Topics Covered
Background
Fine Particle
Technology
Ancient Technologies
Porosity, Particle Size
and Geology
Airborne Particles
Optimum Particle
Size
Surface to Volume Ratio
Porosity
Behavioural Aspects of
Fine Particle Systems
Measuring Particle
Size
Particle
Size
Equivalent
Spherical Diameter
X-ray Sedimentation
Sedimentation Velocity of Suspended
Particles
Sedimentation Analysis
Static Light
Scattering
Scattering
Pattern Characteristics
High-Resolution
Angular Detection
Liquid Solid
Dispersions
Electrical Sensing
Zone
Surface Area
Molecular Level Surface Area
Measurements
Adsorption Theory
Density Functional Theory
Pore Size and Volume
Firing and Sintering
Pore Size
Determining Pore
Volume
Microporous
Material Analysis
Density
Volume and Density Determination
Envelope Density
Bulk
Density
Active Surface
Characterization
Chemisorption
Catalysts
Chemisorption
Instruments
TPD, TPR and TPO
Temperature Programmed
Reduction
Temperature Programmed
Oxidation
Temperature Programmed
Chemisorption
Static Volumetric
Chemisorption System
Automatic Data
Reduction
Nanomaterials
Micropores, Mesopores and Macropores
Conclusion
Background
Since prehistory
man has been aware of the importance of particle size in producing resources and
wares with desired properties. Archeological evidence indicates that paints used
for cave wall paintings are mixtures of finely pulverized pigmenting materials,
predominantly carbon, ochre and hematite. Man came to realize that adding
pulverized materials to clay not only improved its workability, but improved the
drying process, reduced shrinkage and changed the characteristics of the
resulting vessels. There also is evidence of using particles of certain sizes to
control porosity.
For many centuries,
finely divided, calcined lime powder or gypsum mixed with sand was used in
plasters and binders. Then, about 2000 years ago, the Romans improved upon the
formula by adding volcanic (pozzolanic) ash, which produced a superior hydraulic
cement that was used in building many structures that still
stand.
Today, finely
divided particulate materials and objects that incorporate or are produced from
these fine particles are everywhere about us. Frequently encountered powders
include cement, lime, fertilizer, cosmetic powders, table salt and sugar,
detergents, bath and dental powders, coffee creamer, baking soda, and many other
household items. Products in which the incorporation of powders is not so
obvious include paint, toothpaste, lipstick, mascara, chewing gum, magnetic
recording media, many medicinal products, slick magazine covers, floor
coverings, and automobile tires.
Such everyday items
as fused ceramic bathroom fixtures and many small metal objects produced by
powder metallurgy completely obscure their origins as powders. The gold trim of
dinnerware, for example, started as a carefully controlled fine powder. Even
microwave cooking utilizes particle technology. The desire to brown some foods
cooked by microwave was solved by a wrapping of metalized polyethylene
terephthalate (PET) film, a material containing fine grained metallic material
that absorbs microwaves and produces localized elevated
temperatures.
Fine Particle Technology
The applications of
fine particle technology by no means are limited to commercial products, nor is
the need to determine the properties of finely divided materials restricted to
one area of technology. It begins in mining with discovering how fine an ore
must be ground to release the sought-after mineral.
Ancient Technologies
Detailed physical
studies of archeological items indicate that these processes were known in
ancient times. Fine ceramic artefacts indicate a knowledge of processing
naturally occurring rocks and minerals to control purity as well as particle
size distribution in the clays, glazes, and pigments. Plasters used in
decorating the pyramids and mortars used by Roman masons indicate similar
attention to particle size.
Porosity, Particle Size and Geology
Today the porosity
of limestone and sandstone is characterized by quarry source and related to its
expected rate of deterioration in urban environments before it is used in
restoration of historic monuments. The mortars and plasters used in ancient and
modern times owes its characteristics almost wholly to the selection of the
proper sizes of the lime and filler particles.
Earth scientists
use particle technology to solve various mysteries of
nature.
Geologists study
the textural characteristics of clastic rocks to extract clues to the methods of
transportation, sorting, and deposition of the fine materials incorporated in
these rocks. This provides valuable information about the history of natural
events and processes such as water flow, winds, glacier movement, and marine
currents that occurred at the depositional site prior to
lithification.
Petroleum
geologists study the physical characteristics of strata deep within the earth in
order to determine the capacity of the field and to assess the effort required
to remove the petroleum. Oceanographers measure characteristics of marine
sediment for clues to its origin as well as to determine its mechanical
properties for mooring. Soil scientists examine characteristics of near-surface
soils to assess qualities associated with agricultural production. Many of the
physical characteristics of interest to these scientists are dependent upon
characteristics of the fine particles from which the materials are
composed.
Airborne Particles
Climatologists are
concerned with airborne particles that affect weather, and historical
climatologists study particle depositions in ice cores as evidence of weather
patterns over thousands of years. Climatologists, paleontologists and other
natural scientists have found evidence linking mass extinction to an excessive
number of fine particles in the upper atmosphere that shielded solar energy from
the earth’s surface, initiating a chain of events that devastated flora and
fauna world-wide. Civil engineers study the grain size of subsurface soils to
assess load bearing capabilities.
Environmentalists
must know the capacity of adsorbents such as carbon granules in order to prevent
escape of harmful vapors into the atmosphere. They also must characterize soil
to determine the percolation rate, diffusion, and retention characteristics of
hazardous substance spills. These bulk characteristics, too, are dependent upon
the characteristics of the individual particles that compose the
bulk.
Optimum Particle Size
There is an optimum
particle size, or at least a smallest and largest acceptable size, for most
items involving particles.
