Tubular solar collectors

Abstract

A metal composition suitable for originating a joint by means of welding with a borosilicate glass for a solar collector. The composition, expressed in weight percentage, comprises the following alloy elements: TABLE-US-00001 Ni Co Mn Si C Ti Zr Ta Ti + Zr + Ta 28-31 15-18 0.5 0.3 0.05 0.30 0.30 0.30 0.40
and it is such that 45.5(Ni+Co)46.5, and that (Ti+Ta+Zr)4C, the remaining part being made up of iron, apart from the inevitable impurities. Additionally, a metal ring made of the metal composition described above and suitable for originating a metal-glass joint by means of welding; the metal-glass joint thus obtained; and the tubular solar collector thus obtained.

Claims

1. A metal-glass joint obtained by means of welding in order to manufacture a solar collector, wherein the glass is a borosilicate, and wherein the composition of metal, expressed in weight percentage, comprises following alloy elements: TABLE-US-00015 Ni Co Mn Si C Ti Zr Ta Ti + Zr + Ta 28-31 15-18 0.5 0.3 0.05 0.30 0.30 0.30 0.15-0.40 and such that
45.5(Ni+Co)46.5 and
(Ti+Ta+Zr)4C, wherein each of Ti, Zr, and Ta that is present is individually present an amount between 0.15 and 0.30, with remaining part of the composition being made up of iron (Fe) apart from inevitable impurities.

2. The joint according to claim 1, wherein the composition of the glass comprises following components expressed in weight percentage: TABLE-US-00016 B.sub.2O.sub.3 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO Fe 11.8-13.7 5-7.5 5-8 0.1-3 0.1-1.5 <500 ppm remaining part of the glass composition being silicon (SiO.sub.2) apart from inevitable impurities.

3. The joint according to claim 1, wherein the composition of the glass comprises the following components expressed in weight percentage: TABLE-US-00017 B.sub.2O.sub.3 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O Fe TiO.sub.2 ZrO.sub.2 Ta.sub.2O.sub.5 CeO.sub.2 Ba 8-12 5-9 5-9 0-5 <400 ppm 1-5 0.5-1.5 0.5-1.5 0.5-1.5 0.5-2 remaining part of the glass composition being silicon (SiO.sub.2) apart from inevitable impurities.

4. The joint according to claim 3, wherein the composition of the glass suitable to obtain the metal-glass joint further comprises CaO in 0.3-2.0 weight percentage.

5. The joint according to claim 3, wherein the composition of the borosilicate glass comprises the following components expressed in weight percentage: TABLE-US-00018 B.sub.2O.sub.3 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO Fe TiO.sub.2 ZrO.sub.2 Ta.sub.2O.sub.5 CeO.sub.2 Ba 9-11 6-8 6-8 1-3 0.5-1.5 <300 ppm 2-3 0.7-1.1 0.8-1.2 0.7-1.1 0.5-1.0 the remaining part of the glass composition being silicon (SiO.sub.2) apart from the inevitable impurities.

6. The joint according to claim 4, wherein the composition of the borosilicate glass comprises the following components expressed in weight percentage: TABLE-US-00019 B.sub.2O.sub.3 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO Fe TiO.sub.2 ZrO.sub.2 Ta.sub.2O.sub.5 CeO.sub.2 Ba 9-11 6-8 6-8 1-3 0.5-1.5 <300 ppm 2-3 0.7-1.1 0.8-1.2 0.7-1.1 0.5-1.0 the remaining part of the glass composition being silicon (SiO.sub.2) apart from the inevitable impurities.

7. The joint according to claim 1, wherein the following alloy elements, expressed in weight percentage, further comprise: TABLE-US-00020 Ni Co Mn Si C Ti Zr Ta Ti + Zr + Ta 29-30 16-17 0.3 0.2 0.03 0.30 0.30 0.30 0.15-0.40 and such that:
45.5(Ni+Co)46.5 and
(Ti+Ta+Zr)6C, wherein each of Ti, Zr, and Ta that is present is individually present an amount between 0.15 and 0.30, with remaining part of the composition being made up of iron (Fe) apart from inevitable impurities.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

(1) Further characteristics and advantages of the invention will be clear from the description, provided hereinafter, of some embodiments, solely provided by way of non-limiting example with reference to the attached drawings, wherein:

(2) FIG. 1 represents a tubular solar collector in its entirety;

(3) FIG. 2 represents a detail of the joint between the glass tube and the metal ring of the tubular solar collector of FIG. 1;

(4) FIG. 3 represents, in cross-section, the detail indicated with III in FIG. 2 in which the angle is comprised between the direction of the metal-glass tension and the direction of the glass-air tension, tangent to the outer surface of the glass, with the vertex in the glass-metal-air three-phase point; represents the contact angle between the metal and the glass which is formed in the joining step.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(5) The tubular solar collector is indicated in its entirety with it 10. It comprises a first metal inner tube 12 and a second external coaxial tube 14 made of glass. The first tube 12 and the second tube 14 are connected to each other at the ends so as to form a gap 16 adapted to maintain a vacuum condition therewithin.

