Construction and performance test of high stability heat storage microcapsules based on physical regulation of nano-SiO2

Deep underground, where miners extract the resources that power modern life, the earth itself becomes a hazard. In these deep tunnels, the rock is so hot that temperatures can soar past forty degrees Celsius, creating an oppressive environment that threatens the health of workers and the safety of operations. To combat this, engineers are turning to a clever solution: filling the empty spaces left behind by mining with a special kind of concrete that can absorb and store heat, acting like a thermal sponge to cool the mine. The key ingredient in this sponge is paraffin, the same waxy substance found in candles, which is excellent at soaking up heat as it melts. However, paraffin has a fatal flaw for this job: when it gets hot, it turns into a liquid and leaks out, ruining the strength of the concrete and creating a mess. To fix this, scientists wrap the paraffin in a tiny, protective shell, turning it into a microscopic capsule that stays solid and contained even when the heat rises.

A team of researchers from Shandong University of Science and Technology and Henan University of Engineering set out to make these tiny capsules stronger and safer. They knew that the shells, made from a common industrial resin, were often too weak and could leak toxic chemicals during their creation. To solve both problems at once, they introduced a tiny amount of nano-silica, which is essentially microscopic glass dust, into the mixture. They wanted to see if this fine powder could reinforce the shell like rebar in concrete while also acting as a sponge to soak up the unwanted toxic chemicals left over from the manufacturing process. By carefully adjusting how much of this nano-silica they added, they discovered a precise balance that created a superior material ready for the harsh conditions of a deep mine.

The researchers began by mixing the wax, the resin, and the nano-silica in a water-based solution, using a process that builds the shell layer by layer around the liquid wax droplets. They tested five different amounts of the nano-silica, ranging from none at all to a small percentage, to see how the powder changed the final product. When they looked at the capsules under a powerful microscope, the difference was clear. The capsules made without any nano-silica had rough surfaces with tiny holes and dents, like a poorly baked cookie that had shrunk unevenly. These imperfections were dangerous because they provided escape routes for the wax to leak out later. As the researchers added more nano-silica, the surface became smoother and denser. The tiny glass particles filled in the microscopic gaps, creating a tight, seamless skin. However, adding too much powder caused the particles to clump together into hard lumps, which actually created new cracks and weak spots in the shell. The sweet spot was found at a specific, low dosage where the particles were perfectly spread out, filling the holes without clumping.

With the best-looking capsules in hand, the team tested how well they stored heat. They warmed the capsules up and watched how they melted and cooled down again. The capsules with the optimal amount of nano-silica performed exceptionally well. They melted at a temperature just under forty-six degrees Celsius, which is perfect for absorbing the heat in a deep mine. More importantly, they held onto nearly the same amount of heat energy as the pure wax, proving that the shell did not get in the way of the material's main job. The nano-silica also helped the wax freeze back into a solid more quickly when it cooled down, reducing a phenomenon called supercooling where the liquid stays liquid even when it should be freezing. This meant the capsules could react faster to temperature changes, making the cooling system more efficient.

The study also looked at the environmental side of making these capsules. The process of creating the resin shell usually leaves behind water contaminated with free formaldehyde, a toxic chemical. The researchers found that the nano-silica acted like a magnet for this toxin. As the amount of nano-silica increased, the amount of toxic formaldehyde left in the water dropped significantly. At the optimal dosage, the nano-silica removed more than twenty percent of the formaldehyde from the wastewater. This meant that the material was not only better at its job but also cleaner to produce, reducing the pollution associated with its creation.

To prove that these tiny capsules could survive the real world, the researchers mixed them into a batch of concrete designed for mine backfilling. They compared this mixture to one made with plain, unencapsulated wax and a standard control with no wax at all. The concrete containing the raw, unencapsulated wax crumbled under pressure because the liquid wax seeped out and weakened the bonds between the rocks and cement. In contrast, the concrete containing the nano-silica reinforced capsules held its shape and strength almost as well as the standard concrete without any wax. The tiny shells protected the wax inside, preventing it from leaking and damaging the structure, while the reinforced shell itself was strong enough to withstand the crushing weight of the surrounding rock.

The final conclusion was clear: adding a small, precise amount of nano-silica creates a heat-storing capsule that is stronger, more efficient, and environmentally friendlier than previous versions. This material successfully solves the twin problems of leakage and structural weakness, making it a viable candidate for cooling deep mines. By turning a simple wax into a robust, heat-absorbing building block, this research offers a practical path toward safer, more comfortable working conditions in the deepest parts of the earth, where the heat is most intense.