Metal aerogels

Metal-Doped Carbon Aerogels, or, Nano Blueberry Muffins

In the early 2000’s, Dr. Ted Baumann at Lawrence Livermore National Laboratory developed a technique for growing nanoparticles of metals inside carbon aerogels through a very clever technique. In this technique, resorcinol is replaced with 2,4-dihydroxybenzoic acid (DHBA), which is basically resorcinol with a carboxylic acid group attached to it. The DHBA (a weak acid) is then neutralized with potassium carbonate (a strong base) in water to produce potassium 2,4-dihydroxybenzoate (a water-soluble salt). Formaldehyde is added and the solution is cured at 80°C in sealed molds for 1-3 days. The resulting cranberry-sauce-red gels are similar to RF gels used to prepare carbon aerogels, except for that they are laced with potassium ion exchange sites throughout their structure. Soaking these gels in a solution of a metal salt, such as iron(III) nitrate, results in exchange of potassium ions with incoming metal ions which then attach to the gel’s polymer backbone. Think of it like hanging ornaments on a Christmas tree, where the ornaments are metal ions and the Christmas tree is the gel’s framework. The metal-doped gels can then be supercritically dried to make metal-doped organic aerogels and, finally, pyrolyzed to produce carbon aerogels. However, unlike typical carbon aerogels, during pyrolysis the added metal ions attached to the aerogel’s backbone reduce to metal atoms and coagulate into nanosized particles. The resulting material can be thought of kind of like a blueberry muffin morphology in which metal-containing nanoparticles (blueberries) are dispersed throughout a nanoporous carbon matrix (muffin). The presence of these particles does a number of things, for example, improves the electrical conductivity of the aerogel (compared with an undoped carbon aerogel of the same density), and gives the aerogel some catalytic properties characteristic of the dopant metal.


Transmission Electron Micrograph of Iron-Doped Carbon Aerogel

Since the chemistry for doping the gels is done in water, a number of metals can be used to prepare metal-doped carbon aerogels including iron, cobalt, nickel, and copper.

Carbon Nanotubes Grown Directly on an Fe-Doped Carbon Aerogel

In 2005, Aerogel.org’s own Stephen Steiner at MIT and Dr. Ted Baumann at Lawrence Livermore discovered that by using a chemical vapor deposition technique, it was possible to grow carbon nanotubes from the nanoparticles in Fe-doped carbon aerogels, resulting in nanotube growth on the outer surfaces of the aerogels (but not inside the pores). Later the metal-doping technique was extended to virtually any metal on the periodic table and it was found that using metal-doped carbon aerogels it was possible to grow nanotubes from a number of metal catalysts previously not known to catalyze nanotube growth.

Виды аэрогелевого утеплителя

Для строительных нужд продукт выпускается в виде рулонов. Это стекловолокнистый материал, который содержит в себе порошок из аэрогеля. На свойства теплоизолятора влияют: • химический состав материала; • структура основы; • внешнее покрытие изделия.

Выделяют несколько типов аэрогелевых утеплителей. Классификация учитывает температуру применения продукта. Чаще всего используют кремниевые изоляторы с незначительным введением оксида алюминия. Такие материалы могут выдерживать до 450°С. Есть компоненты, которые не боятся температуру в 700°С. Для получения такого продукта прибегают к добавке оксида титана. При увеличении теплотворных показателей у аэрогеля начнут ухудшаться другие важные параметры. Это связано с окислением вещества.

Выпускают композиции и для низких температур. Они обладают многослойной структурой. Качество паропроницаемости у таких материалов отсутствует. Их активно применяют для утепления холодных помещений. Показатели аэрогеля не ухудшатся даже при достижении области абсолютного нуля.

Сегодня производители предлагают несколько видов энергоэффективных изоляторов. Пирогель – материал для утепления промышленных трубопроводов, техники, работающей с высокой температурой. Криогель предназначен для утепления труб и техники, работающей с низкими температурами. Спейслофт создан экспертами для изоляции конструкций, расположенных в разных климатических условиях.

Обособленно в группе теплоизоляторов стоит Спейслофт Сабси. Данный материал используют для утепления системы типа «труба в трубе», которая находится на большой глубине. Чехлы съемные применяют для изоляции промышленных установок, работающих с высокими температурами. Цена теплоизоляции с аэрогелем зависит от ее назначения и толщины. Материал позволяет решать различные задачи. Он утепляет конструкции любых размеров. Кроме трубопроводов, прокладка используется при монтаже: • емкостей: • запорно-регулирующей арматуры; • приборов, контролирующих производственных процессы. Продукт применяют для утепления систем внутри помещения.