(6) As observable in FIG. 1, two bellows 18 are arranged at the ends of collector 10, adapted to compensate the differences between the dilation of the first inner tube 12 and that of the second outer tube 14. A tubular component 20 according to the invention, connected to each bellow 18, is then welded to the end of the glass outer tube 14, obtaining the joint 22 according to the invention.

(7) The tubular component 20 can assume the form of a ring, a collar, a sleeve or the like. In the description hereinafter, the term ring is used to refer to such tubular element.

(8) The metal ring 20 of the joint of the proposed collector 10 is obtained using an alloy having an average coefficient of thermal dilation in the range between 25 C. and 450 C. comprised between 4.510.sup.6 and 5.510.sup.6 K.sup.1.

(9) The authors of the present invention observed that in order to manufacture the abovementioned collectors 10 with excellent characteristics of vacuum sealing reliability in the gap 16, the alloy should contain titanium, in case combined with tantalum and zirconium, within the indicated limits. The presence of such elements in the indicated concentrations avoids that during the physical coupling between the metal rings 20 and the glass 14 at high temperature to obtain the joint 22, there occurs a localized emission of CO and/or CO.sub.2 in the interface zone between the surfaces of the metal and glass. As mentioned previously, the formation of gas bubbles, even extremely small, on the interface between metal and glass, represents a deleterious condition for the mechanical stability of the joint 22 and for the vacuum sealing stability. Thus, the use of the claimed metal alloy allows avoiding the step of decarburization in hydrogen of the metal alloy prior to joining with the borosilicate glasses, since the carbon present in matrix combines in form of titanium, tantalum and zirconium carbides, having a fine dispersion in matrix and a high stability at temperatures exceeding 1000 C.

(10) Thus, the free carbon in matrix is drastically reduced by titanium, tantalum and zirconium possibly present; during the step of joining to the glass, carbon is thus not released by the metal alloy for the formation of CO and/or CO.sub.2 in form of bubbles.

(11) The authors of the present invention observed that, advantageously, titanium and tantalum or zirconium possibly present, exceeding the stoichiometric ratio with carbon. i.e. not combined with the carbon of the matrix, selectively bond with oxygen in proximity of the surface layers of the metal, when the metal ring 20 is heated exceeding 750 C. during the joining procedure and during possible thermal treatments prior to the welding operation. The specific oxides identified according to the present invention, based on titanium and/or zirconium and/or tantalum, chemically interact with the glass to which the metal is coupled generating stable bond bridges between the metal and glass surfaces.

(12) The described chemical reactions occur during the step of forming the joint in the 500-1000 C. temperature range, for 1-60 minutes, in the local softening and fusion processes which occur at the interface between the metal and the glass.

(13) The specific layer of oxide which titanium and/or tantalum and/or zirconium contribute to form, is adherent and rooted both to the matrix of the metal alloy, and to the (glass with which it forms specific oxides and compounds such as boron, aluminium, calcium, barium titanate, thus conferring to the metal-glass bond high affinity and thermo-mechanical resistance.

(14) The presence of nickel and cobalt in the amounts indicated in the present invention allows obtaining an alloy with a coefficient of thermal expansion close to that of borosilicate glasses in the temperature range comprised between the ambient temperature and the glass annealing temperature.

(15) The presence of titanium, and in case of tantalum and zirconium, at the amounts indicated in the present invention, eliminates the need for the step of hydrogen decarburization of the metal alloy prior to joining with the glass, and improves the mechanical resistance properties of the metal and affinity to the glass.

(16) The presence of silicon and manganese, at the amounts indicated in the present invention, confers to the metal alloy a good formability, necessary considering the particular geometries of the products to be obtained.

(17) The entirety of the indicated properties thus makes the metal alloy subject of the present invention particularly suitable for making components having even complex geometry intended for metal-glass joints, easily weldable to borosilicate glasses in an automated manner and without requiring a hydrogen decarburization step prior to welding, or however avoiding complex joining processes.

