1 Introduction

In today's world, global countries, including South Korea, face numerous meteorological challenges, from heavy rainfall to extreme heat waves [1,2,3,4]. These phenomena, intensified by rapid climate change, result in reduced agricultural outputs and considerable human lives and property loss [5,6,7]. Scientific consensus links these anomalies to the significant increase in carbon emissions caused by centuries of industrial activity and extensive use of fossil fuels [8,9,10].

At the 21st Conference of Parties (COP21) in 2015, the Intergovernmental Panel on Climate Change (IPCC) emphasized limiting global warming to below 1.5 °C and called for achieving carbon neutrality by 2050 [11, 12]. In response, South Korea, among other leading nations, has committed to this goal of carbon neutrality [13, 14]. Attaining this target necessitates transitioning to renewable energy sources, supported by various initiatives and technological innovations to reduce energy consumption. Presently, heating and cooling in commercial, industrial, and residential settings consume 40% of global energy and are responsible for 30% of greenhouse gas emissions (GHGEs) [15].

By 2050, the South Korean government is committed to achieving carbon neutrality across various sectors, improving energy efficiency, and shifting towards renewable energy. A key strategy in this endeavor is to enhance building energy efficiency to lower energy consumption, which is highlighted as feasible and impactful in reducing greenhouse gas emissions. As part of this approach, a mandate is set for all new constructions to be fully zero-energy by 2050, alongside stringent carbon emissions regulations for existing structures. Furthermore, recognizing the significant energy loss through windows—responsible for about 40% of a building's thermal energy loss—there is an expected increase in regulations and support for research to boost window insulation capabilities.

Methods such as applying low-emissivity (Low-e) coatings to block infrared light and using double-glazed structures with a gas insulation layer are commonly employed to enhance the insulation of windows in buildings. Figure 1[16] illustrates a typical double-glazed insulating glass, indicating that the insulation performance varies depending on the number of glass panes and the type of gas used for filling. Compared to single-pane glass, air-filled double-glazed glass shows more than a threefold improvement in insulation, with a U-value of 1.4 W/m2·K. Filling the space with argon gas, an inert gas with lower thermal conductivity and a larger molecular weight that reduces convective heat transfer, further improves insulation by approximately 15%, achieving 1.2 W/m2·K. While there has been an interest in using other inert gases like krypton to enhance insulation performance due to these reasons, argon is predominantly used in commercial insulating glass due to its relative abundance and cost-effectiveness compared to the modest increase in insulation performance offered by rarer gases like krypton, which are significantly more expensive and thus limited to specialized applications [17]. Recent advancements have introduced windows featuring a translucent aerogel core structure in double-glazed units [18,19,20] and multilayered plastic structures [21]. Despite these advancements, widespread adoption is hampered by cost and durability issues. The most common high-efficiency insulating window solutions are double-glazed units filled with argon gas and triple-glazed units. Triple-glazed units, when paired with low-e (low-emissivity) coatings, may attain an impressively low U-value of 1.5 W/m2·K [17, 22].

Fig. 1
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An illustration depicting the structure of a double glazing and typical U-values on different glazing types [16]

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However, such double-glazed insulating windows, which are 3 to 5 times thicker and heavier than single-pane windows, present significant challenges for retrofitting existing buildings. Additionally, their insulation performance is generally inferior to that of walls insulated with high-efficiency foam and inorganic materials, indicating a need for further innovation. Vacuum insulating glass (VIG), featuring a vacuum layer between two glass panes, offers a promising alternative. Although cost barriers persist, VIGs provide superior insulation, with more than double the performance and half the thickness (~ 10 mm) of conventional options. Efforts to improve the insulating performance of VIG (U-value below 0.5 W/m2·K) and reduce material and production costs have been ongoing [23]. Furthermore, integrating VIG's insulative and transparent properties with solar energy generation presents an opportunity for groundbreaking applications [24, 25].

This review discusses the principal aspects of the leading transparent insulation material, VIG, explores its technological challenges and limitations, and presents innovative strategies and prospects for VIG technology.

2 Vacuum Insulated Glazing: History and Challenges

To improve insulation, VIG technology has emerged as a groundbreaking alternative to traditional double-glazed windows, typically filled with inert gases like argon or krypton. By creating a vacuum within the gap between glass panes, VIG significantly reduces heat transfer, effectively minimizing convection and conduction losses. The concept of VIG was initially proposed by Zoller in 1913 and later patented in 1924 [26]. Despite its innovative thermal insulation performance, VIG's commercial deployment faced prolonged delays. The primary challenge lies in designing a structure robust enough to withstand the pressures of a high vacuum while maintaining a balance between structural integrity and insulative efficiency. In contrast, double-glazed windows filled with argon achieved commercial success earlier due to their simpler manufacturing processes. Significant advancements in VIG technology were realized in the 1980s, notably in 1989, when a research team led by Collins at the University of Sydney successfully created a vacuum seal by employing low-melting-point glass at the perimeter of the glass panes, which melts at 450 °C. This breakthrough paved the way for NSG in Japan to develop and introduce SPACIA, marking the launch of the world's first commercially available VIG product [26, 27].

