Thermal Analysis of a New Neutron Shielding Vacuum Multiple Glass

sustainability

Article Thermal Analysis of a New Neutron Shielding Vacuum Multiple Glass

Shanwen Zhang 1,* , Min Kong 1, Saim Memon 2 , Hong Miao 1, Yanjun Zhang 1 and Sixing Liu 1

1 College of Mechanical Engineering, Yangzhou University, Yangzhou 225127, China; [email protected] (M.K.); [email protected] (H.M.); [email protected] (Y.Z.); [email protected] (S.L.) 2 London Centre for Energy Engineering, School of Engineering, London South Bank University, London SE1 0AA, UK; [email protected] * Correspondence: [email protected]; Tel.: +86-18361317675

Received: 24 February 2020; Accepted: 2 April 2020; Published: 12 April 2020

Abstract: The neutron shielding glass is widely used in nuclear/fusion plants. To improve its temperature resistance and heat insulation, a Gadolinium (Gd)-containing laminate vacuum multiple glass is proposed by using the vacuum insulation method. A 3D ﬁnite element model validated by theoretical calculation was developed to analyse the heat transfer path and numerical simulation of the multiple glass was carried out to obtain the temperature distribution and the maximum temperatures of the organic glass in relation to dynamic working temperatures, the sealing agent width, view size, and vacuum thermal conductivity. The results show that the vacuum layer between common glasses can make the work temperature of neutron shielding glass increase. The multiple glass has good heat-shielding performance and it is expected to work in a high-temperature environment. In addition, the vacuum layer between the common glasses and the sealing agent width decay with respect to the view size and vacuum thermal conductivity show an increase in the working temperature of the neutron shielding glass. It was concluded that the order of aﬀecting the temperatures of the organic glass follows the pattern of: view size > vacuum thermal conductivity > sealing agent width.

Keywords: neutron shielding glass; vacuum multiple glass; Design; Thermal analysis; high temperature

1. Introduction As a kind of neutral particle, the neutron has a strong penetrating power. It appears widely in nuclear/fusion plants [1–4]. A transparent window (viewing window) is required for visual inspection in nuclear/fusion plants. The neutron shielding glass has good optical properties, easy and cheap production advantages, and it attracts many researchers to study. Gamma and neutron shielding characterisations of the Ag2O–V2O5–MoO3–TeO2 quaternary tellurite glass system were assessed by using the Geant4 simulation toolkit and Phy-X software [5]. Neutron shielding capability of CoO and NiO added bioactive glasses were investigated using MCNPX simulation code [6]. Many eﬀorts have been focused on improving neutron shielding properties for silicate-, borate-, and tellurite-based glasses [7–9]. In mechanical performance, elastic properties of neutron shielding glasses were studied [10–13]. Gd-containing neutron shielding glass has good shielding properties for neutrons, X-rays, and gamma rays. However, its glass transition temperature is lower. When Gd-containing organic glass was prepared with Gd (MA)3 of 30%, the maximum glass transition temperature was reported to be 138.4 ◦C[14]. Neutron shielding organic glass can reduce the damage to the human body next to view glass or machines. It is convenient for workers or robots to observe. However, it has the problem of low-temperature resistance and poor heat insulation, and cannot work in the neutron

Sustainability 2020, 12, 3083; doi:10.3390/su12083083 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 3083 2 of 14 and high-temperature environment of nuclear/fusion plants, such as nuclear accidents associated with ﬁre [15]. Therefore, it is necessary to develop a new neutron shielding glass with high temperature resistance and high heat insulation. Sustainability 2020, 12, x FOR PEER REVIEW 2 of 14 The vacuum has better thermal insulation performance than any material [16,17]. Common glass has highaccidents temperature associatedresistance with fire [15]. and Therefore, a maximum it is necessary glass transition to develop temperature a new neutron which shielding is more glass than 1000 with◦C. So, high vacuum temperature glass resistance formed by and a vacuumhigh heat and insulation. two common glasses has better thermal insulation performanceThe vacuum and high has temperature better thermal resistance insulation performance [18,19]. If vacuum than any glass material and [16,17]. neutron Common shielding glass glass werehas combined high temperature to a new multipleresistance glass, and a this maximum glass will glass have transition the advantage temperature of neutron which is shielding, more than optical properties,1000 °C. thermal So, vacuum insulation glass performance, formed by a andvacuum high and temperature two common resistance. glasses Then,has better it could thermal work in a insulation performance and high temperature resistance [18,19]. If vacuum glass and neutron high-temperature and neutron radiating environment. In this study, Gd-containing laminate vacuum shielding glass were combined to a new multiple glass, this glass will have the advantage of neutron multiple glass is proposed. Its structure and working principle are introduced. A three-dimensional shielding, optical properties, thermal insulation performance, and high temperature resistance. Then, ﬁniteitelement could work model in a high-temperature validated by theoretical and neutron calculation radiating environment. is developed In tothis simulate study, Gd-containing the heat transfer pathlaminate and validated vacuum with multiple the thermal glass is mathematicalproposed. Its structure model. Numericaland working simulation principle are of introduced. the multiple A glass is carriedthree-dimensional out to obtain finite the element temperature model distributionvalidated by theoretical and the maximum calculation temperatureis developed to of simulate the organic glassthe is discussedheat transfer by path changing and validated the work with temperature, the thermal themathematical sealing agent model. width, Numerical view size,simulation and vacuum of thermalthe multiple conductivity. glass is Thiscarried study out to has obtain an importantthe temperature theoretical distribution and an practicald the maximum value fortemperature developing a neutronof the shielding organic glass glass is withdiscussed high by temperature changing the resistance work temperature, and high the heat sealing insulation agent width, as a candidateview windowsize, materialand vacuum for nuclearthermal /conductivity.fusion plants. This study has an important theoretical and practical value for developing a neutron shielding glass with high temperature resistance and high heat insulation 2. Materialsas a candidate and Methods window material for nuclear/fusion plants.