(18) The concentration of the alloy elements and purpose thereof are illustrated hereinafter. Nickel is an austenitizing element and it guaranteesin high concentrations in ironthe reduction of the coefficient of thermal expansion with respect to pure iron.

(19) Cobalt is an austenitizing element and it extendsin high concentrations in the ironnickel alloysthe temperature range within which the coefficient of thermal expansion remains stable.

(20) In order to guarantee complete stability of the austenitic phase, and a coefficient of thermal expansion close to that of borosilicate glasses, stable up to a temperature of 450 C., the overall amount of nickel and cobalt should be comprised between 45.5% and 46.5%, nickel being present at the range between 28 and 31% in weight and cobalt at the range between 15 and 18% in weight.

(21) Manganese is an austenitizing element and it also inhibits the fragilization behaviour of sulphur, but at a higher amount it has a hardening effect for solid solution, hence content thereof should be limited to 0.5% in weight.

(22) Silicon is present as a deoxidizing agent but at high amounts it causes excessive hardening of the alloy and determines insufficient formability, hence the content thereof should be limited to 0.3% in weight.

(23) Carbon is an element inevitably present in solution in iron and in the FeNiCo alloys. For obtaining metal-glass joints, it should be present at amounts close to zero in order avoid the formation CO and/or CO.sub.2 bubbles; however, its amount cannot be excessively reduced without avoiding the use of particularly pure raw materials and/or expensive refining processes. Thus, the amount of carbon should not exceed 0.05% in weight. Titanium, tantalum and zirconium are particularly important elements of the present invention. Such elements have the main role of combining with carbon to prevent the latter from being free in matrix during the step of joining to the glass. Thus, titanium and/or tantalum and/or zirconium should be present at over-stoichiometric amounts with respect to carbon. The amount of such elements, exceeding carbon, also makes the alloy reactive in the joining phase due to the fact that titanium, zirconium and tantalum oxides chemically interact with the glass. The addition of high amounts of titanium, causes surface flaws of the rolled products, due to the appearanceon the surfaceof coarse precipitates and titanium-based aggregates. Furthermore, the addition of high amounts of tantalum and zirconium determines an undesired rising of the recrystalisation temperature and reinforcement of the crystalline structure, to the detriment of formability. Thus, the overall amount of the three elements is limited to below 4 times the amount of carbon with the sum thereof limited to 0.4% in weight; each element, both separately or combined with others, should be present at amounts comprised between 0.15% and 0.30% in weight.

(24) The authors of the present invention also advantageously discovered that borosilicate glass, containing TiO.sub.2, ZrO.sub.2 and Ta.sub.2O.sub.5 within the indicated limits, is particularly suitable for joining with the previously described metal, given that such oxides improve the chemical affinity and wettability of the glass to the metal, in presence of titanium and/or zirconium and/or tantalum oxides on the metal surface. Such condition occurs when the metal ring 20 is heated exceeding 750 C. during the joining procedure and during possible thermal treatments prior to the welding operation.

(25) The glass having the indicated composition, subject of the present invention, is suitable for welding with the previously described FeNiCo alloy given that it has an average coefficient of thermal dilation in the range between 25 C. and 450 C. equivalent to 5.210.sup.6 K.sup.1, and thus generates metal-glass joints with low inner stresses after construction and during operation.

(26) The glass having the indicated composition has an excellent resistance to corrosion of acids and alkaline solutions, a light transmittance in the solar spectrum exceeding 90%, and low costs of manufacture with respect to the family of pharmaceutical borosilicate glasses, given that it is not barium-free. Thus, the glass subject of the present invention is particularly suitable for making reliable and relatively inexpensive tubular collectors 10 for concentrating solar energy.

(27) The present invention has been described generally up to this point. With the help of the following examples, will be given a more detailed description of some specific embodiments thereof, with the aim of better outlining objects, characteristics and advantages.

Example 1

(28) According to the present invention, a metal was provided for a metal-glass joint for a tubular solar collector 10, having the following composition:

(29) TABLE-US-00007 Ni Co Mn Si C Ti Zr Ta Ti + Zr + Ta Ni + Co 28.8 16.8 0.38 0.22 0.048 0.26 <0.01 <0.01 0.26 45.6
the remaining part being made up of iron apart from the inevitable impurities. Such composition meets the conditions:
45.5(Ni+Co)46.5
(Ti+Ta+Zr)4C

(30) An assembly of joints between the proposed metal and a borosilicate glass was thus provided after subjecting the metal to an oxidizing process in air at 900 C. for 20 min without prior decarburization in wet hydrogen. The oxidised metal and glass were directly welded by placing them at contact for 15 minutes at the temperature of 800 C., and cooling the joint down to ambient temperature in controlled conditions with a gradient of 1 C./min.

