also roll very easily off the leaf carrying any collected debris with them as shown in Fig.(2).

The counter example is the “rose petal” effect reported on by Jiang and co-workers.[2]  These investigators looked at water droplets on rose petals which also form very high contact angles but unlike the lotus leaf case these drops also exhibit a very strong hysterisis. An immediate consequence is that these drops do not roll even when held at a steep angle as also shown in Fig.(2).

Example of a hierarchical relief morphology.

Fig.(3). Example of a hierarchical relief morphology.

 A further subtle point brought out in the chapter is the fact that the substrate material does not have to be highly hydrophobic in order to exhibit the superhydrophobic effect. The lotus leaf material is in fact hydrophilic. What gives rise to the superhydrophobic behavior is the hierarchical relief morphology of the surface. An example of such a structure would be the fractal Koch curve shown schematically in Fig.(3). The author goes on to analyze the wetting of these highly variegated surfaces in terms of the Wenzel and Cassie models covered in chapter 5.

In all this volume can be highly recommended to anyone interested in coming up to date on the latest theoretical developments in the rapidly expanding field of contact angle phenomena.

The author invites any inquiries or comments on this article.


 [1] “Purity of the Sacred Lotus or Escape from Contamination in Biological Surfaces”, W. Barthlott and C. Neinhuis, Planta, 202, 1 (1997).<
[2] “Petal effect: A superhydrophobic state with high adhesive force”, L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia and L. Jiang, Langmuir, 24, 4114 (2008).
Recommend