What is the @ symbol used for in material science?
The at symbol @ universally means &quot;at the site&quot;  or &quot;at the rate of&quot;.
It is routinely used in supramolecular chemistry to denote guest@host relationship because

it follows the original designation of denoting the binding site;
its shape resembles a guest (letter &quot;a&quot;) entrapped within a host shell (circle sign &quot;○&quot;) providing a visual cue for an intuitive image (e.g. see endohedral fullerene).

In the context of the paper by Guo et al. [1] you are referring to, paraffin is encapsulated inside $\ce{SiO2}$ shell (obtained in situ), and these silicon dioxide particles were then embedded in silicone rubber matrix.
Here, in the notation SR/Pa@SiO2 the forward slash &quot;/&quot; implies composite material (phase separator), and the at sign &quot;@&quot; refers to host-guest interactions as mentioned before.
Here is a brief infographics I roughly created with Inkscape to illustrate the concept:

An obligatory reference quotation [2, p. 203]:

3.6.17 At Symbol (@)
The at symbol ( @ ) is most commonly used in email addresses. Its modern use is primarily in accounting (where it means “at the rate of”) and should not be used in scientific writing as a substitute for “at.”
Two legitimate uses of the at symbol are

In chemical formulae, the @ is used to denote trapped atoms or molecules. For instance, $\ce{La@C60}$ means lanthanum inside a fullerene cage.
In genetics, an at symbol after a gene symbol indicates that it is part of a gene cluster.


References

Guo, Y.; Yang, W.; Jiang, Z.; He, F.; Zhang, K.; He, R.; Wu, J.; Fan, J. Silicone Rubber/Paraffin@silicon Dioxide Form-Stable Phase Change Materials with Thermal Energy Storage and Enhanced Mechanical Property. Solar Energy Materials and Solar Cells 2019, 196, 16–24. DOI: 10.1016/j.solmat.2019.03.034.
The Manual of Scientific Style: A Guide for Authors, Editors, and Researchers, 1st ed.; Rabinowitz, H., Vogel, S., Eds.; Elsevier/Academic Press: Amsterdam; Burlington, MA, 2009. ISBN 978-0-12-373980-3.