Process for sintering finely divided particulates and resulting ceramic products

Claim 2

IN A PROCESS FOR FORMING SINTERED CERAMIC STRUCTURE INCLUDING THE STEPS OF COMPACTING FINELY DIVIDED PARTICULATE MATERIAL WITH OR WITHOUT AN IGNITABLE BINDER TO FORM A GREEN STRUCTURE AND THEN HEATING THE GREEN STRUCTURE TO THE ONSET OF SHRINKAGE THE IMPROVEMENT COMPRISING SINTERING THE HEATED GREEN STRUCTURE THROUGH THREE SEQUENTIAL RATE REGIMES OF PROGRESSIVELY DECREASING DENSIFICATION RATES FROM SAID ONSET OF SHRINKAGE THE SAID DENSIFICATION RATE IN EACH REGIMES BEING BELONG THE CRITICAL RATE AT WHICH GAS ENTRAPMENT IS EFFECTED AND DISCONTINUOUS GRAIN GROWTH IS INDUCED THE THIRD OF SAID REGIMES BEING CONTINUALLY DECREASING AND THE FIRST AND SECOND OF SAID REGIMES BEING LINEAR SAID FIRST OF SAID REGIMES BEING DEFINED BETWEEN SAID ONSET OF SHINKAGE AND THE ATTAINMENT OF BETWEEN ABOUT 0.70 AND ABOUT 0.75 FRACTIONAL DENSITY SAID SECOND OF SAID REGIMES BEING DEFINED BETWEEN ABOUT 0.70 AND 0.75 FRACTIONAL DENSITY AND THE ATTAINMENT OF BETWEEN ABOUT 0.82 AND ABOUT FRACTIONAL DENSITY AND SAIDD THIRD OF SAID REGIMES BEING DEFINED BETWEEN ABOUT 0,82 AND ABOUT 0,87 FRACTIONAL DENSITY AND THE ATTAINMENT OF FINAL DENSITY.

Claim 3

A process in accordance with claim 1 wherein said finely divided particulate material contains up to 0.5% of an additive.

Claim 4

A process in accordance with claim 2 wherein said additive is a grain-growth inhibitor.

Claim 5

A process in accordance with claim 1 wherein said finely divided particulate material is alumina.

Claim 6

A process in accordance with claim 1 wherein said third of said regimes requires from about 50 to about 65% of the time devoted to said sintering from said onset of shrinkage to the end of sintering heat input.

Claim 7

a process for sintering finely divided ceramic particles to form ceramic structures, comprising the steps of a. precompacting finely divided ceramic particles to form a sinterable structure having a density of between about 48 and 63% of theoretical on a binder-free ignited basis; and b. thereafter subjecting the resulting sinterable structure to a sintering cycle comprising three sequential rate regimes of progressively decreasing densification rates from the onset of shrinkage, the said densification rate in each regime being below the critical rate at which gas entrapment is effected and discontinuous grain growth is induced, the first and second of said rate regimes being linear and the last of said rate regimes being one which is continually decreasing; said first of said regimes being defined between said onset of shrinkage and the attainment of between about 0.70 and about 0.75 fractional density; said second of said regimes being defined between about 0.70 and about 0.75 fractional density and the attainment of between about 0.82 and about 0.87 fractional density; and said last of said regimes being defined between about 0.82 and about 0.87 fractional density and the attainment of final density.

Claim 8

a process for sintering finely divided ceramic particles to form ceramic structures, comprising the steps of a. precompacting finely divided ceramic particles to form a sinterable structure having a density of between about 48 and 63% of theoretical on a binder-free ignited basis; and b. thereafter subjecting the resulting sinterable structure to a sintering cycle comprising three sequential rate regimes of progressively decreasing densification rates from the onset of shrinkage, the said densification rate in each regime being below the critical rate at which gas entrapment is effected and discontinuous grain growth is induced, the first and second of said rate regimes being linear and the last of said rate regimes being one which is continually decreasing; said first of said regimes being defined between said onset of shrinkage and the attainment of between about 0.70 and about 0.75 fractional density; said second of said regimes being defined between about 0.70 and about 0.75 fractional density and the attainment of between about 0.82 and about 0.87 fractional density; and said last of said regimes being defined between about 0.82 and about 0.87 fractional density and the attainment of final density.

Claim 9

A process in accordance with claim 6 including the step of milling said ceramic particles to eliminate agglomerates prior to said precompacting step.

Claim 10

A process in accordance with claim 6 including the step of adding an ignitable binder to said ceramic particles prior to said precompacting step.

Claim 11

A process in accordance with claim 6 including the step of incorporating up to about 0.5% by weight of an additive with said ceramic particles prior to said step of precompacting.

Claim 12

A process in accordance with claim 9 wherein said additive is a grain-growth inhibitor.

Claim 13

A process in accordance with claim 1 wherein said steps of precompacting and subjecting to a sintering cycle are carried out in an air atmosphere.

Claim 14

A process in accordance with claim 6 wherein the densification rate of said second of said regimes is between about one-fifth and one-half the densification rate of said first of said regimes and said third of said rate regimes is logarithmically decreasing.

Claim 15

A process in accordance with claim 6 wherein said third of said regimes requires from about 50 to about 65% of the time devoted to said sintering from said onset of shrinkage to the end of sintering heat input.

