Micro-fine Alumina powder on ebay.com

The secret to amazing refractories

$Micro-fine Alumina powder on ebay.com The secret to amazing refractories$

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How to make lightweight ceramic refractories - The answer is on a chatbot and ebay.com.
Here is an example of the chatbot output. There is much useful info here, just apply some common sense. There are a few proven ways to make lightweight alumina refractories (insulating firebricks, castables, boards, and linings) for high-temperature furnaces. Below I’ll give you the principles, tradeoffs, a couple of concrete workshop recipes (one insulating castable, one fiber board / lightweight block), step-by-step processing, expected properties, and safety notes so you can choose a route that fits your furnace temperature and mechanical needs. Quick overview — design choices & tradeoffs What “lightweight” means: lower bulk density (≈0.3–1.6 g/cm³) vs dense refractories (2.4–3.9 g/cm³). Lower density → much lower thermal conductivity and heat stored, but reduced mechanical strength, abrasion resistance, and sometimes chemical resistance. Max service temperature: depends on alumina source and binder. For service ≲1400–1500°C you can use calcium-aluminate cements or phosphate binders; for 1500–1750°C you need high-alumina aggregates (tabular/fused alumina) and binders that don’t soften (or rely on ceramic bonding after high temp). Lightweighting strategies: add ceramic hollow spheres / alumina bubbles (preformed, good high-temp performance), use organic sacrificial pore formers (EPS beads, starch, sawdust) and burn them out, leaving porosity, use expanded natural minerals (perlite, vermiculite) — lower cost but lower max temp, use ceramic fiber (Al₂O₃–SiO₂ blankets / fibers) or fiber reinforcement for boards, produce foamed castables (gas-forming chemical foaming agents or mechanical foaming / gelcasting). Binders: phosphate (acidic) binders, calcium aluminate cement (CAC), or refractory colloidal silica/sols. Choice affects setting, strength, and max temp. For highest temps use fusion of alumina aggregates and minimal low-melting binders. Recipe A — Insulating alumina castable (workshop scale) Good for furnace linings up to ~1500°C (adjustable). Uses alumina bubbles for low density. Typical properties target: bulk density ≈ 0.6–1.2 g/cm³, thermal conductivity low, compressive strength moderate. Ingredients (by weight, example batch ~10 kg solids): Tabular/fused alumina coarse (T-64 / 90 µm–1 mm) — 3.5 kg Reactive alumina (fine, 1400°C choose fibers and binders rated for that temp (some organic binders will burn out). Result: flexible or rigid board with very low density (0.1–0.8 g/cm³) and very low thermal conductivity, excellent for kilns as backup insulation but mechanically fragile. Alternate / low-cost approaches Expanded perlite / vermiculite + high-alumina binder: cheap and easy but max temp reduced (~1200–1300°C typical). EPS or starch sacrificial pore former: mix polymer beads into mix, cast, then burn beads out — creates lots of porosity but must control pore size and avoid collapse during drying/firing. Foamed castables: chemically foam the slurry (hydrogen evolution, surfactants) for very low density; requires careful control and stabilizers. Processing & firing schedules (typical guidance) Drying: slow. 20–24 h at 60–100°C, increasing slowly to avoid cracks. Organics burnout: hold at 300–500°C to remove organics. Ramp rate 50°C/h or slower if thick sections. First high-temp exposure / sinter: for phosphate/CAC systems you may not need full sinter; otherwise consider a 1200–1600°C soak if using reactive alumina to densify bonding (but this reduces porosity/insulation). Ramp rates: controlled—avoid >200°C/h through critical ranges where bound water evolves (100–600°C). For thick/large items slow to prevent internal cracking. Expected properties & numbers (approximate) Low-density insulating castable with alumina bubbles: density 0.6–1.3 g/cm³, thermal conductivity 0.2–0.8 W/m·K (depending on temp), compressive strength 2–20 MPa. Service temp up to ~1500°C if components are high-alumina. Fiber boards: density 0.1–0.8 g/cm³, very low conductivity 0.05–0.4 W/m·K, low strength, service temp depends on fiber chemistry (1400–1600°C common). (These are ballpark figures — exact values depend on formulation and porosity.) Practical tips & troubleshooting Protect hollow spheres: handle gently; they’re fragile — add them late and stir minimally. Avoid steam blowouts: dry slowly and vent molds; excess water in thick sections causes spalling. Watch binder choice: phosphate binders set fast and give good green strength; CAC needs moist cure. For very high temp, minimize volatile/low-melting phases. Thermal shock resistance: add fibers or small amounts of mullite aggregates to improve. Quality control: cut small test specimens and measure bulk density, cold-crush strength, and do a first heat cycle to check for cracking or collapse. Safety Dust control: alumina and reactive alumina are very fine powders — use NIOSH P2/P3 or N95+ respirator, local exhaust ventilation, dust containment. Binders: some phosphate solutions are acidic — handle with gloves/eye protection. CAC dust is alkaline; handle accordingly. Burnout fumes: burning organics (EPS, binders) can release hazardous vapors — ensure proper ventilation or use a controlled kiln with venting. High-temp hazards: standard kiln/furnace safety — hot surfaces, insulation handling precautions. Final notes & suggestions If you need a specific target (service temp, desired density, geometry, mechanical load), tell me that and I’ll give a tighter recipe and precise binder choices and firing schedule. Sourcing: alumina bubbles and tabular alumina are specialty materials — prices vary; ceramic fiber and colloidal silica are more widely available. Small-scale testing: always make small test blocks (vary bubble content 10–40 vol%) to find the density/strength balance that works for your furnace.

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