SPACIA VIG comprises two glass panes, each ranging from 3-5 mm in thickness, with spacers that are 0.5 mm in width and 0.2 mm in height, placed at intervals of several tens of millimeters. In addition, the product has a unique low-emissivity (low-e) coating on the inner surface, which effectively prevents radiative heat transmission and achieves a U-value of 1.5 W/m2·K [28]. This level of performance is equivalent to the most recent triple-glazed windows, signifying an insulation performance that is three to five times superior to double-glazed windows of the same thickness.

VIG typically consists of two glass panes, with one or both inner surfaces treated with a durable low-emissivity (low-e) coating to prevent radiant heat transfer (Fig. 2) [29]. A notable limitation is that the application of soft low-e coatings, more cost-effective coatings with superior radiant barrier properties, is restricted. This constraint arises from the necessity of a high-temperature baking process, which induced failure of the coating layer. Stainless steel or nickel-alloy spacers are inserted between the two glass panes to maintain a vacuum gap (about 200 um). The vacuum pressure of 1ATM exerts a force equivalent to 1 kgf/cm2, meaning 1 m2 of the surface of the glass pane must support a load of about 10 tons. If the distance between spacers increases, the load on each spacer increases, leading to the risk of breakage due to stress concentration at the spacer-glass contact point. Achieving an optimal spacer interval is, therefore, essential for structural integrity. Hermetic sealing of the edges is indispensable for vacuum retention, typically achieved using low-melting-point glass or sometimes glass-metal alloys, both matched for thermal expansion. An evacuation hole is also needed to extract air between two panes and hold a getter to adsorb residual gases to maintain the vacuum over time [30]. Because of this insulating structure, convection and radiation heat transfer are minimized. Only conductive heat transfers through the minimal spacers and the edge. This structure makes the insulation performance much better than regular insulating glass. Despite these advantages, widespread adoption and mass production of VIG have been impeded by the economic and technical challenges associated with the high-temperature vacuum evacuation process required during its manufacturing. As shown in Fig. 3, addressing these challenges is crucial for making VIG a feasible option for broader markets.

Fig. 2
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© Elsevier 2008 [29]

An illustration depicting the structure of a vacuum glazing,

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Fig. 3
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An Overview of the Structure and Technological Approaches of VIG

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The primary technology under discussion is the hermetic sealing essential for the performance of VIG. As the vacuum level within the VIG is reduced, the mean free path of gas molecules increases, potentially reaching up to 3000 m at a pressure of 0.1 Pa [31]. This expansion significantly decreases molecular collisions, reducing heat transfer via gas molecules. It is, therefore, crucial to sustain an internal vacuum level below 0.1 Pa to optimize the insulation performance of VIG. Using soda-lime glass in VIG is beneficial due to its notably low gas permeability, aiding in maintaining vacuum conditions over time.

Edge seal integrity is paramount, as most leakage in VIG occurs at these points. In some types of vacuum insulation, like vacuum insulation panels(VIPs), hermeticity is achieved by heat-sealing of aluminum laminated polymer films [32, 33]. In vacuum insulation cavities (VIC), a vacuum space is made by welding a thin metal film to the inner or outer metal wall [34,35,36]. On the other hand, VIG cannot rely on polymer adhesives such as epoxy for edge sealing due to their high gas permeability [37]. Instead, low-melting-point sealing materials, often based on glass substrates, are employed. These materials are applied and fused using lasers or other heat sources to ensure a robust seal.

For instance, Collins used lead(Pb) glass with a melting temperature of around 450 °C to join two glass panes effectively [26]. However, the industrial use of lead is severely restricted due to environmental regulations [38, 39]. There has been a shift towards exploring lead-free alternatives and developing new sealing technologies. These include using indium and tin-coated glass or tin-indium alloys for fusion sealing [40].

The second technology under discussion is lowering the baking temperature of VIG. Typically, the production of VIG involves a meticulous low-temperature baking process, segmented into four primary stages: drying, pre-baking, working, and cooling, as depicted in Fig. 4. Drying Stage: This initial phase involves the elimination of binders used to adhere frit powder to the glass surface. The purpose is to prepare the surface by removing organic materials that could interfere with sealing. Pre-Baking Stage: Following drying, this stage focuses on releasing residual gases from the glass surface. The gradual increase in temperature ensures that the glass is adequately prepared for the hermetic sealing process. Working Stage: At this critical juncture, temperatures are elevated to reach the melting temperature of the sealant, enabling the completion of the hermetic seal. Typically, temperatures around 450 °C are required to seal the edges effectively. Cooling Stage: Post-sealing, the assembly is gradually cooled to room temperature. This controlled cooling is essential to prevent thermal stress and ensure the seal's integrity.

Fig. 4
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© Elsevier 2022 [64]