2.1. The2. Materials Structure and of theMethods Gd-Containing Laminate Vacuum Multiple Glass

2.1.A Gd-containing The Structure of laminatethe Gd-Contain vacuuming Laminate multiple Vacuum glass Multiple is proposed, Glass as shown in Figure1. It includes Gd-containingA Gd-containing organic glass, laminate Gd-containing vacuum multiple lined adhesive, glass is proposed, common as glass, shown a vacuumin Figure layer,1. It includes the support, and theGd-containing sealing agent. organic The glass, Gd-containing Gd-containing lined lined adhesive adhesive, is setcommon between glass, the a Gd-containingvacuum layer, the organic glasssupport, and the and middle the sealing common agent. glass. The The Gd-containing vacuum layer lined is adhesive formedby is set sealing between two the common Gd-containing glasses with the sealingorganic agent.glass and To reducethe middle the common high-pressure glass. The diﬀ erencevacuum between layer is formed the vacuum by sealing layer two and common the outside atmosphere,glasses with the supportsthe sealing are agent. set atTo areduce certain the distance. high-pressure This newdifference glass between can solve the the vacuum following layer problems: and the outside atmosphere, the supports are set at a certain distance. This new glass can solve the 1. followingThe problem problems: of neutron shielding: the Gd-containing organic glass contains the Gd element, which can shield the neutron glass; 1. The problem of neutron shielding: the Gd-containing organic glass contains the Gd element, 2. The problemwhich can of heat shield insulation: the neutron the glass; vacuum layer is used to increase the heat resistance, and reduce2. theThe heatproblem exchange of heat betweeninsulation: the the low- vacuum and layer high-temperature is used to increase sides; the heat resistance, and 3. The problemreduce of the high heat temperature exchange between resistance: the low- compared and high-temperature with the neutron sides; organic glass, the common

glass3. hasThe high problem temperature of high temperature resistance (theresistance: top temperature compared with is about the neutron 1580 ◦ C),organic which glass, is set the at the high temperaturecommon glass side. has high temperature resistance (the top temperature is about 1580 °C), which is set at the high temperature side.

FigureFigure 1. The1. The structure structure of of the the Gd-containingGd-containing laminate laminate vacuum vacuum multiple multiple glass. glass.

2.2. Mathematical2.2. Mathematical Modelling Modelling Approach Approach WhenWhen the the vacuum vacuum multiple multiple glass glass is is working,working, its its heat heat transfer transfer includes includes three three ways: ways: convection, convection, conduction,conduction, and and radiation, radiation, as as shown shown in in Figure Figure2 2.. Sustainability 2020, 12, 3083 3 of 14 Sustainability 2020, 12, x FOR PEER REVIEW 3 of 14

FigureFigure 2. The 2. The heat heat transfer transfer paths paths of of the the Gd-containingGd-containing laminate laminate vacuum vacuum multiple multiple glass. glass.

2.2.1.2.2.1. Convective Convective Heat Heat Transfer Transfer Process Process TwoTwo sides sides of the of the vacuum vacuum multiple multiple glass glass have have thethe convection heat, heat, as asshown shown in Figure in Figure 2, and2, andthe the NewtonNewton cooling cooling law law is used is used as as follows: follows: = × − q h (Tw Tf ), (1) q = h (Tw T ), (1) × − f where q (W/m2) refers to the heat flux from the convective heat load. h (W/(m2·K)) refers to the where q (W/m2) refers to the heat flux from the convective heat load. h (W/(m2 K)) refers to the convection coefficient, which depends on the air flow rate past the heat transfer surface,· the air’s convectionthermal coe conductivity,fficient, which air viscosity, depends air specific on the he airat, flow temperature rate past difference the heat between transfer the surface, bulk air theand air’s thermalthe conductivity,heat transfer surface, air viscosity, and so on. air specificTw (K) refers heat, to temperature the temperatures difference of the inside/outside. between the bulk Tf (K) air and the heatis the transfer environment surface, temperature. and so on. T wIn (K)ANSYS14.5, refers to thebecause temperatures the structure of the isinside heterogeneous,/outside. Tthef (K) is the environment temperature. In ANSYS14.5, because the structure is heterogeneous, the convection convection coefficient h was not considered in detail. However, it was assumed that Tw is equal to coefficient h was not considered in detail. However, it was assumed that Tw is equal to T , and the T , and the convection heat was assumed to be the environment temperature. f convectionf heat was assumed to be the environment temperature. 2.2.2. Conductive Heat Transfer Process 2.2.2. Conductive Heat Transfer Process The vacuum multiple glass is also the heat source, because it can absorb the neutron and the The vacuum multiple glass is also the heat source, because it can absorb the neutron and the neutron heat is produced, as shown in Figure 2. Because the neutron is absorbed through the glass, neutronthe heatexponential is produced, function as shownis usedin as Figure the distribution2. Because of the the neutron thermal is energy absorbed [20,21], through and theit can glass, be the exponentialdepicted function as follows: is used as the distribution of the thermal energy [20,21], and it can be depicted as follows: −x x = m m (2) P =ppe−e (2) where P (W/m3) refers to the thermal energy per unit volume after absorbing the neutron. p (W/m3) where P (W/m3) refers to the thermal energy per unit volume after absorbing the neutron. （ refersW/m to the3） thermal refers to the energy thermal per energy unit volume per unit before volume absorbing before absorbing the neutron—it the neutron—it refers refers to the to the natural logarithm.natural xlogarithm.(m) refers x to（ them）depth refers fromto the thedepth top from of the the Gd-containing top of the Gd-containing organic glass organic towards glass the bottom,towards a detailed the bottom, explanation a detailed can explanation been seen in can reference been seen [2 ].inm referencerefers to [2]. the m neutron refers to attenuation the neutron index. Consideringattenuation index. the conduction and the neutron heat, as shown in Figure2, a 3D steady state conductiveConsidering differential the equation conduction with and inner the heat neutron source heat for, as the shown vacuum in multipleFigure 2, glassa 3D cansteady be shownstate as followsconductive [22]: differential equation with inner heat source for the vacuum multiple glass can be shown as follows [22]: ∂ ∂T ∂ ∂T ∂ ∂T (λ ) + (λ ) + (λ ) = Pn, (3) ∂∂x ×∂∂x ∂∂y × ∂∂y ∂z∂ × ∂z ∂ λ × T + λ × T + λ × T = ( ) ( ) ( ) Pn , (3) where T (K) refers to the∂x temperature∂x field∂y variables.∂y λ∂Wz /(m2 K)∂z refers to the heat conductivity · coefficient. P (W/m3) refers to the neutron-generated heat per unit volume. In the ANSYS14.5, λ is the where Tn (K) refers to the temperature field variables. λ （W/(m2·K)）refers to the heat conductivity heat conductivity coefficient3 of the finite element model; x, y, and z are the node positions of the finite coefficient. Pn (W/m ) refers to the neutron-generated heat per unit volume. In the ANSYS14.5, λ element model. P is the neutron generated heat per each element. is the heat conductivityn coefficient of the finite element model; x, y, and z are the node positions of

the finite element model. Pn is the neutron generated heat per each element. Sustainability 2020, 12, x FOR PEER REVIEW 4 of 14

