Aerogel, a nanoscale porous material with a porosity exceeding 90% and a thermal conductivity as low as 0.013 W/(m·K), has been around since 1931 and has always carried a contradictory label - its performance is at the pinnacle, but its cost is prohibitively high. It was once the exclusive thermal protection material for NASA's deep space probes and a delicate scientific wonder in the laboratory, but it has never truly made its way into industrial production and the broad realm of daily life.
The core obstacle hindering the industrialization of aerogel is not its performance, but its cost. The investment required for supercritical drying equipment often amounts to several million dollars, the expensive reagents for organosilane precursors, and the inefficiency of the intermittent production mode have jointly constructed an invisible barrier for aerogel to enter the market. 
The breakthrough comes from a seemingly ordinary industrial raw material - sodium silicate (also known as water glass). By replacing the expensive precursors with this silicon source, which costs only one-tenth of that of organic silanes, and combining it with the atmospheric pressure drying process, Chinese research teams are transforming aerogels from "laboratory luxuries" into "industrial commodities". This article focuses on the unique technical route of the sodium silicate-ammonium sulfate system, and deeply analyzes its scientific principles and process innovations. 
Sol-gel Chemistry: The First Step in the Birth of Aerogels 
To understand the preparation of sodium silicate-based aerogels, one must first return to the basic principles of sol-gel chemistry.
When sodium silicate dissolves in water, there are various silicate ions present in the system. As the pH value decreases, these ions undergo hydrolysis and condensation reactions, gradually forming a three-dimensional network structure with silicon-oxygen bonds as the main chain. This network encapsulates a large amount of solvent, forming what is known as a "wet gel". The reaction seems simple, but it actually hides many complexities - the uniformity of the gel network, the pore size distribution, and the strength of the framework are almost entirely determined by the process parameters at this stage. 
The traditional preparation methods usually use strong acids such as hydrochloric acid and sulfuric acid as gel promoters. These strong acids have extremely fast dissociation rates. If the mixing is not thorough enough, it is very likely to cause drastic fluctuations in local pH values. In the local areas with excessive acidity, the polymerization occurs too quickly, resulting in dense lumps; in the areas with insufficient acidity, the gel is not fully formed, leaving structural defects. The final prepared aerogel is often a mixture of "good structure" and "bad structure". 
This is precisely where the unique value of ammonium sulfate lies. As a weak acid-weak base salt, ammonium sulfate slowly releases ammonium ions and sulfate ions in aqueous solution, and can gently and subtly regulate the pH value of the system in an "effortless" manner. 
This buffering effect prevents drastic fluctuations in local acidity, creating an ideal environment for the uniform condensation of silicate species. The gel network grows calmly under relatively stable chemical conditions, ultimately resulting in a wet gel structure with concentrated pore diameters and uniform framework. 
Pipeline mixer: Upgrading pH control from manual to engineering
If the selection of ammonium sulfate solved the "what to use" problem, then the introduction of the pipeline mixer answered the more engineering-oriented question of "how to adjust".
In traditional batch production, pH adjustment relied on the experience of the operators: drop by drop, acid solution was added to the stirred tank while pH test paper or electrodes were used for monitoring. When approaching the target value, the speed was slowed down, and the final fine-tuning was completed by feel. This operation mode was not only inefficient but also fatally difficult to ensure consistency between batches - differences in judgment by different operators, or even by the same operator at different times, would leave marks on product quality. 
The pipeline mixer has completely changed this situation. In this device, the sodium silicate solution and the ammonium sulfate solution flow into the mixing chamber through their respective pipelines. Under the forced action of static mixing elements or dynamic stirrers, uniform mixing is achieved within seconds or even milliseconds. More importantly, the online pH meter set at the outlet of the mixing pipe and the flow regulating valve form a closed-loop control system - if the pH value is too high, the acid liquid flow is automatically increased; if the pH value is too low, the flow is correspondingly reduced - ensuring that the mixed solution always remains within the target range of 7-8. 
The significance of this automated control goes far beyond the simplistic understanding of "saving manpower". From a scientific perspective, the weak alkaline condition of 7-8 is the "sweet zone" for the gelation of sodium silicate: the polymerization rate is moderate, allowing sufficient time for the gel framework to grow and mature, while avoiding excessive residual sodium ions in the framework under strong alkaline conditions. From an engineering standpoint, the continuous pipeline operation has enabled the production of aerogels to shift from the intermittent mode of "one pot after another" to a continuous and standardized process, laying the foundation for subsequent large-scale mass production. 
Performance Limitations: How Far Can Silica Sodium-Based Aerogels Go
The viability of any technological route depends on whether it can meet the performance requirements of the application scenarios. The achievable performance limit of silica sodium-based aerogels is the focus of industry attention. 
The specific surface area is the core indicator for evaluating the quality of aerogels. Experimental data show that the hydrophobic SiO₂ aerogel prepared by extracting sodium silicate from industrial waste has a specific surface area of up to 750.4 m²/g, with pore diameters ranging from 2 to 15 nm. This data is at the same level as that of high-end products prepared by the organic silane method, proving that the alternative raw materials have not sacrificed the quality of the microstructure. 
The thermal conductivity is the most highly regarded performance indicator for applications. The aerogel prepared by the sodium silicate-based route has a thermal conductivity as low as 0.022 W/(m·K). Even after absorbing air, it still remains within the range of 0.03 to 0.04 W/(m·K), and its heat insulation capacity is 2 to 4 times that of traditional insulation materials. 
The shortcoming that needs to be addressed is the mechanical performance. The trace residual of sodium ions in the gel matrix will make the material slightly more brittle than the products obtained by the alcohol salt method. The compensation strategy comes from composite material technology - a sodium silicate-based composite aerogel reinforced with glass fibers and ceramic fibers. While maintaining excellent thermal insulation properties, it has achieved the engineering characteristics of being cuttable and constructable, which are exactly the necessary engineering properties for industrial applications.