Temperature Curve during VIG Production,

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However, the high temperatures necessary for hermetic sealing can pose significant challenges, particularly affecting low-emissivity coatings. Soft low-e coatings typically applied via Chemical Vapor Deposition (CVD), are susceptible to oxidation at these temperatures. Additionally, the glass may undergo annealing, thereby losing its tempered state. In contrast, hard low-e coatings, which are applied by depositing molten tin on the glass during production, are more resilient to high temperatures. Despite their durability, these coatings are often criticized for their relatively lower emissivity and the complexities of applying them to conventional glass post-manufacture [41]. Consequently, VIGs fabricated through high-temperature exhaust processes are only used with hard low-e coatings. Furthermore, In the production of VIG, a critical issue arises from the exposure of tempered glass to sustained high temperatures, specifically those exceeding 300 °C. Such conditions can lead to annealing, which diminishes the glass's tempered properties, effectively reducing its strength and altering its fracture characteristics. This annealing effect poses a significant challenge, especially given regulatory requirements in many markets for using tempered glass in construction and other applications to ensure safety. Moreover, the aspiration to improve insulation performance by minimizing the number of spacers within VIG units, thereby reducing thermal bridges, is counteracted by the loss of temper due to annealing. This affects the structural integrity of the glass and its insulative efficiency. Additionally, the necessity for high temperatures in the sealing process extends production durations and escalates energy consumption, further complicating mass production and cost-efficiency efforts. Various technological innovations have been pursued to lower the required process temperatures for VIG production in response to these challenges. Efforts have been directed towards developing sealing materials and methods that can be effectively applied at lower temperatures, thereby preserving the tempered state of the glass while ensuring a robust hermetic seal. These endeavors encompass a variety of strategies aimed at enhancing the efficiency of the sealing process, reducing energy consumption, and ensuring the compatibility of VIG with tempered glass requirements. Such technological advancements are crucial for VIG's wider adoption and cost-effective production, aligning with industry and regulatory standards for safety and performance [42,43,44,45].

The third innovative stride is in long-term vacuum maintenance. An essential component in advancing VIG technology is the capability to maintain a stable vacuum pressure below 0.1 Pa for extended periods, often spanning over a decade. Achieving and sustaining such a low pressure requires materials with exceptionally low gas permeability and minimal outgassing properties. Consequently, this requirement significantly limits the choice of suitable materials for VIG construction to primarily glass and metal substrates. These materials are favored for their inherent characteristics that align with the stringent requirements for long-term vacuum retention. Beyond selecting appropriate materials, the innovation in and application of high-capacity gas absorbers or getters play a pivotal role in the longevity of vacuum conditions within VIG units. Getters are substances that absorb or adsorb gases, effectively removing residual gases and those that may permeate into the vacuum space over time. The development of these gas absorbers is critical; they must possess the capacity to adsorb gases throughout the lifespan of the VIG, thereby ensuring that the vacuum's insulative properties remain uncompromised [30, 46].

The fourth aspect under consideration is the assurance of structural integrity in VIG. VIG faces significant challenges in maintaining structural integrity, primarily due to concentrated loads and thermal stresses. These stresses directly result from the intense vacuum pressure exerted within the VIG and the substantial temperature differentials across the glass panes. Specifically, concentrated loads present a critical issue; the small, rigid spacers that separate the two glass panes bear heightened stress in their vicinity due to the vacuum pressure [47]. When VIG units are subjected to external impacts, stress concentration around the spacers can lead to localized points of potential failure, risking glass fracture if the induced stress exceeds the material's inherent strength. To mitigate this issue, reducing the spacer interval is essential, aiming to distribute the load more uniformly across the glass surface. However, a denser arrangement of spacers could inadvertently compromise the VIG's insulation performance by facilitating thermal bridging. Research efforts have been geared toward finding an optimal spacer arrangement that harmonizes insulation efficiency with structural robustness. This pursuit includes the exploration of new spacer geometries designed to augment the strength of the glass panes. These innovative designs aim to disperse stress more evenly, especially in response to external loads, thereby enhancing the glass's resistance to fracture while maintaining or improving the thermal insulation capabilities of the VIG [48, 49].

Thermal stresses in insulating glass, particularly relevant in climates requiring high insulation levels, arise from temperature differences between internal and external environments, causing compression at cooler temperatures and expansion at warmer ones. Addressing this issue, one approach involves using a flexible material, like metal films, between the glass sheets to allow in-plane movement, leveraging their low gas permeability and flexibility. However, creating an airtight seal between the metal film and glass presents a significant challenge, with an effective sealing method still under development [50]. An alternative strategy is the rigid sealing of the glass sheets' edges, preventing relative movement even under substantial temperature changes. While preventing gas exchange, this approach induces significant shear forces at the edges of the glass, requiring a sealing design that can withstand both these forces and atmospheric pressure. The efficacy of such designs is currently being assessed through numerical analysis and experimental testing [27].

The subsequent chapter will delve into specific efforts to overcome these technical hurdles, exploring the advancements and ongoing challenges in achieving durable and effective seals for insulating glass units in demanding thermal environments.

3 New Approaches

3.1 Development of Core Materials

In VIG, the role of spacers is pivotal as they are responsible for withstanding atmospheric pressure and preserving the vacuum gap that ensures the glass's insulative properties. However, the design and placement of these spacers are critical because they can create concentrated loads on the glass surfaces, potentially leading to stress points that may compromise the structural integrity of the VIG. Significant research and development efforts have explored various spacer layouts, structures, and materials to mitigate these challenges. These advancements aim to distribute the load more evenly across the glass surfaces, thereby minimizing the risk of glass fracture and enhancing the overall durability and performance of VIG units. Recent advancements have been made as follows:

3.1.1 Fibers Coated Line Pillar

Tiwari and Kim have introduced a new Triple Pane Vacuum Insulated Glazing (TPVIG) design, incorporating three glass panes separated by two staggered grids [23]. As shown in Fig. 5, Fibers coated with frit glass are arranged in a grid pattern on the glass pane and bonded through heat applied under vacuum conditions. Unlike conventional VIG, which forms a single connected vacuum layer through spacers arranged at regular intervals, intersecting fiber fillers allow for multiple independent vacuum spaces. This design feature ensures that, even if vacuum leakage occurs in one area, the thermal insulation performance of the remaining vacuum spaces is maintained.

