GB2263697A - Ageing-resistant glass for ultrasonic delay - Google Patents

Description

2263697 Ageincr - Resistant Glass for Ultrasonic Delay The invention

relates to a glass with high longterm stability of the acoustic properties for ultrasonic delay, which is suitable in particular for ultrasonic delay lines with excellent resistance to ageing.

Delay lines are required inter alia in radar direction-finding, control electronics, in electronic computers or also in television engineering. For example, they are used to convert an electrical transmission signal on the input side by piezoelectric means using a suitable transducer into a mechanical vibration, preferably an ultrasonic vibration. and to reconvert the acoustic signal obtained, after it has passed through a glass body, into an electric signal, again by piezoelectric means using a transducer. The reconverted electrical signal then possesses the desired delay with respect to the original electrical signal because the speed of propagation of the mechanical vibration in a solid body such as glass is approximately 4 to 5 powers of ten smaller than the electromagnetic waves. In view of the great difference in the speed of propagation, a long delay of an electrical signal can thus be achieved on small delay lines.

In order to prevent tone errors in the PAL colour television process, the chrominance signal of one line must be added to the chrominance signal delayed by 64 gsec of the next but one line.

The purpose of the process is to compensate fully for a phase error occurring in one direction by a signal of the same size but with a phase position rotated by 1800.

2 The purpose of the temperature-stable glass delay lines is to let the input electrical signal reappear at the outlet of the delay line only after the delay time of 64 gsec. To this end, the modulated colour signal is converted to an ultrasonic vibration which passes through the glass body as delay medium.

A further use of temperature-stable delay lines is the use as comb filters in PAL and NTSC colour television systems.

The delay time required in this case is about 64 microseconds with 625 scanning lines and a frequency of 50 hertz. At the frequency in question of 4.43 megahertz, the required band width of 5 megahertz and in view of the above-mentioned difference of about 4 powers of ten between the speed of propagation of sound waves in glass and the speed of propagation of electromagnetic waves in metal conductors, glass is outstandingly suitable as a medium for delay lines.

The glass must, however, fulfil simultaneously a whole bundle of requirements in terms of its properties, particularly with regard to the acoustic properties.

1.

The glass must not exceed a certain minimum attenuation otherwise the ratio of the signal transmitted to the signal received becomes too poor. The minimum attenuation not to be exceeded differs from application to application. In delay lines for delaying colour television signals, for example, the attenuation should not exceed a value of 0.07 dB/cm at 4.4 MHz.

2. The speed of propagation of the ultrasonic waves in the glass must lie within a certain interval and advantageously be adjustable in a controlled manner 3 within said interval. For the reasons described above, said range is 2, 400 to 2,600 m/s for the transmission of colour television signals. Moreover, the speed of propagation in the glass must be adjustable precisely within the given range by means of glass-specific parameters, for example, the composition of the glass.

3.

The delay of the signal in the glass over a given temperature range may not deviate from a nominal value by more than a given extent, otherwise the delay times would alter which, in the case of colour television, could involve a deterioration in the picture quality depending on the temperature. The change in delay of the signal in the glass as a function of the temperature of the glass over a temperature interval of 1VC to 6VC is characterised by the temperature coefficient z/T. According to presentday quality requirements, with a 64 gsec delay time for PAL colour television, a value of 5 nanoseconds over the relevant temperature range which ranges between +100C and +60'C, as indicated, must not be exceeded.

In the literature it is also customary to characterise the temperature coefficient as 1/z.dz/dt = TC, wherein z is the required delay time (i.e., usually 64 Rs for the present field of application), dz is the change in delay in the temperature interval in question (e.g., 5 nanoseconds) and dt is the relevant temperature range (about 50 OC). The values for 1/z. dzMT should lie approximately in the region of 2xl 0-6pC.

The value for z/T should be finely adjustable. As the ultrasonic transducers and solder layers used 4 which, together with the glass, are used to produce the delay line, likewise display a certain temperature dependence, the delay line medium used should likewise be adjustable to the transducers and solder used. To this end it is necessary, for example, to be able to vary the value for z/T in such a way that the delay of the signal either falls or rises with temperature. The fine adjustment of the value of z/T is possible advantageously with a medium such as glass in which the components of the composition and hence also the properties of the glass may be varied over a relatively large range.

5.