2.2.3. Radiative Heat Transfer Process The radiative thermal energy between the vacuum multiple glass, as shown in Figure 2, can be obtained by the following equation [19,22]: T 4 − T 4 = × × 1 2 Q rad A1 C , 1 1 (4) + − 1 ε ε 1 2

Q （ 2） - where rad (W) refers to the radiative heat energy. A1 m refers to the radiative area. C (5.67×10 Sustainability8 2 42020, 12, 3083 4 of 14 W/(m ·K )) refers to the Stefan–Boltzmann constant [23]. T1 and T2 （K） are the environment ε ε temperature and the temperature of the radiative face. 1 and 2 are the radiation coefficient of 2.2.3. Radiative Heat Transfer Process the radiative faces on the high- and low-temperature sides [24]. In the ANSYS14.5, Qrad is the

radiativeThe radiative heat energy thermal from energythe high between temperature the vacuum side to the multiple low temperature glass, as shown side. inA1 Figure is the2 radiative, can be obtained by the following equation [19,22]: face on the high temperature side. T1 is the temperature of the high temperature side. T2 is the 4 4 temperature of the low temperature side. T1 T2 Qrad = A1 C − , (4) × × 1 + 1 1 2.3. Finite Element Model ε1 ε2 − 2 whereTo studyQrad (W)the thermal refers performance to the radiative of the heat vacuum energy. multipleA1 (mglass,) refersa thermal to themodel radiative with a size area. of C70(5.67 mm ×10 70 8mmW/ (mand2 Ka4 sealing)) refers agent to the width Stefan–Boltzmann of 2 mm was built constant by using [23]. ANSYST and 14.5,T (K)as shown are the in × − · 1 2 environmentFigure 3. It temperatureconsisted of andthe theGd-containing temperature or ofganic the radiativeglass, common face. εglass,1 and εGd-containing2 are the radiation lined coeadhesive,ﬃcient ofthe the support, radiative and faces the on sealing the high- agent. and low-temperatureAll of them were sides built [24 with]. In element the ANSYS14.5, Solid70,Q andrad ishexahedral the radiative mesh heat was energy used. fromTarge170 the highand Conta174 temperature were side used to at the the low interface temperature between side. theA common1 is the radiativeglass and face the onsupport. the high The temperature thermal surface side. effectT1 is theelement temperature SURF152 of was the used high at temperature the interface side. betweenT2 is thetwo temperature common glasses of the lowto simulate temperature the side.heat transfer from the high-temperature side to the lower temperature side, as shown in Figure 2. Sharing nodes were used at the interfaces between all of the 2.3.components, Finite Element except Model the support. The total number of nodes and elements was 75,862 and 68,876, respectively.To study The the thermalproperties performance of several materials of the vacuum are shown multiple in Table glass, 1. a thermal model with a size of 70 mm 70 mm and a sealing agent width of 2 mm was built by using ANSYS 14.5, as shown in × FigureTable3. It consisted1. Material of properties the Gd-containing of the comp organiconents employed glass, common in the finite glass,-element Gd-containing modelling lined of vacuum adhesive, the support,multiple and glass. the sealing agent. All of them were built with element Solid70, and hexahedral mesh was used. Targe170 and Conta174Material were used Thermalat the interface conductivity between (Wm-1 theK-1) common glass and the support. The thermal surfaceCommon eﬀect elementglass SURF152 was used 0.75 at the interface between two common glasses to simulate the heatSealing transfer agent from the high-temperature 35 side to the lower temperature side, as shown in Figure2. Sharing nodesSupport were used at the interfaces between18 all of the components, except the support. The total number ofAdhesive nodes and elements was 75,8620.221 and 68,876, respectively. The properties of several materials are shownOrganic in Table glass1 . 0.2

FigureFigure 3.3. TheThe ﬁnitefinite elementelement modelmodel ofof thethe neutronneutron shielded shielded vacuum vacuum multiple multiple glass. glass. Table 1. Material properties of the components employed in the ﬁnite-element modelling of vacuum multiple glass.

1 1 Material Thermal Conductivity (Wm− K− ) Common glass 0.75 Sealing agent 35 Support 18 Adhesive 0.221 Organic glass 0.2 Sustainability 2020, 12, x FOR PEER REVIEW 5 of 14 Sustainability 2020, 12, 3083 5 of 14 2.4. Model Validation

