WO2004050577A1

WO2004050577A1 - Glass composition

Info

Publication number: WO2004050577A1
Application number: PCT/GB2003/004908
Authority: WO
Inventors: Peter Trenton Bishop; Roland Katzbach; Detlef Rehorek; Andrew William John Smith
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2002-11-29
Filing date: 2003-11-13
Publication date: 2004-06-17
Also published as: AU2003283566A1; GB0227843D0

Abstract

A lead-free glass composition comprises 35-55% P O, 10-55 % SnO, 2-25% SrO and 0-10% Li O as calculated in mole percent on an oxide basis or 35-55% P O, 10-55% SnO, 10.1-25% SrO . The compositions are particularly suitable for use as sealing glasses, for example in the manufacture of cathode ray tubes.

Description

GLASS COMPOSITION

This invention relates to low softening point glass compositions, in particular to glass compositions which are lead free, and to the use of such compositions as sealing materials.

Low softening point glasses are particularly useful for sealing applications, for example for the joining of two glass parts, or glass parts to ceramic or metal parts. The glass must be able to wet the parts to be joined at a temperature which is low enough to prevent melting or damage to both the parts to be joined and to regions surrounding the join. This is particularly important in the manufacture of electronic components and in the sealing of cathode ray tubes (CRT). Additionally, glasses used to form seals must have co-efficients of thermal expansion which are comparable to those of the parts to be joined if reliable seals are to be formed. In absolute terms, glasses have low co-efficients of thermal expansion, compared to for example, metals. Glasses used in the manufacture of CRT have co-efficients of thermal expansion of the order of 9-10 x 10^"6 ppm/K, for example 9.85 x 10^"6 ppm/K, between 25 and 300°C. In the context of the present invention, a co-efficient of thermal expansion of 5 x 10^"6 ppm/K would be understood to be a low value, and one of 15 x 10^"6 ppm K would be understood to be a high value.

In practice, glasses in powder form (termed glass frits) are commonly mixed with an organic vehicle, such as amyl acetate to form a flowable or extrudable paste. This paste is applied to the components to be sealed, for example between the funnel rim and panel of a CRT. Typically, the CRT is then heated through a temperature cycle up to ca. 440°C during which the vehicle is volatilised and the glass melts to form the seal. The assembled components are then reheated under vacuum (300 - 400 °C) in an exhaust bake-out process. The sealing material must be able to both form a seal and also remain rigid during the bake-out process in order to maintain the integrity of the seal.

Traditionally, lead based glasses have been used. These provide the required physical properties in terms of softening temperature and thermal expansion. Recently however, health and environmental considerations have highlighted a need to lessen reliance on lead containing glasses. This has prompted research aimed at finding suitable lead-free alternatives. One promising type of lead-free glass is based on the tin-zinc- phosphate system. EP 0 630 867 describes a glass composition consisting essentially of 25-50 mol% P₂O₅, 30-70 mol% SnO and 0-15mol% ZnO, wherein the ratio of SnO:ZnO is greater than 5. The glass also must contain an effective amount (more than 1 mol%) of a stabilising oxide, which may be up to 25 mol% of alkali metal oxide, up to 20 mol% B₂O₃, or up to 5 mol% of Al₂O₃, SiO₂ or WO₃. Examples are shown which indicate that an advantageous reduction in softening temperature is found at high SnO:ZnO ratios, however, this is compromised by a high tendency towards crystallisation and high thermal expansion co-efficients. Glasses according to EP 0 630 867 with very low or no ZnO content are thus not suitable for many sealing applications.

EP 0582 113 describes a tin-zinc-phosphate glass composition consisting essentially of 25-50 mol% P₂O₅ with SnO and ZnO where the ratio of SnO:ZnO is between 1 and 5.

Another system is based on Group IIA oxide-phosphate glass compositions.

The Group IIA metals are Be, Mg, Ca, Sr and Ba. JP 2001-106549 describes a glass composition consisting of 45-60 mol% P₂O₅, 20-35 mol% SnO and up to 10 mol% of at least one of MgO, CaO, SrO and BaO. The glass also must contain at least 10.5 mol% of

Li₂O.

JP 11-130462 describes a glass composition consisting of 50-70 wt% P₂O₅, 20-40 wt% SrO and 1-15 wt% Al₂O₃. The composition does not contain any SnO.