The taste of both
peanut butter and chocolate is affected by the size of their respective
ingredients. Extremely fine amorphous silica is added to tomato ketchup to
control its flow. Pharmaceutical tablets dissolve in our systems at rates
determined in part by particle size and exposed surface area. Pigment size
controls the saturation and brilliance of paints. The setting time of concrete,
dental fillings, and broken-bone casts proceeds in accordance with particle size
and surface area exposure.
Some materials,
gums in particular, do not dissolve in water but absorb water to form viscous
colloidal sols. The particle size of the powder determines the type of
dispersion. Larger particles form a discontinuous mucilage and fine powders
yield homogeneous dispersions. The former is an effective ingredient in
laxatives while the latter finds use in adhesives.
Surface to Volume Ratio
Controlling the
surface-to-volume (surface-to-mass) ratio is one reason for manipulating
particle size. Another is to control interparticle pore size and pore volume for
specialized applications. For example, at the turn of the nineteenth century,
filters having sub-micron pore sizes were constructed from diatomaceous earth
and used to retain bacterium. However, it was demonstrated that infectious
particles far smaller than bacteria could pass through these filters, leading to
confirmation of the existence of unfilterable infectious elements called
‘viruses’.
Porosity
Surface area and
porosity as a function of particle size or surface area and porosity independent
of particle size are other physical characteristics that play an important role
in particle technology. The effectiveness of odor removers depends on the active
surface area of the adsorbent in them. The tightness of the weave of a cloth
raincoat, and therefore its porosity, is adjusted to retard water penetration
but permit air and vapour passage for comfort. Adsorbent towels and tissues, on
the other hand, are made to have pores that readily wick up liquids. The tips of
felt-tip pens have a still different requirement: their pore structure must hold
a viscous ink but release it when compressed.
The pore structure
of prosthetic devices influences whether or not tissue will attach. There is
even a connection between the Sphinx of Egypt’s Giza plateau and porosity. The
Sphinx may be coerced into revealing its true age thanks to the porosity of the
stone from which it is made. A model of the weathering process based on the
porosity of the stone has been suggested that may yield a timeline back to the
date of its creation.
Behavioural Aspects of Fine
Particle Systems
Many behavioural
aspects of fine particle systems come about simply because of the relatively
large amount of surface exposed to its surroundings. As matter is subdivided,
the free energy of the system increases proportional to the amount of new
surface created. The work required to achieve the new surface is equal to or
greater than the increase in free energy. However, the laws of thermodynamics
dictate that a system spontaneously will seek the lowest free energy state that
is possible under the circumstances. The study of the behaviour of the system in
seeking this state, and how it can be manipulated and utilized is the domain of
fine particle technology.
A thought
experiment that exemplifies these principals is as follows. Consider a container
of oil and water, the oil floating above the water, the two liquid phases being
separated by a surface of minimum area and minimum free energy. Adding work to
the system by vigorously shaking the container results in oil droplets being
dispersed in the water and the total surface of the oil-water interface being
greatly increased. However, when allowed to again stand at rest, the droplets
join to form larger and larger drops of oil, each having less surface than the
sum of the surfaces of the individual droplets that formed it, thereby reducing
surface free energy. This behaviour continues until the minimum interface is
achieved, that is, one mass of oil floating above the mass of
water.
The system could be
manipulated by adding a surfactant that would be attracted to the surface of the
oil droplets, thus lowering the free energy of these surfaces and suppressing or
prohibiting their coalescence when the input of agitation energy is
ceased.
The mechanism
employed to achieve minimum energy in the example above is through the mutual
attraction of matter. This non-specific attractive force is commonly referred to
as van der Waals force. It gives rise to the phenomenon termed physical
adsorption (or physisorption) and is also responsible for surface tension and
condensation in liquids. At high temperatures surface energy is likely to be
reduced by electron sharing and valence bonding with gas atoms creating the
phenomenon known as chemical adsorption (or chemisorption). As has been
exemplified, some of the attraction can be reduced by the addition of
surfactants, which give rise to what is called double-layer phenomena.
Measuring Particle Size
Obviously, all the
special attributes relating to particle size, surface properties, and pore
structure could not have been achieved without precise measuring
means.
Particle size
probably was measured crudely first in ancient Egypt. Surviving wall paintings
show ground foodstuff being sieved—possibly through a rough cloth of woven
reeds—to remove the large bits for further grinding. While undoubtedly it was
recognized long ago that grinding to smaller and smaller sizes exposed
progressively more surface area and promoted dissolution, truly assessing the
extent of the exposed area and the consequences thereof got its start only in
the eighteenth century.
This is when it was
discovered that charcoal heated and then cooled without exposure to air would
take up several times its own volume of air upon subsequent exposure. That pores
in the charcoal accounted for much of the gas uptake by its condensation in them
and that all solids exhibited adsorption phenomena to different degrees was
learned by the mid-nineteenth century. From that came the realization that gas
adsorption measurements could yield much information about the physical surface
and pore structure of solids.
Continuing
experimentation early in the twentieth century with gases being first adsorbed
and then removed by heating revealed that more was involved in some instances
than just physical adsorption. Oxygen gas, for example, removed from carbon was
found not to be pure oxygen but to contain oxides of carbon. This suggested that
two processes were involved in gas uptake on solids: one of purely physical
character which, as used above, was given the designation physical adsorption,
and one involving a chemical reaction which is termed chemisorption. Adjacent
chemisorbed atoms become susceptible to reaction with one another to form new
chemical species when the proper surface structures and conditions are present.
This we now know is the action of catalysts. Today, chemists and chemical
engineers tailor the pore size and surface properties of catalysts to produce
everything from shortening to gasoline.
Providing
quantitative measures of the several parameters defining particle size, surface
area, pore size and volume, surface activity, object density, and a few other
more specialized subjects is the purpose of the instruments and services Micromeritics
offers.
Following are
details of just what is being determined when each measurement is made with Micromeritics’
instruments.