2.4. ModelIn order Validation to verify the correctness of the model, the low and high temperatures were 20 ℃ and 170 ℃. The vacuum thermal conductivity was assumed to be 0.023 Wm-1K-1. Through simulated In order to verify the correctness of the model, the low and high temperatures were 20 C and calculation, the heat flow was 18.59 W. The heat flux density was 18.59/0.07/0.07 = 3794 W/m◦ 2. The 170 C. The vacuum thermal conductivity was assumed to be 0.023 Wm 1 K 1. Through simulated simulating◦ equivalent total thermal resistance of the vacuum multiple− glass− was 0.04 (m2·K)/W. 2 calculation,Through theoretical the heat flowcalculation, was 18.59 the W. thermal The heat resi fluxstance density of the was common 18.59/0.07 glass,/0.07 organic= 3794 Wglass,/m . Theand simulating equivalent total thermal resistance of the vacuum multiple glass was 0.04 (m2 K)/W. Through vacuum layers were 0.008/0.75 = 0.011, 0.0044/0.2 = 0.022, 0.0002/0.023 = 0.0087 (m2··K)/W, and the theoretical calculation, the thermal resistance of the common glass, organic glass, and vacuum layers theoretical equivalent total thermal resistance of the vacuum multiple glass was 0.0417 (m2·K)/W. The were 0.008/0.75 = 0.011, 0.0044/0.2 = 0.022, 0.0002/0.023 = 0.0087 (m2 K)/W, and the theoretical equivalent error between the simulating and theoretical results was 4%. ·So, the finite element model was total thermal resistance of the vacuum multiple glass was 0.0417 (m2 K)/W. The error between the reasonable. · simulating and theoretical results was 4%. So, the finite element model was reasonable. 2.5. Boundary Conditions and Distribution of Neutron Energy Loads 2.5. Boundary Conditions and Distribution of Neutron Energy Loads The thermal boundary condition includes the convection heat transfer from the environment, The thermal boundary condition includes the convection heat transfer from the environment, radiation heat from higher temperature side to lower temperature side, neutron heat from the radiation heat from higher temperature side to lower temperature side, neutron heat from the absorption absorption caused by the neutron in the Gd-containing organic glass. In the simulation, the caused by the neutron in the Gd-containing organic glass. In the simulation, the convection heat convection heat transfer was assumed to be the environment temperature of 20 ℃ (lower temperature transfer was assumed to be the environment temperature of 20 C (lower temperature side) and 170 C side) and 170 ℃ (higher temperature side, which was assumed◦ to be the working temperature). For◦ (higher temperature side, which was assumed to be the working temperature). For the radiation heat, the radiation heat, the material emissivity of common glass was 0.84. The emissivity of Gd-containing the material emissivity of common glass was 0.84. The emissivity of Gd-containing organic glass organic glass was not considered in the finite element model. The neutron heat was applied with the was not considered in the finite element model. The neutron heat was applied with the exponential exponential function in the Gd-containing organic glass: 𝑃 = 1E5 ×exp (−x/0.01) Wm-3, and the function in the Gd-containing organic glass: P = 1E5 exp( x/0.01) Wm 3, and the distribution of distribution of the neutron heat energy is shown in Figure× 4. In− the simulation,− 105 Wm-3 was assumed the neutron heat energy is shown in Figure4. In the simulation, 10 5 Wm 3 was assumed to be the to be the maximum neutron heat per element on the top of the Gd-containing− organic glass. x (m) maximum neutron heat per element on the top of the Gd-containing organic glass. x (m) was the depth was the depth from the high-temperature side of the Gd-containing organic glass towards the low- from the high-temperature side of the Gd-containing organic glass towards the low-temperature side, temperature side, as shown in Figure 4. The neutron attenuation index for the Gd-containing organic as shown in Figure4. The neutron attenuation index for the Gd-containing organic glass was assumed glass was assumed to be 0.01. It was assumed for the condition named Case 1. to be 0.01. It was assumed for the condition named Case 1.

Figure 4. The distribution of the neutron heat energy. 3. Results and Discussion 3. Results and Discussion 3.1. Eﬀect of the Work Temperature on the Temperature Distribution of Vacuum Multiple Glass 3.1. Effect of the Work Temperature on the Temperature Distribution of Vacuum Multiple Glass The temperature distribution of the thermal models is shown in Figure5a. The temperature gradientThe directiontemperature of thedistribution model was of fromthe thermal the high-temperature models is shown side in to Figure the low-temperature 5a. The temperature side. Becausegradient the direction vacuum of layer the exists,model thewas temperature from the high-temperature gradient was obvious side atto thethe support low-temperature and the sealing side. Because the vacuum layer exists, the temperature gradient was obvious at the support and the sealing agent around the model. The maximum temperature of the organic glass was predicted to be 98 ◦C, agent around the model. The maximum temperature of the organic glass was predicted to be 98 ℃, which occurred at the corners of the model and was lower than its allowable temperature of 120 ◦C. which occurred at the corners of the model and was lower than its allowable temperature of 120 ℃. Sustainability 2020, 12, 3083 6 of 14

ResultsSustainability show 2020 that, 12 the, x FOR vacuum PEER REVIEW multiple glass has a good heat insulation and can work in6 of the 14 high temperature of 170 ◦C, which is higher than the allowable temperature of 120 ◦C for the organic glass, Results show that the vacuum multiple glass has a good heat insulation and can work in the high and the neutron heat: P = 1E5 exp( x/0.01) Wm 3. Because the multiple glass did not reach the temperature of 170 ℃, which is× higher− than the allowable− temperature of 120 ℃ for the organic glass, allowable temperature of 120 C, the work temperature or the neutron irradiation can be increased. and the neutron heat: 𝑃 =◦ 1E5 ×exp (−x/0.01) Wm-3. Because the multiple glass did not reach the allowableThe working temperature temperatures of 120 ℃, ofthe 190work◦ C,temperature 210 ◦C, or 230 the◦ neutronC were irradiation applied can on be the increased. inner surface. The conditionsThe working were assumedtemperatures for of Case 190 2,℃ Case, 210 3,℃, and 230 Case℃ were 4. Figure applied5 showson the diinnerfferent surface. temperature The distributions.conditions Whenwere assumed the work for temperature Case 2, Case increased, 3, and Case the temperature4. Figure 5 shows of the different multiple temperature glass increased. The sealingdistributions. agent When is the the key work heat temperature transfer element, increased, because the temperature the influence of the mu ofltiple area toglass the increased. sealing agent is largerThe sealing than the agent support is the area.key heat Also, transfer the maximumelement, because temperature the influence of the of organicarea to the glass sealing occurred agent close to theis sealinglarger than agent. the support The maximum area. Also, temperature the maximum of temperature the organic of glass the organic versus glass the occurred work temperature close is shownto the sealing in Figure agent.6. This The maximum figure shows temperature that by of increasing the organic theglass work versus temperature, the work temperature the maximum is shown in Figure 6. This figure shows that by increasing the work temperature, the maximum temperature of the organic glass increased linearly. When the work temperature reached 210 C, temperature of the organic glass increased linearly. When the work temperature reached 210 ℃, the ◦ the maximummaximum temperature temperature of of the the organic organic glass glass was was predicted predicted to be to 118 be 118℃, which◦C, which is lower is lower than the than the allowableallowable temperature. temperature. If If the the multiple multiple glassglass had worked worked in in the the temperature temperature of 230 of 230℃, the◦C, maximum the maximum temperaturetemperature of the of the organic organic glass glass was was predicted predicted to be be 128 128 ℃◦C.. To increaseTo increase the workthe work temperature, temperature, the the influence influenc ofe theof the sealing sealing agent agent width, width, view view size, size, and and vacuum thermalvacuum conductivity thermal conductivity on the temperature on the temperature of the organic of the glass organic was studied,glass was because studied, these because factors these directly affectfactors the heat directly transmission affect the he [23at, 24transmission]. [23,24].

