Lab 102-A — Grow Single Crystals (Alum / Copper Sulfate)
Goes with: Units 3, 4, 6, 9. Time: 1 h setup + 1–3 weeks of growth (10 min/day). Cost: about \(35–\)70 (estimate).
Goal
Grow one large single crystal from a seed by slow cooling/evaporation, and compare with a fast-cooled "polycrystalline crust". Observe facets, measure growth rate, and relate everything to Czochralski crystal growth and thin-film nucleation.
What you will learn
- Nucleation vs growth; supersaturation as the driving force.
- Why slow, controlled growth from a seed gives one crystal (the Cz principle), while fast cooling gives many grains.
- Crystal habit: alum grows octahedra bounded by {111} faces — the same plane family that forms texture pyramids on silicon.
- Defects you can see: inclusions, twinning, spurious nucleation.
Shopping list
| Item | Spec / example | Qty | Approx. cost (estimate) | Where |
|---|---|---|---|---|
| Potassium alum (KAl(SO₄)₂·12H₂O) | food-grade "alum" (e.g., McCormick, spice aisle) or 500 g–1 kg bag | 200–500 g | $8–15 | grocery, Amazon |
| Optional copper(II) sulfate pentahydrate | e.g., 99 % "root killer" grade (check label: copper sulfate pentahydrate) or lab grade | 250–500 g | $10–15 | garden store, Amazon |
| Glass jars | 500 mL, straight-sided, with lids (not used for food afterwards) | 3–4 | $10 | hardware store |
| Fishing line (nylon, thin) + pencils/skewers | — | — | $3 | household |
| Digital kitchen scale (1 g) | — | 1 | $15 | Amazon |
| Thermometer | 0–100 °C | 1 | $8 | Amazon |
| Small insulated cooler or foam box | to slow cooling | 1 | $10 | grocery |
| Coffee filters, nitrile gloves, safety glasses | — | — | $10 | household |
| Magnifier / phone macro lens, protractor | — | — | $10 | Amazon |
Safety
- Alum is low hazard (astringent; avoid eyes; don't eat large amounts).
- Copper sulfate is harmful if swallowed, irritating to eyes/skin, and very toxic to aquatic life: wear gloves and glasses, use dedicated jars and utensils, keep away from children and pets, and do not pour down drains — evaporate leftovers and dispose of solids per local hazardous-waste rules. Read its SDS first (good practice for Unit 11).
- Hot solutions: use oven mitts; don't heat sealed jars.
Procedure
Part 1 — Saturated solution. Heat ~300 mL water to ~70–80 °C (kettle). Stir in alum a spoonful at a time until no more dissolves (roughly 20–30 g per 100 mL at this temperature; approximate). Filter through a coffee filter into a clean jar.
Part 2 — Seeds. Pour a thin layer of the solution into a shallow dish and leave overnight. Small, clear, well-shaped octahedral crystals form — pick the best (3–5 mm) as seeds.
Part 3 — Fast vs slow (the key comparison). - Fast: put one jar of hot saturated solution in the fridge. Within hours, a crust of many small crystals forms — polycrystalline. - Slow: tie a seed with a slip knot on fishing line, suspend it in a fresh jar of solution that has cooled to about 2–3 °C above room temperature (if the solution is too warm the seed dissolves). Put the jar in the insulated cooler, cover with paper (dust), and leave undisturbed.
Part 4 — Grow. Every 1–2 days: measure the crystal (calipers or ruler with a photo), remove any spurious crystals from the line and jar walls (they compete for material), and occasionally transfer the crystal to fresh saturated solution. Log date, room temperature, size.
Part 5 (optional) — Copper sulfate. Repeat with copper sulfate, which grows triclinic blue crystals of a different habit.
What to measure
- Crystal size (edge length or mass) vs time → growth rate (mm/day).
- Angles between facets: an ideal octahedron has 109.47° between adjacent faces (the dihedral angle) and 70.53° as its supplement; measure with a protractor on a photo.
- Number of crystals formed in fast vs slow jars.
- Visible defects: cloudy inclusions, twinned or intergrown crystals, step patterns on faces.
Expected results
- Fast cooling: dozens to hundreds of small crystals (high supersaturation → many nuclei).
- Slow growth from a seed: one crystal reaching ~1–3 cm in 1–2 weeks (estimate; depends on temperature stability).
- Clear octahedra with flat {111} faces when growth is slow; cloudy or skeletal crystals when growth is fast or temperature fluctuates.
- Growth slows as the solution depletes; refreshing solution restarts it.
How to interpret
| Observation | Solar-manufacturing analogue |
|---|---|
| Seed + low supersaturation → one crystal | Czochralski: a ⟨100⟩ seed dipped into a melt held just at the freezing point; slow, controlled pulling keeps one crystal |
| Fast cooling → many grains | casting multicrystalline ingots; rapid thin-film deposition giving small grains |
| Spurious crystals on walls/line | unwanted nucleation (e.g., on crucible walls); "loss of structure" in Cz |
| Inclusions from temperature swings | growth-rate fluctuations causing defects (striations, voids) |
| {111} octahedral faces | slow-growing/slow-etching {111} planes → pyramid texture on (100) silicon |
Extensions
- Add a little food dye: does it get incorporated into the crystal or rejected to the solution? (Segregation, Unit 6.)
- Grow at two controlled temperatures and compare growth rates (kinetics, Unit 7).
Sources
- Holden & Morrison, Crystals and Crystal Growing (classic hobby/teaching text).
- Copper sulfate SDS (any supplier); alum SDS.
- Octahedron geometry: dihedral angle arccos(−1/3) = 109.47°.