(31) From the assembly of metal-glass joints thus provided, one was subjected to metallographic analysis through cross-sectional observation to verify possible presence of gas bubbles in the glass and measure the contact angle between metal and glass, and another was subjected to a vacuum tightness test to verify the air-tightness of the joint, through spectrometric measurement of the permeation of helium. In order to verify reliability thereof, another joint was subjected to a series of thermal cycles constituted by heating up to 350 C. and maintaining such temperature for 20 minutes, followed by cooling at ambient temperature for 5 minutes. After 10.sup.4 cycles, the joint was subjected to the vacuum sealing test.

(32) The metallographic analysis revealed a structure of the joint free of hubbies in the glass, with a contact angle between the glass and the metal equal to 55.

(33) The vacuum sealing test revealed a gas permeation equal to 6.7110.sup.10 mbar*l/s. After thermal cycling, the measured vacuum tightness amounted to 9.5210.sup.10 mbar*l/s.

(34) Such example proves the advantages obtained by means of the joint of a metal according to the present invention and a borosilicate glass. The joint revealed to be free of flaws caused by the formation of CO and/or CO.sub.2 in the glass without requiring the introduction in the process of a metal decarburization step, a satisfactory wettability was also observed of the borosilicate glass with respect to the surface of the metal; the joint thus obtained revealed a vacuum tightness reliable over time, since the values of permeability were lower than the threshold of 110.sup.9 mbar*l/s, deemed critical for the resistance of metal-glass joints. The metal-glass joint 22 thus obtained is therefore suitable for use in concentrating solar collectors 10.

Example 2

(35) According to the present invention, a metal was provided for a metal-glass joint for a tubular solar collector, having the following composition (introduce composition actually used in the example):

(36) TABLE-US-00008 Ni Co Mn Si C Ti Zr Ta Ti + Zr + Ta Ni + Co 30.6 15.3 0.43 0.27 0.036 <0.01 0.19 0.17 0.36 45.9
the remaining part being made up of iron apart from the inevitable impurities.

(37) Such composition meets the conditions:
45.5(Ni+Co)46.5
(Ti+Ta+Zr)4C.

(38) A glass was also provided for a metal-glass joint for a tubular solar collector, having the following composition:

(39) TABLE-US-00009 B.sub.2O.sub.3 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO Fe 12.3 6.8 6.8 2.5 0.5 200 ppm
the remaining part being SiO.sub.2 apart from the inevitable impurities.

(40) An assembly of joints was thus provided between the proposed metal and glass, after subjecting the metal to an oxidation process in air at 950 C. for 1.0 minutes, without prior decarburization in wet hydrogen. The oxidised metal and glass were directly welded by placing them at contact for 15 minutes at the temperature of 850 C., and cooling the joint down to ambient temperature in controlled conditions with a gradient of 1.5 C./min.

(41) From the assembly of metal-glass joints thus provided, one was subjected to metallographic analysis through cross-sectional observation to verify possible presence of gas bubbles in the glass and measure the contact angle between metal and glass, and another was subjected to a vacuum tightness test to verily the air-tightness of the joint, through spectrometric measurement of the permeation of helium. In order to verify reliability thereof, another joint was subjected to a series of thermal cycles constituted by heating up to 350 C. and maintaining such temperature for 20 minutes, followed by cooling at ambient temperature for 5 minutes. After 10.sup.4 cycles, the joint was subjected to the vacuum tightness test.

(42) The metallographic analysis revealed a structure of the joint free of bubbles in the glass, and with a contact angle between the glass and the metal equal to 48.

(43) The vacuum tightness test revealed a gas permeation equal to 5.9510.sup.10 mbar*l/s. After thermal cycling, the measured vacuum tightness amounted to 8.9610.sup.10 mbar*l/s.

(44) Such example proves the advantages obtained by means of a joint of a metal and a glass according to the present invention. Also in this case, the joint revealed to be free of flaws caused by the formation of CO and/or CO.sub.2 in the glass, without requiring a metal decarburization step; the borosilicate glass subject of the present invention revealed an optimal wettability with respect to the surface of the metal; the joint thus obtained revealed a vacuum tightness reliable over time, given that the values of permeability were lower than the threshold of 110.sup.9 mbar*l/s, deemed critical for the resistance of metal-glass joints, and better with respect to the values measured for the metal-glass joint of example 1. The metal-glass joint 22 thus obtained is therefore suitable for use in concentrating solar collectors 10.