(a) Case 1 (170 ℃)

(b) Case 2 (190 ℃)

Figure 5. Cont. Sustainability 2020, 12, x FOR PEER REVIEW 7 of 14

Sustainability 2020, 12, 3083 7 of 14 Sustainability 2020, 12, x FOR PEER REVIEW 7 of 14

(d) Case 4 (230 ℃) (d) Case 4 (230 ℃) FigureFigure 5. The 5. The temperature temperature distributions distributions of of multiple multiple and and organic organic glass glass for for thethe thermalthermal modelmodel at ((a)a) Case FigureCase 5. 1The (170 temperature ℃), (b) Case 2 distributions (190 ℃), (c) Case of 3multiple (210 ℃), and (d)organic Case 4 glass (230 ℃ for). the thermal model at (a) 1 (170 ◦C), (b) Case 2 (190 ◦C), (c) Case 3 (210 ◦C), and (d) Case 4 (230 ◦C). Case 1 (170 ℃), (b) Case 2 (190 ℃), (c) Case 3 (210 ℃), and (d) Case 4 (230 ℃). 130 130 120 120 110

110 100 Max Temperature(℃) Max 10090

Max Temperature(℃) Max 170 190 210 230 90 Work Temperature (℃) 170 190 210 230

Figure 6. The maximum temperatureWork of the Temperature organic glass versus (℃) the work temperature.

Figure 6. The maximum temperature of the organic glass versus the work temperature. Figure 6. The maximum temperature of the organic glass versus the work temperature. Sustainability 2020, 12, x FOR PEER REVIEW 8 of 14

Sustainability3.2. Effect of2020 the, 12Sealing, 3083 Agent Width on the Temperature Distribution of Vacuum Multiple Glass 8 of 14 SealingSustainability agent 2020, 12widths, x FOR PEERof 1.5 REVIEW mm, 1.0 mm, and 0.5 mm were used in the finite element model8 of 14 and 3.2.a work Effect temperature of the Sealing of Agent 230 ℃ Width was onapplied the Temperature on the inner Distribution surfaces. of The Vacuum conditions Multiple were Glass assumed for 3.2. Effect of the Sealing Agent Width on the Temperature Distribution of Vacuum Multiple Glass Case 5, Case 6, and Case 7. Figure 7 shows dynamic temperature distributions. When the sealing Sealing agent widths of 1.5 mm, 1.0 mm, and 0.5 mm were used in the finite element model and a agent widthSealing decreased, agent widths the temperature of 1.5 mm, 1.0 on mm, the and left 0.5 of mm the were sealing used agent in the decreased.finite element The model temperature and work temperature of 230 ◦C was℃ applied on the inner surfaces. The conditions were assumed for Case gradienta work of thetemperature edge was of controlled 230 was byapplied the sealing on the inneragent. surfaces. The maximum The conditions temperature were assumed of the fororganic 5, CaseCase 6, 5, and Case Case 6, and 7. FigureCase 7.7 Figureshows 7 dynamic shows dy temperaturenamic temperature distributions. distributions. When When the the sealing sealing agent glass versus the sealing agent width is shown in Figure 8, which shows that with a decrease of the widthagent decreased, width decreased, the temperature the temperature on the left on ofthe the left sealing of the sealing agent decreased.agent decreased. The temperature The temperature gradient sealing agent width, the maximum temperature of the organic glass decreased linearly. When the of thegradient edge was of the controlled edge was controlled by the sealing by the agent.sealing Theagent. maximum The maximum temperature temperature of of the the organic organic glass sealing agent width reached 1.0 mm, the maximum temperature of the organic glass reached 117 ℃, versusglass the versus sealing the agent sealing width agent is width shown is inshown Figure in 8Figure, which 8, which shows shows that with that with a decrease a decrease of the of the sealing which is lower than the allowable temperature of 120 ℃. Decreasing the sealing agent width can agentsealing width, agent the maximumwidth, the maximum temperature temperature of the organic of the organic glassdecreased glass decreased linearly. linearly. When When the the sealing increasesealing the agentwork width temperature reached of1.0 the mm, multiple the maximum glass. temperatureHowever, it of is the different organic to glass seal reached the common 117 ℃, glass agent width reached 1.0 mm, the maximum temperature of the organic glass reached 117 C, which in thewhich manufacturing is lower than process. the allowable So, a reasonable temperature sealing of 120 agent ℃. Decreasing width should the sealing be obtained agent width in the◦ can sealing is lower than the allowable temperature of 120 C. Decreasing the sealing agent width can increase process.increase the work temperature of the multiple glass.◦ However, it is different to seal the common glass the workin the temperature manufacturing of process. the multiple So, a reasonable glass. However, sealing agent it is diwidthfferent should to seal be obtained the common in the glasssealing in the manufacturingprocess. process. So, a reasonable sealing agent width should be obtained in the sealing process.

FigureFigure 7. TheThe 7. The temperature temperature distributions distributions ofof multiple glass glass for for different different diﬀerent sealing sealing agent agent widths. widths.

130130

120120

110110

100 100

Max Temperature(℃) Max 90 Max Temperature(℃) Max 90 21.510.5 21.510.5Sealing agent width (mm) Sealing agent width (mm) .