Group IIA metal oxides are also used by Byun et al., Jn. Non-crystalline Solids (1995) which discloses glasses according to the formula (45-x)RO.xNa₂O.2.5Al₂O₃.52.5P O₅ where R represents a Group IIA metal oxide and 0 < x < 31.

JP 2001-010843 describes a glass composition comprising 20-45 mol % P₂O₅, 45-75 mol% SnO, 0.5-10 mol% WO₃, at least 0.1 mol% of both ZrO₂ and Nb₂O₅ and optionally up to 10 mol% of SrO, although none of the examples given include any SrO.

SU 876573 describes a glass composition consisting of 58-70 wt% P₂O₅, 10.8-20 wt% ZnO, 1-3 wt% Al₂O₃, 0.2-2 wt% SnO₂, 0.5-3 wt% Li₂O and 7-25 wt% SrO.

JP 2001-322832 A discloses tin-phosphate glasses, the majority of which contain 60mol% or more of SnO. Only two examples are given which contain SrO. One of these contains only lmol% SrO and the other, 2mol% SrO combined with 68mol% of SnO.

Similarly, JP 09-235136 A discloses glasses with high SnO content. One example contains 53mol% SnO but only 0.7mol% of SrO. Only one other example contains any SrO at all, at 0.4mol%.

JP 09-175833 A discloses tin-phosphate glasses, some of which contain SrO. The maximum amount of SrO used is 1.5mol%.

Glass compositions with high P₂O₅ contents are often hygroscopic making them unsuitable for many sealing applications. Seals produced from such glasses are liable to degradation over time when exposed to the atmosphere, leading to eventual failure of the seal.

Many of the known lead-free sealing glasses have high thermal expansion coefficients when compared to the glasses normally used to form the component parts of e.g. cathode ray tubes. Using such sealing glasses without modification leads to poor sealing characteristics. To overcome this problem, filler materials such as cordierite are often used in admixture with the powdered sealing glass/vehicle paste. These filler materials lower the effective thermal expansion co-efficient of the sealing paste, allowing closer matching with the thermal expansion co-efficient of the parts to be sealed. The filler materials are not part of the glass, instead they constitute separate phases introduced as mill additions. In some cases, quantities of filler material in the region of 30-40 wt% and more are required. It is an aim of the present invention to provide glass compositions which are substantially non-hygroscopic. It is a further aim of the present invention to provide glass compositions for use in sealing pastes, the thermal expansion co-efficients of which can be matched to those of the materials commonly used in the manufacture of devices such as cathode ray tubes, using minimal or preferably, no filler materials.

In accordance with the present invention, a lead-free glass composition comprises 35-55% P₂O₅, 10-55 % SnO, 2-25% SrO and 0-10% Li₂O as calculated in mole percent on an oxide basis.

In an alternative embodiment the composition comprises 35-55% P₂O₅, 10-55 % SnO, and 10.1-25% SrO as calculated in mole percent on an oxide basis. This embodiment may further comprise up to 15 mol% of Li₂O.

The total amount of P₂O₅, SnO and SrO in the compositions is preferably at least

60 mol%, more preferably, at least 70 mol%.

The compositions may further comprise a total of up to 15 mol% of R₂O where R is Na, K or any mixture thereof. Preferred are mixtures of Na O and K₂O.

Other optional additions to the compositions include up to 5mol% of Al₂O₃ and up to 10 mol%, preferably up to 5mol%, of one or more of BaO, MgO, B₂O₃, Nb₂O₃,

MoO₃, WO₃, ZrO₂ and TiO₂. Some of these additional components, for example, ZrO₂ and TiO₂ have been found to lower the thermal expansion co-efficient of the glass compositions.

The compositions may also optionally include up to 25mol% SiO₂.

The glass compositions according to the present invention typically have glass transition temperatures of between 250 and 400°C and preferably, between 290 and 330°C. Typical process temperatures used to form seals, for example in the manufacture of CRT, are in the range from 420 to 440°C. It is important in the manufacture of for example, CRT and other electronic components, that any glass compositions used are sufficiently electrically isolating. Preferably, the glass compositions according to the present invention are electrically isolating above ca. 10 kN/mm.

Preferably, the glass compositions are combined with a vehicle to form a sealing paste. Suitable vehicles are known in the art and include organic vehicles such as amyl acetate.