The processability of the glasses, particularly melting, must be sufficient for production in continuous units, and the glasses must have good crystallisation stability.

Some special glasses have now become known which fulfil the requirements described above to a greater or lesser extent.

DE-AS 15 96 908, for example, discloses glasses based on Si02-PbO-alkali oxides as delay medium f or acoustic delay lines, where the glasses have the following composition in mole percent: Si02 70-78, PbO 1530, where at most 5 mole percent PbO may be replaced by at least one of the oxides MgO, BaO, CaO and SrO, Na20+120 0-7, of which Na20 is at most 0.5, Sb203+ As203 at most 0.5.

Although it is to be assumed that the glasses according to the examples of DE-AS 15 96 908 all fulfil the requirements in respect of attenuation and speed of propagation, the properties concerning the ageing resistance cannot be derived from DE-AS 15 96 908. Moreover, for the illustrative glasses of DE-AS 15 96 908, a TC value of -4 to -5xl 0-6/OC is given, the value being about a factor of 2 higher than the required value of approx. 2xl 0-6 /OC for 1/z.dz/dT, i.e. the identical quantity.

Further glasses for delay lines in colour televisions or video recorders can be derived from published specification JP 88-166,735. The glasses according to the published Japanese specification are also of the Si02PbO-alkali oxide type. The glasses melted according to said publication have a 0.1 to 50% by wt. total content of the alkali oxides Cs2O+Li2O+ Na2O+K20+Rb20. where more specifically it is imperative that 0.1-50% by wt. CS20 be contained in the mixture. The glasses usually have a sufficiently adjustable speed of propagation for ultrasonic vibrations and also have a satisfactory coefficient for the temperature dependence of the speed of propagation. It can be deduced from the examples in said publication that in fact, with few exceptions, about 10-20% by wt. on average or even more CS20 is included in the melt. As caesium-containing raw materials are usually relatively expensive, the glasses presented have high mixture prices. At the same time, the composition ranges.of the glasses of the published Japanese specification extend over such large ranges that, when operations are carried out according to the JP patent, glasses may also be obtained that have high values for attenuation.

Moreover, glasses are obtainable commercially that are composed essentially only of the components SiO2, PbO and K20. Although said glasses do essentially fulfil the requirements in terms of attenuation, speed of propagation and temperature dependence of the speed of 6 propagation, both these and the glasses described above are on the whole subject to substantial ageing. As a rule, it is observed that the delay time of a delay line is subject to a certain decrease over time. For this reason, another requirement has to be met by the glass which is to be used successfully for delay lines:

For long-term use in electronic circuits and the like, the delay should hardly alter, otherwise premature replacement of the component will be necessary. The change in delay time with time (ageing) is measured directly on the whole line composed of glass, solders and transducers and characterised by a logarithmic time scale. A parameter for ageing is the change in the delay time per decade (10 years) z/t, the decade being standardised in days. A value of 2 nanoseconds per decade should not be exceeded.

In view of the state of the art discussed, the invention is based on the aim of providing glasses for ultrasonic delay which make it possible to comply with the narrow tolerances required by the electronics industry for the speed of propagation of sound and which have not only an improved temperature stability but.also an improved long-term stability of the speed of propagation of sound, where the glasses should simultaneously possess low attenuation and good crystallisation stability, so that production in continuous units is possible. Moreover, the mixture prices for the glass to be melted should be relatively low.

According to the present invention, there is provided an ageing resistant glass for ultrasonic delay, characterised by the following composition in % by wt.:

7 Si-02 B203 A1203 P205 Li20 Na20 K20 Alkali oxides Mgo CaO SrO BaO ZnO Alkaline earth oxides + ZnO La203 Ta205 W03 Zr02 Ti02 PbO F La203+Ta20_5+ W03+Zr02+TiO2+F - 50 0 - 10 0.1 - 3 0 - 2 0 - 1 0 - 1 1 - 5 - 5 0 2 0 2 0 2 0.5 0 - 2 0.5 - 5 0 2 0 2 0 2 0 2 0 2 55 0 1 0 - 2 and refining agent(s) up to at most 0.5% by wt.