Example 3

(45) According to the present invention, a metal was provided for a metal-glass joint for a tubular solar collector, having the following composition:

(46) TABLE-US-00010 Ni Co Mn Si C Ti Zr Ta Ti + Zr + Ta Ni + Co 30.2 15.8 0.40 0.25 0.033 0.16 0.15 <0.01 0.31 46.0
the remaining part being made up of iron apart from the inevitable impurities.

(47) Such composition meets the conditions:
45.5(Ni+Co)46.5
(Ti+Ta+Zr)4C

(48) A glass was also provided for a metal-glass joint for a tubular solar collector, essentially consisting of:

(49) TABLE-US-00011 B.sub.2O.sub.3 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O Fe TiO.sub.2 ZrO.sub.2 Ta.sub.2O.sub.5 CeO.sub.2 Ba 9.8 6.2 5.9 3.8 200 ppm 1.2 1.3 1.2 1.5 1.0
the remaining part being SiO.sub.2 apart from the inevitable impurities.

(50) An assembly of joints was thus provided between the proposed metal and glass, after subjecting the metal to an oxidation process in air at 850 C. for 41) minutes without prior decarburization in wet hydrogen. The oxidised metal and glass were directly welded by placing them at contact for 10 minutes at the temperature of 900 C., and cooling the joint down to ambient temperature in controlled conditions with a gradient of 2.5 C./min.

(51) From the assembly of metal-glass joints thus provided, one was subjected to metallographic analysis through cross-sectional observation to verify possible presence of gas bubbles in the glass and measure the contact angle between metal and glass, and another was subjected to a vacuum tightness test to verify the air-tightness of the joint, through spectrometric measurement of the permeation of helium. In order to verify reliability thereof, another joint was subjected to a series of thermal cycles constituted by heating up to 350 C. and maintaining such temperature for 20 minutes, followed by cooling at ambient temperature for 5 minutes. After 10.sup.4 cycles, the joint was subjected to the vacuum tightness test.

(52) The metallographic analysis revealed a structure of the joint free of bubbles in the glass, and with a contact angle between the glass and the metal equal to 35.

(53) The vacuum tightness test revealed a gas permeation equal to 4.4710.sup.10 mbar*l/s. After thermal cycling, the measured vacuum tightness amounted to 7.7910.sup.10 mbar*l/s.

(54) Such example proves the advantages obtained by means of the joint of a metal and a glass according to the present invention. Also in this case, the joint revealed to be free of flaws caused by the formation of CO and/or CO.sub.2 in the glass, without requiring a metal decarburization step; the borosilicate glass subject of the present invention revealed an optimal wettability with respect to the surface of the metal; the joint thus obtained revealed a vacuum tightness reliable over time, given that the values of permeability were lower than the threshold of 110.sup.9 mbar*l/s, deemed critical for the resistance of metal-glass joints, and better with respect to the values measured for the metal-glass joint of example 1. The metal-glass joint 22 thus obtained is therefore suitable for use in concentrating solar collectors 10.

Example 4

(55) According to the present invention, a metal was provided for a metal-glass joint for a tubular solar collector, having the following composition:

(56) TABLE-US-00012 Ni Co Mn Si C Ti Zr Ta Ti + Zr + Ta Ni + Co 29.1 16.8 0.28 0.11 0.021 0.22 <0.01 0.16 0.38 45.9
the remaining part being made up of iron apart from the inevitable impurities.

(57) Such composition meets the conditions:
45.5(Ni+Co)46.5
(Ti+Ta+Zr)4C

(58) A glass was also provided for a metal-glass joint for a tubular solar collector, essentially consisting of:

(59) TABLE-US-00013 B.sub.2O.sub.3 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO Fe TiO.sub.2 ZrO.sub.2 Ta.sub.2O.sub.5 CeO.sub.2 Ba 9.2 7.3 6.7 2.8 0.5 300 ppm 2.3 0.7 0.8 0.7 0.5
the remaining part being SiO.sub.2 apart from the inevitable impurities.

(60) An assembly of joints was then provided between the metal and the glass described previously, after subjecting the metal to an oxidation process in air at 800 C. for 60 minutes, without prior decarburization in wet hydrogen. The oxidised metal and glass were directly welded by placing them at contact for 5 minutes at the temperature of 900 C., and cooling the joint down to ambient temperature in controlled conditions with a gradient of 1.5 C./min.