Particle Size
If all fine
particles were spheres, their size would be defined explicitly by their diameter
or radius. If cubical, the length along one edge would be characteristic; if of
some other regular shape, another equally appropriate dimension could be chosen.
Unfortunately the great majority of particles are quite irregular and an
arbitrary definition of “size” is the only resort, short of detailed examination
of each particle. Moreover, every collection of particles contains particles of
many different sizes, commonly referred to as the particle size distribution.
Hence a practical definition of particle size must permit a great number of
particles to be examined in a relatively short time.
Equivalent Spherical Diameter
What is termed an
equivalent spherical diameter best meets the requirement for a non-specific
measure. Equivalence of size means that the “diameter” assigned to an
irregularly shaped particle is the same diameter as that of a sphere which
behaves identically when both are exposed to that same
process.
There are numerous
manual and automated techniques by which to determine the mass vs. equivalent
size distribution of a collection of particles. Selecting the most appropriate
technique is critical in attaining reliable data. No single technique is
superior in all applications.
X-ray Sedimentation
Micromeritics’
SediGraph particle size analyzer measures the distribution of
equilibrium velocities of particles settling through a liquid under the
influence of gravity. Stokes’ law relates these velocities to particle diameters
for spherical particles. The instrument determines the settling velocity of the
particles and applies Stokes’ law to determine diameters. It thus measures
non-spherical particles in terms of the diameter of a sphere of the same
material that settles at the same velocity, i.e., it determines equivalent
spherical diameters.
Most powders used
in manufacturing processes are at some point mixed into or compounded with a
liquid. Predicting the behaviour of such a mixture is more likely to be
successful if particle diameters are known. Since the sizing of particles by the
sedimentation technique also involves dispersing powders in a liquid, the
analysis essentially is performed in situ. This benefit also extends to studies
of marine silts and sediments, the deposition of the solids being dependent upon
their sedimentation velocity in a liquid, a fundamental measurement when sizing
by the sedimentation technique.
Sedimentation Velocity of Suspended Particles
The sedimentation
velocity of suspended particles can be obtained by measuring the quantity of
sediment produced as a function of time or by measuring the concentration of
particles remaining in suspension with time. The latter approach is preferable
mathematically and is employed by Micromeritics.
The instrument design in which this approach is implemented utilizes a beam of
low-energy X-rays to measure mass concentration in terms of the transmittance of
the suspension relative to the suspending liquid. The transmittance to X-ray
wavelengths is a function only of the mass concentration of the suspended
particles. The X-ray beam is extremely narrow in the vertical dimension, and
because it does not disturb the suspension it constitutes an ideal measuring
probe.
Small particles
settle quite slowly under gravity. To avoid the long settling times that would
be required to measure both the larger, faster-settling particles and the
smaller, slower-settling ones, the cell containing the particles is moved
downward with time relative to the X-ray beam. The entire cell is thus scanned
in a matter of minutes and particle size resolution is achieved as rapidly as
could have been obtained by centrifuging the cell but without the mechanical
complications of a rotating element.
Sedimentation Analysis
Most of the
analysis processes are automated to reduce or eliminate operator error, thus
assuring repeatability and reproducibility of results. As examples, the movement
of the cell is computer controlled as is the introduction of sample and the
flushing away of it when the test is done. An accessory unit permits multiple
samples to be selected and then analyzed automatically in any order
desired.
Powdered materials
having diameters from 0.1 to 300 mm (micrometers) can be measured with a
precision of 1 mass percent over the entire size range provided three criteria
are met: the particles must be more dense than the liquid in which they are
suspended; the particles must disperse, or break free of one another, in the
liquid; and the particles must absorb more X-rays than the liquid so that
adequate contrast with the liquid is created. The last criterion generally means
that the materials must contain elements having atomic numbers greater than 11
(sodium).
Powders,
particularly fine ones, often are difficult to disperse, i.e., separate into
individual entities with each particle free and not attached to one or more
other particles.
Unless the
dispersed state is achieved, particle size measurement by sedimentation, or any
other method for that matter, can be greatly misleading. Micromeritics
has developed a series of liquids which greatly facilitates the dispersion of
difficult-to-disperse powders.
These liquids are
available in both aqueous and organic formulations.
Static Light Scattering
The size of
particles also can be determined from the manner in which they scatter
light.
The most common
application of this technique is low angle light scattering (LALS) in which an
assemblage of particles is illuminated by a source of monochromatic, coherent
light. This is the technique employed by Micromeritics’
DigiSizer.
In this instrument
design, a lens is positioned in such a way that light scattered at a specific
angle from any particle in the illuminated zone will intersect the focal plane
at a specific distance relative to the focal point. The intensity of scattered
light is measured at a number of predetermined positions corresponding to a set
of scattering angles. Using these intensity vs. forward angle measurements, Mie
or Fraunhofer theory (a special case of Mie theory) can be applied to extract
particle size information. Mie theory predicts the intensity of scattered light
over a 180-degree range of scattering angles. Using intensities measured only at
low angles (<90 degrees), the sizes of particles over a range of about 0.1 to
1000 mm can be determined. Mie theory, in the strictest sense, applies only to
spherical, isotropic particles with specific and known optical properties.
However, Mie theory most often is applied to particle systems that do not
exactly conform to the theoretical model. As with the sedimentation technique,
particle size is reported as equivalent sizes. In the case of light scattering,
the reported quantity vs. size distribution is that of spherical particles that
most closely reproduce the same scattering pattern as that of the particle
assemblage being analyzed.
Scattering Pattern
Characteristics
All information
about particle size and quantity resides in the intensity versus angle
characteristics of the scattering pattern; therefore, precise measurement of the
light scattering characteristics are fundamental to obtaining good particle size
data. A unique design feature of Micromeritics’
DigiSizer is the use of a high-resolution detector array (a
charge coupled device or CCD) to measure scattered light. The spatial density of
detector elements is so great that several million measurements are collected
between 0 and 36 degrees of scattering angle and an angular resolution of a few
thousandths of a degree is achieved. Due to the symmetry of the scattering
pattern in the area of measurement, many of the intensity measurements are for
the same scattering angle and these redundant measurements provide real-time
signal averaging.
