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Cathode ray tube (CRT) panel glass is processed into glass foams by using carbon and Mn 3 O 4 as foaming agents. We investigate the foaming ability and foaming mechanism of the mixture by varying the composition and the temperature. Carbon load of 5 wt% inhibits the sintering of the glass powder, while 2 wt% carbon promotes the coalescence of pores during foaming. Carbon load of ≤0.4 wt% is suitable for achieving low density foams (0.15–0.23 g cm-3) with closed pores (75–100%). Foaming is initiated by the reaction between carbon and Mn 3 O 4 and this is the main source of melt expansion. Reduction of cations in the glass melt, including Mn3+ dissolved from Mn 3 O 4 , makes an important contribution to the melt foaming at >800 °C. The main gaseous product is CO 2 (67–95 vol%). CO appears above 800 °C and the concentration increases with temperature. [ABSTRACT FROM AUTHOR]

• Low density open-porous-type foamed glass was prepared. • Nucleation agent TiO 2 secured low content of closed pores. • Contributions to the effective thermal conductivity were evaluated. • Solid thermal conductivity contributes 50% to the effective thermal conductivity. Sustainable development focused on energy and material savings is driving attempts to decrease the thermal conductivity of heat insulation materials. Foamed glass is one of the most sustainable thermal insulation materials and has unique, load-bearing properties. In this study, contributions to the effective thermal conductivity were quantified for an open-porous-type foamed glass. Foamed samples were prepared by heating a mixture of waste flat glass, the foaming additives carbon and Mn 3 O 4 , and the nucleation agent TiO 2. The density of the foamed glass was 117 kg m−3 (total porosity 95.8 vol%) with a closed porosity of only 5.8 vol%. The latter was minimized by promoting partial crystallization. The thermal conductivity was 54.3 mW m−1 K−1 at atmospheric pressure. Fitting the pressure dependence of the thermal conductivity gave a base thermal conductivity of 26.7 ± 0.4 mW m−1 K−1, and a gaseous contribution of 27.7 ± 0.5 mW m−1 K−1 with a pore size of 0.77 ± 0.07 mm. Thus, the base thermal conductivity, which is attributed mainly to the thermal conduction in the solid phase, is much higher than in the case of fibrous glass insulation products. [ABSTRACT FROM AUTHOR]

The process of foaming glass is very dependent on the chemical composition of the glass. In this study we used a foaming-agent/oxidizing-agent couple and a crystallization inhibitor to foam cullets of flat, container and CRT-panel glass. Foamed glass with a density of 110–120 kg m –3 , a thermal conductivity of 50–52 mW m –1 K –1 and a homogeneous pore structure was obtained from a mixture of panel glass, 0.33 wt% carbon and 4.45 wt% Fe 2 O 3 . We also showed that it is possible to fabricate foamed glass with the same density or pore structure as mentioned above by adding up to 50 wt% container cullet or 70 wt% flat glass to the mixture. In the foamed samples with a low content of panel glass, crystals form, resulting in an increased open porosity, density and inhomogeneous pore structure. The crystallization can, however, be inhibited by adding calcium phosphate, so enabling the preparation of high-quality foamed glass from flat glass or flat/container-glass mixture. The pore gas is predominantly CO 2 and the pressure inside the pores is 0.36–0.47 bar. The reduced effect of the composition on the foaming process suggests that there is a great potential for stabilizing the production of foamed glass and ensuring the product's quality. [ABSTRACT FROM AUTHOR]

The gas-releasing reaction is the most important process in the preparation of foam glass. In this paper we investigated the gas-releasing reactions by means of thermogravimetry coupled with mass spectrometry. We used carbon (activated charcoal and carbon black) and/or manganese oxides (MnO 2 , Mn 2 O 3 , and Mn 3 O 4 ) as the foaming additives. We show that manganese oxides have different functions in the foaming process. The thermal decomposition of MnO 2 below the sintering temperature has a negative impact on the foaming process as it shifts the foaming to higher temperatures, increases the mass-loss rate, leading to open pores, and burns out the carbon. When foaming in an oxidizing atmosphere, the carbon is burnt out by the oxygen from the atmosphere. Instead, Mn 2 O 3 can be used as the foaming agent in an oxidizing atmosphere. In the oxygen-free atmosphere, Mn 3 O 4 can be used as the oxidizing agent , supporting the oxidation of carbon and the foaming process. The redox equilibrium of manganese (Mn 2+ /Mn 3+ ), influenced by the oxygen partial pressure in the pores and physically dissolved oxygen in the glass, shows the strongest influence on the foaming process. The CO/CO 2 ratio in the evolved gases depends on the carbon source and the temperature. [ABSTRACT FROM AUTHOR]