A Note About Silica vs. Silicon vs. Silicone

Silica refers to the oxide of silicon, which has the empirical formula SiO2.IT IS NOT SILICON.Silicon is a semiconductor metalloid used in microchips, whereas silica is an insulating glassy material.They are very different!Unfortunately, you will see “silicon aerogel” mentioned in the media from time to time (even on one of NASA’s websites!), however this is very incorrect, and should instead read “silica aerogel” (even Aerogel.org’s own co-founder Will Walker confuses the terms from time to time, but we forgive him).There are no reports of silicon aerogels yet, but they would certainly be interesting materials!

Silicones, on yet another hand, are polymers composed of silicon and oxygen, usually containing carbon and hydrogen as well.They are rubbery solids or liquids at room temperature and are very different from both silica and silicon. Think implants and temperature-resistant rubber. Silicone aerogels can be and have been prepared and possess many properties characteristic of silicone rubbers.This said, if someone says “silicone aerogel” they probably just mean “silica aerogel”.

Here’s a dumb pneumonic to help keep it straight:when you hear silicA think glAss, when you hear silicOn think cOmputers, and when you hear siliconE think rubbEr.

Synthesis of Resorcinol-Formaldehyde Aerogels

By far, RF aerogels are the most researched organic aerogels and so we’ll use them as an example of how organic aerogels are prepared. Like other kinds of aerogels, RF aerogels are prepared through a sol-gel process, starting with a solution containing small molecules with the ability to link together (polymerize) to form larger molecular clusters that can eventually grow into nanoparticles dispersed throughout the solution (a sol). These nanoparticles can then be coaxed into interconnecting (or as they say in polymer chemistry, cross-linking) to form a continuous network of interconnected nanoparticles that spans the volume of the liquid solution, namely, a gel.


Polymerization of Resorcinol and Formaldehyde

In the case of RF aerogels, the chemistry that is used is the polymerization of resorcinol (properly called 1,3-dihydroxybezne) with formaldehyde (properly called methanal) in water. This reaction is impractically slow at room temperature and so a small amount of a basic catalyst such as potassium or sodium carbonate is usually added. Over the course of about 24 hours at room temperature, polymer chains of resorcinol-formaldehyde polymer will form throughout the solution, tangling up into RF nanoparticles. The solution is then put into an oven (in a sealed bottle or mold!) at 80°C for 1-3 days (or just left at room temperature for a couple weeks), during which the nanoparticles cross-link together to form a mesoporous network across the solution, resulting in a gel.

Gel Formation in the Resorcinol-Formaldehyde System

The resulting RF gel has a consistency much rubberier than silica aerogels, more like canned cranberry sauce, and can even be sliced as such. Just like other aerogels, the RF gel must be purified before supercritical drying. For RF gels, this entails a few soakings in water, followed by a few soakings in a polar organic such as acetone or ethanol to prepare for supercritical drying. Lastly, the gel is supercritically dried to produce RF aerogel.

Resorcinol-Formaldehyde Gel Looks and Cuts Like Cranberry Sauce from a Can

A resorcinol-formaldehyde polymer aerogel (left) and a carbon aerrogel (right)

The General Process

This is how subcritical drying is typically done.

  1. A gel is made using a standard sol-gel process like those in the Make section of Aerogel.org
  2. The gel is purified with an organic solvent such as alcohol or acetone as it would be in preparation for supercritical drying
  3. The pore fluid in the gel is exchanged with an aprotic solvent such as pentane, hexane, or toluene (generally the aprotic solvent used is also a low-surface-tension solvent as well that can be evaporated later)
  4. The gel is chemically modified to replace its polar surface groups with non-polar groups by diffusing a solution of aprotic solvent and waterproofing agent into its pores
  5. The gel is purified by exchanging into pure aprotic solvent
  6. The gel is optionally exchanged into a different low-surface tension solvent if the current pore fluid is not suitable for evaporative drying
  7. The liquid in the gel is gently evaporated, causing the gel to partially collapse
  8. The gel springs back once the liquid has finished evaporating from it pores (on its own or sometimes under the assistance of gentle heating or vacuum)

Reading step 7, you may be able to appreciate why monoliths made by this technique have shape and size limitations–when the gel collapses, although only partially and temporarily, sharp corners and other stress concentrators can become aggravated and cracks can form.  Additionally, a complex three-dimensional stress state can arise in a thick parts as they collapse, since the outer edges dry exponentially faster than the inner volume.  This stress state can cause the gel to crack as it dries, limiting crack-free monoliths to small dimensions.