Another benefit
gained by use of a CCD is a means of accommodating a wide range of light
intensity. This is because the CCD is inherently an integrating device rather
than a current-generating device such as a photodiode. The charge accumulated by
a CCD element is proportional to the product of the intensity of incident light
and the exposure time. Very low light intensities are measured by allowing long
exposure times, and very high light intensities are measured using microsecond
exposures. This capability is important in measuring a scattering pattern in
which light intensities can vary over a range of ten orders of
magnitude.
High-Resolution Angular
Detection
The high-resolution
angular detection allowed by the area array permits the position of the optical
axis (the position of the central, unscattered light beam) to be determined
within a fraction of one pixel element, that is, a few thousandths of a
degree.
This point
represents the origin of the polar axis about which the scattering pattern is
centered. Relative to this point, a scattering angle can be assigned by software
to all other detector element. If any mechanical or optical deviations cause the
optical axis to move from the zero point, it is promptly determined by software
and the detector array is dynamically remapped, thus, mechanical alignment is
unnecessary.
Once the scattering
pattern has been characterized by a set of angle vs. intensity data, the final
step is to determine the sizes and quantities of particles that will most
closely reproduce the measured scattering pattern. This is accomplished by an
iterative process of fitting theoretical models to the data using a non-negative
least square method.
Liquid Solid Dispersions
The same caveat
about liquid-solid dispersions that applies to the sedimentation technique and
the SediGraph applies also to particle sizing by static light
scattering.
Unless the
particles are separated, a true mass vs. size distribution cannot be
achieved.
However, in some
applications, the objective may be to study dispersion or flocculation
characteristics. In this case, a sample recirculating system such as the DigiSizer’s Liquid Sample Handling System provides a means by
which the size distribution characteristics of the same sample can be measured
repeatedly as the process under study evolves.
Electrical Sensing Zone
The electrical
sensing zone (ESZ) technique, also known as the Coulter principle, analyzes the
sample particle by particle rather than examining an assemblage of particles as
done in the two techniques discussed previously. Micromeritics’
Elzone analyser utilizes this technique to count and size
particles.
To analyze a sample
by the ESZ technique, a homogeneously dispersed suspension of sample material is
prepared in an electrolytic solution. A tube with a small aperture of short path
length is submerged in the suspension, an electrode being positioned on both
sides of the aperture. A pump establishes a flow of electrolyte through the
aperture, providing a conductive path between the two electrodes and a small
electrical current is established between them. Both electrolyte and particles
pass through the aperture. The particles, being non-conductive, impede the
electrical current flow as they enter the orifice. This creates an electrical
signal proportional to the volume of the particle in the aperture. Each
individual particle is counted and classified according to volume, thus
producing a volume frequency distribution. If the particles are considered to be
spherical, then particle diameter can be determined from
volume.
The concentration
of particles in the electrolyte is very dilute since two or more particles
entering the orifice in close succession will cause an erroneous signal.
However, statistical probability dictates that a coincidence of particles will
occur in the orifice now and then, so a coincidence correction routine is built
into the software to correct for such events.
The ESZ technique
is applicable to a wide range of sample materials including plant and animal
cells. It is particularly useful when the number distribution of particles by
size needs to be determined. ESZ also is a very high-resolution method of
particle sizing.
Surface Area
There is an inverse
relationship between particle size and surface area. A cube one centimeter on an
edge has a surface area of 6 cm2. If the cube were fragmented into smaller
cubes having edges of 0.1 cm there would be 1000 of the smaller cubes and the
total surface area would have become 60 cm2 . This ideal relationship is unlikely ever
to be encountered because irregular particles break into smaller particles with
a range of sizes and shapes. Actual particles of whatever size, if examined on a
molecular scale, display planar regions, but they also are likely to include
lattice distortions, dislocations, and cracks. This means that the actual
exposed surface of particles is greater – sometimes very much greater - than
would be calculated assuming any one geometric shape.
Molecular Level Surface Area
Measurements
Micromeritics’
provides several types of surface area instruments that permit determination of
surface areas at the molecular level by measurement of a low temperature
isotherm. At the upper end of the range are multipurpose, sophisticated units
capable of providing timely results for both quality control and research and
development needs. Midrange instruments include those for high throughput,
round-the-clock, reliable service for quality and production control purposes.
At the lower end are inexpensive, semi-automatic and manual instruments for
occasional usage. Properties other than surface area can be determined from the
data provided by these instruments – chemical activity and pore structure are
examples; only the surface area function is described in this
section.
All such
instruments first free the sample of moisture and atmospheric vapours by
application of heat and either evacuation or purging with a non-adsorbing gas,
usually helium or nitrogen (nitrogen may adsorb at room temperatures on some
materials). Then the sample temperature is reduced to that of liquid nitrogen,
liquid argon, or another coolant appropriate for the gas or vapour to be
adsorbed. The adsorbing gas is admitted in incremental doses in one instrument
design (static volumetric technique), continuously as the sample itself permits
in another design (adaptive rate technique), and as a component in a flowing
mixture with nonadsorbing helium in still another design (dynamic, or continuous
flow technique). The accumulated gas quantity adsorbed vs. gas pressure data at
one temperature are then graphed to generate what is called an adsorption
isotherm.
The data are then
treated in accordance with gas adsorption theories to arrive at a specific
surface area value for the sample.