Preferably, the glass has the following composition in % by wt., where the sum of the chosen constituents amounts to 100%:

8 S102 PbO K20 BaO A1203 42 48 47 53 2 4 1 3 0.1 - 1 and refining agent(s) up to at most 0.5% by wt.

Preferably also, the glass is characterised in that sound waves propagate in the glass at a speed between 2400 and 2600 m/s and that it has in the temperature interval from 10 'C to 60 'C a temperature coefficient of the delay 1/z.dzMT of less than 2 X 10-6 /OC.

The present invention also resides in the use of an ageing-resistant glass according to the present invention as a delay medium in an acoustic delay line which is preferably used for signal delay in a colour television set or a video recorder.

With the invention it is possible for the first time to provide glasses that are markedly improved in comparison with known glasses whilst simultaneously complying with all the given basic values for the production of ultrasonic delay lines, also in terms of ageing, i.e., the long-term stability of the propagation properties of ultrasound in the glass medium.

The glasses according to the invention and the glasses according to the state of the art are glasses of the Si02-PbO-alkali oxide type. The glasses according to the invention therefore have essentially between 40 and 50% by wt. Si02 and between 45 and 55% by wt. PbO. In a preferred embodiment, the glasses according to the invention contain 42-48% by wt. Si02 together with 47-53% by wt. PbO. The variation in the Si02 and PbO proportions 9 in the glass system advantageously makes it possible, inter alia to adjust the ultrasonic delay glasses of the invention to the transducers and solders used in ultrasonic delay lines.

As a necessary constituent. the glasses according to the invention contain 1-5% by wt. alkali oxides, where the sum of the alkali oxides contained in the mixture may be composed of 0-1% by wt. Li20. 0-1% by wt. Na20 and 1-5% by wt. K20. In a particularly preferred embodiment, the glass according to the invention contains only K20 in a quantity of between 2 and 4% by wt., based on 100% by wt. glass quantity. In comparison with the glasses according to the state of the art, the glasses according to the invention have a markedly reduced alkali content. In relatively high contents, the alkali oxides in the glass system have a negative effect on the ageing behaviour but reduce the attenuation of the glass. Contents of more than 5% by wt. alkali oxide lead to ageing values which can no longer be tolerated. If, in contrast, the alkali content is reduced below the minimum value according to the invention of 1% by wt., the ageing behaviour improves but the attenuation of the glass reaches values that are no longer satisfactory. Of the alkali oxides, K20 is particularly preferred because it has a particularly advantageous effect on the processing properties of the melt and because the glass in a mixture of several alkalis may be susceptible to unwanted, abrupt changes in its properties (so-called "mixed alkali effect"). Replacing K20 by up to 1% by wt. Na20 or up to 1% by wt. Li20 is, however, possible without disadvantages.

As a further essential constituent, a glass according to the invention contains a quantity of between 0.1 and 3% by wt. A1203. where in a particularly preferred embodiment the quantity of A1203 does not exceed 1% by wt.

In addition to the reduction in the alkali content which is particularly preferred for the ageing of the glass, the invention has brought the surprising discovery that additions of small quantities of A1203 likewise have a positive effect on the ageing properties. The quantity of A1203 may obviously not be too small in order to achieve the effect, i.e., it may not be less than 0.1% by wt. If, on the other hand, a content of 3% by wt. A1203 is exceeded, the values for the attenuation of the glass increase to an extent that can no longer be tolerated and, as a further disadvantage, higher A1203 contents entail an increase in the energy consumption during production because the melting point of the glass increases as the A1203 content rises and increasingly high temperatures have to be used to melt the glasses.

A further essential component for the glasses according to the invention is barium oxide which is contained in the glass according to the invention in a quantity of between 0.5 and 5% by wt., in a preferred embodiment in a quantity of between 1 and 3% by wt. Barium oxide is essentially added to improve the processing properties of the glass in the continuous units. If less than 0.5% by wt. barium oxide is added, the quantity is no longer sufficiently effective for the desired purposes. At the same time, barium oxide in the range according to the invention has a positive effect on the ageing resistance of the glass. If, however, the range according to the invention is exceeded, the devitrifying tendency of the glass increases and the production capability of the glasses suffers. Except for 0.5% by wt., the quantity of barium oxide can be replaced up to the upper limit of 5% by wt. by other divalent glass oxides, for example magnesium oxide between 0 and 2% by wt., calcium oxide between 0 and 2% by wt., strontium oxide between 0 and 2% by wt., or also zinc 11 oxide between 0 and 2% by wt., without obtaining disadvantages in the glasses.