Figure 8. The maximum temperature of the organic glass versus the sealing agent width. Figure 8. The maximum temperature of the organic glass versus the sealing agent width. Figure 8. The maximum temperature of the organic glass versus the sealing agent width. 3.3. Effect of the View Size on the Temperature Distribution of Vacuum Multiple Glass 3.3. Effect of the View Size on the Temperature Distribution of Vacuum Multiple Glass 3.3. Effect Theof the view View sizes Size of on 50 the × 50 Tempera mm, 35ture × 35 Distribution mm, and 20 of× 20Vacuum mm were Multiple utilised Glass in the finite element Themodel, view with sizes a work of 50temperature50 mm, of 35 230 ℃35 applied mm, and on the 20 inner20 surface. mm were The utilised conditions in were the finite assumed element The view sizes of 50 ×× 50 mm, 35 × 35 mm, and 20 × 20 mm were utilised in the finite element model,for with Case a 8, work Case temperature 9, and Case 10, of 230respectively.◦C applied Figure on the 9 shows inner surface.different Thetemperature conditions distributions. were assumed model, with a work temperature of 230 ℃ applied on the inner surface. The conditions were assumed for CaseWhen 8, Casethe view 9, and size Case decreased, 10, respectively. the temperature Figure9 showsof the dimultiplefferent glass temperature decreased. distributions. Temperature When thefor viewCasegradient size8, Case decreased,around 9, andthe view theCase temperature area 10, and respectively. support of the was multiple Fi clgureearer glass9than shows other decreased. different areas. The Temperature temperature point of the gradient maximum distributions. around theWhen viewtemperature the area view and forsize support the decreased, organic was glass clearer the changed, temperature than otherfrom the areas. ofcorner the The ofmultiple pointthe multiple of glass the glass maximum decreased. to the support temperature Temperature area, for thegradient organicwhen around decreasing glass the changed, theview view area from sizes. and the The support corner maximum of was the temper cl multipleearerature than glass of other the to organic the areas. support glass The versuspoint area, whentheof the view decreasingmaximum size thetemperature viewis shown sizes. for in Figure Thethe organic maximum 10, which glass shows temperature changed, that with from of th theethe decrease organiccorner in of glassview the multiplesize, versus the themaximum glass view to the sizetemperature support is shown area, in Figurewhen decreasing 10, which showsthe view that sizes. with The the maximum decrease in temper view size,ature the of maximumthe organic temperature glass versus of the the view organic size glassis shown decreased in Figure dramatically. 10, which shows When that the viewwith sizethe decrease reached 20in view20 mm,size, thethe maximummaximum temperaturetemperature × Sustainability 2020, 12, x FOR PEER REVIEW 9 of 14

of the organic glass decreased dramatically. When the view size reached 20 × 20 mm, the maximum ℃ Sustainabilitytemperature2020 of, 12the, 3083 organic glass reached 44 , which is much lower than the allowable temperature9 of 14 of 120Sustainability ℃. Decreasing 2020, 12, xthe FOR view PEER REVIEWsize can further increase the work temperature of the multiple9 of glass.14 of the organic glass decreased dramatically. When the view size reached 20 × 20 mm, the maximum of thetemperature organic glass of the reached organic 44 glass◦C, reached which 44 is ℃ much, which lower is much than lower the allowablethan the allowable temperature temperature of 120 ◦C. Decreasingof 120 ℃ the. Decreasing view size the can view further size increasecan further the increase work temperaturethe work temperature of the multiple of the multiple glass. glass.

(a) Case(a) Case8 (50 8 × (50 50 ×mm) 50 mm) (b()b Case) Case 9 9 (35 (35 ×× 35 mm) (c)( cCase) Case 10 (2010 (20× 20 × mm) 20 mm)

FigureFigure 9. The 9. The maximum maximum temperatures temperatures ofof the or organicganicganic glass glass versus versus the the view view size. size.

140140

120120 100 100 80 80 60

60Temperature(℃) Max

Max Temperature(℃) Max 40 40 20×20 35×35 50×50 70×70 20×20 35×35View Size (mm)50×50 70×70 View Size (mm) Figure 10. The max temperature of the organic glass versus the view size.

The viewFigure size of 10. 20 The × 20 max mm temperature was used in of the the fin organicite element glass model versus at the work view temperatures size. of 600 ℃, 1200 ℃, 1580 ℃, applied on the inner surfaces, and such conditions were assumed for Case 11, The viewview sizesize ofof 2020 ×20 20 mmmm was was used used in in the the ﬁnite finite element element model model at workat work temperatures temperatures of 600of 600◦C, Case 12, and Case 13,× respectively. The maximum temperature of the organic glass versus the work 1200℃, 1200◦C, 1580℃, 1580◦C, applied℃, applied on the on inner the inner surfaces, surfaces, and such and conditions such conditions were assumed were assumed for Case for 11, Case Case 11, 12, andCase Case 12, and 13, respectively.Case 13, respectively. The maximum The maximum temperature temperature of the organic of the glass organic versus glass the work versus temperature the work (20 20 mm) is shown in Figure 11, which shows that with an increase in the work temperature, the × max temperature of the organic glass increased linearly. When the work temperature reached 1200 ◦C, Sustainability 2020, 12, x FOR PEER REVIEW 10 of 14

Sustainabilitytemperature2020 (20, 12 ,× 3083 20 mm) is shown in Figure 11, which shows that with an increase in the 10work of 14 temperature, the max temperature of the organic glass increased linearly. When the work temperatureSustainability reached 2020, 12, x1200 FOR PEER ℃, theREVIEW maximum temperature of the organic glass reached 11810 ℃ of, 14which isthe lower maximum than the temperature allowable oftemperature. the organic This glass is reachedvery important 118 ◦C, whichfor the isorganic lower thanglass theworking allowable in a temperature (20 × 20 mm) is shown in Figure 11, which shows that with an increase in the work high-temperaturetemperature. This environment. is very important for the organic glass working in a high-temperature environment. temperature, the max temperature of the organic glass increased linearly. When the work temperature reached 1200 ℃, the maximum temperature of the organic glass reached 118 ℃, which is lower than the allowable temperature.140 This is very important for the organic glass working in a high-temperature environment. 120

100140

80120

60100

Max Temperature(℃) Max 80 40 60 230 600 1200 1580

Max Temperature(℃) Max 40 Work Temperature (℃) 230 600 1200 1580 Work Temperature (℃) Figure 11. TheThe max max temperature temperature of of the the organic organic glass glass versus versus the the work work temperature temperature of the of theview view size size (20

