Photovoltaic glass furnace lip bricks—how does such an "unassuming" brick determine the life or death of a furnace?

The Lip Brick: The "Gatekeeper" at the Furnace Outlet
To understand why the lip brick is so critical, one must first grasp the role it plays in photovoltaic (PV) glass production.
The core process of PV glass manufacturing follows this sequence: batching, melting, rolling, annealing, and cutting. During the melting stage, the raw material mixture transforms into a homogeneous molten glass at temperatures exceeding 1,500°C. This molten glass then flows through the furnace's longitudinal and transverse channels, eventually converging at the overflow outlet. Positioned precisely between this outlet and the rolling rollers, the lip brick acts like a refined "lip," guiding the molten glass evenly and steadily onto the rollers to be pressed into a glass ribbon of a specific thickness.
What does this location imply? It means the lip brick must simultaneously withstand three "hellish" challenges:
The first: high-temperature corrosion. It remains immersed for extended periods in alkaline molten glass at temperatures around 1,100°C, subjecting it to constant chemical erosion.

The second factor is mechanical erosion: the flowing molten glass acts like a scorching river, scouring the surface of the brick day after day.
The third factor is thermal stress shock: during every machine change—specifically the replacement of the lip brick—the brick undergoes rapid heating and cooling; the stresses caused by thermal expansion and contraction are sufficient to crack the fragile material.
Once the lip brick deforms, erodes, or cracks, the consequences are immediate and severe: uneven glass sheet thickness, the formation of stones and cords, incomplete bubble removal, and a precipitous drop in yield rates. In industry parlance, the lip brick is a "small component with a massive impact."

First Generation: Clay-based Lip Bricks—Affordable but Short-Lived
In the early days of the photovoltaic industry, there were no specialized suppliers of lip bricks in China, making clay-based lip bricks virtually the only option.
These bricks were primarily composed of natural aluminosilicates (Al₂O₃-SiO₂); their raw materials were readily available, the firing process was simple, and the cost was low—these were their greatest advantages. Additionally, clay bricks offered decent thermal shock resistance, capable of withstanding a certain degree of temperature fluctuation.
However, they suffered from fatal flaws. The silica (SiO₂) content in the clay bricks reacted readily with the alkaline substances in the molten photovoltaic glass, causing the brick body to gradually erode and dissolve. This not only contaminated the glass melt—leading to defects such as stones and cords—but also caused the lip brick itself to deform, directly compromising the flatness and thickness uniformity of the glass sheets. Even more frustrating was the fact that early imported clay bricks had a service life of only about a month yet cost as much as 100,000 yuan apiece, with delivery lead times stretching up to six months—a situation that caused immense hardship for domestic photovoltaic glass manufacturers.
In short: clay bricks were a product of an era where they were "affordable to buy but too costly to replace and too damaging to use." As quality requirements for photovoltaic glass rose, they were quickly relegated to the dustbin of history.

The Second Generation: Sillimanite Bricks—The Rise of the Best Value-for-Money Option
To address the inherent weakness of clay bricks—specifically their poor corrosion resistance—materials engineers turned their attention to a higher-purity aluminosilicate mineral: sillimanite.
Sillimanite bricks are primarily composed of natural sillimanite (Al₂O₃·SiO₂), typically with an alumina (Al₂O₃) content ranging from 55% to 65%. This compositional shift brought about a qualitative improvement: the higher alumina content enhanced the brick's chemical stability and significantly boosted its resistance to molten glass. Even more advantageously, during the firing process, sillimanite undergoes a mullitization reaction, forming a stable crystalline network of mullite (3Al₂O₃·2SiO₂); this imparts superior high-temperature strength and dimensional stability to the brick.
In 2006, with the introduction of German-style rolling machines to the domestic photovoltaic industry, sillimanite lip bricks made their debut. In 2014, Zhejiang Rongxing New Material Technology Co., Ltd. pioneered two core technologies—vacuum casting and precision kiln firing—and combined them with nano-coating processes to launch the first generation of sillimanite lip bricks. These bricks reduced bubble elimination time from 2–3 days to just 6 hours and extended service life from one month to three months, all while remaining far more affordable than imported alternatives. This breakthrough shattered the overseas monopoly; sillimanite lip bricks rapidly became the dominant choice for photovoltaic applications between 2006 and 2012, at one point commanding over 80% of the market share.
The significance of sillimanite bricks lies in their exquisite balance between performance and cost. They enabled numerous small and medium-sized photovoltaic glass production lines to utilize—for the first time—domestically produced lip bricks that were both durable and affordable, thereby laying the material foundation for the industry's large-scale expansion.