Fig. 5
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Schematic for the construction of TPVIG, which involves fibers coated with a line pillar, ©Elsevier 2022 [23]

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Additionally, the bond between the grids and the adjacent glass panes serves to distribute stress on the spacers and the edges, thereby enhancing the structural stability of the VIG. The independent vacuum spaces also allow the VIG to be cut into smaller sizes post-production without compromising individual vacuum spaces, thus maintaining insulation performance. Post-production flexibility offers a cost-effective method for manufacturing VIGs of various sizes, potentially reducing production costs significantly. However, this approach presents challenges that must be addressed, including the absence of specific methods for evacuating multiple independent vacuum spaces and the lack of established techniques for edge sealing and evacuation processes.

3.1.2 Full-filled aerogel

Due to the superior thermal performance of aerogel, many researchers have tried to embed it into double-glazing windows. So many types of aerogel glazing are developed, such as full-filled, partial-filled, and hollow silica-based [51,52,53]. Beyond filling the double glazing with aerogel, there have been attempts to evacuate air in it. Schultz et al. [54] have ventured beyond traditional design, proposing an alternative form of VIG that incorporates monolithic silica aerogel between two glass panes under a vacuum that is 1,000 to 10,000 times less intense than standard VIG. Filling the vacuum with a porous substance like aerogel, due to its intricate pore structure, traps gas molecules, curtailing heat transfer by gas molecules and delivering superior insulation even at these low vacuum.

This structure obviates the need for high-vacuum processes, marking a pivotal advancement for mass production. Moreover, silica aerogel's light scattering and reflective properties render additional low-e coatings unnecessary, presenting another cost-saving advantage. However, challenges still need to be addressed, like the high cost, low mechanical strength of monolithic silica aerogels, and poor transparency caused by light scattering in the aerogel [55]. Figure 6 depicts a vacuum-sealed aerogel glazing enclosed within a frame. Monolithic silica aerogel has a thermal conductivity of 0.017 W/m·K under atmospheric conditions and transmits almost 90% of solar energy. It has been demonstrated that achieving thermal conductivity as low as 0.010 W/m·K is feasible under a modest vacuum of 100 to 500 Pa [54].

Fig. 6
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Picture of a vacuum-sealed aerogel glazing within a highly insulated frame. The view appears undistorted. However, a minor haziness is noticeable, ©Elsevier 2008 [54]

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3.1.3 Partial-filled aerogel

Diverging from the technique of fully infusing the interstitial space with silica aerogel, Büttner et al. [56]. have proposed employing highly porous monolithic silica aerogel as spacers, covering merely 10% of the total area. While monolithic silica aerogel exhibits good optical clarity and insulation properties, they are accompanied by substantial cost implications for large-scale applications. Büttner's approach, which drastically reduces the amount of aerogel used, has yielded both a reduction in material costs and an augmentation of insulation performance by preventing convection between the spacers and the glass panes. Nonetheless, this mechanical load increase in aerogel could compromise structural stability. So, Cuce and Riffat [57] numerically investigated to find the optimum density of aerogel spacers and evaluate their thermal performance compared with conventional pillars. When considering the actual properties of aerogel spacers, an improvement in insulation performance of up to 46% was observed. The U-value is calculated at 0.67, while 1.2 W/m2·K of commercial VIG of Pilkington.

Due to cost-related issues, innovative research to minimize the use of aerogels, such as those mentioned above, has yet to be effectively integrated with practical studies on long-term vacuum maintenance, including evacuation and getter activation processes.

3.1.4 Mesh-type spacer

Research has also been conducted to enhance insulation performance by attaching transparent vacuum insulation panels (TVIP) to glass windows [58]. As seen in Fig. 7, TVIP is created by inserting a lattice core structure into a transparent gas barrier envelope, then vacuum evacuating and heat sealing it. These TVIPs differ in material and structure from traditional VIPs, made by sealing a porous core structure like glass fiber and fused silica with metal-deposited films. The paper verified the insulation performance of various core structures through analysis and experimentation. Based on these validations, a panel with a thickness of 3 mm achieved the lowest thermal conductivity value of 6.5 × 10–3 W/m·K under a vacuum pressure of 1 Pa. Furthermore, the vacuum pressure was successfully maintained for a considerably long time thanks to getters inserted during the VIP production process to remove residual gases. Its thermal transmittance was determined for mesh and traditional silica aerogel spacers, yielding values of 1.14 and 1.84 W/m2·K, respectively. Despite these advantages, unlike structures supported by two layers of rigid glass panes, those relying on thin polymer films to withstand atmospheric pressure can experience issues with structural warping due to minor imbalances and challenges in maintaining flatness. Additionally, unlike VIPs that utilize films with opaque aluminum deposition layers to improve polymer gas permeability, TVIPMs must maintain light transmissibility, prohibiting metallic gas barrier layers. Therefore, this indicates the need for further long-term performance studies over several years.