Adsorption Theory
Classical
adsorption theory that has been in use since the 1930’s and is still in use
assumes that gas molecules admitted under increasing pressure to a clean, cold
surface form a layer one molecule deep on the surface before beginning a second
layer. The data treatment technique finds the quantity of gas forming this first
layer, and then the area covered is calculated from the number of molecules of
the gas and gas molecule dimensions. Actually, adsorbed gas molecules do not
attach to a solid surface and thereafter remain attached while other molecules
build upon them. In the first place, there are regions on all surfaces that are
more attractive to gas molecules and regions that are less so. What we call
adsorption is really the manifestation of a continual exchange between gas
molecules temporarily residing on a solid surface and those nearby in the gas
phase. The number of molecules attached to a solid at any instant increases as
gas pressure increases until a point is reached where statistically it is
reasonable to consider a monolayer to have formed. Only in the sense of an
average condition does an adsorbed monolayer ever really exist,
however.
Many modifications
to the classical model have been offered over the years, some based on empirical
or semi-empirical foundations and others derived from either thermodynamics or
kinetic theory. All of these data reduction methods have the common attribute of
applying only to a certain segment of the isotherm rather than over the full
range. A more modern approach is to begin with basic principles of statistical
thermodynamics, combine these building blocks with newly available computational
techniques, and seek a single or unified theoretical model applicable over the
total range of the isotherm. This approach utilizes density functional theory,
and an efficient implementation of this theory by Micromeritics
enables rapid calculations of this once computationally intractable data
reduction task.
Density Functional Theory
Density functional
theory is a means by which the exact population density of a system of molecules
at a specific temperature and pressure can be expressed mathematically. When the
expression is solved for the state of minimum energy, the population density
profile at equilibrium is described. Since the energy of the system must take
into regard surface energies of a solid substrate exposed to the system of
molecules, the population density profile reveals how various layers of
molecules have formed on and near the solid surface. This method allows the
acquisition of a family of profiles that describes gas adsorption over a range
of pressures from near zero to the saturation pressure.
The less
sophisticated Micromeritics instruments start with classical theory, but
others, through their software computational capability, can report results
obtained by classical theories as well as by recent, popular adsorption theories
with applicability limited to specific conditions and ranges. Density functional
theory can be utilized with all adsorption isotherms. However, it is best used
with the more sophisticated instruments that are capable of collecting high
resolution low-pressure data, thus, providing the highest quality
results.
Accessory equipment
is provided for minimizing operator involvement and speeding sample preparation.
This includes units for degassing samples by the flowing gas method or by
applying heat and vacuum. A liquid cryogen storage and transfer system also is
provided for making more convenient the supplying of sample coolant for all
instruments.
Pore Size and Volume
Solid particles
from crushing or grinding operations and weathering or leaching processes often
will be found to have cracks, cavities, and holes (collectively called pores)
within their structure. Solid particles produced by condensation or
crystallization processes may contain, or acquire after a period of time, cracks
along grain boundaries and at positions where impurities are occluded. Fine
particles also tend to stick together, or adhere to form aggregates or larger
secondary particles, giving rise to another level of pore
sizes.
Firing and Sintering
Adhesion is
accelerated at elevated temperatures and with the mechanical application of
pressure. Industrial processes that make use of this property are called firing
in the case of pottery manufacture and sintering in powder metallurgy. Many of
the pores in industrial products thus are comparable in dimensions to the
primary particles themselves. In these cases the walls of the pores are the
exposed surfaces of the particles, and, not surprisingly, these pores are likely
to exhibit interconnectivity and great tortuosity. A few natural materials such
as kaolin clay and mica occur as more or less orderly stacks of thin plates;
graphite also can be produced with a similar structure. The pore dimensions are
very small in one direction and relatively large at right angles to that
direction.
Pore Size
Pore size is
expressed either in terms of the diameter (or radius) of the opening, assuming
it cylindrical, or simply as the width in a more general sense. Pores of widths
less than 2 nanometers (nm), or 20 Angstrom units (Å), are referred to as
micropores.
Pores having widths
from 2 nm to 50 nm (500 Å) are called mesopores, and pores of larger widths are
referred to as macropores. The volume of all cracks, fissures, holes, channels,
etc., within the body of particles or of larger objects is the total pore
volume.
Micromeritics
products acquire detailed pore information in two ways:
- Gas adsorption,
and
- Mercury
intrusion.
Determining Pore Volume
The first technique
for pore volume assessment condenses a gas in the pores and derives pore volumes
from the quantities of gas (converted to condensed liquid volume) required. In
the presentation above on surface area evaluation, it was described that an
inert gas admitted to clean, cold surfaces first adsorbs to a monomolecular
layer according to the classical viewpoint. Admitting more gas causes the layer
to thicken to a depth of several molecules and, ultimately, to a layer of
infinite thickness, i.e., condensation to bulk liquid when the saturation vapour
pressure is reached. If, however, the solid is porous so that it has internal
surface area, condensation of the gas will begin in the smallest pore spaces and
progressively fill larger and larger pores prior to bulk
condensation.
Equipment for
measuring pore sizes and volumes is identical to that for determining surface
area in most instances. What is required of the instruments is that gas
admission to the cooled sample be continued beyond the first adsorbed gas layer
to the point at which bulk condensation begins. Computation of results now also
must account for the added complexity that gas continues to be adsorbed on
exterior surfaces while condensation is occurring in the central core of pores
which already had adsorbed gases on their walls before condensation
started.
This basically
means that the computation must proceed in reverse, as it were, after all pores
have been filled. This is so because the assumption that all pores are filled
can be made only for the last increment of added gas before bulk condensation
occurs. These calculations incorporate the Kelvin capillary condensation
equation which holds for pores down to about 2 nm (20 Å) diameter, i.e., down to
the micropore region.
Microporous Material
Analysis
There are numerous
modifications to classical theory and additional theories which encompass
microporous material analysis. Precise measurements down to quite low pressures
are required. The Micromeritics volumetric physical adsorption instruments are
equipped to carry out pore diameter and volume analyses and to report results by
most of these techniques. Those units with high vacuum manifolds are most
applicable for thorough analysis of micropore structure and for reporting
density functional theory calculations.