(20× 20 mm).20 mm). ×Figure 11. The max temperature of the organic glass versus the work temperature of the view size (20 3.4. Effect Effect ×of 20 the mm). Vacuum Thermal Conductivity on thethe Temperature DistributionDistribution ofof VacuumVacuum MultipleMultiple GlassGlass 1 1 1 1 1 1 The3.4. Effect vacuum of the thermal thermalVacuum Thermalconductivity conductivity Conductivity of of 0.003 0.003 on th Wm Wme Temperature-1−K-1K, 0.002− , 0.002Distribution Wm Wm-1K-1−, of0.001 KVacuum− ,Wm 0.001 Multiple-1K Wm-1 and −Glass theK− viewand sizethe viewof 20 × size 20 mm of 20 were20 used mm in were the 3D used finit ine theelement 3D finite model element at the work model temperature at the work of temperature 600 ℃ applied of The vacuum× thermal conductivity of 0.003 Wm-1K-1, 0.002 Wm-1K-1, 0.001 Wm-1K-1 and the view 600 C applied on the inner surfaces, and the conditions were assumed for Case 14, Case 15, and on the◦size inner of 20 surfaces, × 20 mm were and usedthe cond in theitions 3D finit weree element assumed model for at Casethe work 14, temperatureCase 15, and of Case600 ℃ 16. applied Figure 12 showsCaseon 16. different the Figure inner 12 surfaces,temperature shows and diff theerent distributions. cond temperatureitions were When distributions.assumed the vacuum for Case When thermal14, Case the vacuum 15,conductivity and Case thermal 16. decreased, Figure conductivity 12 the temperaturedecreased,shows different the of the temperature multiple temperature glass of thedistributions. decreased. multiple WhenThe glass infl decreased.theuence vacuum of area thermal The on influence the conductivity support of area becamedecreased, on the larger, supportthe and thebecame maximumtemperature larger, temperature and of the the multiple maximum of theglass organic temperature decreased. glass The ofincreased, infl theuence organic whichof area glass onoccurred increased,the support on the whichbecame support occurred larger, area, and onas the vacuumsupportthe maximumarea,thermal as the conductivity temperature vacuum thermal of increased. the organic conductivity The glass maxi increased, increased.mum temperaturewhich The occurred maximum of on the the temperature or supportganic glass area, ofthe asversus the organic the vacuumglassvacuum versus thermal thethermal vacuum conductivity conductivity thermal is shownincreased. conductivity in FigureThe ismaxi shown 13,mum which in temperature Figure shows 13 that, whichof withthe or shows anganic increase thatglass with versusof the an vacuumthe increase thermalof thevacuum vacuum conductivity, thermal thermal conductivity the conductivity, maximum is shown the temperature maximumin Figure 13, of temperature which the organicshows ofthat glass the with organic increased an increase glass linearly. of increased the vacuum When linearly. the thermal conductivity, the maximum temperature of the organic1 glass1 increased linearly. When the Whenvacuum the thermal vacuum conductivity thermal conductivity reached 0.002 reached Wm-1 0.002K-1, the Wm maximum− K− , the temperature maximum of temperature the organic of glass the vacuum thermal conductivity reached 0.002 Wm-1K-1, the maximum temperature of the organic glass reachedorganic glass119 ℃ reached, which 119 is lower◦C, which than isthe lower allowable than the temperature allowable temperatureof 120 ℃. Decreasing of 120 ◦C. the Decreasing vacuum reached 119 ℃, which is lower than the allowable temperature of 120 ℃. Decreasing the vacuum the vacuum thermal conductivity, that is, increasing the vacuum pressure, can increase the work thermalthermal conductivity, conductivity, that that is, is,increasing increasing the the vacuumvacuum pressure,pressure, can can increase increase the the work work temperature temperature of of thetemperature multiplethe multiple glass. of the glass. multiple glass.

Figure 12. Cont. Sustainability 2020, 12, x FOR PEER REVIEW 11 of 14 Sustainability 2020, 12, 3083 11 of 14 Sustainability 2020, 12, x FOR PEER REVIEW 11 of 14

-1 -1 -1 -1 -1 -1 (a) Case(a )14 Case (0.003 14 (0.003 Wm WmK )-1 K-1) (b)( bCase) Case 15 15 (0.002 (0.002 Wm Wm-1KK-1) (c(c) )Case Case 16 16 (0.001 (0.001 Wm Wm-1K-1) K )

FigureFigure 12. The 12. temperatureThe temperature distribution distribution of ofmultiple multiple and and organicorganic glasses glasses in in which which vacuum vacuum thermal thermal conductivities of 0.003 Wm-11 -1-1K1-1, 0.002 Wm-1 -1-1K1 -1, 0.0011 Wm-1-1K-1 and1 a 1view size of 20 × 20 mm were conductivities of 0.0030.003 WmWm− KK,− 0.002, 0.002 Wm WmK− , K0.001− , 0.001 Wm WmK −andK −a viewanda size view of size 20 × of 20 20 mm20 were mm used at the work temperature of 600℃ ℃ were applied on the inner surfaces and the conditions× were wereused at used the atwork the worktemperature temperature of 600 of 600 were◦C wereapplied applied on the on inner the innersurfaces surfaces and the and conditions the conditions were assumed for Case 14, Case 15, and Case 16. wereassumed assumed for Case for Case14, Case 14, Case15, and 15, Case and Case16. 16.

140 140 120 120 100 100 80 80 60

60Temperature(℃) Max 40 Max Temperature(℃) Max 0 0.001 0.002 0.003 40 Vacuum Thermal Conductivity (Wm-1K-1)) 0 0.001 0.002 0.003 Vacuum Thermal Conductivity (Wm-1K-1)) Figure 13. The maximum temperature of the organic glass versus the vacuum thermal conductivity.

Figure3.5. Overall 13. TheThe Discussions maximum maximum temperature temperature of of the the organic organic glas glasss versus the vacuum thermal conductivity.