We prepared low-density foam glasses from cathode-ray-tube panel glass using carbon and MnO 2 as the foaming agents. We investigated the influence of the carbon and MnO 2 concentrations, the glass-powder preparation and the foaming conditions on the density and homogeneity of the pore structure and the dependence of the thermal conductivity on the foam density. The results show that the moderate foaming effect of the carbon is greatly improved by the addition of MnO 2 . A density as low as 131 kg m −3 can be achieved with fine glass powder. The foam density has a slight dependence on the carbon and MnO 2 concentrations, but it is mainly affected by the foaming temperature and the time. The thermal conductivity of the foam-glass samples is lower than that of commercial foam glasses with the same density. The lowest value was determined to be 42 mW m −1 K −1 for a foam glass with a density of 131 kg m −3 . A further improvement in the closed porosity could potentially decrease the thermal conductivity even further, and thus our approach has great potential in terms of a thermal insulation material. [ABSTRACT FROM AUTHOR]

Vermiculite materials are widely used as insulation material in the industry, fire protection systems, and fire stoves. The high dimensional stability of vermiculite boards ensures that thermal runways in the insulation lining are avoided. Vermiculite boards for high-temperature applications are produced from thermally expanded vermiculite and alkali silicate binder. Here, we show the impact of potassium silicate binder and vermiculite source on the dimensional stability of vermiculite boards. The vermiculite boards expand at 500–830 °C and shrink stepwise at 1000–1250 °C and 1400 °C, respectively, when heated at 10 °C min−1 in ambient air. The expansion process is discussed in relation to the boards containing non-expanded particles and metal carbonates. The evolved gas analysis indicates that the CO 2 release from metal carbonates is the potential cause of the expansion of some vermiculite types. The shrinkage is initiated by the dehydroxylation of the clay structure. The binder promotes crystallisation during the shrinkage, ceasing the shrinkage process at higher temperatures. The expansion and shrinkage of vermiculite boards are anisotropic and depend on the board density and vermiculite origin. These findings provide new insights into the expansion and shrinkage behaviour of vermiculite boards. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the kinetics and dynamics of the reaction between Na2CO3 and the cathode ray tube panel glass powder at 923–1173K. The reaction causes foaming of the glass melt. After the reaction, the T g decreases with increasing Na2CO3 content and reaches a minimum value of T g. However, this T g value is even lower than that of the homogeneous bulk glass with the same chemical composition. The lower T g of the foam glass could be attributed to inhomogeneous incorporation of Na in the glass, leading to Na-rich domains that cause an overall decrease of T g. Remarkably, after 5min treatment at 1073K, the T g drops by 120K, indicating that the reaction between Na2CO3 and glass is very fast. Increasing treatment duration causes a slight increase of T g likely due to both a more homogeneous Na distribution and the compositional change of the glass as a result of Na2SrSi2O6 crystal formation. [Copyright &y& Elsevier]

Abstract: We prepared foam glasses from cathode-ray-tube panel glass and CaCO3 as a foaming agent. We investigated the influences of powder preparation, CaCO3 concentration and foaming temperature and time on the density, porosity and homogeneity of the foam glasses. The results show that the decomposition kinetics of CaCO3 has a strong influence on the foaming process. The decomposition temperature can be modified by varying the milling time of the glass–CaCO3 mixture and thus for a specific CaCO3 concentration an optimum milling time exists, at which a minimum in density and a homogeneous closed porosity are obtained. Under the optimum preparation conditions the samples exhibit a density of 260kg/m3. The thermal conductivity of the foam glass was measured to be 50–53mW/(mK). The observed dependence of the foaming process on the decomposition kinetics of the foaming agent can be applied as a universal rule for foaming processes based on thermal decomposition. [Copyright &y& Elsevier]

