Patent of the Month – Sol Gels
Editor | On 12, Jan 2018
Aerogels. Long touted as one of the ‘holy grail’ technologies, aerogels or sol-gels have never really made the transition to a successful commercialization. On the plus side, the properties of the materials as thermal insulators are unparalleled. On the negative side, aerogels continue to be very expensive to produce, in no small part due to the problems of shrinkage and spring-back. Now, granted as of November 7 as US9,808,864, it finally seems like these issues have benefited from a step-change in the right direction, thanks to a trio of inventors at Michigan State University.
Here’s what the inventors have to say about the problem needing to be solved:
Conventional aerogels have insulating properties. Such aerogels are highly porous silica materials which retain a low thermal conductivity (i.e. less than about 0.1 Wm*K), even at temperatures in excess of about 300.degree. C. Conventional aerogels are produced from a liquid mixture that solidifies at room temperature to form a sol gel, which can allow the aerogel to be cast in place on a structure or preformed using a mold. The sol gels are then supercritically dried to extract (i.e., remove) the solvent from the sol gel.
However, the supercritical drying used to extract the solvent from the sol gel to form the aerogel can be an expensive and energy intensive process. Alternative methods have not proven satisfactory, as conventional non-supercritical drying methods produce aerogels that shrink in volume to a significant degree upon being dried and then expand, a phenomenon known as “springback,” which results in an aerogel having a final volume which is larger than the smallest volume obtained by the gel during drying.
Excessive springback (e.g., more than about 20% linear shrinkage, which corresponds with an approximately 49% reduction in volume) can cause a number of problems. For example, excessive springback can cause a gap to form between the gel and the structure (e.g., solid object, encasement, further including any of the structures noted herein such as, but not limited to, heat engines, microelectronics, building, clothing, equipment, pipelines, etc.) in which the aerogel is in contact with. As such, the aerogel is not able to form a proper bond, seal or other connection with the structure, resulting in separation of the aerogel from the structure. Movement of a structure containing a separated aerogel can also cause damage to the structure or otherwise cause the structure to fail. Excessive springback can also cause an aerogel to crack at the interface between the aerogel and the object, a result that is highly undesirable in most applications and particularly with encapsulation of thermoelectrics. Excessive springback can also prevent the ability of an aerogel to be cast around or in a structure altogether.
Here’s what the springback problem looks like when mapped on to one of our contradiction Bubble Maps:
And here’s what happens when we map the conflict pair onto the Contradiction Matrix wizard:
And here’s how the US9,808,964 patent solves the problem:
A method of forming a sol gel from a sol gel-forming composition comprising: forming the sol gel from the sol gel-forming composition, wherein the sol gel-forming composition comprises a silane solution and a catalyst solution, wherein the catalyst solution is added to the silane solution at a rate from about 1% to about 50% catalyst solution per volume per second (Principles 5, 15); and non-supercritically drying (Principle 35) the sol gel to provide a dried silicon-based porous sol gel having no springback, wherein the dried silicon-based porous sol gel is cast in place on a structure or preformed using a mold (Principle 30)…
…the various embodiments described herein provide porous sol gels which do not need to be supercritically dried… the lack of springback is due to the morphology of the products described herein, all of which include macropores or mesopores. (Principles 3, 31)
…Also, in contrast to conventional aerogels, some of which can have a rough, irregular substantially oblong or ellipsoid shape or can otherwise be a type of aggregate that forms an agglomerate, embodiments of the dried porous sol gels described herein have solids which are substantially spherical or “bead-like” in appearance. Specifically, the dried porous sol gels can form a repeating series of solids in a pattern that can be referred to as a “pearl necklace” (Principle 30) …which can be oriented in a curved configuration (Principle 14). In one embodiment, the solids in the macroporous or mesoporous sol gels have the “pearl necklace” appearance. In one embodiment, the solids in the outer larger macroporous or mesoporous sol gel of a hybrid aerogel can have this appearance.
The resulting silica insulators provide advantages which are surprising and unexpected. For example, some embodiments allow the direct casting of a porous gel around a rigid object, including large or small objects, to form a porous dried porous sol gel that has no cracking or limited cracking, as well as no gap between the porous dried porous sol gel and the solid object or a limited gap. In some embodiments, the gel can be dried using ambient conditions, avoiding the cost and inconvenience of for example, supercritical drying or other drying methods. The method of making the aerogel is far more easily scalable than other methods of making aerogels. The methods described herein are also more efficient and more cost effective on a large scale, primarily as a result of the ability to avoid use of supercritical drying conditions. In some embodiments, the dried porous sol gels can be ambiently dried. In one embodiment, an opaque material can be added, such that the dried porous sol gel can better block certain types of radiation, such as thermal radiation. In some embodiments, the aerogel includes inorganic compounds that can withstand extremely hot or cold temperatures. As a silicon-based aerogel, embodiments of the dried porous sol gel can withstand higher temperatures than organic-based aerogels, such as resorcinol/formaldehyde-based aerogels.
The dried porous sol gels produced herein are useful in many applications, including, but not limited to, next-generation thermoelectric power generation, superconductors, heat engines, such as otto cycle engines (e.g., car engines), diesel cycle engines, brayton cycle engines (e.g., jet turbines), sterling cycle engines (e.g., NASA advance radioisotope sterling generator), rankine cycle engines (e.g., classic steam power plant), microelectronics, including for microelectronics manufacturers interested in channeling heat or thermal isolation, insulation for consumer electronics, biomedicine, cryogenic or low temperature insulation, packaging, aerospace or space insulation, automotive insulation, heavy industry/equipment insulation, home insulation, petrochemical pipeline insulation, and new building construction and retrofits for improved energy efficiency.