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Archive Article: Spodumene for Low Thermal Expansion

Table1

 

Clay bodies suitable for use in thermal shock situations require low or zero coefficients of thermal expansion. When spodumene (Li20 Al2035Si02) is included in the body, it has the effect of altering thermal expansion behavior.Spodumene occurs naturally as a monoclinic pyroxene known as alpha spodumene. When heated to above 1082°C (1980°F), it undergoes an ir­reversible volume increase of approxi­mately 30% to form the tetragonal phase known as beta spodumene.Within the past decade, a large de­posit of spodumene ore in Western Aus­tralia was opened for commercial exploitation. It is claimed that this de­posit is the largest source of high-grade spodumene in the world. The com­pany, Gwalia Consolidated, Ltd., pro­vided samples for the following tests:Plastic Body TrialsAnalyses of spodumene ores from this deposit are detailed in Table 1, but because of time constraints only alpha spodumene concentrate was selected for this study.Four clays of very different charac­ter were selected for blending trials. Adialyses of these clays are also given in Table 1. Two blending ratios were used for each clay, namely 40% spodumene and 50% spodumene by weight, pro­ducing eight test bodies.Two additional bodies were included in these trials: Body 9, a minor varia­tion on Body 1; and Body 10, a synthet­ic bone china body without spodumene (42% synthetic bone ash, 37% potash feldspar, 21 % kaolin and 2% Ben tone 38). Table 2 lists the compositions of the other test bodies.The individual compositions were batched, then ball-milled with water for three hours. The milled body was then de-watered by pouring it into plas­ter molds. Upon reaching a plastic state, it was removed and stored in a sealed container to age for one week.

Next, half of each body was rolled into a slab of 10mm (less than ½ inch) thickness and cut into test bars of 30 x 130mm (approximately 1 x 5 inches), which were immediately marked at 100mm centers and allowed to air dry. When fully dry, they were measured to determine shrinkage.The remaining half of the body was evaluated for its throwing properties using a potters wheel. Two standard bowl shapes, 120mm in diameter and 80mm in height, were made from each body. When leather hard, one bowl from each body was burnished to seal the surface and evaluate any benefits.All the test bowls and bars (with the exceptions of B3 and B7) were fired together in a natural gas-fired kiln of 1- cubic-meter capacity to a temperature of 1300°C (2372°F). Firing atmosphere was maintained at neutral throughout the firing using an oxygen probe. Total firing time was 6 hours. Bodies B3 and B7 were fired to 1140°C (2084°F) in an electric kiln.When the bars were cool, tests to measure the clays physical properties (including drying and firing shrinkage, flexural strength, water absorption, ap­parent porosity, bulk density and ther­mal expansion characteristics) were con­ducted. Five bars were tested for all bodies and results averaged.

Slip-Casting Body Trials
A terra-cotta body was selected as the basis for evaluating the influence of spodumene on casting slips. A line blending procedure was used to pro­vide bodies with 1% increments in spodumene up to a total of 10%.

Eleven casting bodies were prepared and allowed to age for three days. The slip-casting body compositions are listed in Table 3. Slips were individually poured into plaster molds. Casting time was 10 minutes, after which all molds were emptied and incline-drained for an additional 10 minutes. The castings were removed from the molds after 24 hours, air dried, then collectively fired to 1140°C in an electric kiln.

Measurements were then taken for drying and firing shrinkage, water ab­sorption and thermal expansion char­acteristics.

Plastic Body Results
A summary of all physical test re­sults is presented in Table 5. Drying shrinkages were not excessive and all fell within the range 3%—7%; how­ever, firing shrinkages were unusual. Indeed, there were firing expansions in seven cases—the result of the irrevers­ible expansion of alpha to beta spod­umene. The bone-china body BIO shows a typically high-firing shrinkage and contrasts the unusual behavior of alpha spodumene.Flexural strength (modulus of rup­ture) was acceptable for stoneware ap­plications. Most of the compositions showed strengths in this region. Bodies B5 and B8 were the strongest of the spodumene compositions.All the bodies had high water-ab- sorption figures, demonstrating the ef­fect of the volume expansion of alpha spodumene at 1082°C. The lowest ab­sorption was obtained from body B8.

Apparent porosity results were also high, the lowest porosity being obtained from body B8.Bulk density results were low, as would be expected from bodies with high porosity values. The highest result was obtained from body B8.The bodies achieved low and in most cases linear rates of expansion. The low­ est thermal expansion was achieved by body B4 with a coefficient of expan­sion of 1.88 X 10E-6 cm/cm between 20°C and 800°C (68°F and 1472°F).Subjective evaluations of the throw­ing and burnishing behavior of the plas­tic bodies are presented in Table 4. The best bodies for throwing were Bl, B5 and B9, all containing ball clay, BBR and spodumene. Addition of the com­mercial plasticizer Bentone 38 improved the properties required for throwing.Slip-Casting Body ResultsDrying shrinkage values for all slip- casting bodies was 8% of dry length. The spodumene content had no no­ticeable effect.Firing shrinkage for all slip-cast bod­ies was 15% of dry length. Again, the spodumene had no noticeable effect.Water absorption steadily decreased from 4.93% to 0.66% with increasing spodumene content. The fluxing ac­tion of the lithium in the spodumene is clearly beneficial.The thermal expansion of the base terra-cotta casting slip was progressively decreased with increasing spodumene content. The quartz anomaly at 573°C (1063°F) was notably reduced.ApplicationsAs a modifier of existing bodies, spodumene can provide better thermal expansion characteristics and reduced porosity. In this study, the addition of only small quantities of spodumene toa terra-cotta body reduced the porosity. Also, the sensitivity to rapid firing and cooling cycles was improved, thus per­mitting a faster rate of cooling and consequently a quicker kiln turnaround.Furthermore, the addition of only small quantities of spodumene could assist in the precise lowering of the thermal expansion of a body to achieve a better fit between the body and some glazes. It would be of particular benefit where glaze shivering was a problem.The unusual firing expansion en­countered with alpha spodumene bod­ies could be useful in the production of products requiring zero or minimal di­mensional change in firing; for example, prototypes for mold making.Another suitable application would be for raku firing. Raku trials using spodumene-based bodies have demon­strated that they are superior to the conventional heavily grogged fireclay bodies. The loss rate was less, the bod­ies were structurally sound and, in many instances, could be used functionally. Another benefit was the improved glaze colors, which was attributed to the pres­ence of lithium compounds in the body.This color enhancement was also achieved from the same spodumene bodies without any glaze when they were in a wood-firing kiln together with a range of conventional bodies.

The author M. Murray is a senior lecturer in ceramics at Monash Univer­sity in Victoria., Australia.

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