Third Generation: Zirconia-Mullite Bricks—The Current "All-Round Champion"
As photovoltaic glass furnaces rapidly evolve toward higher capacities and greater efficiency, the performance limitations of sillimanite bricks have become apparent. The market demanded a new generation of material offering superior corrosion resistance, extended service life, and better overall performance.
Zirconia-mullite bricks emerged to meet this need.
These bricks incorporate zirconia (ZrO₂) into a mullite matrix, typically with a ZrO₂ content ranging from 10% to 40%. The addition of zirconia—a true "warrior" material—was a masterstroke. It offers three core advantages:
First, exceptional resistance to corrosion and penetration. Zirconia effectively withstands the chemical corrosion and physical erosion caused by high-temperature molten glass, ensuring the long-term dimensional stability of the lip brick and, consequently, the uniformity of the glass flow.
Second, a phase-transformation toughening mechanism. Zirconia undergoes a phase transformation when subjected to thermal stress, absorbing the energy of crack propagation; this makes the brick less prone to cracking and significantly extends its service life.
Third, superior thermal stability. During the rapid heating and cooling cycles associated with repeated furnace adjustments, zirconia-mullite bricks demonstrate thermal shock resistance far superior to that of earlier products.
Currently, sintered zirconia-mullite bricks have become the preferred lip brick material for new, large-scale, high-efficiency photovoltaic glass production lines. Although the unit price is higher than that of sillimanite bricks, the high product yield and extended service life they deliver make their total lifecycle cost highly competitive. According to industry data, using high-quality zirconia-mullite lip bricks can boost the finished glass yield from 70% to 85%—a difference that translates into astronomical economic gains in an industry where profit margins per square meter are measured in mere fractions of a cent.

Fourth Generation: Sintered Mullite Bricks—The "Pure-Blooded Aristocrats" at the Pinnacle
If zirconia-mullite bricks are the battle-hardened warriors of the field, then sintered mullite bricks are the pristine "pure-blooded aristocrats."
These bricks are manufactured using high-purity synthetic mullite (3Al₂O₃·2SiO₂) as the raw material; they typically feature an alumina content exceeding 70% and possess an exceptionally pure, uniform microstructure. Their core value lies in "zero contamination": because their composition closely matches the secondary crystalline phases found in the molten glass, even trace amounts of erosion do not introduce new impurities (such as zircon stones), thereby fundamentally eliminating glass defects caused by the brick material itself.
Furthermore, sintered mullite bricks offer superior refractoriness, high-temperature structural strength, and creep resistance, remaining virtually undeformed during prolonged exposure to high temperatures. However, the trade-off is evident: the use of high-purity synthetic raw materials and demanding manufacturing processes makes them the most expensive option. Consequently, they are typically reserved for high-end applications requiring stringent glass quality—such as premium electronic glass and ultra-thin photovoltaic glass—rather than being the standard choice for conventional photovoltaic production lines.

From clay to sillimanite, and from zirconia-mullite to pure sintered mullite—behind every material evolution lies the photovoltaic industry's relentless pursuit of higher quality, lower costs, and longer service life.

Conclusion
A single "lip brick"—measuring no more than two meters in length and weighing just a few hundred kilograms—bears the weight of a furnace's production capacity, a line's yield rate, and an industry's baseline for quality. From clay to mullite, from import dependence to independent control, and from a replacement cycle of "once a month" to "three months or longer," the evolution of the lip brick embodies the tenacity and glory of China's photovoltaic industry as it transitioned from a follower to a global leader.

When we gaze at a lip brick resting quietly at the furnace exit, we see more than just the physicochemical beauty of a refractory material; we witness an industry’s unwavering commitment to uncompromising quality. On this journey toward excellence, the quality of every brick, every layer of insulation fiber, and every inch of thermal barrier begins at the source. With the same dedication to craftsmanship maintained for fifteen years, Zibo Heru Energy-Saving Materials Co., Ltd. is committed to laying a solid, indispensable thermal insulation foundation—invisible yet vital—for the efficient operation of every photovoltaic furnace.