Fig. 7
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The structured-core and transparent vacuum insulation panel (VIP) for existing windows are shown, including manufacturing methods, real photographs, schematic description, and a proposed practical field application, ©Elsevier 2019 [58]

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3.1.5 Elastomeric layer on pillar

For the mass production of VIG, extensive research has primarily focused on ensuring the hermetic seal of the glass panes' edges and materials and bonding methods that allow for low-cost and rapid assembly. However, the design of spacers that support the vacuum space is also a crucial area of research from a long-term reliability perspective. The placement and design of spacers, constrained by material limitations and the simplicity of two-dimensional arrangements, have established optimal design conditions (e.g., metallic spacers, square or diamond lattice arrangements). Yet, the VIG structure, comprising rigid metal spacers and comparatively soft glass compressed by atmospheric pressure, inherently possesses instability that can lead to easy damage from unspecified external impacts. It is noted that substantial compressive stress is exerted at the periphery of the spacer during contact. Generally, spacers are fabricated from metallic materials resistant to compression-induced damage. Hence, damage typically manifests on the opposing glass surface, not the spacer itself. Figure 8 elucidates the phenomenon of cracking and breakage occurring in proximity to a stainless steel spacer. To address these issues, recent research by Son and Song has focused on modifying the geometry and adding reinforcements to spacers to alleviate stress concentration problems [59]. They proposed four typical metallic spacer designs, as shown in Fig. 9, and evaluated the maximum stress using a numerical analysis approach. As anticipated, the results indicated that the smoother the contact shape of the spacer with the glass and the lower the stiffness of the contact material, the more the stress concentration was mitigated. Notably, rounding the edges of the spacers improved the shear stress by up to 9 times, and applying an elastic coating to the contact surface improved the shear stress by more than 13 times. However, challenges remain, such as the difficulty of machining curved surfaces on spacers of 250 μm size, outgassing issues with elastic coatings, and the complexities of the coating process, posing several technical challenges that need to be overcome.

Fig. 8
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The fracture pattern of glass being pressed with a stainless steel spacer, ©Elsevier 2019 [59]

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Fig. 9
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Four representative spacer shapes and the stress concentration ratio for each spacer shape. ©Elsevier 2019 [59]

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3.2 Edge sealing technics

The initial manufacturing procedure for VIG involved positioning two glass panes along their perimeters with frit glass, which has a low melting point. This setup was sealed, and the air within the intervening space was extracted in a high-temperature furnace. This method aimed at creating a hermetic seal to maintain the vacuum, thus enhancing the insulative properties of the glass. Figure 4 displays the temperature profile within the high-temperature oven, designed to facilitate a multi-stage temperature increase and decrease for various processes: a drying process for the sublimation of the binder within the solder, a baking process to remove residual gases from the glass surface and interior, and a sealing process that melts the low-melting-point glass. As previously discussed, the high-temperature sealing process significantly extends the production time due to the time required for temperature ramping, thus increasing production costs. To solve this problem, an effort is made to heat the edges in a specific area by applying a hermetic seal and compressing with a steady force. This method is done to minimize energy usage while heating. The technique involves utilizing a hydrogen gas-fueled torch [60]. However, these methods encountered issues such as glass breakage due to temperature differentials with the surroundings. To address these difficulties, ongoing research is being carried out to create sealing materials with reduced melting points and to change the heat sources used for joining. These tries will effectively enhance the sealing process for the fabrication of VIGs.

3.2.1 Low melting point sealant

The demand for an effective low-temperature hermetic sealing process for joining glass panes has driven extensive research into alternative bonding agents. Initially, the use of flexible and low-melting-point indium and its alloys proved successful for edge sealing [45], but the high cost of this semi-precious metal impeded commercialization. To address the issue, Memon et al. [61] proposed using ultrasonic soldering with Cerasolzer cs186 (Sn 56%, Pb 39%) for hermetic sealing., followed by secondary bonding with an epoxy adhesive. Utilizing this advanced technology, triple vacuum glazing was successfully manufactured at a relatively low process temperature of 200 °C. This development has led to remarkably low thermal transmittance, 0.33 W/m2·K. Figure 10 shows a fabricated triple vacuum glazing with a glass and Cerasolzer composite edge seal and its SEM image without a crack on the interface. However, the environmental concerns regarding the lead content used to lower the melting point have prompted further research into lead-free sealing materials or bonding methods, as many countries now strictly limit the lead content in products.

Fig. 10
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(a) A triple vacuum glazing (300 mm × 300 mm) with a composite edge seal. (b) Scanning Electron Microscope (SEM) view of the front surface of the glass-Cerasolzer bond, revealing the absence of any fractures inside the bond, ©Elsevier 2015 [42]

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In 2020, Memon et al. [40] developed a new concept of glass-metal fusion vacuum hermetic sealing technology. Figure 11 shows two required Pilkington K-glass panes, glass coated with hard low-e tin oxide. The edges of the glass panes were heat coated at 450 ℃ with a primary coating layer. Various combinations of materials, excluding Pb, such as B2O3, Sn, Bi, or Zn, were trialed, and an optimal coating layer was chosen at Sn62-B2O338 wt%. This result is presumed due to the ease of binding between the tin oxide in the binding mixture and the tin layer on the low-e-coated glass. After the primary coating was applied to both glass panes, research on a secondary metal sealing agent was conducted, and it was possible to secure vacuum hermeticity and mechanical strength at a heating condition of 450 ℃ with Sn90-In10 wt%. The Fusion edge-sealed vacuum glazing (FSVG) produced in this manner achieved a very high vacuum level of 8.2 × 10–4 Pa. The thermal transmittance in the middle of the glass pane was measured to be 1.24 W/m2·K, while the total thermal transmittance was measured to be 1.79 W/m2·K.