The second way Micromeritics
products determine pore size information is by mercury intrusion - forcing
liquid mercury into the pores and keeping inventory of the quantity penetrating
them. Mercury has an exceptionally high interfacial tension and wets only a few
materials, the resistance to wetting being quantified by a parameter known as
the contact angle. When the contact angle is greater than 90° - mercury against
most solids usually registers between 130° and 150° - increasing external
pressures must be applied to cause mercury to penetrate progressively decreasing
size pores. Quite high pressures are necessary to fill very small pores. Micromeritics
manufactures two instruments, one capable of pressures to 207 MPa (30,000 psia),
causing pore diameters to be filled down to 6 nm (60 Å) and the other with the
capability of attaining pressures to 414 MPa (60,000 psia), filling pore
diameters down to 3 nm (30 Å). Also offered is a device for measuring contact
angles.
Sample material
first is evacuated and then inundated with mercury in a mostly glass device
termed a penetrometer. Pressure is applied hydraulically to both the mercury in
the penetrometer and also about the sample. Subsequent penetration into pores is
tracked by a change in electrical capacitance, which registers the volume of
mercury penetrating pores; pressure transducers measure the corresponding
pressure. Pore diameters and respective volumes are calculated from this
information and from the value of contact angle. Each porosimeter instrument
presents results as total pore volume, pore area, median and average pore
diameters, percent porosity, incremental and cumulative pore volumes as a
function of pore diameter, and more in tabular form. Various forms of graphical
data also are presented.
Density
Density is a
prosaic property of all matter. It is simply the mass of a quantity of matter
divided by the volume of that same quantity. Accurately determined, density
reveals much about the composition of an alloy, provides information with which
to keep a process under control, reveals the richness of a mineral body, and
much more. There are three densities associated with powders. The absolute
density (also termed the true or skeletal density) excludes the volumes of pores
and of the interparticle spaces; the envelope density (sometimes called the
apparent density) includes the pore volume but excludes interparticle spaces;
and the bulk density includes both pore volume and interparticle spaces. For a
powder, the latter changes with vibration and applied forces and is not an
intrinsic property of the material.
The absolute and
envelope densities of a nonporous object are identical. If the object is a
relatively large cube, sphere, or other regular geometric shape, its volume is
not difficult to determine nor is its absolute (and envelope) density difficult
to calculate.
Difficulty in
measurement becomes apparent when the material in question is of irregular
shapes and especially when it is also in small bits or granules. The difficulty
increases if, in addition, the material also has pores, cracks, crevices, or
deep concave regions. The absolute and envelope densities differ in this case,
and require separate techniques to assess. Absolute density by definition
excludes all pore volumes that have access to the outside. Envelope density
includes pore spaces up to the plane of the surface.
Volume and Density Determination
Micromeritics
provides multiple instruments, manual and automatic, specifically for
determining absolute density. They accept a wide range of sample sizes and
operate at a range of gas pressures. All use helium gas as the standard working
medium but other gases can be employed. In both, a sample of the material in
question first is placed in a sealed chamber of known volume and then exposed
to series of elevated and
then released gas pressures to flush away atmospheric gases and vapors. Next,
instead of venting the gas at elevated pressure to atmosphere, it is released
into another chamber of known volume. The pressures in both chambers are
determined both before and after the expansion of the gas.
This permits
calculating the volume of the sample, and division of this volume into the
sample weight gives the density. The result is an absolute density value because
the helium fills all open spaces including that of the pores. By cutting
materials that have closed pores into smaller pieces and thus exposing more of
the pores, an absolute density pycnometer also can be employed to evaluate the
proportion of open and closed pores.
Envelope Density
Micromeritics
also produces an instrument for determining envelope density. It operates on the
principle of immersing the object, or objects, to be evaluated in a fluid medium
of known volume and measuring the displaced volume. The medium, instead of being
a liquid as Archimedes used, consists of free-flowing, fine spherical particles.
To ensure that beads conform to the external contour of the object being
measured, the object is tumbled freely in a cylinder containing both it and the
beads. Gradually the space is reduced by an intruding plunger until a prescribed
force is achieved. Where the plunger stops when the beads are compacted about
the object is a measure of the volume of the object and the pores not intruded
by the beads. Dividing this volume into the object weight yields the envelope
density of the object.
The envelope
density of an object in and of itself is sometimes of great utility, for
instance in controlling a sintering operation. Other useful information can be
calculated from the envelope and absolute density values for the same object,
viz., the porosity and specific pore volume of the object. These latter
parameters indicate many things from the suitability of a catalyst substrate or
the yield potential of an oil-bearing formation.
Bulk Density
Bulk density is the
parameter defining how granular, fibrous and powdery materials pack or
consolidate under a variety of conditions. Knowing its value is useful in
packaging, handling, and shipping all manner of products from breakfast cereal
to cement. The Micromeritics instrument for measuring envelope density
determines bulk densities as well. The weighed, granular test sample alone is
tumbled in the cylinder and the volume it occupies is measured at any
pre-selected force applied by the advancing plunger. Dividing sample weight by
the volume now yields bulk density. Thus the compacting behavior of a material -
measured in terms of its bulk density – is established.
Active Surface Characterization
Physical adsorption
was described previously under ‘Surface Area’. It is a relatively weak
attraction between the gas and the surface molecules. Chemisorption, in
contrast, involves stronger solid-gas attractions.
Chemisorption
Chemisorption is
the basis from which has been developed an array of man-made materials called
heterogeneous catalysts. Without catalysts the modern world would be in short
supply of fertilizers, pharmaceuticals, synthetic fibers, solvents, surfactants,
gasoline, and other fuels, for deep within the tiny galleries, pores and
cavities of catalysts occur the chemical reactions that support our industrial
society. As a specific example, the metal rhodium exposed on the surface of a
ceramic honeycomb structure is the heart of the exhaust system of
automobiles.