Special Properties of Aerogels

Many aerogels boast a combination of impressive materials properties that no other materials possess simultaneously. Specific formulations of aerogels hold records for the lowest bulk density of any known material (as low as 0.0011 g cm-3), the lowest mean free path of diffusion of any solid material, the highest specific surface area of any monolithic (non-powder) material (up to 3200 m2 g-1), the lowest dielectric constant of any solid material, and the slowest speed of sound through any solid material. It is important to note that not all aerogels have record properties (in fact most don’t, although they may have very good values for many properties)!

By tailoring the production process, many of the properties of an aerogel can be adjusted. Bulk density is a good example of this, adjusted simply by making a more or less concentrated precursor gel. The thermal conductivity of an aerogel can be also be adjusted this way, since thermal conductivity is related to density. Typically, aerogels exhibit bulk densities ranging from 0.5 to 0.01 g cm-3 and surface areas ranging from 100 to 1000 m2 g-1, depending of course on the composition of the aerogel and the density of the precursor gel used to make the aerogel. Other properties such as transparency, color, mechanical strength, and susceptibility to water depend primarily on the composition of the aerogel.

For example, silica aerogels, which are the most widely researched type of aerogel (and the type people typically see in photographs), are usually transparent with a characteristic blue cast due to Rayleigh scattering of the short wavelengths of light off of nanoparticles that make up the aerogel’s framework. Carbon aerogels, on the other hand, are totally opaque and black. Furthermore, iron oxide aerogels are just barely translucent and can be either rust-colored or yellow. As another example, low-density (<0.1 g cm-3) inorganic aerogels are both excellent thermal insulators and excellent dielectric materials (electrical insulators), whereas most carbon aerogels are both good thermal insulators and electrical conductors. Thus it can be seen that by adjusting processing parameters and exploring new compositions, we can make materials with a versatile range of properties and abilities.


The Flower, the Mona Lisa of aerogel pictures, dramatically demonstrates the superinsulating properties of silica aerogel by insulating a delicate, moist flower from the raging heat of a Bunsen burner (image credit Lawrence Berkeley National Laboratory)

Aerogels of all sorts hold records for different properties. Here are some:

Records held by some specially-formulated silica aerogels:

  • Lowest density solid (0.0011 g cm-3)
  • Lowest optical index of refraction (1.002)
  • Lowest thermal conductivity (0.016 W m-1 K-1)
  • Lowest speed of sound through a material (70 m s-1)
  • Lowest dielectric constant from 3-40 GHz (1.008)

Record held by a specially-formulated carbon aerogel:

Highest specific surface area for a monolithic material (3200 m g-1)

A more in-depth discussion of the properties of silica aerogel and other historically underrepresented types of aerogel can be found in the Flavors of Aerogel section.

Плюсы и минусы аэрогелевой изоляции

Среди достоинств утеплителя выделяют: • незначительную теплопроводность; • гидрофобность; • универсальность; • стабильность к деформациям. Изделия возможно применять в разных конструкциях и в сочетании с любыми строительными материалами.

Несмотря на вышеперечисленные положительные стороны, аэрогель имеет один существенный недостаток. Изоляция не выдерживает открытой кислородной среды. Попадая в нее вещество мгновенно растворяется.

На сегодняшний день уже есть позитивные отзывы о теплоизоляции аэрогелем. Отечественный институт, занимающийся научными исследованиями, активно использует инновационное изделие листового типа для внутреннего и внешнего утепления оборудования. При этом температура агрегата достигает 310°С.

Evaporation

Once the chemically-modified gels have been purified and exchanged into a low-surface-tension solvent, the solvent can be evaporated.  The solvent should be evaporated slowly, for example, by leaving the gel in a jar with a bit of pentane and then putting the lid on but not screwing it on tightly. Shrinkage that results may partially reverse itself after the gel is dry or can be assisted by heating the resulting dry material and/or placing under vacuum (this is called the “spring-back method”).