Fig. 11
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A graphic illustrating a new type of vacuum glazing called fusion edge-sealed vacuum glazing. This glazing is made using a bonded combination of 62% tin (Sn) and 38% boron oxide (B2O3) for the surface texture, which is fused with a 90% tin (Sn) and 10% indium (In) alloy at high temperatures, ©Elsevier 2020 [40]

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This glass-metal fusion seal technology, which does not include lead, ensures environmental stability and has the advantage of being able to produce sealants at a price 75 ~ 85% lower than the hermetic sealants currently on the market due to the low amount of indium used and the relatively cheap materials. This result implies that it is a good technical means to reduce the high material cost that has been a stumbling block for vacuum insulation glass. However, while sealants containing lead can achieve low sealing temperatures, the glass-metal sealant requires a sealing temperature of over 450 ℃, limiting the application of soft low-e coatings and increasing manufacturing costs. In addition, there is still the problem of glass tempering release due to the manufacturing process at temperatures over 300 ℃, making the distribution of commercial and domestic vacuum-insulated glass still challenging, and research to lower the sealing temperature remains a task for the future.

3.2.2 Glass frit bonding by laser source

Recent advancements have introduced an alternative to the traditional high-temperature vacuum bonding process by utilizing the high transmissivity of glass and the thermal properties of laser technology [62,63,64]. This innovative approach employs laser beams to precisely heat and seal the edges of glass panes, creating a vacuum seal at lower temperatures and with greater control. This method significantly reduces the risk of thermal stress and damage to the glass, offering a more efficient and delicate way for VIG units. Figure 12 shows the laser source and laser bonding apparatus. Utilizing a laser as a heat source, known for its high instantaneous output, requires careful selection of materials and establishing precise process conditions for bonding glass frit. Table 1 presents the types of glass frits considered in laser bonding processes. Typically, glass frits with a high Coefficient of Thermal Expansion (CTE), which expands significantly upon heating, are processed at relatively high temperatures. Here, the process temperature refers to the ambient temperature controlled during the laser bonding. The rapid heating of the glass frit to melting temperatures (above 400 ℃) by the laser, followed by swift cooling, can cause fractures in the bond. Figure 13 depicts the different failure modes and microscope images encountered in laser-assisted soda-lime substrates with glass frit.

Fig. 12
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(a) Schematic view of laser scanning system (b) Automatic laser sealing stage, ©Elsevier 2017 [63]

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Table 1 shows the types of glass frits considered in laser bonding processes
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Fig. 13
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Failure mode of the glass frit during laser sealing process, ©Elsevier 2017 [63]

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It's necessary to control the process temperature based on the thermal expansion rate to prevent failures. In essence, the lower the CTE of the edge sealant, the lower the process temperature can be for laser bonding, and vice versa for glass frits with a high CTE. Meanwhile, recent research has been moving towards using relatively inexpensive soda lime glass frits to reduce manufacturing costs.

Among these studies, Emami et al. [63] utilized 2.2 mm thick FTO (fluorine-doped tin oxide) flat glass and attempted bonding at a process temperature of 120 ℃ by controlling laser output and speed. They measured the quality of the bond based on laser conditions. They determined the optimal bonding conditions to be an output of 47 W, speed of 250 mm/s, and beam diameter of 0.6 mm, confirmed through helium leak and thermal shock tests.

Similarly, Zhang's research investigates the relationship between laser parameters, such as power and speed, and their effects on heat penetration and bonding temperatures in glass substrates. The study identified an optimal bonding speed of 0.4 mm/s, a power of 31 W, and a beam diameter of 3 mm for effective glass fusion. Despite the formation of micropores and micro-cracks during the experimental process, the integrity of the bonds was confirmed through liquid penetration tests, showing adequate shear strength exceeding 5.68 MPa. However, the study recognizes the limitation of not incorporating helium leak detection for a more precise assessment of seal integrity [64].

3.2.3 Direct edge sealing by microwave source

Conventional glass bonding methods employing furnaces confront challenges associated with high processing temperatures and slow bonding speeds. Microwave-assisted heating has been proposed as a solution, as depicted in Fig. 14 [65]. The process begins with the cleaning of soda-lime glass surfaces to eliminate contaminants. Non-metallic spacers, particularly zirconia alloys, are strategically placed to prevent arc phenomena in the microwave field. Carbon blocks, which absorb microwaves to heat the edges, are then tightly affixed, and microwaves are emitted until a temperature of 550 ℃ is reached. Upon completion of the bonding, a cooling process finalizes the production of VIG. This production eschews the need for additional solders or frit glass, reducing material costs, potentially increasing production speed, and lowering manufacturing expenses. Moreover, eliminating lead from the sealing materials ensures compliance with environmental regulations. However, the produced samples exhibited a thermal transmittance of 3.5 W/m2·K, indicating a lower insulation performance, potentially due to the absence of low-e coatings or inadequate vacuum levels. Addressing these issues could lead to VIG with improved insulation performance in future developments.