Abstract: The dependence of the dielectric permittivity on the uniaxial compressive stress was investigated for the case of Na0.5Bi0.5TiO3–KTaO3 ceramics. Special attention was focused on the reversibility of the permittivity–stress dependence. The results were connected with the transition from the ferroelectric to the relaxor state in the concentration region between 5 and 10mol% of KTaO3. With this transition, the permittivity–stress dependence changes from non-linear, irreversible and time dependent to linear, instantaneous and reversible. In samples with 10–30mol% of KTaO3 some minor relaxations were observed, which were suppressed under a small pre-stress. The stress sensitivity was higher at a lower measuring frequency and the reversible change of the permittivity was the highest in the sample with 20mol% of KTaO3 (the absolute and relative changes of permittivity were 196.5 and 10.3%, respectively, at a pressure change between 8 and 219MPa). The obtained results are discussed in terms of the macrodomains/microdomains’ reorientation under applied pressure and the transition from one to another with a change of the KTaO3 concentration. The use of Na0.5Bi0.5TiO3–KTaO3 ceramics for device applications is described. [Copyright &y& Elsevier]

Abstract: A novel (1− x)Bi0.5Na0.5TiO3–xKTaO3 system was characterized using X-ray powder diffraction, scanning electron microscopy, as well as dielectric and ferroelectric measurements. The results showed the formation of solid solutions across the whole concentration range; however, using a solid-state reaction method it was not possible to obtain single-phase ceramics. The secondary phases formed in the system were alkali-hexatitanate and -tetratantalate. The formation of the solid solutions initially starts with the formation of the Bi0.5Na0.5TiO3- and KTaO3-rich phases, which then react towards the nominal composition at higher temperatures. We observed that the structural and dielectric properties are strongly influenced by the heat-treatment conditions. Typical relaxor properties with a frequency dispersion of the dielectric maximum were obtained only after annealing at a higher temperature, which considerably improved the homogeneity of the perovskite phase. In accordance with the decreasing temperature of the permittivity maximum, ferroelectric measurements showed a changing of the properties from ferroelectric through relaxor to paraelectric with an increasing content of KTaO3. [Copyright &y& Elsevier]

• Development of low-density open- and closed-porous foamed glasses. • Homogeneous pore structure with >90% of open or closed pores. • Open porosity achieved by partial crystallization of the glass. • Low thermal conductivity (37.5–65.6 mW m−1 K−1) measured using heat-flow meter. • Analysis of the difference in the thermal conductivity. Future developments in foamed glass should focus on improving the thermal insulation properties, controlling the foamed structure and the type of porosity. In this study, we report on the synthesis and properties of foamed glasses with open-porous and closed-porous structures prepared from waste cathode-ray-tube (CRT) panel and soda-lime-silica glasses with the addition of carbon black, and manganese or iron oxides. The samples exhibit a homogeneous structure with an average pore size in the range 0.5–1.35 mm and open porosity above 91% or closed porosity above 92%. The open porosity was obtained by partial crystallization of the glass during the foaming process. The thermal conductivity, measured using a heat flow meter at 10 °C, is 57.2–65.6 mW m−1 K−1 at 116–143 kg m−3 for the open-porous samples and 37.5–56.2 mW m−1 K−1 at 107–245 kg m−3 for the closed-porous samples. The conduction in the solid phase contributes the major part to the effective thermal conductivity (52–72%). The conduction in the gaseous and solid phases, respectively, is higher in the open-porous foamed glass due to the different gas composition (air) and the different solid composition (glass composition and crystalline content). The developed foamed glasses also have high mechanical strength, making them suitable for load-bearing applications. [ABSTRACT FROM AUTHOR]