3.5. OverallThe Discussions results are further comparatively discussed in this section. Based on the validated 3D finite 3.5. Overallelement Discussions model simulations the vacuum layer between common glasses can make the work Thetemperature results are of furtherneutron comparatively shielding glass discussed increase. The in this multiple section. glass Based had on a goodthe validated heat-shielding 3D finite finite elementperformance modelmodel simulations simulations and it is expected the the vacuum tovacuum work layer in alayer betweenhigh-t emperaturebetween common common environment. glasses canglasses makeThe resultscan the work makeimplicate temperature the that work the vacuum layer between the common glasses and the sealing agent width decay with respect to the oftemperature neutron shielding of neutron glass shielding increase. Theglass multiple increase. glass The had multiple a good heat-shielding glass had a performancegood heat-shielding and it is view size and vacuum thermal conductivity show an increase in the working temperature of the expectedperformance to work and it in is aexpected high-temperature to work in environment.a high-temperature The results environment. implicate Th thate results the vacuumimplicate layer that betweenneutron the commonshielding glassesglass. The and results the from sealing Figure agent 5 are width further decay discussed with here, respect where to theit is viewimperative size and the vacuumto mention layer that between the sealing the common agent contributed glasses andto the the heat sealing transfer agent more width than decaythe support with arearespect of the to the vacuum thermal conductivity show an increase in the working temperature of the neutron shielding view sizesample. and Thus, vacuum the maximum thermal temperatureconductivity resulted show arou an ndincrease the sealing in the area, working which further temperature resulted ofin the glass.neutronFigure The shielding results 6, and fromitglass. shows Figure The a linear results5 are relationship further from Figure discussed between 5 are here,an further increase where discussed of it the is work imperative here, temperature, where to mention it i.e., is imperative 210 that ℃, the sealingto mentionand agent the that maximum contributed the sealing temperature to agent the heat contributed of the transfer organic to moreglass, the predicted thanheat transfer the to support be more118 ℃ area ,than which of the the is foundsupport sample. to be area Thus,lower of the maximumsample.than Thus, the temperature allowablethe maximum temperature. resulted temperature around The effect theresulted sealingof sealing arou area, agentnd whichthe width sealing further is shown area, resulted whichin Figure further in 7, Figure and resulted it6 ,was and itin showsFigurefound a6, linear and that it relationship shows the seal-width a linear between was relationship proportional an increase between to of the the tean workmperature increase temperature, onof thethe workleft i.e., side 210 temperature, of◦ C,the and sealing the i.e., maximumagent, 210 ℃, temperatureand theand maximum it could of the be temperature organic controlled glass, with of predicted the the orsealingganic to agent. beglass, 118 This predicted◦C, phenomenon which to is be found 118 was to℃ further, be which lower examined isthan found the in to Figure allowable be lower 8. The view size relationship with temperatures of the multiple glass is shown in Figure 9 and it was temperature.than the allowable The eff temperature.ect of sealing agentThe effect width of is sealing shown inagent Figure width7, and is itshown was found in Figure that the 7, seal-widthand it was found that a decrease in view size decreased the temperature of the multiple glass panes. The wasfound proportional that the seal-width to the temperature was proportional on the leftto the side temperature of the sealing on agent,the left and side it of could the sealing be controlled agent, withand it the could sealing be controlled agent. This with phenomenon the sealing was agent. further This examined phenomenon in Figure was8 .further The view examined size relationship in Figure with8. The temperatures view size relationship of the multiple with glasstemperatures is shown of in the Figure multiple9 and glass it was is foundshown that in Figure a decrease 9 and in it view was sizefound decreased that a decrease the temperature in view of size the multipledecreased glass the panes.temperature The maximum of the multiple temperature glass of panes. the organic The Sustainability 2020, 12, 3083 12 of 14 glass versus the view size is shown in Figure 10, and it shows that with the decrease in view size the maximum temperature of the organic glass decreased dramatically. Based on the above work, the result comparison for 16 cases is shown in Table2. In the table, comparative results are presented that show the effect of different factors on the maximum temperature of the organic glass. As work temperature, view size, and vacuum thermal conductivity increase, the maximum temperature of the organic glass increases. If the work temperature is very high, a smaller view size, sealing agent width, and vacuum thermal conductivity are better. The order of affecting the temperature of the organic glass is as follows: view size > vacuum thermal conductivity > sealing agent width. Based on the work temperature, different values of the view size, vacuum thermal conductivity, and sealing agent width could be selected. The sealing agent width and vacuum thermal conductivity are limited by the manufacturing conditions. The view size is the best method to improve the work temperature.

Table 2. Results comparison for 16 cases.

Sealing Vacuum Thermal Work Maximum View Size Case Agent Width 2 Conductivity Temperature Temperature of the (mm ) 1 1 (mm) (Wm− K− ) (◦C) Organic Glass (◦C) 1 2.0 70 70 0 170 98 × 2 2.0 70 70 0 190 108 × 3 2.0 70 70 0 210 118 × 4 2.0 70 70 0 230 128 × 5 1.5 70 70 0 230 123 × 6 1.0 70 70 0 230 117 × 7 0.5 70 70 0 230 107 × 8 2.0 50 50 0 230 71 × 9 2.0 35 35 0 230 50 × 10 2.0 20 20 0 230 44 × 11 2.0 20 20 0 600 77 × 12 2.0 20 20 0 1200 118 × 13 2.0 20 20 0 1580 138 × 14 2.0 20 20 0.003 600 132 × 15 2.0 20 20 0.002 600 119 × 16 2.0 20 20 0.001 600 104 ×

4. Conclusions The temperature distribution characteristics of the vacuum multiple glass was obtained and discussed, and the feasibility of the vacuum multiple glass was veriﬁed by numerical calculation. A Gadolinium (Gd)-containing laminate vacuum multiple glass was proposed by using the vacuum insulation method in this paper. A mathematical model was used to analyse the heat transfer process. A 3D ﬁnite element model validated by the theoretical calculation was developed to analyse the heat transfer path and numerical simulation of the multiple glass to obtain the temperature distribution and the maximum temperatures of the organic glass in relation to dynamic working temperatures, the sealing agent width, view size, and vacuum thermal conductivity. Following is a summary of conclusions:

1. The vacuum layer between common glasses can make the work temperature of neutron shielding glass increase. The multiple glass has good heat-shielding performance and it is expected to work in a high-temperature environment; 2. The vacuum layer between the common glasses and the sealing agent width decay with respect to the view size and vacuum thermal conductivity show an increase in the working temperature of the neutron shielding glass; 3. The order of aﬀecting the temperature of the organic glass is as follows: view size > vacuum thermal conductivity > sealing agent width. The sealing agent width and vacuum thermal conductivity are limited by the manufacturing conditions. The view size is the best method to improve the work temperature. Sustainability 2020, 12, 3083 13 of 14

Author Contributions: Conceptualization, S.Z., H.M., M.K. and H.M.; methodology, validation, S.Z., H.M., Y.Z. and S.L.; formal analysis, S.Z., M.K. and S.M.; investigation, S.Z., M.K. and Y.Z.; writing—original draft preparation, S.Z., M.K. and S.M.; writing—review and editing, S.M., S.Z. and M.K. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge the funding support of the National Natural Science Foundation of China (51672241), Project funded by the 14th batch of "Six Talents Peak" High-level Talents (XCL-092), the Province Postdoctoral Foundation of Jiangsu (1501164B), Yangzhou science and technology project (YZ2017275), the Technical Innovation Nurturing Foundation of Yangzhou University (2017CXJ024) and Project funded by China Postdoctoral Science Foundation (2016M600447). Acknowledgments: This work supported by the research collaboration between London South Bank University, UK, and Yangzhou University. Authors would like to thank the reviewers and editors for their suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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