Fig. 14
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(a) The process of fabricating vacuum glazing using microwaves (b) the complete setup of the system used for making vacuum glazing, ©Elsevier 2023 [65]

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3.3 Vacuum maintenance

In vacuum insulation applications like VIG and VIP, keeping a vacuum below a specific pressure is critical for their insulating performance. As shown in Fig. 15, Loss of vacuum arises from several sources, such as the permeation of low molecular weight gases through the glass, desorption of organic adsorbates such as water or nitrogen due to residues from incomplete evacuation during manufacturing, photo-fragmentation reactions from large adsorbate molecules due to oil contamination in the production process, and leakage through the edge seal [66].

Fig. 15
figure 15

A diagram illustrating factors contributing to the increase in the internal pressure of VIG: 1) Gas (He) permeation, 2) Leakage from the edge, 3) Desorption by thermal and optical, 4) Organic decomposition by light, ©Elsevier 2023 [66]

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Figure 16 [67] illustrates how thermal conductivity varies with vacuum level and the pore size of the filler material. As the size of the pores decreases and their quantity increases relatively, the probability of disrupting the heat transfer caused by gas molecule collisions becomes higher, thereby enhancing the insulation performance. Illustratively, the thermal conductivity in a vacuum environment is determined by the barrier to the molecular collisions of gases, indicating that insulation performance depends on the disruption of these collisions. VIPs, with their glass fiber interiors harboring micropores smaller than a few micrometers, demonstrate high insulation performance even at relatively low vacuums. In contrast, VIGs lack a gas barrier, requiring high vacuum levels to achieve target insulation performance, and typically need a vacuum 100 times greater than VIPs.

Fig. 16
figure 16

(a) Variation in thermal conductivity according to pore size and gas pressure, ©ASCE 2008 [67]

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Over time, VIPs tend to experience a steady increase in internal pressure due to continuous gas permeation through their thin polymer-based envelopes, including the surfaces and heat-sealed areas. This is why VIPs often utilize a significant amount of calcium oxide-based getters.

On the other hand, VIGs are sealed with inorganic materials like glass or metal, which have very low gas permeability. However, to achieve and maintain a much higher vacuum, a long and high-temperature exhaust process above 400 ℃ is needed to minimize residual gases. Furthermore, small amounts of getters composed of titanium, zirconium, vanadium, etc., are used to absorb the minimal outgassing over long periods, unlike VIPs, which can absorb larger amounts of gas due to their porous materials. VIGs, having less internal surface area, produce less outgassing, enabling the long-term maintenance of vacuum with only a minimal amount of getter material.

Most vacuum failures in VIGs are more likely due to leaks from mechanical damage rather than continuous outgassing. Therefore, to ensure long-term vacuum maintenance in VIGs, development efforts focus on detecting seal defects with precise optical systems [68], identifying initial micro cracks with helium-leak tests, and reducing risk factors for long-term vacuum maintenance through high-strength thermal stress testing [69].

4 Synergy with other glazing technologies

Recent research has expanded beyond enhancing VIG's performance and production methods to incorporate multifunctional features. A pioneering approach involves developing an active insulation glass system that synergizes polymer dispersed liquid crystal (PDLC) technology with VIG. As delineated in reference [70] and illustrated in Fig. 17, this innovative combination leverages the application of 20 V of alternating current (AC) to PDLC. This electrical input aligns the liquid crystals, thereby facilitating the passage of sunlight through the glass. In the absence of AC, the disruption of this alignment causes an increase in light scattering and a reduction in solar transmission, enabling active control over solar heat gain.

Fig. 17
figure 17

Illustration depicting the polymer-dispersed liquid crystal glazing in both its translucent OFF state and transparent ON state, ©Elsevier 2023 [70]

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The fabrication of a vacuum-PDLC composite and its thermal performance evaluation are discussed in detail. The composite's U-value, a measure of thermal conductivity, was determined to be less than 1.1 W/m2·K, indicating its high insulation efficiency. This capability to dynamically regulate solar heat gain according to seasonal variations underscores a significant leap in optimizing energy efficiency in building envelopes.

The installation of solar panels has predominantly taken place in areas with lower utility value, such as rooftops of buildings, deserts, and water surfaces [71]. Recently, research has been advancing toward integrating solar panels into the exterior walls and windows of buildings. Integrating photovoltaic (PV) systems with VIG represents a forward-thinking approach to energy-efficient architectural solutions, specifically in energy-producing windows. In a notable experiment conducted by Ghosh et al., referenced as [72] and illustrated in Fig. 18, the performance of PV cells integrated with VIG (termed Solar Photovoltaic Vacuum, or SPV, configuration) is thoroughly investigated. This setup sequentially aligns a single glass pane, PV cells, and VIG, optimizing the system to leverage solar energy for electricity production during daylight hours. Conversely, VIG's inherent superior insulation characteristics are harnessed to conserve energy used for heating during nocturnal periods.

Fig. 18
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An exploded view of semi-transparent PV vacuum glazing, ©Elsevier 2018 [72]

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The research delineates that strategically placing the VIG panel on the interior side, thus exposing PV panels directly to solar irradiance, significantly augments the system's electricity generation efficiency. This configuration promotes heat dissipation, effectively curbing the temperature escalation within the PV cells. The culmination of this study reveals that the PV-vacuum glazing configuration achieves a U-value of 66% lower and a solar factor of 42% lower than that of conventional PV-double glazing systems.