As non-essential constituents, the glasses according to the invention may have either La203, Ta205, W03, Zr02, TiO2 and/or F up to an upper limit of a total of 2% by wt., where each of the constituents listed may be contained in the glass according to the invention up to an upper limit of 2% by wt. with the exception of F, whose content should be a maximum of 1% by wt. because with higher contents the glass will possibly become cloudy, but the total sum of the components listed may likewise not exceed 2% by wt. The rare earth oxides listed, like the other glass components generally known in glass engineering, may be added up to the maximum content according to the invention without disadvantages being expected for the properties of the glasses. As said components increase the mixture price considerably, however, the addition of said components is dispensed with in a preferred embodiment.

As the last constituent, finally, the glasses according to the invention may contain the customary refining agents known in glass engineering up to an upper limit of a maximum of 0.5% by wt. As the glass according to the invention is refined with refining agents known in the state of the art, where the refining agents e.g., Sb203 and nitrates, sulphates and others may normally be used, their remaining in the glass according to the invention up to the upper limit according to the invention cannot be excluded. It is assured, however, that the refining auxiliaries listed as well as the generally known refining auxiliaries do not adversely affect the properties of the glass, particularly the acoustic properties.

12 A glass melted only from the components S102. PbO, K20, BaO and A1203 according to the preferred embodiment with the above-mentioned preferred proportions of said substances in the glass and having up to a maximum of 0.5% by wt. of the customary refining agents is characterised quite particularly advantageously in conjunction with suitable solders and transducers by a sound wave propagation speed of 2400 to 2600 m/s and a temperature coefficient of the delay 1/z.dzMT in the temperature interval 10 OC to 60 OC of less than 2xl 0-6 /OC.

Moreover, it is preferred that the ageing-resistant glasses in accordance with the invention normally have a change in delay time per decade of less than or equal to 2 nanoseconds.

The invention is explained in more detail below on the basis of glasses melted by way of example and the acoustic properties measured.

Table 1 gives the composition of 5 glasses melted by way of example in accordance with the invention and of one glass known as NN which corresponds to the state of the art. Moreover, for each of the 5 illustrative glasses and for the known glass NN, table 1 contains the results of measurements of the speed of propagation of sound in the glass in metres per second, the density of 3 the glass in grammes per cm and details about the delay coefficient, which describes the change in delay z as a function of the temperature T in an interval between +10 OC and + 60 OC.

The glasses according to the invention are produced as follows:

13 The raw materials (oxides, carbonates etc.) are weighed out and mixed well. The mixture is melted at 1,480 OC, then refined and homogenised'well. Casting takes place at about 1,480 OC in pre-heated moulds, it being possible therefore to produce the glass in production in continuous melting tanks.

The speed of propagation of sound in the finished glass bodies is determined in the following way:

Test specimens are cut from the test melts, ground and milled. A specimen is obtained with a reflection of ultrasound. Metallising of the glass surfaces onto which the piezoelectric transducers are applied is normally carried out by treatment in reducing solutions and also by vacuum evaporation or sputtering of metal layers. Piezoelectric transducers made of the material Sonox SP-2 were soldered onto the metallised glass surfaces.

Measurement of the absolute delay time and the change in delay time with temperature were carried out with the customary methods of measurement. The alternating current signal generated by a generator is transmitted at the frequency of 4 433 618 Hz once directly and once through the delay line to be tested. Both the incoming signals are recorded by a vector voltmeter and their phase difference measured. From a knowledge of the exact geometry of the test parallelepiped, exact frequency measurements and a knowledge of the number of periods in the delay line, the absolute delay time and the change in delay time with temperature can be calculated.

In order to cover the temperature range from 10 to 60 'C, the glass bodies are attached to a temperaturecontrollable test sample holder.