Claim 16

A process of forming a dense alumina structure of fine, uniform grain size, comprising the steps of a. precompacting finely divided alumina particles to form a sinterable structure having a density of between about 48 and 63% of theoretical on a binder-free ignited basis; and b. thereafter subjecting the resulting sinterable alumina structure to a sintering cycle comprising three sequential rate regimes of progressively decreasing densification rates from the onset of shrinkage, the said densification rate in each regime being below the critical rate at which gas entrapment is effected and discontinuous grain growth is induced, the first and second of said rate regimes being linear and the third rate regime decrEasing logarithmically; said first of said regimes being defined between said onset of shrinkage and the attainment of between about 0.70 and about 0.75 fractional density; said second of said regimes being defined between about 0.70 and about 0.75 fractional density and the attainment of between about 0.82 and about 0.87 fractional density; and said third of said regimes being defined between about 0.82 and about 0.87 fractional density of final density.

Claim 17

A process of forming a dense alumina structure of fine, uniform grain size, comprising the steps of a. precompacting finely divided alumina particles to form a sinterable structure having a density of between about 48 and 63% of theoretical on a binder-free ignited basis; and b. thereafter subjecting the resulting sinterable alumina structure to a sintering cycle comprising three sequential rate regimes of progressively decreasing densification rates from the onset of shrinkage, the said densification rate in each regime being below the critical rate at which gas entrapment is effected and discontinuous grain growth is induced, the first and second of said rate regimes being linear and the third rate regime decrEasing logarithmically; said first of said regimes being defined between said onset of shrinkage and the attainment of between about 0.70 and about 0.75 fractional density; said second of said regimes being defined between about 0.70 and about 0.75 fractional density and the attainment of between about 0.82 and about 0.87 fractional density; and said third of said regimes being defined between about 0.82 and about 0.87 fractional density of final density.

Claim 18

A process in accordance with claim 14 including the step of milling said finely divided alumina to eliminate agglomerates prior to said precompacting step.

Claim 19

A process in accordance with claim 14 including the step of adding up to about 0.5% by weight of MgO, Y2O3 or a mixture of MgO and Y2O3 to said alumina particles prior to said precompacting step.

Claim 20

A process in accordance with claim 14 including the step of adding an organic ignitable binder to said alumina particles prior to said precompacting step.

Claim 21

A process in accordance with claim 14 wherein said alumina particles range in size between about 0.1 and 0.25 microns and are characterized as having a specific surface area of from 6 to 11 square meters pre gram.

Claim 22

A process in accordance with claim 14 including the step of thermally treating the sintered structure at temperatures to render it transluent or transparent.

Claim 23

A process in accordance with claim 14 wherein the densification rate of said first of said regimes is no greater than 0.01 minute 1, and the densification rate of said second of said regimes is between about one-fifth and one-half the densification rate of said first of said regimes.

Claim 24

A process in accordance with claim 14 wherein said third of said regimes requires from about 50 to about 65% of the time devoted to said sintering from said onset of shrinkage to the end of sintering heat input.

Claim 25

A process in accordance with claim 14 wherein said first of said regimes is defined between the onset of shrinkage and the attainment of about 0.75 of theoretical density with a densification rate of about 0.0075 minute 1and said second of said regimes is defined between the attainment of about 0.75 and the attainment of 0.85 theoretical density with a densification rate of about 0.0025 minute 1.

Claim 26

A process of forming a dense ceramic structure of fine, essentially uniform grain size, comprising the steps of a. precompacting finely divided ceramic particles to form a sinterable structure having a density of between about 48 and 63% of theoretical on a binder-free ignited basis; b. prefiring said sinterable structure to the onset of shrinkage; c. thereafter subjecting the resulting prefired structure to a sintering cycle comprising three sequential rate regimes of progressively decreasing densification rates, the said densification rate in each regime being below the critical rate at which gas entrapment is effected and discontinuous grain growth is induced, the first and second of said rate regimes being linear and the third of said rate regimes being one which is continually decreasing; said first of said regimes being defined between said onset of shrinkage and the attainment of between about 0.70 and 0.75 fractional density; said second of said regimes being defined between about 0.70 and about 0.75 fractional density and the attainment of between about 0.82 and 0.87 fractional density; and said third of said regimes being defined between about 0.82 and about 0.87 fractional density and the attainment of final density; d. monitoring the change in temperature during said three sequential regimes thereby to obtain a time-temperature curve corresponding to the desired time-densification program; and e. using said time-temperAture curve to program the repetition of said process.

Claim 27

A process of forming a dense ceramic structure of fine, essentially uniform grain size, comprising the steps of a. precompacting finely divided ceramic particles to form a sinterable structure having a density of between about 48 and 63% of theoretical on a binder-free ignited basis; b. prefiring said sinterable structure to the onset of shrinkage; c. thereafter subjecting the resulting prefired structure to a sintering cycle comprising three sequential rate regimes of progressively decreasing densification rates, the said densification rate in each regime being below the critical rate at which gas entrapment is effected and discontinuous grain growth is induced, the first and second of said rate regimes being linear and the third of said rate regimes being one which is continually decreasing; said first of said regimes being defined between said onset of shrinkage and the attainment of between about 0.70 and 0.75 fractional density; said second of said regimes being defined between about 0.70 and about 0.75 fractional density and the attainment of between about 0.82 and 0.87 fractional density; and said third of said regimes being defined between about 0.82 and about 0.87 fractional density and the attainment of final density; d. monitoring the change in temperature during said three sequential regimes thereby to obtain a time-temperature curve corresponding to the desired time-densification program; and e. using said time-temperAture curve to program the repetition of said process.

Claim 28

A process in accordance with claim 23 wherein said third of said rate regimes is one which is logarithmically decreasing.