(61) From the assembly of metal-glass joints thus provided, one was subjected to metallographic analysis through cross-sectional observation to verify possible presence of gas bubbles in the glass and measure the contact angle between metal and glass, and another was subjected to a vacuum tightness test to verify the air-tightness of the joint, through spectrometric measurement of the permeation of helium. In order to verify reliability thereof, another joint was subjected to a series of thermal cycles constituted by heating up to 350 C. and maintaining such temperature for 20 minutes, followed by cooling at ambient temperature for 5 minutes. After 10.sup.4 cycles, the joint was subjected to the vacuum tightness test.

(62) The metallographic analysis revealed a structure of the joint free of bubbles in the glass, with a contact angle between the glass and the metal equal to 26.

(63) The vacuum sealing test revealed a gas permeation equal to 2.6710.sup.10 mbar*l/s. After thermal cycling, the measured vacuum tightness amounted to 3.4210.sup.10 mbar*l/s.

(64) Such example further proves the advantages obtained by means of the joint of a metal and a glass according to the present invention. Also in this case the joint revealed to be free of flaws caused by the formation of CO and/or CO.sub.2 in the glass, given that the metal decarburization step was avoided and the glass revealed an excellent wettability with respect to the surface of the metal; the joint thus obtained revealed a vacuum tightness particularly marked and reliable over time since the values of permeability were lower than the threshold of 110.sup.9 mbar*l/s deemed critical for the resistance of metal-glass joints, and even lower than the values measured regarding the joints of the previous examples. The metal-glass joint 22 thus obtained is therefore particularly suitable for use in concentrating solar collectors 10.

Example 5

(65) Lastly, a metal was provided for a metal-glass joint for a tubular solar collector, having the following composition:

(66) TABLE-US-00014 Ni Co Mn Si C Ti Zr Ta Ti + Zr + Ta Ni + Co 28.6 17.5 0.30 0.19 0.045 <0.01 <0.01 <0.01 <0.01 46.10
the remaining part being made up of iron apart from the inevitable impurities. Such composition meets the condition:
45.5(Ni+Co)46.5
but does not meet the condition:
4C(Ti+TaZr)0.40

(67) An assembly of joints was then provided between the proposed metal and a borosilicate glass, after subjecting the metal to an oxidation process in air at 850 C. for 30 minutes, without prior decarburization in wet hydrogen. The oxidised metal and glass were directly welded by placing them at contact for 10 minutes at the temperature of 850 C., and cooling the joint down to ambient temperature in controlled conditions with a gradient of 1 C./min.

(68) From the assembly of metal-glass joints thus provided, one was subjected to metallographic analysis through cross-sectional observation to verify possible presence of gas bubbles in the glass and measure the contact angle between metal and glass, and another was subjected to a vacuum tightness test to verify the air-tightness of the joint, through spectrometric measurement of the permeation of helium. In order to verify reliability thereof, another joint was subjected to a series of thermal cycles constituted by heating up to 350 C. and maintaining such temperature for 20 minutes, followed by cooling at ambient temperature for 5 minutes. After 10.sup.4 cycles, the joint was subjected to the vacuum tightness test.

(69) The metallographic analysis revealed a structure of the joint affected by bubbles in the glass, with a contact angle between the glass and the metal equal to 70.

(70) The vacuum tightness test revealed a gas permeation equal to 8.4510.sup.7 mbar*l/s. After thermal cycling, the measured vacuum tightness amounted to 6.3410.sup.7 mbar*l/s.

(71) Such example proves the fact that a FeNiCo alloy beyond the limits of the present invention cannot be welded to a borosilicate glass generating reliable joints in absence of a specific composition of metal and glass and a metal decarburization step. The joint revealed flaws caused by the formation of CO and/or CO.sub.2 in the glass, given that carbon was free to spread during the step of implementing the joint from the metal matrix in the borosilicate glass, reacting with the oxides of the latter. In absence of titanium, and/or of tantalum and/or of zirconium, the wettability of the borosilicate glass on the surface of the metal revealed to be poor. The joint thus obtained revealed an unreliable vacuum tightness over time, since permeability, even though initially lower than the threshold of 110.sup.9 mbar*l/s, considerably increased due to the thermal cycles up to extremely high and unacceptable values. The metal-glass joint thus obtained is therefore not suitable for use in concentrating solar collectors.