Catalysts
How it transforms
the deadly exhaust gases of nitric oxide (NO) and carbon monoxide (CO) into
harmless nitrogen (N2) and carbon dioxide (CO2) is typical of the action of a catalyst. At
the high temperature of an automobile exhaust, carbon monoxide binds to the
rhodium surface. When nitric oxide does the same, it dissociates into oxygen and
nitrogen, and the bound oxygen reacts with the carbon monoxide to form carbon
dioxide. Then when other molecules of nitric oxide and carbon monoxide land
close to the remaining bound nitrogen a second carbon dioxide and a nitrogen
molecule are formed.
Practical catalysts
are characterized by having a high specific surface area, i.e., area per unit
mass. They may consist of finely divided metal dispersed on the surface of a
nonreactive, high surface area, refractory oxide such as alumina or silica.
Other metal catalysts have an open, skeleton-like structure as a result of
leaching away one metal of a bimetallic alloy. The newest and most exciting
catalysts are termed zeolites. They consist primarily of aluminum, silicon, and
oxygen but host an assortment of other elements.
They are highly
porous crystals veined with submicroscopic channels. The assorted other elements
can be moved about or replaced and the channels can be altered in size to make
zeolites very useful indeed.
The surface area
and pore structure of catalysts obviously are critical to their behaviour. Both
parameters can be measured by the instruments described
previously.
These tests are
conducted basically as described before and their description will not be
repeated. However, because catalyst surfaces are highly reactive and can be
altered by exposure to the atmosphere as when transferred from one instrument to
another, Micromeritics chemisorption instruments incorporate provisions
for making these measurements in situ. Critical parameters for chemisorption
measurement are: the area of the active element; metal dispersion, i.e., what
proportion of the active element is actually exposed; surface acidity; and the
strength of acid sites.
Chemisorption Instruments
The simplest
chemisorption instruments utilize the titration (dynamic, or continuous flow)
technique in which small, reproducible volumes of a reactive gas such as
hydrogen, oxygen, carbon monoxide, sulfur dioxide, or ammonia are injected into
a flowing carrier gas such as helium that passes over the sample catalyst. Gas
composition downstream is detected by matched thermal conductivity detectors.
Repeated injections of identical reactive gas quantities are made from which
all, most, some, and then none of each pulse is chemisorbed. The cumulative
chemisorbed quantity is derived by summing the proportions of all pulses
consumed. Metal surface area, dispersion, acidity, and other important
parameters are derived from the chemisorbed quantity, taking into account
stoichiometric factors and the nature of both the gas and metal involved.
Single-injection calibration or testing can be accomplished using either a
syringe or a built-in injection loop. The instrument’s two-port design permits
high throughput. While sample preparation is being conducted on one port, an
analysis can be performed on the other.
TPD, TPR and TPO
By adding to this
instrument an accessory package which contains among other things a programmable
furnace, temperature-programmed desorption (TPD), temperature programmed
reduction (TPR), and temperature-programmed oxidation (TPO) tests can be
accomplished. Temperature-programmed desorption evaluates the gas being desorbed
from a catalyst as its temperature is increased. First, the catalyst is
outgassed, reduced, or otherwise prepared. Then reactive gas is chemisorbed onto
the surface active sites of the sample surface. Ever increasing temperature is
applied to the sample. At a certain temperature, the heat energy will exceed the
bonding energy and the chemisorbed species will be released. If different active
metals are present or if the active sites have more than a single activation
energy, the chemisorbed species will desorb at different
temperatures.
The desorbed
molecules enter the inert carrier gas stream and are swept to the detector which
measures gas concentration. The volume of gas desorbed combined with
stoichiometric factors and the desorbing temperature yield the number and
strength of the active sites. Temperatures exceeding 1100°C can be
attained.
Temperature Programmed
Reduction
Temperature-programmed reduction determines
the number of reducible species and the temperature at which reduction occurs.
This analysis begins by flowing hydrogen, usually at 10% concentration in an
inert gas stream, over the sample; the system is usually at ambient temperature.
The amount of hydrogen consumed in reduction of oxide species with increasing
temperature is monitored.
Temperature Programmed
Oxidation
Temperature-programmed oxidation examines
the extent to which a catalyst can be reoxidized. First, metal oxides in the
sample are reduced to the basic metal with hydrogen. Then the reactive gas,
typically 2% oxygen, is applied to the sample as a steady stream while the
sample temperature is increased.
Again, the amount
of oxygen consumed is monitored by the thermal conductivity detectors and
quantified; these tests determine the extent of the reduction and the nature of
the reoxidized species.
Temperature Programmed
Chemisorption
Temperature-programmed chemisorption
reactions can be studied in greater detail by use of another Micromeritics
instrument type. As in the instruments just described, reactive gas in an inert
carrier stream flows over the catalyst sample after it has been outgassed,
reduced, or otherwise prepared. Temperature- programmed desorption, reduction
and oxidation studies are conducted as described before. This instrument design,
however, permits testing from subambient to over 1000°C.
The significant
difference between this instrument design and the one described previously is
that operation of this latter instrument type is automated. The valves,
detector, and other critical internal components of the analysis system are
designed and engineered for minimum dead volume, maximum response, and high
resolution.
Furthermore, by
incorporating programmable heaters into the valves and internal gas lines,
liquid vapors such as pyridine and quinoline can be used as chemisorbate probe
molecules without loss due to condensation. This design feature also facilitates
attaching a mass spectrometer or other external detector, thus permitting the
identity of the reaction species to be determined. The latest model of this
design is computer controlled from start to finish and results are reported as
graphs and data tabulations.