One of the most promising candidates to replace lead-based compounds in actuator applications are Na 0.5 Bi 0.5 TiO 3 (NBT)-based materials. K 0.5 Na 0.5 NbO 3 (KNN)-modified NBT-BaTiO 3 (NBT-BT) solid solutions exhibit giant large-signal strain–electric-field coefficients (S max /E max) exceeding 500 pm V−1. However, despite the promising properties of the ceramics reported in the literature, the synthesis of these materials remains challenging, leaving gaps in the understanding of the synthesis-property relationship. In this contribution, we investigate the microstructure and the electrical properties while changing the composition to destabilize the ferroelectric order in the material, which is the key to achieve large strain response. Measurements of dielectric and ferroelectric properties reveal that Na- or Ti-deficiency or excess of Bi decrease the ferroelectric-to-relaxor transition temperature and remnant polarization, indicating a destabilization of the ferroelectric order. Additionally, the use of KNO 3 instead of K 2 CO 3 as the potassium source in KNN results in an additional destabilizing effect on the ferroelectric order, which can be attributed to better incorporation of K+ into the perovskite structure. The results identify the key aspects of the synthesis of NBT-BT-KNN ceramics to obtain high S max /E max values. [ABSTRACT FROM AUTHOR]

Foamed glass preparation is a complex combination of reactions, greatly influenced by the composition of the used glass and often hindered by simultaneous crystallization. The crystallization phenomenon is undesirable in foamed glass production since it decreases the quality of the final product. In this work the influence of different types of additives (foaming agents, flux agents, crystallization inhibitors and nucleating agent) on the crystallization of waste container glass and properties of the sintered samples (density and thermal conductivity) was studied. Results of our study confirmed partial crystallization during sintering stage. We found that waste container glass manifests complex crystallization with the formation of four main crystalline phases, which can be inhibited with the addition of fluxing agents (B 2 O 3 and borax). Moreover, here we show that prevention of the crystallization can lead to a significant decrease of the thermal conductivity. [ABSTRACT FROM AUTHOR]

Polyurethane foam (PUR) is a lightweight, thermally insulating, widely used, and highly flammable material that has after its use an adverse effect on the environment, i.e., PUR disposal is considered hazardous. Its flammability can be mitigated using various fire retardants, but they do not change the hazardous nature of waste PUR. Therefore, in the current study, waste PUR with and without flame retardants based on N and P was incorporated into a geopolymer, the alkali-activated material (AAM) based solely on metakaolin, to evaluate the potential recycling route of waste PUR while taking into account its flammability, so it can enter safely into the circular economy through the building industry. To enhance the mechanical properties of the composite, a fresh mixture was irradiated with microwaves. However, the irradiation of geopolymer containing PUR negatively influenced mechanical performance, which led to the evaluation of the behaviour of the complex dielectric constant of PUR and fire retardants. Materials and composites were evaluated regarding their chemistry, mineralogy, microstructure, and porosity to connect the structure with extrinsic properties like geometrical density, thermal conductivity, and fire properties. Nonetheless, positive influences of PUR being encapsulated in the geopolymer were lowered density (from 1.8 to 1.6 kg/l) and improved thermal insulation ability (from 940 to 860 mW/(m·K)) of the composites: with the inclusion of <5 % of PUR, thermal insulation improved by nearly 10 %. However, the contribution of PUR to the composite originated from its skeleton, which has more than 15 times bigger geometrical density (0.81 kg/l) compared to the density of the skeleton (0.047 kg/l). This offers an open field for further advancements of thermal properties, but would also lead to a decrease of the compressive strength, which was already lowered from 90 MPa for 30 % with <5 % of added grated PUR. Furthermore, the flammable nature of PUR and its other drawbacks can be controlled by permanent embedding in the noncombustible structure of geopolymer, making the envelope of sustainable buildings green and safer. Overall, including grated waste PUR in geopolymer represents a promising, easy, cost-effective recycling path with low energy consumption, where the composite cannot develop fire on a scale of pure PUR, even in the worst-case scenario, but only if the composite is designed in a way, that flammable materials cannot join flames during their combustion. This paper gives prospects to other flammable waste materials to be safely used in the circular economy, and to porous materials to shape properties of the composite by their intrinsic and/or extrinsic properties. [Display omitted] • Density of the composite is affected by PUR grating quality more than by its mass. • Well-grated PUR shifts pore sizes of the geopolymer composite to smaller values. • Less than 5 % of grated PUR improves insulation ability of geopolymer by up to 10 %. • Upon fire-safe combustion of PUR, remaining voids become available for gas release. • Voids ensure fewer crack formation during exposure to high temperatures. [ABSTRACT FROM AUTHOR]