14 Table 1 % by wt. 1 2 3 4.5 NN Si02 46.33 45.32 45.69 45.83 45.76 48.26 PbO 48.32 49.33 48.50 M. 83 48.81 43.53 K20 3.40 3.40 2.88 3.39 3.45 8.20 BaO 1.80 1.80 2.33 1.80 1.83 - A1203 0.15 0.15 0.61 0.15 0.15 v (m/S) 2522.6 2501.6 2528.5 2510.8 2509.5 2515 (g/CM3) 3.73 3.78 3.76 3.75 3.75 3.48 Z/T in ns (10-60'C) <5 <5 <5 1/z.dzMT X10-6/OC 1.2 (10-60-C) <5 1.2 0.8 1.0 1.0 <5 <5 The results of the measurements for some of the illustrative glasses and for a known glass are reproduced graphically in figures 1 and 2. In the figures, Figure 1 shows a graph of the change in the delay time in nanoseconds as a function of temperature in 'C in the temperature interval between +10 and +60 'C in 101 stages for the glasses 1 to 3 melted by way of example; and Figure 2 shows a semi-logarithmic graph of the change in the delay time in nanoseconds plotted against ageing in days for the three illustrative glasses 1 to 3 and a known glass NN, in the graph example 4 from table 1.

In the case of illustrative glasses 1 to 3, the ratio of Si02 and PbO varies in the composition range of is the invention. In figures 1 it can be seen that, depending on the SiO2 and PbO content at one and the same temperature, the change in the delay time z/T may be varied without an upper limit for z/T of 5 nanoseconds being exceeded in the chosen temperature interval between +10 and +60 OC, so that the ultrasonic delay glass may advantageously be adjusted to certain transducers by means of the variation of Si02 and Pbo.

Figure 2 illustrates how clearly the additions according to the invention of A1203 and BaO and the reduction in the K20 content in the glass according to the invention affect the ageing resistance of the glasses. In addition to the change in delay of examples 1 to 3, the change in delay of a known glass from table 1 is also plotted. It is clear from the graph of figure 2 that the known glass is subject to markedly greater ageing than the glasses according to examples 1 to 3.

The examples described above have shown that with glasses according to the invention which are based on the glass system Si02-Pbo-alkali oxide, it is possible, by adding small quantities of A1203 and reducing the alkali content. to obtain glasses that possess not only the required sound speed with sufficiently low attenuati on but also a low temperature coefficient z - T with an ageing resistance that is simultaneously markedly increased in comparison with the glasses according to the state of the art.

Consequently, an ageing-resistant glass according to the invention is used to particular advantage as delay medium in an acoustic delay line. In particular, the glasses melted in the examples and the glasses melted according to the preferred embodiments of the invention are outstandingly suitable for said use. The acoustic 16 delay lines produced with a glass according to the invention as delay medium are particularly advantageous for use for signal delay in colour teldvision sets or video recorders.

17

Claims (5)

1. An ageing-resistant glass for ultrasonic delay, characterised by the following composition in % by wt.:

   Si02 B203 A1203 P205 Li20 Na20 K20 Alkali oxides Mgo CaO SrO BaO ZnO Alkaline earth oxides + ZnO La203 Ta205 ZrO2 Ti02 PbO F La203+Ta205+ 0 - 10 0.1 - 3 0 - 2 0 - 1 0 - 1 1 - 5 1 - 5 0 2 0 2 0 2 0.5 - 5 0 - 2 0.5 - 5 0 2 0 2.

   0 2 0 2 0 2 - 55 0 - 1 0 - 2 18 and refining agent(s) up to at most 0.5% by wt.
2. 2. An ageing-resistant glass according to claim 1, characterised in that it has the following composition in % by wt., where the sum of the chosen constituents amounts to 100%:

   Si02 PbO K20 BaO A1203 42 - 48 47 - 53 2 - 4 1 - 3 0.1 - 1 and refining agent(s) up to at most 0.5% by wt.
3. 3. An ageing-resistant glass according to claim 2, characterised in that sound waves propagate in the glass at a speed between 2400 and 2600 m/s and that it has in the temperature interval from 10 OC to 60 OC a temperature coefficient of the delay 1/z.dzMT of less than 2 X 10-6/C)C.
4. An ageing-resistant glass according to any one of claims 1 to 3, characterised in that the change in the delay time per decade is less than or equal to 2 nanoseconds.
5. 5.

   An ageing-resistant glass according to claim 1, substantially as hereinbefore described in any one of Examples 1 to 5.

   The use of an ageing-resistant glass according to any one of the preceding claims as a delay medium in an acoustic delay line.

   19 The use according to claim 5, characterised in that the acoustic delay line is used for signal delay in a colour television set or a video recorder.