Traditionally, the development of solar collectors has focused on creating coatings that efficiently absorb radiant heat. Recently, this has been significantly enhanced by nanomaterials, which amplify the absorption capabilities and cooling performance to prevent overheating [73, 74]. Meanwhile, other advancements in solar thermal technology have seen the integration of VIG into non-concentrating solar collectors, traditionally employed for low-temperature applications under 120 °C in both residential and industrial contexts. The inefficacy of these systems in operating efficiently at intermediate temperatures ranging from 120 to 300 °C prompted the development of an innovative solution: the evacuated aerogel solar collector (EASC), as depicted in Fig. 19 [75].

Fig. 19
figure 19

The diagram and cross-sectional representation of the EAFC, ©Elsevier 2022 [75]

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The EASC design ingeniously utilizes evacuated aerogel to maximize the absorption of incident solar energy on the collector panel, significantly reducing the re-radiation of absorbed energy back into the atmosphere. This strategic approach ensures a substantially high total energy absorption rate, thereby overcoming the limitations observed in traditional solar thermal collection systems within the specified temperature range.

In pursuing more sustainable and energy-efficient building solutions, a significant shift has been observed from passive energy-saving strategies, such as chromic windows that primarily block solar energy, towards active energy conversion methodologies. A key development in this area is the advent of thermoelectric glazing (TEG), which capitalizes on the temperature differences across the glass to generate electrical energy, as illustrated in Fig. 20.

Fig. 20
figure 20

(a) The transparency and flexibility of the thin films made of CuI, ©Springer Nature 2017 [79] (b) The glass container contains 12 thermoelectric modules based on Bi2Te3, generating solar-thermal-electric power, ©WILEY 2021 [77]

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Thermoelectric materials are central to the operation of TEGs, with bismuth telluride (Bi2Te3) being a prominent example due to its well-documented thermoelectric properties [76, 77]. This material, alongside transparent and electrically conductive metals such as zinc oxide (ZnO) [78], copper iodide (CuI) [79], and indium oxide (In2O3) [80], has been the subject of intensive research efforts. Additionally, there has been a growing interest in organic-based thermoelectric materials, with devices based on poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonate (PSS) gaining attention for their flexibility and transparency [81].

The efficiency of TEG systems is intrinsically linked to the temperature differential induced by the absorption of solar energy on the glass surface [82]. Consequently, highly insulating glass, capable of sustaining large temperature differences, is anticipated to result in enhanced generation efficiency. The potential to absorb ultraviolet and infrared rays, thereby establishing a significant temperature differential between the interior and exterior surfaces through vacuum insulation, suggests a dual benefit: minimizing heat loss while maximizing thermoelectric generation performance.

Despite the promising prospects of TEG technology, the research landscape indicates a necessity for more comprehensive investigation and development [83]. This underscores the imperative for continued research and innovation in this field to fully realize the potential of thermoelectric glazing in contributing to the energy efficiency of buildings.

5 Summary and Future work

Enhancing building energy efficiency is identified as a crucial and practical approach to reduce energy use and greenhouse gas emissions. To this end, a policy will require all new buildings to achieve zero-energy status by 2050, with rigorous emission controls on existing buildings. Moreover, with windows being a major source of energy loss, accounting for nearly 40% of a building's heat loss, there is an expectation for stronger regulations and increased research funding to improve window insulation performance. Aligning with global trends, leading window manufacturers worldwide are developing VIG, and due to the lack of standardized specifications, they are employing various manufacturing technologies to produce VIG with remarkable insulation performance, achieving levels around 0.5 W/m2·K shown in Table 2. This innovation represents a significant advancement in insulation technology, offering the potential for substantial energy savings in buildings by further reducing heat transfer compared to traditional double-glazed or gas-filled insulating glass.

Table 2 Companies that specialize in Vacuum Insulated Glazing and their products [88, 89]
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Nevertheless, the adoption of high-efficiency windows faces economic barriers. Standard double-glazed IGUs are priced at approximately $10/ft2, and windows of passive house standards with triple-glazing start at $30/ft2. In comparison, the cost for VIG stands at an estimated $66/ft2, significantly hindering its widespread implementation [84].

While the specific cost details of materials and processes for VIG are proprietary secrets of companies, it is generally known that the production process can take up to 75% of the total cost due to the long evacuation time. To reduce the price of VIG, it's necessary to shorten the processing time and develop technical methods to lower the processing temperature. As shown in Table 3, many researchers have been exploring the use of solders made from combinations of lead, indium, and tin to allow for hermetic sealing at lower temperatures. Additionally, efforts have been made to heat only the edges that require joining locally, thereby reducing the overall processing time using technologies such as hydrogen torches, microwaves, and laser heating.

Table 3 Summary of recent research on VIG for mass production
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Despite these research efforts, most VIGs produced today are made using methods similar to the original manufacturing techniques, typically involving the application of glass-based low-melting-point sealants and heating in high-temperature ovens. However, as interest and research in VIG continue to grow, developing low-melting-point sealants below 300 ℃ or new local heating processes could potentially disrupt the current insulating glass market dominated by argon-filled double glazing, offering superior insulation performance.

The impact of such high-efficiency insulating glass is expected to be substantial in achieving carbon neutrality goals. Moreover, if VIG becomes more cost-competitive, it could contribute to the mass production of next-generation insulating glass by enabling integration with technologies such as liquid crystals, photovoltaics, and thermoelectric, further enhancing energy efficiency and functionality in building materials.