For thermal insulation to be sustainable, its performance and production efficiency must be considered. Foamed glass prepared from the mixture of waste cathode ray tube panel glass (CRT), Mn 3 O 4 and carbon could become such a material assuming that its production efficiency could be improved. In light of this, the aim of the study was to engineer the transfer of the foaming process from inert to air atmosphere without drastically disturbing the primary mechanism of expansion. Foaming of carbon-containing mixtures in air atmosphere is normally a challenge due to premature oxidation of carbon by the oxygen from the air. Here, we systematically investigate how the addition of water glass (WG) affects the process by thermogravimetry coupled with mass spectrometry (TG/MS) and heating-stage microscopy analysis. Further, we propose an explanation about how WG protects the carbon and show that the addition of WG allows for the process to be successfully performed in air atmosphere. Two direct sources of expansion were identified (carbon-Mn 3 O 4 reaction and WG) and quantitatively evaluated, allowing determination of an optimal addition of WG, 12 wt %, for the foaming temperature of 800 °C. The obtained foamed glass samples have a relatively low density and degree of open porosity, which reflects in their low thermal conductivity (λ). The lowest λ obtained was 39 mW m−1 K−1 at a density of 145 kg m−3, which is comparable to the samples prepared in inert atmosphere and best commercial products. Image 1 • Water vapours displace the air and form less oxidative atmosphere within the sample. • Main expansion source identified from different combinations of additives. • Water glass content with minimum effect on primary foaming mechanism determined. • Obtained product with similar properties as in a controlled atmosphere. • Effective use of the process in the air atmosphere provides environmental benefits. [ABSTRACT FROM AUTHOR]

Glass foams are attractive thermal insulation materials, thus, the thermal conductivity (λ) is crucial for their insulating performance. Understanding the foaming process is critical for process optimization. Here, we applied high-speed synchrotron X-ray tomography to investigate the change in pore structure during the foaming process, quantifying the foam structures and porosity dynamically. The results can provide guidance for the manufacturing of glass foams. The 3D pore structures were also used to computationally determine λ of glass foams using image-based modelling. We then used the simulated λ to develop a new analytical model to predict the porosity dependence of λ. The λ values of the glass foams when the porosity is within 40% to 95% predicted by the new model are in excellent agreement with the experimental data collected from the literature, with an average error of only 0.7%, which performs better than previously proposed models. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Low thermal conductivity foams prepared from recycled panel glass have been produced. • The mechanisms of heat transfer through glass foams of varying density and microstructure are evaluated. • A new model accounting for the scattering of radiation in foams with bimodal cellular structures is proposed. • The contribution of each heat transfer mechanism and the thermal conductivity of glass foams at different temperatures are predicted by the model. • The model is validated using experimental measurements and the deviation is below 2%. Glass foams are highly sustainable materials with enormous potential to replace traditional thermal insulators. Current research on glass foams is focused on improving thermal insulation by reducing density, but this is somehow a limited approach. Selecting the best approach to reduce thermal conductivity relies on establishing a detailed knowledge on the heat transfer mechanisms playing the most significant role. We have studied the mechanisms of heat transfer in a set of cathode-ray-tube (CRT) panel glass foams. The influence of relative density, cellular structure, gas composition, glass distribution between cell walls and struts, and radiation on heat transfer is addressed. From a detailed characterisation and state-of-the-art models, we have created a new analytical model adapted to the peculiarities of the foams under analysis. Additionally, the thermal conductivity for varying temperature conditions has been predicted. The obtained results point out to a significant contribution of the heat transfer by radiation for foams with porosities higher than 0.96. The predicted values of thermal conductivity indicate the existence of a plateau region at high porosities. Temperature is proven to shift the contributions of heat transfer, increasing the weight of radiation and gas conduction. The predictions of the new analytical model have been contrasted with experimental measurements of the thermal conductivity at different densities and temperatures obtaining differences below 2%. [ABSTRACT FROM AUTHOR]