Static Volumetric Chemisorption
System
The Micromeritics
static volumetric chemisorption system is a version of the static volumetric
physisorption unit noted in conjunction with surface area
measurement.
When employed as a
chemisorption analyzer, an accessory permits the sample to be prepared on the
analysis port. This eliminates the necessity of moving the sample holder between
preparation and analysis ports, which would expose the sample to atmospheric
contaminants. Not only can this unit be used for surface area and pore size and
volume distribution determinations, but it also determines automatically active
metal surface area and percent metal dispersion for catalyst
materials.
Preparation and
analysis of samples are directed through a graphical user interface to the
computer system. Sample preparation uses the flowing gas techniques with
hydrogen gas, either pure or in an inert carrier, to reduce completely the
oxides on the catalyst. Complete removal of residual hydrogen after preparation
is accomplished by applying heat and a high vacuum. Analysis is made by the
static volumetric technique to obtain precise dosing of the reacting gas and
rigorous equilibration following the dose.
The first analysis
measures both strong and weak interactions in combination. A repeat analysis
after evacuation measures only the weak, or reversible, uptake of
reactant.
Automatic Data Reduction
Automatic data
reduction provides complete information about active metal surface area and
percent metal dispersion. An analysis log reports pressures, temperatures, and
volumes chemisorbed plus an elapsed-time record for each data point. Plots
containing both the initial and repeat analysis curves are generated. A
difference plot shows the strong component of chemisorption. It is fitted to a
single straight line for computations of a single uptake number and subsequent
computation of the percent metal dispersion.
Nanomaterials
Nanotechnology is
anticipated to lead to a wide array of technical innovations in the near
future.
The prefix “nano”
indicates a scale factor of 10-9(one billionth). A particle of nano size has at
least one linear dimension in the nanometer range. Since it requires about 3
to10 atoms (depending on the element) to span one nanometer, a few hundred atoms
is about the limit of the dimension of a nanoparticle.
Nanoscience seeks
to gain knowledge and understanding of nanoscale phenomena, while nanotechnology
employs this knowledge in the development of new products. These products can be
improved catalysts or materials with enhanced strength, wear-resistance,
corrosion resistance or high temperature endurance but, in general, they are
materials with enhanced performance. On the whole, nanostructured materials are
providing novel opportunities in wide range of scientific
fields.
But, being of
nanosize is not what attracts such great interest in nanomaterials, it is their
properties. The properties of nanomaterials are different from those of the same
material at the macro scale. When materials are reduced to sufficiently small
sizes, typically less than 50 nanometers (a few molecules), novel physical,
chemical, and biological properties arise that provide opportunities for new
applications. Furthermore, these surface properties can be optimized for
particular applications through molecular modification. The fundamental reasons
for the changes or enhancements of characteristics are the increased
surface-to-volume ratio and the increase dominance of quantum effects that
determine the material’s optical, magnetic, and/or electronic properties.
Working with nanoscale systems requires special tools for manipulating,
measuring and controlling size and properties. A different knowledge set also is
required since nanoscale phenomena involve quantum mechanics rather than
classical mechanics as is the case with materials of larger
scale.
Micropores, Mesopores and Macropores
Micromeritics
instruments have been used in the investigation of nanomaterials for over a
decade. As previously discussed, pores are classified according to diameter
where micropores have diameters less than about 2 nm, mesopore sizes range from
about 2 nm to about 50 nm and macropores have diameters greater than about 50
nm.
In addition to pore
volume distribution, total surface area also can be determined from gas
adsorption. Not only does increasing surface-to-volume ratio enhance reactivity,
as previously noted, it also enhances the efficiency of the material in trapping
or storing adsorbed gases and vapors.
Being a
nondestructive test, gas adsorption is the preferred method of determining pore
characteristics and surface area of nanomaterials. However, mercury porosimetry
also is capable of measuring pores of nano dimensions. At 30 kpsi pressure,
mercury intrudes into pores of 5nm diameter and at 60 kpsi, pores of 2 nm can be
probed.
Sample density also
is a valuable determination in the characterization of
nanomaterials.
A surprising amount
of information can be gleaned from this seemingly simple
measurement.
For example, if the
specific surface area of a mono-sized dispersion has been determined, the
particle size of the material can be calculated provided that each particle is
of the same regular geometry (typically spherical)l and without any porosity.
The degree of crystallization of a material also may be inferred by comparing
the measured density to the theoretical crystalline density of the
material.
Particle sizing
perhaps poses the greatest challenge in characterizing
nanomaterials.
Recall that the
sought after properties of nanoparticles are size-dependent and usually do not
prevail until size has been reduced to less than 50 nm. While most of the novel
properties are size-dependent, many of the common methods used for generation of
nanopowders result in different size distributions. Not only is there need for
tighter control of size in the production process, there also is need for a
fast, high resolution method of measuring distributions in the lower end of the
nanoscale in order to control production.
Conclusion
Fine particles play
essential roles in determining the characteristics of both natural and manmade
materials and have considerable influence on processes such as dissolution,
adsorption and reaction rate. In the majority of cases, these effects are a
function of either the size, shape, surface area or porosity of the individual
particles or of an agglomeration of particles. These particle-related
characteristics must be controlled in order to optimize the desired effects, and
efficient control requires measurement. These same particle characteristics are
either the causes of, the results of, or a determining factor in natural
phenomena.
In this category,
understanding or exploitation rather than control is more likely the objective
and, again, measurements provide fundamental information used in achieving the
objective.
As this article has
illustrated, there likely are multiple techniques for determining the same
particle dimension and each has its advantages and
disadvantages.
Selecting a
technique that is inappropriate for the application can have a profound impact
on the quality of the measurement you obtain.
Source: Micromeritics Instrument Corporation
For more information on this source please visit Micromeritics
Instrument Corporation.