Now that you’ve learned about how subcritical drying of aerogels works, try making them for yourself!

What Silica Gels Look and Feel Like

Well-made silica gels are generally transparent exhibiting no color and little scattering effect.Dense gels (0.1-0.5 g cm-3) can be very hard and rubbery—tapping a beaker filled with this density of gel one can feel elastic vibrations resonate in hand.Lower density (0.01-0.1 g cm-3) gels are more like gelatin dessert in consistency, but don’t bend as well.Ultralow density gels (0.001-0.01 g cm-3) are very delicate and do not hold up well under their own weight without a mold.All silica gels tend to fracture or crumble when poked or cut, much more so than gelatin dessert.

Poorly made silica gels will be opaque white or opaque-ish (not totally transparent), and depending on how poorly made they are, slushy with poor monolithicity—more like a slurry than a gel.These properties result from overhydrolysis of reactants or nanoparticles, which causes them to fall out of solution instead of joining up with other particles to form a gel framework.Gelation can also occur unevenly across a container and part of a gel will be well-formed while another part is slushy or even liquid still.

Свойства и преимущества аэрогеля:

– высокая пористость. На 99,8% состоит из воздуха,

имеет рекорд по самой малой плотности у твердых тел — 1,9 кг/м³, это в 500 раз меньше плотности воды и всего в 1,5 раза больше плотности воздуха (кварцевые аэрогели),

– уникальный теплоизолятор. Имеет низкую теплопроводность – λ = 0,013 ~ 0,019 Вт/(м К) (в воздухе при нормальном атмосферном давлении) меньшую, чем теплопроводность воздуха (0,024 Вт/(м К) (кварцевые аэрогели). Как утеплитель в 2-5 раз эффективнее традиционных утеплителей,

температура плавления составляет 1200°C (кварцевый аэрогель),

– аэрогель является прочным материалом. Он выдерживает нагрузку в 2000 раз больше собственного веса,

– имеет низкий модуль Юнга,

– не сжимается, устойчив к деформации, имеет высокую прочность на растяжение,

скорость распространения звука имеет самое низкое значение для твердого материала, что является важным преимуществом при создании шумоизоляционных материалов. Скорость звука в нем ниже скорости звука в газах,

– некоторые виды аэрогеля являются отличным сорбентом. Они в 7-10 раз эффективнее популярных современных сорбционных материалов,

– является устойчивым пористым веществом. Объем пор внутри аэрогеля в десятки раз превышает объем, занятый самим материалом. Данное свойство позволяет использовать аэрогель определенного состава в качестве катализатора в химических процессах с целью получения органических соединений. С другой стороны, его большая внутренняя емкость может быть использована для безопасного хранения определенных веществ, например, ракетного топлива , окислителя и пр.,

– отличная гидрофобность. Не впитывает влагу,

– обладает высокой жаропрочностью и термостойкостью. Имеет широкий рабочий температурный диапазон использования – от -200 °С до +1000 (1200) °С. Без потерь сохраняет теплоизоляционные и механические характеристики при нагревании до не менее 1000°С,

– является негорючим материалом. Может использоваться также для огнезащиты различных конструкций,

– прозрачен (кварцевый аэрогель). Имеет показатель преломления света от 1,1 до 1,02. Из него можно изготавливать различные виды стекол ,

– обладает достаточно высокой твердостью,


– долговечность,

– экологичен и безопасен для человека и окружающей среды,

– имеет большую удельную площадь внутренней поверхности. Она составляет порядка 300-1000 м 2 /г,

– химический состав аэрогеля можно регулировать, легко вводить в его состав различные добавки, что открывает новые возможности для его использования,

– устойчив к кислотам, щелочам, растворам,

– в тоже время является хрупким материалом.

How is Silica Aerogel Made?

Like most other aerogels, silica aerogel starts its life out as a gel (see How Aerogel is Made).A silica gel serves as the precursor in this case and can be prepared through a number of chemical processes.It is important to note that the “silica gel” which comes in little packets marked “SILICA GEL DO NOT EAT” (usually found in consumer electronics packaging) is actually pellets of silica xerogel—a dense, dry, solid form of silica used to absorb moisture.Silica gels used for preparing silica aerogels, on the other hand, are wet gels close to gelatin dessert in consistency (but a bit crumblier).Silica gels are composed of two components—a solid, nanoporous silica-based framework which gives the gel its rigidity and solid form, and a liquid which permeates the pores of the framework.