Although not required, the glass compositions of the present invention may be admixed with filler materials and/or other mill additions in order to improve or alter both the properties of sealing pastes and those of seals formed therewith. Commonly used fillers and mill additions will be known to those skilled in the art and include materials such as silica, cordierite and other aluminosilicates, mullite, zircon, zirconia, β-spodumene and eucryptite. Also suitable as fillers are low thermal expansion glasses and ceramics. If desired, pigments may be added to colour the glass compositions. Suitable pigments include inorganic pigments commonly used in the colouration of glasses. Such pigments are known to those skilled in the art. Methods for the manufacture of sealing pastes incorporating the glass compositions of the present invention, including particle sizes of components, mixing methods, and also methods for their use in sealing applications, are similarly known in the art.

The invention will now be described by way of example only.

EXAMPLE 1.

Glass Sample Preparation

Ammonium dihydrogen phosphate, tin (II) oxide and strontium carbonate were weighed out in the required quantities and dry mixed by tumble blending in a plastic container. Optional components were added prior to blending if required; alkali metals were added as carbonates and all other components as oxides. After mixing, the mixture was transferred to a platinum crucible and placed in an electric furnace. The mixture was heated at 30°C/minute to between 850 and 950°C and left to soak at this temperature for 30 minutes. Heating was in air and no lid was placed on the crucible. The glasses were then quenched by pouring onto a brass plate and dry ground to produce a frit powder of particle size < 20μm. Twenty-one glass samples were produced, the compositions of which (in mol%) are shown in Table 1.

Table 1.

EXAMPLE 2

Adhesion and Flow Testing

Samples of some of the glasses prepared above were evaluated for their ability to adhere to glass and flow on heating. Powdered glass was pressed into pellets and re-fired on a piece of float glass. The firing cycle used comprised a 10°C/minute ramp up to 420 or 440 °C followed by a dwell at temperature for 35 minutes, a ramp down at 2°C/minute to below the Tg (glass transition temperature) and finally, cooling to room temperature at 5°C/minute. Tg was measured using differential scanning calorimetry in air. Results are given in Table 2 below.

Adhesion to float glass was assessed and ranked from 1 to 3 where l=little or no adhesion, 2 = some adhesion, removed by firm finger pressure, 3 = strong adhesion to glass. It should be noted that the behaviour of the glasses on float glass is not necessarily directly comparable to the behaviour which would be found on e.g. a glass used in CRT manufacture. Float glass was used in the present experiments for convenience.

Glass flow was assessed visually and ranked from 1 to 3, where 1 = little or no flow, 2 = rounding of the pellet towards a semi-sphere , 3 = considerable flow, pellet has flattened out.

Table 2.

EXAMPLE 3 Phase Analysis by Powder X-ray Diffraction

Powder x-ray diffraction (Cu-k radiation) was used to determine phases present in some of the glass samples prepared in example 1 above. Glasses were analysed either as quenched, after re-firing at 440°C or under both conditions. Crystalline phases were identified by comparing measured data with JCPDS standard data. Results are given in table 3 below. Table 3.

(* - not measured, f - where a crystalline phase is indicated this is present as a minor phase in a largely amorphous major phase)

Claims

1. A lead-free glass composition, the composition comprising 35-55% P₂O₅, 10-55 % SnO, 2-25% SrO and 0-10% Li₂O as calculated in mole percent on an oxide basis.

2. A lead free glass composition, the composition comprising 35-55% P₂O₅, 10-55% SnO, and 10.1-25% SrO as calculated in mole percent on an oxide basis.

3. A composition according to claim 2 further comprising up to 15 mol% of Li₂O.

4. A composition according to any preceding claim, wherein the total amount of P₂O₅, SnO and SrO comprises at least 60 mol% of the composition

5. A composition according to any preceding claim further comprising a total of up to 15 mol% of R O; wherein R is Na, K or any mixture thereof.

6. A composition according to any preceding claim further comprising up to 5mol% ofAl₂O₃.

7. A composition according to any preceding claim further comprising one or more of the following components which when present, individually comprise up to 10 mol% of the glass; BaO, MgO, B₂O₃, Nb₂O₃, MoO₃, WO₃, ZrO₂, TiO₂.

8. A composition according to any preceding claim further comprising up to 25 mol% SiO₂.

9. A composition according to any preceding claim having a glass transition temperature of between 250 and 400°C.

10. The use of a glass composition according to any preceding claim as a sealing glass.

11. A sealing paste, the paste comprising at least one glass composition according to any of claims 1 to 9 and a vehicle.

12. A paste according to claim 11 further comprising at least one filler material and/or mill addition.