Silica aerogel is made by extracting the liquid from the framework of the silica gel in a way that preserves at least 50% (but typically 90-99+%) of the gel framework’s original volume.This is typically done by supercritically drying the gel, but can also be done a number of other ways (see How Aerogel is Made).

Aerogels: Enhancing Carbon Nanotube Growth Since 1994

The first report describing a combination of aerogels and carbon nanotubes appeared in the Journal of Non-Crystalline Solids in 1994 by Aerogel.org contributors Dr. Arlon Hunt and Dr. Mike Ayers and their associate Dr. Wanqing Cao at Lawrence Berkeley National Laboratory.

In their work, CVD (they call it “chemical vapor infiltration”) of carbon into the pores of silica aerogels and silica aerogels doped with iron or nickel was explored. Although the paper focused on deposition of uniform layers of carbon into silica aerogels, they describe the appearance of “carbon rings, fibers, and tubes” as seen under transmission electron microscopy (TEM) in silica aerogels after CVD with a flow of 25% acetylene 75% argon at a rate of 100-300 sccm (standard cubic centimeters per minute) at temperatures of 550-580°C. They describe that the decomposition of the carbon-containing gases they used occurred at much lower temperatures in the presence of silica aerogels than without, indicating that the silica aerogel catalyzes decomposition of the gas. This paper is interesting because it’s not clear that if the nanotubes they observe grow from catalyst nanoparticles or if they just seem to self-assemble from confinement.

Reference: Arlon J. Hunt, Michael R. Ayers, Wanqing Cao, “Aerogel composites using chemical vapor infiltration”, Journal of Non-Crystalline Solids, 185, 227-232 (1995).

Nanotube “Aerogels” in Nanotube Fiber and Sheet Production

Carbon nanotube yarns and sheets are an exciting area of research. These are materials made completely of entangle carbon nanotubes.

Unfortunately, the term “aerogel” has been used to describe smokes of carbon nanotubes which form during production of nanotube yarns and even thin sheets of carbon nanotubes drawn from carbon nanotube forests. This is not an appropriate use of the word aerogel, since these materials do not have defined mesoporosity and especially in the case of nanotube smokes, are generally not monolithic materials. The terms “elastic smoke” or “ethereal film” is more appropriate.

Nonetheless, these are interesting materials and are worth reading more about themselves. Read more at:

  • Nanocomp Technologies
  • Artificial muscles made of carbon nanotube sheets
  • Carbon nanotube yarns at the University of Cambridge

Purifying and Aging Silica Gels

Once a silica gel has been formed, the liquid within the pores of the gel (or the “gel liquor”) usually contains lots of stuff other than just solvent.Gels produced through the alkoxide technique will contain alcohol, water, catalyst, and other random organic compounds.Gels produced through waterglass gelation contain sodium ions, acid, and lots of water.

If the gels are to be supercritically dried to produce aerogels (see How Aerogel is Made), these miscellaneous components (especially water and sodium salts) are not supercritically extracted as easily as the solvent being extracted (CO2, ethanol, etc.), meaning they’ll be stuck in the pores of the resulting aerogel.This can cause reduced transparency and mechanical strength, shrinkage and/or cracking of the monolith, and off-gassing and/or residual wetness after the aerogel is made.Impurities are more significant in Hunt Process (CO2) drying than in direct supercritical extraction of an organic solvent (such as methanol or ethanol) since CO2 is very non-polar, and doesn’t dissolve water or sodium salts well even in its supercritical state.

As a result, the gel liquor must be purified.This is done by simply soaking the gel in a pure solvent, usually an organic solvent such as methanol, ethanol, or acetone.As the gel soaks in the pure solvent, impurities will diffuse out and pure solvent will diffuse in until an equilibrium concentration of species is reached.Depending on the size and shape of the gel, this can take anywhere from hours to days.The process is repeated with fresh solvent a number of times to ensure adequate removal of impurities from the gel.

During this purification process, some final chemical reactions which contribute to the strengthening of the gel’s structural framework may occur as well.This is referred to as aging.

Once the gel is purified it can be dried through supercritical drying or subcritical drying to produce silica aerogel.


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